Electron Spin Resonance. Vol. 9. 9780851868318, 0851868312

565 120 17MB

English Pages [402]

Report DMCA / Copyright

DOWNLOAD FILE

Polecaj historie

Electron Spin Resonance. Vol. 9.
 9780851868318, 0851868312

Table of contents :
BK9780851868318-FX001
BK9780851868318-FP001
BK9780851868318-FP005
BK9780851868318-FP007
BK9780851868318-00001
BK9780851868318-00016
BK9780851868318-00087
BK9780851868318-00139
BK9780851868318-00166
BK9780851868318-00223
BK9780851868318-00246
BK9780851868318-00291
BK9780851868318-00324
BK9780851868318-00361

Citation preview

Electron Spin Resonance Volume 9

A Specialist Periodical Report

Electron Spin Resonance Volume 9 A Review of the Literature Published between December 1982 and May 1983 Senior Reporter P. B. Ayscough, Department of Physical Chemistry, University o f L eeds Reporters N. J. Blackburn, UMIST, Manchester Ching-San Lai, Medical College of Wisconsin, Milwaukee, US.A. N. J. F. Dodd, Christie Hospital, Manchester D. Gatteschi, Universita degli Studi di Firenze, Italy D. J. T. Hill, University of Queensland, Brisbane, Australia A. Hudson, University of Sussex T. J. Kemp, University of Warwick J. H. O'Donnell, University of Queensland, Brisbane, Australia P. J. Pomery, University of Queensland, Brisbane, Australia M. C. R. Symons, University of Leicester B. J. Tabner, University of Lancaster

The Royal Society of Chemistry Burlington House, London, W l V OBN

ISBN 0-85 186-831-2 ISSN 0305-9578

Copyright 0 1985 The Royal Society of Chemistry

All Rights Reserved No part of this book may be reproduced or transmitted in any form or b y any means - graphic, electronic, including photocopying, recording, taping or information storage and retrieval systems - without written permission from The Royal Society of Chemistry

Printed in Great Britain at t h e Alden Press, Oxford, London and Northampton

Foreword

The series of Specialist Periodical Reports continues to evolve in response to changes in emphasis in chemical research and to economic pressures. The volumes concerned with e.s.r. are not exempt from these pressures and most of the changes in Volume 9 reflect decisions based on financial constraints rather than academic judgement. The overall size of the volume has been slightly reduced and some chapten have been omitted on this occasion in order to achieve this. In particular, to my considerable regret, it has not been possible to include a special review chapter of the kind which has appeared in the last five volumes. Despite these lorages I believe that the quality of the contributions to Volume 9 will ensure that it provides a unique and comprehensiveaccount of recent developments in e.s.r. I am once again most grateful to all Reporters for their splendid contributions. October 1984

P. B. Ayscough

Contents Chapter 1

Theoretical Aspects of E.S.R. By A. Hudson

1 Introduction 2 Numerical Methods and Spectral Analysis 3 Spin-relaxationand Lincbroadening Effects 4 cmm 5 Applications of Quantum Chemistry References Chapter 2

1 1 3 7 9 12

Transition-metal Ions By D.Gatteschi

1 Introduction 2 General Experimental Techniques Ligand Field and Molecular Obital Models Jahn-Teller Spin Hamiltonian, Analysis of Spectra, and Computing Oligonuclear Complexes Metal Ion-Organic Radical Interactions Mixed Valence Magnetic Materials Conductors Phase Transitions Application to Mineral Systems and Glasses Linewidths and Relaxation Studies 3 s=i d 1 Configuration Tervalent Titanium Tetravalent Vanadium, Niobium, and Tantalum Quinquevalent Chromium, Molybdenum, and Tungsten d5 Configuration Tervalent Iron, Ruthenium, and Osmium, Tetravalent Cobalt d7 Configuration Bivalent Cobalt, Rhodium, and Iridium and Tervalent Nickel, Palladium, and Platinum d9 Configuration Bivalent Copper and Silver Univalent Nickel, Palladium, and Platinum

4 S=Q

d3 Configuration Tervalent Chromium, Molybdenum and Tungsten Divalent Vanadium Quadrivalent Manganese and Rhenium d5 Configuration Tervalen t Iron d7 Configuration Bivalent Cobalt Vii

16 17

17 20 21 23 24

30 32 33 36 37 40 43 44 44 44

45 50 52 52 53

53 58

58 62 63 63 63 65 65

65 65 66 66

...

Con tents

Vlll

5 S=$ Univalent Chromium Bivalent Manganese References Chapter 3

67 67 67 70

Inorganic and Organometallic Radicals By M . C. R . Symons

1 Introduction 1.1 Books and Reviews 1.2 Techniques 2 Trapped and Solvated Electrons 2.1 Electrons in Solvents 2.2 FCentres 3 Monatomic Radicals and Clusters 3.1 Trapped Hydrogen Atoms 3.2 Helium Cations 3.3 Chlorine Atom Centres 3.4 Group 1 Metal Atoms and Clusters 3.5 Other Metal Atoms and Clusters 3.6 Lead Cations and Anions 3.7 Metal Atom-Hydrocarbon Complexes (i) Complexes with ethylene and acetylene 4 Diatomic Radicals (AB) 4.1 Hydroxyl Radicals 4.2 Superoxide Anions 4.3 N; andCO+ 4.4 The F-C- Radical and Related Species 4.5 LiH+ and HCl' Centres 4.6 CdOH and MnS 5 Triatomic Radicals (AB, ) and Related Species 5.1 Metalcentred Radicals 5.2 CarbonCentred Radicals 5.3 NitrogenCentred Radicals 5.4 OxygenCentred Radicals 5.5 HalogenCentred Radicals 6 Tetraatomic Radicals (AB3 ) and Related Species 6.1 Boron and Gallium 6.2 Carbon, Silicon and Tin 6.3 Nitrogen and Phosphorus 6.4 Sulphur and Halogen 7 Penta-atomic Radicals ( AB4 ) and Related Species 7.1 M 0 4 Centres 7.2 Boron- and CarbonCentred Radicals 7.3 5nH: and Related Radicals 7.4 Phosphorus and ArsenicCentred Radicals 8 Other Radicals 8.1 'B2H; and B3H; Radical Atoms 8.2 The Radical Anion of Tetra-t-butyl-diborane 8.3 C20;Cations 9 Radicals in Inorganic Materials 9.1 Magnetic Centres in Si02 9.2 Carbon, Silicon and Germanium 9.3 Other Materials 10 Environmental Factors 11 The Use of Spin Traps 1 1.1 Hydroxyl Radicals 1 1.2 Superoxide and Peroxy Radicals 1 1.3 Other Radicals

87 88 89 90 90 91 92 92 93 94 94 95 96 97 97 100 100 100 101 101 102 102 103

103 104 104 106 106 106 106 108 108 109 110

110 110 111 111 117 117 118 118 119 119 121 123 125 126 126 128 128

ix

Contents 12 Transition Metal Alkyls, Carbonyls and Related Species

12.1 Metal Carbonyls 1 2.2 Cyclopentadienyl Derivatives 12.3 Metal Alkyl Derivatives 13 Radicals in the Gas Phase 13.1 Neutral Radicals and Triplet-State Molecules 13.2 The Methoxy Radical 13.3 Radical Cations 13.4 Analytical and Mechanistic Applications References Chapter 4 1

2 3

4

5

6 7 Chapter 5 1

2 3

4

5 6

7 8

128 128 130 131 131 132 132

133 133 134

Organic Radicals in Solids By T. J. Kemp Introduction and Bibliopphy Technical and General Spectroscopic Aspects 3.1 Alkane Cation Radicals 3.2 Carboncentred Radicals from Saturated Molecules 3.3 Oxygen-centred Radical Cations 3.4 Heteroatomcentred Radical Cations: Atoms other than Oxygen 3.5 Heteroatomcentred Neutral Radicals 3.6 Charged Radicals from Unsaturated Systems 3.7 Neutral Radicals from Unsaturated Systems Mechanistic Studies 4.1 Alkanes and their Radicals 4.2 Saturated Systems Containing Heteroatoms 4.3 Unsaturated Systems Containing Heteroatoms Molecules of Biologicd interest 5.1 Aminoacids and Peptides 5.2 Pyrimidines, Purines and their Derivatives Radicals at Surfaces and in clathrates Radicals in Semiconductors References

139 139 141 141 142 143 146 146 147 149 149 149 150 152 154 154 155 157 158 161

Organic Radicals in Solution By B. J. Tabner Introduction Carbon-centredhdiclrls 2.1 Alkyl Radicals 2.2 Delocalized Radicals Nitrogencentred Rdkals 0xygen-ctntredR.diuls Nitroxides Sulphurcentred Radicals Radicalcations Radical-anions References

166 167 167 179 185 189 193 197 198 206 215

Applications of E.S.R. in Polymer Chemistry By D. J. T.Hill, J. H. O’Donnell, and P. J. Pomery 1 Introduction 2 Polymer Degradation 2.1 Ionizing Radiation 2.2 Photodegradation 2.3 MechanoChemical Degradation 2.4 Thermal Degradation

Chapter 6

223 224 225 228 230

23 1

Contents

X

3 Polymerization 3.1 Homogeneous Chain Growth 3.1.1 Solid State Polymerizations 3.1.2 Liquid State Polymerizations 3.1.3 Spin Trapping 3.1.4 Emulsion Polymerization 3.1.5 Resins, Composites and Coatings 3.2 Heterogeneous Chain Growth 3.2.1 Graft Copolymerization 3.2.2 Polymerization Catalysis 4 Polymer Structure, Interactions and Properties 4.1 Conductive Polymers 4.1.1 Polyacetylenes 4.1.2 Poly(p-phenylene) 4.1.3 Polypyrroles 4.1.4 Other Systems 4.2 Polymer/Metal Interactions References Chapter 7

232 232 232 234 235 236 237 237 238 238 239 239 239 240 240 24 1 24 1 242

Spin Labels: Biological Systems By ChingSan Lai

1 Introduction 2 Recent Developments 2.1 Instrumentation 2.2 Techniques 3 Protein 3.1 Membranes 3.2 Blood 3.3 Enzymes 3.4 Muscle 3.5 Others 4 Nucleic Acid 4.1 DNA 4.2 Chromatin 4.3 RNA 5 Properties of Phospholipid Bilayers 5.1 Lateral Diffusion 5.2 Phase Transition and Phase Separation 5.3 Oxygen Diffusion 5.4 Membrane Potential and ApH 5.5 Membrane Permeability 6 Lipid-Protein Interaction 6.1 Integral Proteins 6.2 Peripheral Proteins 7 Membrane Fluidity of Cells 7.1 Proliferating Cells 7.2 Nonproliferating Cells 8 Modification of Membrane Functions by Drugs 8.1 Anesthetics 8.2 Others 9 Immunology 10 Miscellaneous 11 Synthesis 1 1.1 Lipid Spin Labels 11.2 Protein Spin Labels 1 1.3 Nucleic Acid Spin Labels 11.4 Others References

246 246 246 247 249 249 25 1 25 1 25 3 254 254 254 255 256 256 256 25 7 258 25 9 260 260 260 263 264 264 265 266 266 267 270 273 273 273 274 276 277 284

xi

Contents Chapter 8

Metalloproteins By N. J. Blackburn

1 CopperProteins 2 CytochromeOxida8e 3 Iron Sulphur Proteins 4 HaemRoteins 5 Nonhaem Iron Proteins 6 MolyWenumProteins 7 Nickelhteins 8 Manganese and Other Metals References

Applications of E.S.R. in Medicine By N. J. F. Dodd 1 Introduction 2 Tissues 2.1 Blood and Serum 2.2 Soft Tissues 2.3 Hard Tissues 2.4 Melanin 3 Radiation Effects in Biological Molecules 3.1 DNA and Related Bases 3.2 Amino Acids and Peptides 3.3 Lipid, Carbohydrate and other Biomolecules 3.4 Radiation Dosimetry 3.5 Photochemical Reactions 4 Radical Reactions of Dmg~and Toxic Chemicals 4.1 Carcinogens 4.2 Antitumour Drugs 4.3 Vitamins 4.4 Semiquinones 4.4.1 Ortho-Semiquinones 4.4.2 Para-Semiquinones 4.5 Other Drugs 5 Enzymes 6 Oxygen Radicals References

29 1 297 301 306 31 1 312 316 317 319

Chapter 9

Author Index

324 325 325 326 329 330 33 1 33 1 333 335 337 337 338 338 339 342 344 344 345 345 348

350 352 36 1

1 Theoretical Aspects of E.S.R. BY A. HUDSON

1 Introduction

The t h e o r y o f E . S . R . is w e l l understood and t h i s chapter w i l l be l a r g e l y

concerned w i t h a p p l i c a t i o n s r a t h e r t h a n new developments. There has been a s i g n i f i c a n t i n c r e a s e i n t h e number o f papers d e a l i n g w i t h s p i n - p o l a r i z e d ( C I D E P )

spectra. W r e o v e r , s u c h experiments are now b e i n g perfonned on c h e m i c a l l y i n t e r e s t i n g systems. There w a s a t i m e when CIDEP seemed t o be l a r g e l y c o n f i n e d t o t h e p h o t o l y s i s o f duroquinone.

The development o f e x p e r i m e n t a l t e c h n i q u e s f o r t h e p r o d u c t i o n o f radical c a t i o n s from s a t u r a t e d compounds h a s provided e x p e r i m e n t a l d a t a on a l a r g e number o f i n t e r e s t i n g new radicals. The i n t e r p r e t a t i o n o f the spectral p a r a m e t e r s i n tenus o f molecular g e o m e t r i e s h a s l e d t o a number of c o n t r o v e r s i e s and b o t h ab tnttto and semi-empirical molecular o r b i t a l c a l c u l a t i o n s have been used i n a n attempt t o r e s o l v e some o f t h e problems i n v o l v e d . 2

A

Numerical Methods and S p e c t r a l A n a l y s i s .

new j o u r n a l devoted t o t h e use of computers i n s p e c t r o s c o p y c o n t a i n s

d e s c r i p t i o n e of microprocessor based d a t a accumulation and r e d u c t i o n systems’’

2.

O t h e r r e c e n t l y d e s c r i b e d data system i n c l u d e one based on a n Apple X I P l u s (48 K)3

and a n o t h e r ba8ed on S-100 bus components*

Morton and P r e s t o n 5 have g i v e n a d e t a i l e d account o f t h e u s e o f a 6

computer-assisted

tmrcircle goniometer t o assemble the h y p e r f i n e - i n t e r a c t i o n

and g2 t e n s o r s i n t h e c r y s t a l l o g r a p h i c axis system. An e x p l a n a t i o n is g i v e n f o r each of the seven c r y s t a l classes. e x t r a c t i o n o f t h e g-tensor

A

g e n e r a l method h a s been d e s c r i b e d for

from s i n g l e c r y s t a l d a t a 6 . A simplified method h a s 1

T o r references see p - 1 2

2

Electron Spin Resonance

b e e n d e v e l o p e d for d e a l i n g w i t h s p e c t r a showing s t r o n g q u a d r u p o l e i n t e r a c t i o n s a n d a p p l i e d t o the case of Ir2+ i n .'-M

Stevenson'

has p u b l i s h e d a n e x t e n s i v e

theoretical d i s c u s s i o n o f t r i p l e t s t a t e spectra. The computer s i m u l a t i o n of powder spectra i s v i r t u a l l y e s s e n t i a l i f s e v e r a l h y p e r f i n e i n t e r a c t i o n s are i n v o l v e d . A good r e c e n t example i n v o l v e s t h e spectrum of NF3+ t r a p p e d i n a r i g i d matrix a t 2.5 K . The program d e s c r i b e d is a c c u r a t e t o

s e c o n d order i n a l l h y p e r f i n e tenus and copes w i t h up t o f o u r nuclei'.

Rieger

10

h a s w r i t t e n a program which p e r f o r m s a least s q u a r e s analysis o f powder p a t t e r n s w i t h n o n c o i n c i d e n t p r i n c i p a l axes of t h e g and h y p e r f i n e t e n s o r s . Graphs h a v e b e e n p u b l i s h e d which may be used for t h e i n t e r p r e t a t i o n o f powder spectra from axial sites w i t h e f f e c t i v e s p i n s r a n g i n g frornS=l t o S = 5 / 2 . The r e s o n a n c e f i e l d s

are p r e s e n t e d as f u n c t i o n s of t h e z e r o - f i e l d s p l i t t i n g ' ' .

A

powder p a t t e r n

analysis h a s also b e e n d e s c r i b e d f o r cubic sites of Fe3+ i n MqO".

In disordered

s o l i d s i t is n e c e s s a r y t o i n c l u d e t h e d i s t r i b u t i o n of s p i n H a m i l t o n i a n

parameters. -1s

a n d K l i a v a 1 3 h a v e a i m u l a t e d spectra for d'

i o n s i n c l u d i n g Mo

5+

in a phosphate glass. A d e t a i l e d s t u d y of t h e powder spectrum of a n 15N e n r i c h e d n i t r o x i d e has

led t o t h e c o n c l u s i o n that s u c h probes are s i g n i f i c a n t l y better t h a n 1 4 N

n i t r o x i d e s i n p o l y c r y s t a l l i n e o r amorphous s y s t e m s 1 4 s i n c e the h y p e r f i n e f e a t u r e s are w e l l r e s o l v e d a t X-band.

An

a l t e r n a t i v e s o l u t i o n i s t o work a t

Q-band a n d s i m u l a t i o n s h a v e b e e n p r e s e n t e d o f f i r s t and s e c o n d d e r i v a t i v e

spectra for a v a r i e t y of s p i n labels in f r o z e n s ~ l u t i o n s ~A ~k e. y factor i n the s u c c e s s f u l s i m u l a t i o n o f f i e l d swept spectra is t h e Aasa and Vanngard l / g

factor. P i l b r o w h a s c o n s i d e r e d t h i s p o i n t i n some d e t a i l i n a d i s c u s s i o n of l i n e s h a p e s for frequency-swept a n d f i e l d - s w e p t spectral6. P h i l l i p s and H e r r i n g have investigated''

the u s e of d i s p e r s i o n v e r s u s a b s o r p t i o n p l o t s € o r d e t e c t i n g

l i n e s h a p e d i s t o r t i o n s a t t r i b u t a b l e t o e i t h e r t h e spectrometer or the s a m p l e . An i n t e r a c t i v e method allows f o r the e l i m i n a t i o n of b a s e l i n e d r i f t . A f a s t d e c o n v o l u t i o n p r o c e d u r e h a s b e e n d e s c r i b e d f o r inhomogeneous r e s o n a n c e l i n e s

18

The d e c o n v o l u t i o n o f h y p e r f i n e s p l i t t i n g 8 is n e c e s s a r y i n the i n v e s t i g a t i o n of

.

1 Theoretical Aspects of E.S.R

3

paramagnetic s p i n d i s t r i b u t i o n s by E.S.R. imaging”. There have been f u r t h e r d e v e l o p a e n t s i n t h e s i m u l a t i o n of isotropic s o l u t i o n spectra u s i n g fast F o u r i e r t r a n sf o r m mathoda20’21.

A

general i t e r a t i v e

l e a s t - s q u a r e s procedure has been described f o r f i t t i n g complex dynamic l i n e s h a p e s i n t h e faat motional r e g i o n 2 2 .

nJ0

papers have been concerned w i t h

the problem of superhyp e r f i n e s t r u c t u r e when s i m u l a t i n g the E.S.R. spectra of

n i t r o x i d e s p i n probes i n s ~ l u t i o n ~ ~ ’This ~ * . is r e l e v a n t t o the measurement of s p i n exchange rates and t h e u s e o f s p i n probes € o r mo n i t o r in g the c o n c e n t r a t i o n o f d i s s o l v e d oxygen i n biological samples. B a l e s has e x p e r im e n ta lly v e r i f i e d a p r e v i o u s l y published procedure €or e x t r a c t i n g s p i n exchange rates from inhomogeneously broadened l i n e s 2 5 . A

procedure based on c o r r e l a t i o n methods has been applied t o the a n a l y s i s

of weak spectra. X t is p a r t i c u l a r l y u s e f u l for l o c a t i n g satellites due t o ”C or 29Si

i n n a t u r a l abundance26.

A

p r o d u c t f u n c t i o n produced by c o r r e l a t i o n of t h e

d i g i t i z e d e x p r i m e n t a l spectrum wi t h a test spectrum is used aa a c r i t e r i o n of t h e goodness of f i t . The test spectrum is i n i t i a l l y a s i n g l e l i n e b u t becomes i n c r e a s i n g l y complex as t h e analysis procedes and additional c o u p lin g c o n s t a n t s

are located. 3 Spi n-rel axat i o n

An

and Line-broadening E € € e c t s

i n t e r e s t i n g review article by Kurreck and h i s coworkers i n c l u d e s a good

account of the v a r i o u s r e l a x a t i o n processes involved i n EM)(IR and its e x t e n s i o n to triple resonance e q e r i m e n t s

27

.

There have been f e w d e v e l o p n e n t s i n the t h e o r y of s p i n r e l a x a t i o n and li nes hapee. Baram has d i s c u s s e d n o n se c u l a r l i n e s h a p e s i n t h e slow motion region”.

Exact

s o l u t i o n s have been o b t a i n e d f o r the modified B l o c h e q u a t i o n s

w i t h 3-site chemical exchange2’ and a new formalism ha8 been applied t o t h e t w o- s it e problem i n t h e i n t e r m e d i a t e exchange region3’.

ltooser and Rssing have

i n v e s t i g a t e d the e f f e c t s of two-dimensional r e o r i e n t a t i o n i n p a r t i a l l y ordered ~ y s t e m e ~A ~g.e n e r a l i d t r e a t me n t € o r slow d i f f u a i o n a l r e o r i e n t a t i o n of axially

Electron Spin Resonance

4

symmetric h y p e r f i n e c e n t r e s 3 ' has b e e n applled t o t h e spectra of n r t r o x i d e probes i n amorphous polymers33. Pormu Lae h a v e b e e n d e r i v e d f o r a v e r a g e d t e n s o r components when a dynamic process o c c u r s between t w o synrmetry related sites i n a s i n g l e crystal34. R e l a x a t i o n processes h a v e b e e n c o n s i d e r e d i n trimeric c l u s t e r s i n c l u d i n g both i s o t r o p i c and a n i s o t r o p i c exchange i n t e r a c t i o n s 3 ' . s t u d i e d the e f f e c t o f a n t i s y m m e t r i c exchange on E . S . R .

that t h e a n g u l a r dependence c a n h a v e a period of

360°

%pel

has

l i n e w i d t h s and h a s shown i n s t e a d of 180° 36. The

l i n e s h a p e s of a l k a l i metal b i p h e n y l salts h a v e p r o v i d e d e v i d e n c e f o r s p i n d i f f u s i o n 3 7 i n these quasi-two-dimensi.ona1

magnetic systems.

S p i n exchange i n s o l u t i o n has been i n v e s t i g a t e d f o r n i t r o x i d e s i n n e m a t i c l i q u i d crystals38 a n d i n h y d r o c a r b o n s o l v e n t s 3 9 .

In t h e latter case p a r t i c u l a r

emphasis w a s g i v e n to the i n t e r m e d i a t e exchange r e g i o n . A u s e f u l i n s i g h t i n t o

the e f f e c t s of s p i n exchange and r o t a t . i o n a 1 d i f f u s i o n h a s b e e n o b t a i n e d by comparing the E . S . R .

l i n e w i d t h s for t h e d i f f e r e n t m a g n e t i c isotopes i n a

molybdenum complexw. A w i d e r a n g e o f r o t a t i o n a l c o r r e l a t i o n times has b e e n found €or v a n a d y l s i s o l a t e d from o i l shale4?

Linewidth v a r i a t i o n s i n t h e

s o l u t i o n spectra of d i t h i a z o l y l and r e l a t e d r a d i c a l s h a v e b e e n a n a l y s e d t o c a l c u l a t e hydrodynamic r a d i i 4 ' .

A pronounced t e m p e r a t u r e dependence i n t h e

E.S.R. s p e c t r u m of the pentafluorocyclopentadienyl radical o r i g i n a t e s from a v e r a g i n g of the 19P anisotropic h y p e r f i n e t e n s o r 4 3 .

We s h a l l n o t d e a l i n t h i s c h a p t e r w i t h t h e numerous a p p l i c a t i o n s OF s p i n probes i n biological s y s t e m s . However, t h e i r u s e i s n o t r e s t r i c t e d t o i n v e s t i g a t i n g t h e d y n a m i c a l properties o f b i o m o l e c u l e s . M t h n e u t r a l and p o e i t i v e l y c h a r g e d n i t r o x i d e s h a v e b e e n employed i n a s t u d y o f micelles formed

from s u l p h a t e

surf act ant^^^.

L a r g e and n e a r l y c y l i n d r i c a l p r o b e s are h i g h l y

o r d e r e d w i t h i n t h e c h a n n e l s of t h i o u r e a - c y c l o h e x a n e i n c l u s i o n compounds, whereas s m a l l e r and n e a r l y spherical p r o b e s r e o r i e n t i s ~ t r o p i c a l l y ~A n~ .i n v e s t i g a t . i o n

of s p i n - l a b e l e d

r o d l i k e p o l y ( b e n z y l g l u t a m a t e ) s u g g e s t s t h a t t h e macromolecule

b e h a v e s h y d r o d y n a m i c a l l y as a v e r y por'ws c y l i n d e r w i t h a n i m p e n e t r a b l e core4? M e i r ~ v i t c hreports ~~ on s t r e t c h i n g - i n d u c e d m o l e c u l a r mobil i t y and the

I TheoreticalAspects of E.S.R

5

p a r t i t i o n i n g o f s p i n probes among d i f f e r e n t sites i n s e m i c r y s t a l l i n e l w - d e n s i t y p o l y e t h y l e n e f i l m a . Another c o n t r i b u t i o n d e a l s w i t h t h e i n t r i n s i c f l e x i b i l i t y g r a d i e n t found i n hydrocarbon chains i n l i p i d bilayers".

S t u d i e s of l i q u i d

crystals i n c l u d e a n i n v e e t i g a t i o n of molecular dynamics a t t h e nematic to emectic A t r a n s i t i o n 4 9 and a n a n a l y s i s , u s i n g p a r a l l e l - e d g e l i n e s , of t h e o r i e n t a t i o n a l d i s t r i b u t i o n o f a s p i n probe i n r i g i d MBBA50. The E.S.R. spectra

of n i t r o x i d e s i n some m a g n e t i c a l l y a l i g n e d l i q u i d crystals formed from s u r f a c t a n t s have been i n t e r p r e t e d i n terms o f c y l i n d r i c a l micelle and disc-shaped micelle

structure^^^.

A

combination of ELDOR, s a t u r a t i o n and E . S . R .

l i n e w i d t h measurements has been used t o s t u d y d e v i a t i o n s from the Brownian motion model. A d d i t i o n a l r e l a x a t i o n terms have been formulated i n terms o f a s l o w l y r e l a x i n g local s t r u c t u r e mechanisms2. The motion o f midchain peroxy r a d i c a l s i n poly(tetrafluoroethy1ene) is c o n s i s t e n t w i t h a model i n v o l v i n g h e l i c a l t w i s t i n g o f t h e polymer axiss3. lrwo

simple methods o f d e t e r m i n i n g t h e microwave f i e l d s t r e n g t h i n E.S.R.

should be u s e f u l i n s a t u r a t i o n t r a n s f e r experiments54. Recent a p p l i c a t i o n s of the latter t e c h n i q u e i n c l u d e a s t u d y o f m u l t i p l e m t i o n a of t h e s p e c t r i n - a c t i n

ooarplexS5 and an i n v e s t i g a t i o n o f maleimide s p i n - l a b e l e d ccmpea chlorotic mottle (15N]nitroxides have been employed t o d e t e r m i n e the e f f e c t 6 of non-coincident magnetic and d i f f u s i o n t e n s o r axes when t h e r e is a n i s o t r o p i c r o t a t i o n a l d i f fusions7. Pajer and Harsh58 have a n a l y s e d the s e n s i t i v i t y of X-band

ST

spectra to a n i s o t r o p i c r o t a t i o n . The i n f l u e n c e of e x p e r i m e n t a l

parameters hae been c o n s i d e r e d b y Delmelle5';

the e f f e c t s of overmodulation are

d i f f i c u l t to i n c l u d e i n s i m u l a t i o n 8 because of v e r y l o n g computation times and Robinson h a s s u g g e s t e d v a r i o u s approximations t o overcome t h i s p m b l e n ~ ~ Th ~e. r e s u l t s o f v a r y i n g t h e modulation frequency have also been the subject of r e c e n t a t t e n t i o n 6 1 . Improvements i n c a v i t y d e s i g n have l e d t o renewed i n t e r e s t i n t h e d i s p e r s i o n mode method o f r e c o r d i n g ST E . S . R .

A

t e c h n i q u e h a s been

d e s c r i b e d f o r d e a l i n g w i t h multicomponent spectra and a p p l i e d t o membrane S a t u r a t i o n t r a n s f e r h a s also been s t u d i e d i n p u l s e e x p e r i m e n t s u s i n g

6

Electron Spin Resonance

an e l e c t r o n s p i n - e c h o ~ p e c t r o m e t e r ~ The ~ . m o t i o n of n i t r o x i d e r a d i c a l s i n d i b u t y l phthalate is best a c c o u n t e d for b y a l a r g e - a n g l e

jump model. Very s l o w

m o t i o n can also be d e t e c t e d by a method i n v o l v i n g d o u b l e m o d u l a t i o n of t h e

E.S.R. spectrum66. Another p o s s i b i l i t y is t o u s e a n E L W R t e c h n i q u e based upon s p i n echoes a n d r a p i d s t e p p i n g o f the m a g n e t i c f i e l d 6 7 . Baram h a s d e v e l o p e d a t h e o r y of s p i n e c h o e s i n t h e s l o w motion regime6'. P u l s e t e c h n i q u e s f e a t u r e i n c r e a s i n g l y in t h e l i t e r a t u r e a n d t h i s t r e n d w i l l c o n t i n u e as t h e n e c e s s a r y equipment becomes commercially a v a i l a b l e . The d i f f u s i o n of s p i n s i n t h e r a d i c a l c a t i o n s a l t ( f l u o r a n t h e n y l )2 A s F 6 - has b e e n +

measured by o b s e r v i n g e l e c t r o n s p i n e c h o d e c a y s i n a m a g n e t i c f i e l d g r a d i e n t 6 ' . I r r a d i a t i o n of m e t h a n o l a b s o r b e d on z e o l i t e s g e n e r a t e s hydroxyrnethyl r a d i c a l s . E l e c t r o n s p i n e c h o spectra o f t h e s e r a d i c a l s h a v e t h e n b e e n used t o e l u c i d a t e t h e geometrical a r r a n g e m e n t o f s u r r o u n d i n g methanol m o l e c u l e s 7 0 , A t r e a t m e n t of n u c l e a r q u a d r u p o l e e f f e c t s on e l e c t r o n s p i n e c h o m o d u l a t i o n b y Shubin and Dikanov''

has b e e n e x t e n d e d t o t h r e e p u l s e s e q u e n c e s by Kevan and coworkers72

wbo p o i n t o u t a number of l i m i t a t i o n s .in t h e e x p r e s s i o n s o b t a i n e d by

p e r t u r b a t i o n t h e o r y . Q u a d r u p l e s p l i t t x n g parameters h a v e b e e n e s t i m a t e d for 1 4 N nuclei i n a nitroxide biradicalr13.

The d e t e r m i n a t i o n of k i n e t i c parameters f r o m l i n e w i d t h v a r i a t i o n s is l o n g s t a n d i n g and w e l l e s t a b l i s h e d . S t e v e n s o n and his coworkers74 have i n v e s t i g a t e d t h e hydrogen b o n d i n g of e t h a n o l t o t h e radical a n i o n o f p - c y a n o n i t r o b e n z e n e in hexamethylphosphoramide and report f o r t h e f i r s t time a c t i v a t i o n parameters c o n t r o l l i n g hydrogen bond f o r m a t i o n t o a n a n i o n i c species. F u r t h e r work has b e e n r e p o r t e d on i n t e r m o l e c u l a r exchange of sodium ions i n d i n i t r o b e n z e n e i o n

pairs7=. A d e t a i l e d s t u d y of r e s t r i c t e d r o t a t i o n of t h e a c e t y l g r o u p i n m- and p n i t r o a c e t o p h e n o n e a n i o n radicals haa been u s e d t o e s t a b l i s h an optimum p r o c e d u r e f o r o b t a i n i n g thennodynamic parameters f r o m dynamic E . S . R .

76

spectra

.

I o n pair f o r m a t i o n h a s a s i g n i f i c a n t i n f l u e n c e on t h e h i n d e r e d r o t a t i o n of t h e p h e n y l r i n g s in n i t r o b e n z o p h e n o n e r a d i c a l anions77. The dynamic b e h a v i o u r of t h e

5-hydro-6-methyl-6-yl-uracil r a d i c a l ha:j b e e n s t u d i e d in s i n g l e crystals,

7

I Theoretical Aspects of E.S.R

p o l y c r y s t a l l i n e powders and aqueous glasms7'. The detected v a r i a t i o n s can be accounted f o r by d i f f e r e n t molecular packing6 i n the respective m a t r i a e s . Barriers t o r o t a t i o n determined by E.S.R. have been used t o estimate

s t a b i l i s a t i o n e n e r g i e s i n a mi n o a l l y l , aminopropynyl, and aminocyanonrethyl Unusually large barriers t o methyl group r o t a t i o n have been found i n

radicals7'.

c a t i o n radicals f o n m d from methyl and e t h y l estera".

Barriers t o r o t a t i o n can

also be estimated from t h e t e mp e r a t u r e dependence o f p-hyperfine c o u p l i n g constants. radicals''

A

r e c e n t unusual example i n v o l v e s muonic-substituted

ethyl

.

E l e c t r o n s p i n t r a n s f e r between naphthalene n - sy st e m ,

which w a s first

s t u d i e d i n 1961 by A t he r t o n and Weissman, is s t i l l the subject o f i n t e r e s t i n g r e s e a r c h . A r e c e n t s t u d y involved a c a r e f u l a n a l y s i s , u s i n g E.S.R. and ENDOR, o f i n t r a m o l e c u l a r t r a n s f e r between naphthalene r i n g s separated by a v a r i a b l e number of spirobonded cycl obut a n e r i n g s . With 3 or 5 o f the latter, the i n t r a m o l e c u l a r

s p i n transfer w a s slaw on the h y p e r f i n e t i m e scale, b u t f a s t exchange was observed f o r

( 1)

i n media o f h i g h s o l v a t i n g power. The E.S.R. and ENDOR spectra

a c t u a l l y c o n s i s t e d o f a s u p e r p o s i t i o n o f fast and slow exchange spectra and these have been assi gned t o the sun- and a n t t - c o n f o r ma t i o n s of (1) s i n c e the d i s t a n c e between the naphthalene u n i t s is s i g n i f i c a n t l y shorter i n the formmr82.

The problem o f dev e l o p i n g a c o n s i s t e n t mathematical t h e o r y for a s p i n 1/2

system undergoing f irst-order s p i n - s e l e c t i v e r e a c t i o n s , has b e e n c o n s id e r e d by P o t t i n g e r and LendiS3, who d e r i v e q e n e r a l i s e d Bloch e q u a t i o n s employing the

8

Electron Spin Resonance

theory of quantum Markovian master e q u a t i o n s . The r e s u l t s o f a t h e o r y o f CIDEP

and Heisenberg s p i n exchange have also been p r e s e n t e d i n terms o f Bloch-type equationse4.

An

unusual p a t t e r n o f o s c i l l a t i o n s i n the time-resolved C I D E P

s i g n a l h a s been observed i n a s t u d y o f p h o t o e l e c t r o n s i n Rb/TEJF s o l u t i o n s 8 5 . Baer and P a u l have observed s t a t i o n a r y n u t a t i o n s i n t h e spectra of b e n z y l

r a d i c a l s produced b y p h o t o l y s i s of methyl b e n z y l k e t o n e w i t h modulated UV l i g h t e 6 . This experiment y i e l d s b o t h t h e r e l a x a t i o n times and t h e CIDEP o f t h e s p i n system. Another n o v e l experiment i n v o l v e s u s i n g t h e extra s e n s i t i v i t y o f f e r e d by chemically induced p o l a r i z a t i o n e f f e c t s t o d e t e c t the E N W R of short l i v e d radicalse7. I f radicals are g e n e r a t e d by a laser p u l s e i n the absence o f a microwave f i e l d which is later a p p l i e d i n a c o n t i n u o u s wave f a s h i o n , t h e e v o l u t i o n and o b s e r v a t i o n o f t h e m a g n e t i z a t i o n c a n be separated i n a two-dimensional experiment",

which y i e l d s the s p i n - l a t t i c e r e l a x a t i o n t i m e and

the p o l a r i z a t i o n ratio. Basu and McLauchlaneg have shown how a non-uniform c o n c e n t r a t i o n o f f r e e r a d i c a l s produced by a l i g h t p u l s e can a f f e c t t h e subsequent kinetic behaviour of the system. I t has been p o i n t e d o u t t h a t o v e r l a p p i n g spectra can o f t e n be separated i n a time-resolved elrperiment by u t i l i z i n g t h e d i f f e r e n t t e m p o r a l v a r i a t i o n s o f t h e two s i g n a l s g o . A t h e o r y based on t h e Bloch e q u a t i o n s h a s been used t o account f o r t h e i n f l u e n c e o f e l e c t r o n t r a n s f e r r e a c t i o n s on CIDEP spectrag1. The Bloch e q u a t i o n approach has also b e e n used t o a n a l y s e a t i m e - i n t e g r a t i o n method the E.S.R.

which removes s p u r i o u s s i d e b a n d s i n

spectra of s p i n - p o l a r i z e d radicals produced by laser f l a s h

photolysi~~ ~ . same paper also c o n t a i n s a u s e f u l d i s c u s s i o n o f numerical The r e s o l u t ion-enhancement methods

.

The t i m e i n t e g r a t i o n method h a s been used t o s t u d y t h e spectra of

a-aminoalkyl radicals produced by f l a s h p h o t o l y s i s o f

benzene-1,2:3,4-tetracarboxylic d i a n h y d r i d e i n t h e p r e s e n c e o f tertiary m i n e s g 3 . The Oxford group have also reported on t h e p h o t o p h y s i c s and photochemistry of d i a z a n a p h t h a l e n e s g 4 and m e t h y l p y r a ~ i n e s ~I~n .t e r e s t i n g r e s u l t s have been found f o r r a d i c a l s d e r i v e d from a l i p h a t i c ketonesg6. It has been shown

I Theoretical Aspects of E.S.R

9

that t h e zero-f i e l d s p l i t t i n g c o n s t a n t i n t h e photoexcited t r i p l e t state of such

molecules is normally p o s i t i v e g 7 . The 700 G doublet s p l i t t i n g o f phosphonyl r a d i c a l s shows a marked E-A p o l a r i z a t i o n when generated by p h o t o l y s i s o f a d i a l k y l p h o s p h i t e i n di-t-butyl

peroxideg*. T h i s is t h e f i r s t example o f CIDEP

f o r a r a d i c a l i n which t h e unpaired e l e c t r o n is l o c a t e d on phosphorus. Wan and h i s coworkers have a l s o r e p o r t e d on CIDEP s t u d i e s involving chKHnone and chromanoneg9, ascorbic acid''',

and some sulphur c a t i o n radicalslol. A laethod o f

d i s c r i m i n a t i n g between P-pair p o l a r i z a t i o n and t h e triplet mechanism h a s been applied t o t h e durosemiquinone r a d i c a l i n a v a r i e t y of s o l v e n t s l o 2 . I t h a s been sh0wnlo3 using time resolved E.S.R. t h a t t r i p l e t s p i n

p o l a r i z a t i o n is conserved d u r i n g energy t r a n s f e r between t r i p l e t carbonyl compounds and aromatic hydrocarbons i n e t h a n o l g l a s s e s a t 77 K. P h o t o l y a i s o f a s i n g l e c r y s t a l of benzoyl formic a c i d a t

77

K shows CIDEP from a k e t y l r a d i c a l

i n a d d i t i o n t o t h e spectra o f t h r e e trapped r a d i c a l pairs1'*. 5 . Applicatione of Quantum Chemistry

There i a an i n c r e a s i n g tendency f o r ab t n t t t o c a l c u l a t i o n s t o be included i n t h e d i s c u s s i o n of new experimental r e s u l t s . The IN00 method remain8 the most popular of semi-empirical procedures b u t MNDO c a l c u l a t i o n s also f e a t u r e prominently. New procedures f o r t h e production of radical i o n s i n s o l u t i o n have

led t o renewed a t t e n t i o n being given t o conjugated n-electron

systems. The

biphenyl r a d i c a l anion has been known f o r over twenty years b u t t h e corresponding r a d i c a l c a t i o n h a s only r e c e n t l y been preparedlo5.

me

spin

d e n s i t y d i s t r i b u t i o n s i n t h e two syatema are very similar a 8 expected from t h e p a i r i n g t h e o r e m , and t h e r e is no evidence t h a t t h e s e species are anything b u t planar. Many i n t e r e s t i n g new r a d i c a l c a t i o n a have been prepared i n t h e s o l i d state

by ? i r r a d i a t i o n

of Freon s o l u t i o n s at 77 K or belaw. Moet o f t h e major

c o n t r i b u t o r s t o t h i s area were p r e s e n t at t h e r e c e n t Paraday Discuseion on 'Radicals i n Condensed Phaaee'lo6. The s t r u c t u r e s of a number o f t h e s e species

Electron Spin Resonance

10

have been the subject o f some c o n t r o v e r s y . I t now appears t o be a g r e e d that t h e

radical c a t i o n o f e t h y l e n e and its d e r i v a t i v e s is t w i s t e d .

r e c e n t paper by

A

I w a s a k i and coworkers c o n t a i n s l e a d i n g r e f e r e n c e s L o 7 . Ab tnttto

calculations10E’ log s u p p o r t a proposal that a r a d i c a l c a t i o n fonned from

is t h e r i n g opened P-oxa-trimethylene

e t h y l e n e oxidell’

symmetrical planar

cation with a

s2% s t r u c t u r e .

The radical c a t i o n s o f c y c l o a l k a n e a are o f p a r t i c u l a r i n t e r e s t b e c a u s e o f their tendency t o exhibit l a r g e J a h n - T e l l e r d i s t o r t i o n s . A good

su~~~[liity of the

work of I w a s a k i and h i s group appears i n t h e aforementioned Paraday Discussionlo6. There have been t w o a b tnttto c a l c u l a t i o n s on cyclopentane11‘”12.

The paper by S h i d a ’ s group a l s o treats t h e radical c a t i o n s

of cyclopropane and c y c l o b u t a n e . An e x t e n s i v e m u l t i c o n f i g u r a t i o n SCP s t u d y haa

been made o f e l e c t r o n i c EyImIIetry b r e a k i n g i n t h e c y c l o p r o p e n y l radica1113. C o n s i d e r a b l e e f f o r t w a s n e c e s s a r y to reduce a s p u r i o u s s e p a r a t i o n found between the 2A 70

and 2B

components which should be d e g e n e r a t e f o r -3h D

Bymmetry.

Gaussian

c a l c u l a t i o n e a t t h e STO-3G l e v e l s u g g e s t that the stable molecular

conformation o f t h e 1 . 4 - d i t h i a n e r a d i c a l c a t i o n is a boat fom114.

Many new species have been prepared by Knight and h i s group of which t h e most i n t e r e s t i n g is C H ~ + ~ ” .R e s u l t s f o r c202+l16 and H2CO+117

have b e e n

compared w i t h ab tnttto c a l c u l a t i o n s . T h e Gaussian 76 molecular orbital program h a s been employed t o s t u d y t h e 1,3,2-dithiazolidin-2-yl s t r u c t u r e ( 2 )I1’. Ab

t n t t t o methods have a l s o been used t o i n v e s t i g a t e t h e s p i n - d e n s i t y d i s t r i b u t i o n i n t h e phenoxyl r a d i ~ a l l ’ ~ the , geometry of t h e t r i m e t h y l s i l y l radical12’, the e f f e c t s o f a l k y l s u b s t i t u t i o n on n i t r o x i d e radicals1”.

eet h a s been used t o treat

IiNSH which

A

and

double-zeta basis

is a model system f o r E-alkyl-??-

11

I Theoretical Aspects of E.S.R

(alky1thio)aminyl radica1slz2. Symmetry adapted c l u s t e r t h e o r y h a s been a p p l i e d t o BeH, CH3, CH3CHz, and

HCO,

An e x t e n s i v e s t u d y of t h e methyl r a d i c a l h a s been performed u s i n g

UHF SCXP w a ~ e f u n c t i o n s+ l ~ Taking ~ i n t o account v r b r a t i o n a l a v e r a g i n g , g i v e s

isotropic h y p e r f i n e c o u p l i n g c o n s t a n t s t o w i t h i n 4% of experiment. S t r a t t and D e s j a r d i n ~ ~ ~d i' s c u s s s o l v a t i o n i n t h e methyl radical based on the i d e a of

and t h e

v i b r a t i o n a l p o l a r i z a b i l i t y . The s p i n d e n s i t y d i s t r i b u t i o n i n Li3126 p e n t a g o n a l b i p y r a m i d a l geometry of Li7127

are i n good agreement w i t h t h e

p r e d i c t i o n e of ab t n t t t o c a l c u l a t i o n s . Recent system s t u d i e d b y t h e Xa method i n c l u d e PO 2-128 and a model f o r t h e a c t i v e s i t e i n 2-Fe Ferrodoxinl". B r u ~ n b y lhaa ~ ~ applied the INDO method t o primary alkyl radicals s u c h aa

propyl and b u t y l and proposes a r e i n t e r p r e t a t i o n of some earlier r e s u l t s . The

INDO method h a s also been a p p l i e d t o BP4131, the radical a n i o n s of *halogen s u b s t i t u t e d a c e t a m i d e ~ i l ~the ~ , radical a n i o n o f i o d o a ~ e t a m i d e l ~the ~ , radical anion134 and radical cation13'

of t e t r a f l u o r o e t h y l e n e , the 1,l - d i f l u o r o e t h y l

r a d i c a l l S 6 , and t h e o c t a f l u o r o c y c l o o c t a t e t r a e n e radical

Similar

c a l c u l a t i o n s s u p p o r t t h e i d e a of i n t e r n a l hydrogen bonding i n t h e

IT

c a t i o n s of

(hydroxymethyl ) u r a c i l and (hydroxymethyl )cytosine13'. The "m method

Lzr

f ' n :is&

'

i n v e s t i g a t e alkyl( t r i a l k y l s i l y 1 ) a m i n y l

cycloalkenylmethyl radicalslW, h e p t a t r i e n y l and polyetiyl

radicals d e r i v e d from thiole, s u l p h i d e s , and d i ~ u l p h i d e s l ~and ~, c a t i o n r a d i c a l s d e r i v e d from ethers143 and [ 3 . 3 . 3 ] p r 0 p e l l a n e l ~ ~ . The s p i n - d e n s i t y d i s t r i b u t i o n i n the p e n t a l e n e anion'"

is well accounted

for b y McLachlan's method, b u t i t does n o t give good agreement for naphth-1-yl phenyl nitroKide'*.

Honeybourne has applied t h e McLachlan approximations i n

c a l c u l a t i o n s on columnar s t a c k s o f macrocyclic radical ions147.

The n - e l e c t r o n

s p i n d i s t r i b u t i o n s and 9-values i n s e m i q ~ i n o n e s ~' ~ ' and the s i g n s of c a r b o n y l 13C h y p e r f i n e s p l i t t i n g e l s o have been t h e subject o f f u r t h e r d i s c u s s i o n a8 has

been t h e mechaniem o f s p i n - t r a n s f e r t o P-protons i n n - r a d i c a l ~ l ~ ~ .

Electron Spin Resonance

12

References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

45 *

J . D . Lipscomb and R.W. S a l o , camp. Enhanced S p e c . , 1983, 1, 11. F.Momo , G . A . R a n i e r i a n d A. S o t g i v , C o w . Enhanced S p e c . , 1983, 1, 79. G.J. Rormann and B.M. Peake, J . Magn. Reson., 1983, 53, 121. R . S h u l t z , G. Hurst, T.E. Thieret, and R.W. Kreilick, J . Magn. Reson., 1983, 3, 303. J . R . Morton and K.F. P r e s t o n , J . Magn Reson., 1983, 457. M.P. Byrn and C.E. S t r o u s e , J . Magn. Reson., 1983, 53, 32. J . Barak, A. Raizman, and J . T . S u s s , J . Maqn. Reson., 1983, 53, 23. R .C .S tevenson, J- Magn. R e s o n . , 1984, 22, 24. A.M. Mauri ce, R.L. B e l f o r d , I.B. Goldberg, and K . O . C h r i s t e , J . Am. Chem. Lo-., 1983, 105, 3799. P.B. R i e g e r , J . Maqn. Reson., 1982, 50, 485. R.S. d e B i a s i and J . A . M . Mendonca, J . Magn. R e s o n . , 1983, 53, 4 6 2 . J.L. Boldu, E. Munoz P . , Y. Chen, a n d M.M. Abraham, J . Chem. P h y s . , 1984, 80, 574. Bals and J . K l i a v a , J. Magn. Reson., 1983, 12, 243. S. Lee, T.C. S a n d r e c z k i , a n d I . M . Brown, J . Chem. P h y s . , 1984, 3983. M. P a s e n k i e w i c z - G i e r u l a , J . S . Hyde, a n d J . R . P i l b r o w , J . Magn. Reson., 1983, E, 255. J.R. P i l b r o w , J . Maqn. Reson., 1984, 58, 186. P.S. P h i l l i p s and F.G. H e r r i n g , J . Magn. Reson., 1984, 22, 43. R. B o s c a i n o a n d J . - P . K o r b , J . Maqn. Reson., 1984, 57, 127. K . Ohno, J. Magn. Reson., 1982, 145. J.C. Evans and P.H. Morgan, J . Maw. Reson., 1983, 52, 529. S. Brumby, J . Magn. Re=., 1984, 204. M. B a r z a g h i and M . S i m o n e t t a , J. Maqn. Reson., 1983, 175. M. R o m a n e l l i , J. P h y s . Chem., 1984, 88, 1063. J.S . Hyde and W . K . S u b c z y n s k i , J . Magn. Reson., 1984, 56, 125. B.L. Bales a n d D. W i l l e t t , J . Magn. R e s o n . , 1983, 51, 138. R.A. J a c k s o n , J , Chem. SOC. P e r k i n T r a n s . [I, 1983, 523. H . K u r r e c k , B. K i r s t e , and W. L u b i t z , Anqew. C h e m . I n t . Ed. E n q l . , 1984, 23, 173. A. Baram, J. P hys. Chem., 1983, E, 1676. J. S c h o t l a n d and J.S. Leigh, J . Magn. Reson.,1983, 51, 48. K.Kimura, J. Magn. Reson., 1983, 52, 13. R.G. K o o s e r and H . A . R e s i n g , J . P h p . Chem., 1983, E, 2564. s. Lee a n d D.P. Ames, J . Chem. Phyg., 1984, 80, 1766. S. Lee, I.M. Brown, and D.P. Ames, J.Chem. PhyS., 1984, 3948. L.K.Bof€mann and L.S. S z c z e p a n i a k , J. Magn. Reson., 1983, 52, 182. B.S. T e u k e r b l a t , M.I. B e l i n s k i i , B.Ya. Kuyavakaya, and V.E. F a i n z i l ' b e r g , a e m . P h y s . Lett., 1983, 98, 149. C.E. Zkepel, J. Chem. P h y s . , 1984, 3978. R. Murugeaan and E. d e Boer, Chem. P h p . L e t t . , 1983, 95, 301. N.M. A t h e r t o n and M.C.B. Shohoji, J. Chem. Soc. P a r a d a y T r a n s . 2, 1983, 29, 1243. B.L. Bales a n d D . W i l l e t t , J . Chem. P h y s . , 1984, 2997. N.M. A t h e r t o n and C. O l i v a , J . Chem. Soc. F a r a d a y Trans. 2, 1983, 79, 167. 6 . B . Garrett a n d W.M. G u l i c k J r . , J. Chem. Soc. F a r a d a y Trans. 1, 1983, 3, 1733. S.R. H a r r i s o n , R.S. P i l k i n g t o n , a n d L.H. S u t c l i f f e , J . Chem. Soc. P a r a d a y T r a n s . 1, 1984, 8 0 , 669. T. Chen. F. G r a f , a n d H8.H. G u n t h a r d , Chem. P h y s . , 1983, 3, 165. M . F . O t t a v i a n i , P . B a g l i o n i , a n d G . M a r t i n i , J. Phys. Chem., 1983, g , 3146. E. W i r o v i t c h , J. Phys. Chem., 1983, 87, 3310.

z,

A,

e,

so, s,

z,

-

so,

e,

s,

13

1 Theoretical Aspects of E.S.R

C.C. Wu, W.G. M i l l e r , and R.P. Maeon, J. Phvs. Chem., 1983, 46. J.M. 2-1, 87, 5435. 47. E. Meirovitch, J. Phya. Chem., 1984, 88, 2629. 48 * M.S. Broido and E. Meirovitch, J. Phva. Chem., 1983, 87, 1635. 49 * S.A. Yager and J . H . Freed, Chem. P h ~ a .L e t t . , 1984, 109, 270. 1. 50. K. Ohno and J. S o b , J. Haqn, Reeon., 1984, 51. B.J. F o r r e s t and J. Mattai, J. Phva. Chem., 1984, @a 1720. , 52. E. van d e r D r i f t , B.A.C. Rouaaeeuw, and J. Smidt, J. P h w . Chem., 1984, 88, 2275. 53. S. S c h l i c k and B.R. nccarvey, J. P h w . Chem., 1983, 87, 352. 54. A.1. Vietnes and L.R. Dalton, Mam. Re=., 19838 5 3 78. 55. L.W.-M. Fung and M.E. Johnson, J. Kaqn. Reeon., 1983, S l , 233. 56. G . Vriend, J . G . S c h i l t h u i a , B.J.M. Verduin, and M.A. Bemninga, J . naPn. Reeon., 1984, 58, 421. 57. A.B. Beth, K. Balaaubramanian, B.H. Robineon, L.R. Dalton, S.D. Venkataramu, and J . B . Park, J. W e . Chem., 1983, 87, 359. 58. P. F a j e r and D. Mareh, J.Nam. Reeon., 1983, Q, 446. 59. M. Delmelle, J. m q n . Reeon., 1983, S l , 245. 60. B.H. Robineon, J. Chem. P h w . , 1983, 7 8 , 2268. 61. M.A. m i n g a , J.H. Minders, and P.A. de J a g e r , J. Magn. Reeon., 1984, 58, 428. 62. P.A. Sehr, C. Mailer, and P.F. Devaux, J . m n . Reeon., 1983, 52, 23. 63. P. F a j e r and D. Narah, J. Ham. Reeon., 1983, 55, 205. 363. 64. L.I. Aorvath and D. Marsh, J. Maan m a o n . , 1983, 65. S.A. Deuba, A.G. Maryasov, K.M. Salikhov, and Yu. D. Tavetkov, J. mqn. Remn., 1984, 58, 95. 66. B.Rakvin, Chem. P h m . Istt., 1984, 109, 280. 67. J.P. Bornak and J . B . Freed, Chem. Phw. Lett., 1983, 101, 115. 1695. 68. A. Baram, J. Phw. Chem., 1984, 69. G.G. m s c h , M. Llshring, J.U. von Schuta, and H.C. Wolf, Chem. Phye., 1984, 85, 333. 70. C.S. N a r a a i m h a n , M. Narayana, and L. Icevan, J. Phya. Chem., 1983, 87, 984. 71. A.A. Shubin and S.S. Dikanov, J . Maan. Reaon., 1983, 52, 1. J. chep. phys., 1984, 80, 4044. 72. M. i(#aanelli, M. Narayana, and L. -van8 73. S.A. Dikanov, A.V. Astashkin, and Yu.D. Tavetkov, Chem. Phw. I m t t . , 1984, 105, 451. 74. G.R. Steveneon, J.B. Sedgwick, and R.C. miter, J. Ph~e.Chem., 1984, 88, 1347. 75. M. Branca, A. Ganba, and C. Oliva, J. m g n . Reaon. , 1983, 54, 216. 76. M. Branca, A. Gamba, C. Oliva, and M. Simonetta, Chem. Phya., 1983, 75, 253. 77. M. Barzaghi, S. Mlertus. C. Oliva, E. O r t o l e v a , and M. Simonetta, J. phys. 1983, 87, 881. 78. H.-D. Aberle, R. Icnopp, and A. WUllet, -1. PhW., 1984, 518 1193. 79. D. G r i l l e r , D.C. Nonhebel, and J . C . Walton, J. Qlem. Sac. P e r k i n Trana. If, 1983, 1373. 80. M.D. S e v i l l a , 0. Becker, C.L. S e v i l l a , and S. Swarte, J. PhY8 Chem., 1984, 88, 1701. 81. K J . ~araoa, D. m n n a , and B.C. webater, J. mem. SOC. p a r d a y mahe. 1, 1984, 8 0 , 267. 82. P. Geraon, W. Ruber, W.B. Martin Jr., P. Caluwe, T. Pepper, and M. Szwarc, mlv. chirp. m a , 1984, 67, 416. 83. J. P o t t i n g e r and It. I s n d i , J. Wasn Reaon., 1984, 5 8 , 502. chea. PhW. Istt. , 1984, 108, 266. 84. J.A. 85. U. E l i a v and J.H. Freed, J. PhVe. Chem., 1984, 88, 1277. 86. R. Baer and B. Paul, Chem. PhYs., 1984, 87, 73. 87. R.Z. Sa@eev, W. Wl, and K . W i u a , J . Phw. Chen., 1983, 87, 3183. 88. K.A. E L a u c h l a n and G.R. Sealy, -1. Phve., 1984, 52, 783. 89. s. Baeu and K.A. Ilclauchlan, J. Chem. Soc. Perkin Tram. 11, 1983, 855. 90. S. Barru and K.A. BWLauchlan, J. lFasn. lbeeOn., 1983, 5 1 8 335.

x,

J.

.

a,

e,

-

e.,

.

wage,

.

Electron Spin Resonance

14

91.

S . m u , K.A. McLauchlan,

and A . J . D . Ritchie, Chem. Phye. Iatt. , 1984, 105,

447. 92. s. m u , K . A . McLauchlan, and G.R. s e a l y , -1. m e . , 1984, 52, 431. 93. K.A. McLauchlan and A . J . D . Ritchie, J . Chem. Soc. Perlcin Trans. 11, 1984, 275. 94. 5 . m u , A.A. WLaUchlan, and A . J . D . Ritchie, C h a . P h Y e . , 1983, 79, 95. Phm., 1984, 86, 323. 95. C.D. Buckley and K.A. McLauchlan, Ch-. 96. S. m u , A . I . Grant, and K.A. McIauchlan, Chem. Phye. L e t t . , 1983, 94, 517. L e t t . , 1983, G, 120. 97. A . I . Grant and A.D. McLauchlan, C h a . Ph-. 98. M. Anpo, K.U. Ingold, and J . K . S . Wan, J. PhF. Chm. , 1983, 87, 1674. B.B. Meleke, D. W e i r , M.C. Depew, and J . K . S . W a n , Can. J . Chem., 1984, 62, 99. 117. loo. M.C. Depew, M. Chan, and J.A.S. Wan, J. H+%3n Beon., 1984, 57, 297. 101 Y.C. Depew, L. Zhongli, and J . K . S . W a n , J. Am. am. S oc., 1983, 2480. 102. A. Icanemoto, S Niieuma, S. Iconishi, and 8 . Kokubun, Bull. Chem. Soc. Jpn., 1983, 55, %. 103. D. Weir and J . K . S . Wan, J . Am. Chem, S o c . , 1984, 106, 427. 104. S. Emori, J.P. Colpa, and J . K . S . W a n , Chem. Phw. Istt., 98, 1983, 142. 105. J . L . Courtneidge, A.G. Daviea, T. Clark, and D. Wilhelm, J . Chem. Soc. Perkin Trane. XI, 1984, 1197. 106. Paraday Discweion No. 78, Leicester, 4th-6th Sept. 1984. 107, It. Toriyama, K . Nuname, and M. Xwasaki, Chem. Ohm. L e t t . , 1984, 107, 86. 108. W . J . Bouma, D. Poppinger, 5 . Saebo, J . K . Macleod, and L. Man, Chem. PhW. L e t t . , 1984, 101, 198. S o c . , them. Commun., 1984, 666. 109* T. C l a r k , J. Ch-. 110. L.D. Snow, J . T , Wang, and P. W i l l i a m s , Chem. Phm. L e t t . , 1983, 100, 193. M.B. Hung, 5 . Lunell, and A. Lund, Chem. m e . I&&,, 1983, 99, 201. 111 112. K . ohta, ti. N a k a t s u j i , ti. Kubodera, and T. Shida, Chem. Phvs., 1983, 76, 271. 113. M.R. IIoffmann, W.D. Laidig, K.S. Kim, D . J . Pox, and 8.P. Schaefer 111, J. Chem. Phye., 1984, 80, 338. L e t t . , 1984, 107, 568. 114. T . l&mo8e, T . S u u k i , anf T. Shida, C h a . Ph-. 115. L.B. Knight Jr., J . Steadman, D. P e l l e t , and E.R. Davidemn, J. Am. ChePI. 1984, 106, 3700. 116. L.B. Knight,Jr., J . Steadman, P.K. Miller, D.E. Bowman, E.R. Davidson, and D. F e l l e r , J. Chem. Rive., 1984, 80, 4593. 117. L.B. ICnight,Jr. and J . Steadman, J . C h a . Phva., 1984, 8 0 , 1018. 118. S.A.Fairhuret, R.S. Pilkington, and L.H. S u t c l i f f e , J. Chem. S o c . , paraday Trane. 1, 1983, 79, 925. 119. D.R. Amstrong, C. Cameron, D.C. N o n h e b e l , and P.G. Perkins, J. Chem. Soc. Perkin Trane. X I , 1983, 569. 120. F.K. c a r t l e d g e and R.V. Piccione, Orsanaraetallice, 1984, 2, 299. 121. B. Gillon, P. Becker, and Y . E l l i n g e r , M o l . Phve., 1983, 40, 763. 122. Y. Miura, 8 . Asada, M. Kinoahita, and K. Ohta, J. ohye. Chem. , 1983, 87, 3450. 123. El. N a k a t w j i , K . Ohta, and T. Yonezawa, J. M e . Chem. , 1983, 87, 3068. 124. P. Botschwina, J . Plesch, and W. Meyer, Chem. Phva., 1983, B,321. 125. R.M. Stratt and S.G.Deejardin8, J. Am. Chem. Soc. , 1984, 106, 256. 126. D.A. Garland and 0.1. Lindeay, J , Chem. Phw., 1983, 3,2813. D.A. Garand D.M. Lindeay, 'J. chem. Erhya 1984, 80, 4761. 127 128. J . Weber, Y . Corioley, and M. Geoffmy, Chem. Phvs. L e t t . , 1983, 9 6 8 636. 129. L , Noodleman and E.J. Baerende, J. Am. chem. Soc., 1984, 106, 2316. 130. S. BrunJly, J . Phw. am., 1983, 8 7 , 1917. 131. A. Itaseg-, ti. m a , M. Hayashi, and W.C.R. Symons, ml. PhYS., 1983, 50, 1273, Samekag and L.D. ICiBpert, J. W e . Chem., 1984, 88, 1385. 132. 133. p . 4 . samskog, L.D. mapert, and 8. m l y a n a r a m n , J . chen~.soc., paraday Tr-. 2, 1984, 80, 267.

.

e.,

.,

r-0.

m,

1 : Theoretical Aspects of E.S.R

and M.C.R. Symbns, J. Chem Soc.. P a r a d a y Tr?ms. 1, 1983, 7 9 8 1565. 135. A. Basegawa and M.C.R. sym0ti88 J. Chm Soc.. Paradav Tr?ms. I, 19838 7 9 8 93. 136. A. Hasegawa, T. ooakabaykehi, M. m a S h 1 , and M.C.R. Symone, J. chem Sot., F a r a d a Y T r a n s . 1, 1983, 7 9 , 941. 8 137. B.W. Walther, F. W i l l i a m s , and D.M. Lmal, J. Am. Chem. m . 8 19848 E 134.

A. Basegawa

548 *

138. M.D. S e v i l l a and H. McGlaehen, J. Ph~8.Chem., 1983, 87, 634. 139. J.C. Brand, M.D. Cook, and B.P. m r t s , J. Chem. Soc. P e r k i n Trans. 11, 1984, 1187. c. Roberts and J.C. Walton, J. Chem. Soc. P e r k i n Trans. XI, 19838 879. I . G . Green and J.C. Walton, J. Chem. Soc. P e r k i n T r a n s 11, 1984, 1253. C. G l i d e w e l l , J.Chem Soc. P e r k i n Trans. I f , 1984, 407. C. G l i d e w e l l , J. Chem. Soc. P e r k i n T r a n s . I f , 1983, 1285. 143. 144. C. G l i d e w e l l , J. Chem. S o c . P e r k i n Trans. 11, 1984, 1175. 145. D. Wilhelm, J.L. Courtneidge, T. Clark, and A.G. D a V i e S , J. Chen~. S oc., Chem. Camaurn., 1984, 810. , Brunton, A.R. F o r r e s t e r and J.D. F u l l e r t o n , J, cheoa. Soc. 146. ~ . ~ . B e n n e t tG. P e r k i n Trans. I f , 1983, 1481. 147. c.L. Honeybourne, H o l . Phva., 19838 5 0 8 1045. PhW., 1983, 79, 5752. 148. B . S . Prabhananda, J. Ch-. r J. them. PhYS., 19848 8 0 8 3078. 149. C . C . F e l i x and B.S. 8-P 150. C.C. P e l i x and B.S. Prabhananda, J. Mam. Reson. , 1984, 57, 146. 151. M. B r u e t o l o n and A.L. Maniero, J. Uagn * r(Stwxt.8 1983, =# 1%.

15

2 Transition- metal Ions BY D. GATTESCHI

1 Introduction

The general considerations which I made at the beginning of the last report still apply, therefore the general structure has not been changed, with the exception of the addition of a paragraph regarding the EPR spectra of metal ion complexes with organic radicals. Indeed the EPR spectra of transition metal complexes are now

rarely

of

great

importance when

used

E, but

interest per to

clarify

the

they

structure

may

of

acquire

biological,

catalytic, mineral systems, or the electronic structure of exchange coupled species. General reviews have been r e l a t i v e l y r a r e i n t h e p e r i o d covered

by

this

report.

Buckrnaster

reviewed

the

EPR

literature

on

transition metal, lanthanide and actinide ions in solids for the year

1978l

while

a

useful

source

of

references

to

the

literature can be found in the annual review reported by Wasson

EPR

.

2

Several review articles dealt with various applications of EPR spectroscopy to biological systems. The role of the manganese(I1) ion as a magnetic relaxation probe in the study of biomechanisms and of biomolecules was reviewed by Tiezzi et a13. Both EPR and NMR data were

discussed, particular

emphasis being placed

on

the

manganese(I1)-ATP complex. Reference to EPR data can be found in a 4 review on metalloproteins with phenolate coordination , in one on 5

the study of zinc enzymes through metal substitution , and in one about

modeling

the

metal

centre

of

molybdenum

Dickinson and Symons exhaustively reviewed the ESR 7

haemoglobin and myoglobin

6

hydroxylases

.

spectra of

. 16

[For references see p . 70

17

2: Transition-metalIons

Rhodium carboxylates, including also complexes in which the metal ions are bound to organic radicals, were described, and EPR 8

data given a relevant place

.

Bramley and Strach, in a comprehensive review, described the theory, instrumentation, and applications of EPR spectroscopy at zero magnetic field. They showed that it is now fairly easy to use this technique which is particularly well suited for systems with

sal

9

which undergo significant zero field splitting

.

2 General

E x p e r l m e n t a l T e c h n i q u e s . - New techniques were developed to increase

the resolving power of the traditional EPR experiment. One of these is based on the use of paramagnetic resonance absorption of Zeeman modulation

energy''.

This method,

together with

a

super-high

frequency saturation was employed to separate overlapping spectra 11,12 It is even possible to of different paramagnetic centres

.

solve the problem of two species with close g values by using both a

modulation

spectrometer

concentrations and

the

a

and

conventional

spin-lattice

relaxation

one,

if

the

times

of

the

paramagnetic centres are very different from each other13. The method

of

plotting

dispersion

z.

absorption for

a

spectrum

(DISPA), already used to diagnose overlapping Lorentzian lines, was 14

extended to detect and characterize overlapping gaussian peaks A

.

difficult problem in single crystal EPR spectra is that of

disentangling the signals belonging to different magnetically non equivalent sites. Schweiger recently described a new ESR technique, called

'orientation modulation

suppression

of

over lapping

ESR' single

which

allows

crystal

selective

spectra

and

characterization of anisotropic spin systems by a smaller set of 15 crystal orientations than required in conventional ESR

.

Methods and apparatus for the quantitative determination of spin

concentration16

and

spin

density

distribution17

were

described. Schults and Gullikson presented a method for measuring the

dc

magnetization

of

a

sample by

using

conventional ESR

Electron Spin Resonance

18

spectrometer. Magnetization is obtained simultaneously with the ESR spectrum of the sample. The method can also be used for samples not exhibiting an ESR signal18. Methods in which the time evolution of transient magnetization is observed, including delayed transient nutation, free induction decay, and electron spin echoes, were 19

reviewed

.

Two simple methods for determining the microwave magnetic field at the sample position in ESR spectroscopy were described2'.

The

first method is based on the magnetization hysteresis spectrum obtained from a combination of spectra.

The

other method

is

the based

in-phase on

the

and

out-of-phase

power

dependent

linewidth. Morton and Preston reported the

application of a computer

assisted two circle goniometer, previously described2'

to the study

of the anisotropy of EPR spectra observed in single crystals of 22 various symmetry classes

.

A

new pulsed microwave acoustic method is suitable for the

detection of ESR signals. Calculations demonstrate that temperature gradients within the sample are important for generating large signal amplitudes. Hence this technique may be of special interest for samples with an inhomogeneous distribution of paramagnetic centres or for the study of

interface^^^.

The problems associated

with the formation of gaps during the pulsed action of super high frequency power

on

inhomogeneously

broadened

ESR

lines

were

discussed24. An ESR spectrometer was described with a HCN laser and an optically pumped far-IR laser as radiation sources. The latter permits measurements of magnetic resonances at many frequencies 25 throughout the far IR range

.

Heat pulses were used for spin system excitation. The time variation of the ESR signal was studied for different energies of the exciting pulses26. A new ESR

transmission spectrometer is

described, which is capable of obtaining by a single bridge and a single scan all the resonance information contained in the spin sample under the form of four separate resonance modes, e.g. the two components of the spin-magnetization vector either in Cartesian

2: Transition-metalIons

19

or polar coordinates. Its fixed frequency feature allows one to have a calibrated field scale and the system can perform multiple 27

scan averaging of weak signals buried in the noise

.

A digital phase-sensitive detector for a modified Bruker ESR spectrometer 28 described

equipped

with

an

Aspect

2000

minicomputer

is

.

The construction and

the design equations of

a

microwave

resonator which can be used as a sample cavity in a low-frequency ESR spectrometer2’ and a balanced cavity scheme for saturation 30 transfer dispersion EPR were described

.

Modifications to a commercial ESR spectrometer are described which allowed data to be obtained from 64 spectra with scan times of 0.01-0.9

s

and with 0.02-900

delay times between

s

scans.

Performance of the system was demonstrated by the determination of the kinetics of rearrangement of a bis (dipeptide) nickelate(II1) complex31. A spectrometer employing 2MHz magnetic field modulation was constructed for the study of photochemical processes involving free radical triplet species32. A rapid response ESR spectrometer operating in a new mode involving time integration of transient signals,

entirely

controlled

by

a

home-built

computer, was

described33. A few systems for interfacing ESR spectrometers with different computers were reported together with the process for handling the output data34’35’36’37’38. Short time domain ESR and 39

double resonance techniques were exhaustively reviewed

.

A novel technique involving simultaneous modulation of

the

magnetic field and the radiofrequency allows each transition to be assigned to the corresponding type of nucleus and often reduces the 40

number of transitions in complicated spectra

.

Schweiger et al. described the combination of two spectroscopic techniques, double ENDOR and ENDOR with a circularly polarized radio frequency field (CP-ENDOR) to induce selective excitation of different types of nuclei and of different, paramagnetic species. 41

The set-up of the CP-double ENDOR spectrometer is described Double

ENDOR

was

also

applied

to

improve

the

.

orientation

selectivity of paramagnetic species diluted into powders or frozen

20

Electron Spin Resonance

42 liquid crystals . An apparatus for pulse studies in magnetic fields of 50-200 Oe was described43. A modulation scheme for ELDOR spectrometers which makes the suppression of false ELDOR lines relatively easy was 44 described

.

Kasanskii discussed the advantages of the optical detection of ESR and ENDOR of transition-metal ions in conditions of powerful super high frequency field. The possible mechanisms of absorption of

the

low

frequency

field

in

dielectric

crystals

are

also

discussed45946. A deconvolution method for hyperfine patterns in ESR imaging is discussed. An exact expression is derived for the 47 total inverse filter function

.

Ligand Field and Molecular Orbital Models.-Ligand Field models are now less generally used and several different MO approaches are now gaining in poularity. Soviet '

authors

advocate

the

use

of

the

Angular

Overlap

Model48y49. Crystal Field calculations were used to monitor the 50

energy levels in distorted tetrahedral copper(I1) complexes

.

A

simple crystal field treatment yielded satisfactory results when applied to the interpretation of the EPR and Mossbauer spectra of high-spin ethylene bis(2-hydroxy phenyl glycinato) iron(II1)

51

.

Accurate & initio SCF CI calculations were performed on CuF 2' The calculated CI g values compare well with the experimental ones: g =1.93 I1

s.1.91, gL-

2.76 5.2.6052. EPR

data were

used

to

confirm the nature of the single occupied molecular orbital in dirhodium tetracarboxylate cations 53 calculations . INDO

obtained

through

ab

initio

calculations were performed on eight-coordinate sulfur

chelate complexes of molybdenum(V) and molybdenum(1V). The observed 95M0 and

97M0 hyperfine

splittings have

been compared 54 coefficient of the xy metal orbital in the HOMO .

to

the

Multiple scattering Xa methods have been used to some extent, due

to

the

ease with

which

consideration. Weber and

heavy

metal

coworkers showed

ions how it

are

taken

into

is possible to

2: Transitionmetal Ions

21 55

calculate hyperfine couping constant within this framework the method was applied also to Co(acaacen) 56

bis

acetylacetoneiminato)

relativistic X

.

An

, and

(acacen=N,"-ethylene

interesting

application

calculations has been made on IrC16

2-

of

for which

the spin orbit coupling constants were computed in the various MO's rather than assumed as it is generally done.at the non-relativistic The low copper hyperfine splitting in pseudo-tetrahedral CuC14

2-

ions has been attributed to covalency effects, and not to 58

the 4p metal orbital mixing on the basis of X Noodleman and Baerends used LCAO-Xa

calculations

.

valence bond calculations

to describe the electronic structure of [Fe S (SH) J2-'3- as models 2 2 4 2-Fe ferredoxins. They calculated the

for the active site of

isotropic exchange coupling constant and also the g and A tensors, 59

in good agreement with experimental data The

validity

of

the

point-dipolar

. approxiamtion to the 2+ was confirmed

electron-proton hyperfine interaction in [VO(H 0) ] 2

5

by MO calculations60. Chinese authors used MO models to calculate EPR spin hamiltonian parameters for high spin chromium(II1) and 61,62 cobalt(I1) ions

Jahn-Teller.

-

Theoretical treatments of the Jahn-Teller effect

have been less numerous during the period relevant to this report. An

interesting

treatment has

appeared

regarding

the

magnetic

properties of pairs of JT centres63. A review has appeared dealing with JT centres in sernicondu~tors~~ and spin-lattice relaxation data for Mn+ centres in silicon were reported65. Calculations were 66

made for the vibronic reduction factors in the ESR of JT centres

.

Octahedral copper(I1) is still one of the favourite subjects of investigation, but lower coordination numbers are also now analyzed in terms of JT o r pseudo JT effects. Reinen suggested that the trigonal bipyramidal structure of [CO(NH ) 1LCuC1 1 seen at room 3 6

5

temperature is the result of a dynamic averaging over elongated square pyramids, as a consequence of SeCGnd order JT effects67. A similar

model

comp1exes68 ' ".

was

used

also

for

other

five-coordinate

A two-dimensional dynamical JT effect was assumed

Electron Spin Resonance

22

to be responsible for the reversed spectrum (i.e. with g,, i gL) observed

above

128 K

for

CU~(BTA)~(RNC)~ (R=tert-butyl; BTA=

c)n

benzotriazolato(1-)

I".

Correlations of distortions of JT copper(I1) centres in K ZnF 2 4 crystals at distances exceeding the radius of indirect exchange interactions were observed".

CuZrF

6

and CrZrF

were

found to

undergo phase transitions between 100 and 450 K which are induced 72 static JT distortions by changes from dynamic to 2+ Cu(im)6 (im=imidazole) was shown to undergo a strong JT coupling.

.

The

three wells

energies,

in

but

the potential

two

lie

surface have

lower

and

not

one

the

same 73

higher

.

copper(I1) complexes are still actively

Hexakis-pyridine-!-oxide

investigated: a neutron diffraction and EPR study showed the loss of the room temperature trigonal symmetry and the formation of three domains associated with static JT distorted

structure^^^.

An

Extended Huckel MO treatment was used to correctly predict the sign of the JT

distortion^^^.

Finally the EPR spectra of pairs

of

76 77,78

Cu( pyNO)62+ were studied

The distortions of hexa-aquo copper(I1) ions were observed in 79,80 81 The ZnTiF6.6H20, ZnGeF6.6H20, ZnZrF6.6H 20, and ZnSiF6.6H20

.

ferroelastic properties of the orthorhombic phase of Rb2PbCu(N0 ) 2 6 were attributed to cooperative JT effects82. Nitrogen hyperfine splitting could be observed in63Cu doped Rb Cd (N02)6 confirming 2 2 83 the presence of tetragonally elongated CuN6 chromophores

.

The

nickelocenium

cation

doped

into

several

diamagnetic

lattices showed dynamic JT effects. Extended Huckel and MS Xa calculations were

used

to

estimate

the covalency

effects

and

compare them with those of c ~ b a l t o c e n e ~A~ .variational approach was used to calculate the relevant EPR spin hamiltonian parameters of the latter compound85. Dynamic JT effects were invoked also for 86 1,l-dimethyl-chromocene 2Re04 ions, formed in X-irradiated KC1 and KBr, and

.

palladium(1) doped 87,a8,89 distortions

in

AgCl

were

shown

to

undergo

JT

2: Tmnsition-metalIons

23

Spin Hamiltonian, Analysis of Spectra and Coqut1ng.-ESR

and ENDOR

transition probabilities for a general spin hamiltonian and various modulation schemes, explicitely containing zeroth- and first-order contributions,

were

expressed

90

transform technique

using

a

generalized

operator

.

The effect of small distortions from cubic symmetry on the EPR line positions

for 2=3/2 was calculated using

a

perturbative

approachg1. Low symmetry effects on the line intensities were also taken into considerationg2. Closed form solutions for the resonant 93 fields of triplet state EPR spectra were given

.

The ambiguity inherent in the fitting of the spin hamiltonian parameters

using

94

discussed

.

numerical

diagonalization

techniques

was

The method of determining the parameter errors in a

rigorous least-squares fitting procedure was described and applied to the single crystal spectra of C2N3H3.CuC1295. nuclear quadrupole interaction on the EPR treated within a

The effect of

spin hamiltonian was

second order perturbation, yielding

accurate

results in a much shorter time compared to exact diagonalization 96

techniques

.

It is now more and more clearly realized that single crystal spectra are really needed to fully understand the spin hamiltonian parameters of transition metal complexes, and the articles dealing with the technical details are multiplicating. In one of these

g . advocate a procedure which uses rotations around 97 three axes determined with a single crystal X-ray diffractometer

Strouse

.

Polish authors examined the conditions under which two magnetically 98 non equivalent sites yield one line in a single crystal spectrum

.

A

review was published dealing with computer simulation of EPR

and NMR

spectra".

described'

Computer generated plots, obtained

procedureloo

were

reported

to

with

interpret

a

the

polycrystalline powder EPR spectra of systems with 1LSh5/2 in axial crystal fields"'. patterns described'**.

using

A

a

computer program for analyzing EPR powder non-linear

least-squares

procedure

was

The conditions under which extra divergence peaks due

to angular anomalies occur in the EPR spectra of axial systems with

Electron Spin Resonance

24

S=x

were discussed!03

and

applied

to

the

analysis

of

copper(I1)

s p e c t r a l o 4 . A method o f s i m u l a t i n g t h e EPR s p e c t r a o f S-=% 105 disordered s o l i d s w a s described .

on t h i s s u b j e c t a r e s t i l l i n c r e a s i n g

O l i g o n u c l e a r Complexes.-Papers i n number.

ions i n

I n o r d e r t o cope w i t h t h e i n c o n v e n i e n c e s d e t e r m i n e d by

t h e use of d i f f e r e n t forms f o r t h e i n t e r a c t i o n s p i n h a m i l t o n i a n i n the

literature

we

give

will

here

the

form

of

the

bilinear

h a m i l t o n i a n we w i l l u s e t h r o u g h o u t t h i s r e p o r t :

+ S .D.S

H = J S S -1'-2

-1

and w i l l r e f e r

+ d.S x s

-2

t o the

-1 -2

t h r e e t e r m s as i s o t r o p i c ,

anisotropic,

and

a n t i s y m m e t r i c exchange r e s p e c t i v e l y . T a b l e s o f n u m e r i c a l v a l u e s f o r t h e r e l e v a n t c o e f f i c i e n t s needed to write

the

effective

spin operators

spanning

each

total

spin

m u l t i p l e t were p r o v i d e d l a 6 . The t a b l e s a r e b a s e d on a s e c o n d o r d e r 107 s p i n h a m i l t o n i a n formalism

.

The

effect

of

antisymmetric

exchange

on

the

spectra

EPR

of

c o u p l e d p a i r s o f t r a n s i t i o n m e t a l i o n s h a s been c o n s i d e r e d a g a i n . to

Suggestions d i r e c t i o n s of given

108

estabilish

the

presence

its

through

z e r o f i e l d s p l i t t i n g t e n s o r of

the the

principal couple

are

.

When a d i a m a g n e t i c m e t a l i o n i s c l o s e t o a p a r a m a g n e t i c one t h e

EPR s p e c t r a c a n show t h e p r e s e n c e of a f r a c t i o n of

the

unpaired

e l e c t r o n on t h e f o r m e r n u c l e u s . T h i s w a s o b s e r v e d f o r 25Mg c l o s e t o 3+ 2+109 Cr and N i A method u s i n g t h e i n t e n s i t y o f t h e A M = 2

s p e c t r a of

systems of

two

s=x s p i n s

d i s t a n c e between t h e two s p i n s

110

was

t r a n s i t i o n i n t h e EPR

described t o

obtain

the

.

The i n t e r e s t i n t h e c h a r a c t e r i z a t i o n o f s i m p l e m o l e c u l e s formed i n matrices

i s continuing,

s h i f t i n g now t o more complex

systems.

Sc3 and Y3 m o l e c u l e s were i d e n t i f i e d , and t h e geometry a s s i g n e d on t h e b a s i s o f EPR s p e c t r a . tentatively

described

as

E v i d e n c e w a s a l s o found f o r Sc 13'11

.

Cu5,

Ag5,

and

Mn5

a species have

been

2: Transition-metal Ions

tentatively

2s

identified

on

the

basis

113y114. While for Cu5 and Ag5

spectra

of

their

EPR

trigonal bipyramidal

geometries were suggested, Mn5 is proposed to be planar pentagonal. Cu3 is found to be linear on the basis of the EPR spectra which consist of sixteen sets of quartets. The two terminal copper atoms show a much larger hyperfine splitting (A=625.5G) than the terminal one (A=55.6GI1l5. Conversely Au3 is suggested to have a slightly bent structure116. In the CrCu molecule the ground state is while it is

6r

for CrAg and CrAu, and

7r

4r

,

for CrZn. The zero field

splitting increases from -0.005 I^or CrCu to +0.084 for CrZn, to 0.44 for CuAg to =2 cm-'for

CrAu, showing the effect of increasing

spin orbit coupling1179118. A

list of diatomc molecules which

cannot be observed through EPR was also provided. In the Mn2 molecule the zero field splittings in the S=1, 5=2, and 5=3 multiplets were found to follow the Judd-Owen theory. Assuming that the spin-spin determined D value is entirely due to the through space coupling the Mn-Mn distance is calculated to be 3.4 ill4. It must be noted that the justification to assume

that the exchange

contribution to D is negligible is based on the J value relative to the ground state, and not to that invol.ving excited states as it should be. Therefore it

possible

is

that

the

long calculated

distance is the result of the neglect of exchange contributions. EPR

spectra of

[Tc2Clg]

'-,

Tc2(CH COO)4C1, 3

Tc2(CH C00)4Br, 3

[Tc2(CH3COO) C121- were used to discuss the nature of the ground 4

state

wave functions 119 complexes

in

these

multiple

metal-metal

bonded

.

Some paramagnetic metal clusters have also been studied. The Au9(PFh3)8

2+

species, formed by mixing Au (PPh ) 3+ and Aug(PPh3)8+ 3 8 9 in equimolar amounts, is characterized by g =1.923,gL=2.011120. An " 3Of CRh12i CO 13 (p2-CO)lo(C) 2J [COl,(CO) 12 ESR Study

( p -CO)12(C)2] 2

4-

and [Co,(CO),(p,-CO),C]-

was used to describe the

HOMO'S of these high nuclearity clusters121. In electrochemically generated Mo6Cl14 the EPR saectrum was assigned to an axially symmetric

g=l/,species

with g

ll

=

2.0

and g

I-- 2.10122.

EPR spectra

were also obtained for clusters based on R E C O ~ ( C O )(R= ~ methyl,

Electron Spin Resonance

26

phenyl; E = Ge, P ) , and RECo2M(CO) Cp (R=methyl, phenyl; E=C, Ge; M

a

Mo,

=Cr,

W;

Cp=cyclopentadienyl) 123 phenyl; M=Fe,; M'=Mo, Ni)

and

RPCoMM'(C0) Cp(R=methyl,

a

.

Copper(I1) dinuclear and oligonuclear species are by far the most studied, as usual. In a brief review the factors which affect the appearance of the EPR spectra of exchange coupled pairs of transition metal ions were resumed, with examples taken from Cu 2 124 species . An

EPR

unusual

[M(en)3]2(Cu2C18)C12.2H

spectrum 0 (.M=Co,

was

observed

for

dimeric

Rh, Ir; en= 1,2-diaminoethane).

2 The powder spectra show four features which are reminiscent of a triplet spectrum, but the single crystal spectra show that the four features

are

Interdimer

due

to

exchange

turning

points

interactions

of

a

average

doublet

the

spectrum.

transitions

of

different sites in the crystal in two of the principal planes, but not in the third intermolecular temperature, results

125

.

An analysis of the line shapes yielded an

exchange

decreasing

were

found

interaction 1 inearly

of

was

[Cu(Et3en)Ci2I2

cm

at

with

for

room

Similar

inter-dimer

bis(diethyldithiocarbamato)copper( I d z 7 .

features

-1

0.072

exchange

No evidence of

in

triplet

found for the dichloro bridged complex 128 (Et en= N,N,N'-triethylethylenediamine) . 3

A correlation between anisotropic exchange and structure in a series

of

di-p-hydroxo bridged

copper(I1)

complexes

has

been

suggested on the basis of single crystal and polycrystalline powder spectra. The zero-field -1

cm

splitting parameters

are

large

(D) 1.1

) , with the largest value observed parallel to the direction of

maximum g splitting, and orthogonal to the Cu-Cu direction. The zero

field

splitting tensor has

been

attributed

mainly

to a 2 2 ferromagnetic exchange interaction involving the ground x -y 129 orbital of one ion with the excited xy orbital of the other ion

.

Similar comp1exes

conclusions 130,131

were

reached

also

for

A small zero field splitting of 7 6 ~ 1 0 -cm~

some 1

di-p-methoxo

was attributed to

a dipolar interaction between two copper ions in the complex shown

27

2: Tmnsition-metal Ions

below

132

,

while D=0.42 copper( I1 )

ern-' was

found for a ferromagnetic di-p-azido bridged In an

complex133.

adenine bridged

species,

with

a

structure similar to that of copper acetate hydrate D was found to -1 134 be 0.10 cm EPR was used to show the pH dependence of the formation of imidazolate

bridged

binuclear

aminocarboxylates135y136. copper(I1) 137 species Both

peptide

Similar

systems,

copper(I1) studies

showing

the

complexes

were

of

performed

formation

of

on

dimeric

.

homo-

porphyrins

were

and

hetero-dimers

found

to

be

of

copper(I1)

formed

in

and

silver(I1)

solution

by

EPR

Mono- , di- and polymeric species were found to form

when

water

soluble

copper(I1)

and

oxovanadium(1Vj

phthalocyanine and porphyrazine chelates are absorbed on Sephadex 139 resins . The

EPR

spectra

oxovanadium(1V) oxalato

and

of

copper(I1)-copper(II),

copper(I1)-oxovanadium(1V)

ligands showed

in

any

case

a

very

oxovanadium(1V)pairs

bridged

small

zero

by

field

splitting140. EPR was used to show the presence of Cu-Cu and VO-VO species absorbed on silica surfaces141.

The formation of mixed

copper(I1) oxovanadium(1V) and copper(I1)-manganese pairs in borate glasses was observed through EPR spectra142,143. EPR spectra were reported also for the following compounds: Itetrakis(p-crotonato)bis(quinoline)dicopper(II)

144

Itetrakis(p-N-benzoylvalinate(i-!)bis(ligand)dicopper(II)

145

ligand=pyridine, 3 - , 4-methylpyridine di-p-azido-bis((N,N,N'N'-tetramethyethylenedi~ine)azido copper(I1) 146

Electron Spin Resonance

28

tetrakis(p-aminoacidato)bis(aquo)dicopper(II)

147

N-benzoyl-DL-alanine;

aminoacid=g-ncetyl-DL-alanine;

N-benzoyl-alanine N-3-methyl-salicylidene-

(L=g-salicylidene-L-methionine;

CuL

L-methionine; 2-salicylidene-L-leucine) 149 copper mandelate various copper acetate type complexes

148

150-156

CuL

H L=Schiff bases derived from (+)-(hydroxymethy1)menthone; 2 (+)-(hydroxymethy1)camphor; acetylacetone; benzoylacetone;

salicyladehyde;

2-hydroxy-1-naphthaldehyde 157

and

F12NROH :

R=trimethylene; 2-phenylene

158 [Cu(NMIz) (pnf) ] pnf=_p-N02-phenolato 2 2 2 Oligonuclear species involving iron

atoms

are

actively

investigated, because of their relevance to naturally occurring enzymes and proteins. Holm and coworkers reexamined the electronic p;.operties of single and double MoFe3S4 cubane type clusters. As far as the EPR spocki'a are concerned, double cubanes including

{M=Mo, W ; R'= alkyl; cat= catecholate) (SR) (3,6-R' 4 2 display complex powder spectra, presumably due to weak subcluster coupling. The spectra of single cubanes can be readily interpreted within a 2 = 3 / 2 spin hamiltonian formalism, split by a large zero field splitting interaction

159

.

The EPR spectra of the [Fe(MoS4)2]

-S = 3 / 2

3-

species correspond to a

ground state, largely split in zero field. The data are

interpreted describing the ground state as originating from the antiferromagnetic coupling of two

s=x spins

on the molybdenum(V) 160 . Similar results

ions with the 2=5/2 of the central iron(II1) 3- 161 were obtained for [Fe(WS4)2] The

EPR

spectra of the chemically reduced 2(Y = C1, H , CH3 ) complexes [Fe2S2(SC6H4Y-p)4]

forms

of

the

29

2: Transition-metal Ions

r

1

2-

L

of

and

[Fe2S2 [ ( s c H ~ ) ~ c ~ 2] H ~2-- ~ Jshowed

s=% s p e c t r a ,

anisotropic

with

g values, g =2.001-2.002; g =1.952-1.958; g3= 1 2 1.911-1.915. The t e m p e r a t u r e dependence of T w a s used t o e v a l u a t e -1 t h e e n e r g y of t h e e x c i t e d q u a r t e t , o r i g i n a t i n g from t h e i n t e r a c t i o n

2

of t h e

= 2 and S =5/2 i n d i v i d u a l

'162 mechanism

, 163 -2

s p i n s t a t e s , assuming an Orbach

The EPR s p e c t r a o f ( E t N ) [Fe S (S-p-C H B r I 4 ] i n t h e s o l i d and 4 4 6 4 4 3 i n s o l u t i o n have been used t o c h a r a c t e r i z e a s t r u c t u r a l change which is d i s c u s s e d i n the frame o f t h e problem o f e l e c t r o n t r a n s f e r 164 i n ferrecioxins , pairs

Cr3+-Cr3+

in

through t h e i r EPR s p e c t r a The EPR s p e c t r a o f Co were

obtained

using

[ A l ( H 0 ) ] (N03)3.3H30 2 6 165

were

characterized

.

2+

p a i r s i n cadmium c h l o r i d e t y p e c r y s t a l s

thermally

detected

EPR.

No

spectrum

a t t r i b u t a b l e t o n e a r e s t n e i g h b o u r p a i r s were o b s e r v e d 166. The EPR spectra

of

nickel(I1)

pairs

in

KZnF3

were

i s o t r o p i c exchange c o u p l i n g c o n s t a n t , J= 83 crn given

for

indirect

exchange

used -1 167

to

.

of

interactions

derive

the

Formulae were vanadium(II), 168

m a n g a n e s e ( I I ) , and n i c k e l ( I 1 ) i o n s i n c u b i c c r y s t a l l i n e f i e l d s parameters

EPR

were

determined

for

impregnated on p o l y c r y s t a l l i n e Ti02 c a t a l y s t s

molybdenum(V) 169

.

pairs

.

Although t h e t e m p e r a t u r e dependence of t h e EPR s i g n a l i n t e n s i t y in

principle

splitting

in

allows pairs

the of

determination

interacting

of

the

s=?$s p i n s ,

singlet-triplet

the

r a r e l y u s e d due t o s e v e r a l e x p e r i m e n t a l d i f f i c u l t i e s .

et al.

technique

is

Hendrickson

measured t h e EPR s i g n a l i n t e n s i t y i n a s e r i e s o f c a r b o x y l a t e

bridged dicyclopenta dienyl titaniurn(II1)

complexes t o e v a l u a t e J

Electron Spin Resonance

30

values of 0.74, 0.84, 1.12 cm-', as

0.6 cm-l

EPR

170.

but the estimated error was as

spectra

also

used 171 characterization of analogous vanadium(II1) complexes

high

were

for

the

.

Tris(~-hydroxo)bis[l,4,7-trimethyl-1,4,7-triazacyclononane)

chromium(III)]tripe~chlorate

trihydrate

was

found

to

display

antiferromagnetic coupling between the two metal ions, with J= 128 -1

cm

, but the fit of the temperature dependence of

susceptibility required

-

j(S,.S,)',

the

with j=-1.8 cm-'.

inclusion of

the magnetic

biquadratic

exchange,

The polycrystalline powder EPR spectra

were simulated to give D=0.34 cm-l

and D =0.18 cm-l where

the

former is the single ion and the latter is the interaction zero 172 field splitting parameter . Manganese(I1) pairs in tetramethylammonium trichloro cadmate were described by a spin harniltonian for strong exchange, including 173 exchange striction effects . Dialkoxy and dihydroxy bridged iron(II1) complexes yield broad 174,175 EPR signals in the g=2 region

Metal Ion-Organic Radical Interactions.-The number of articles in which

the

EPR

spectra

are

used

to

monitor

the

spin-spin

interactions between transition metal ions and organic radicals is increasing, mainly as a consequence of the interest in this area for several biological applications. In fact most of the studies are about nitroxide and serniquinones: the former as a consequence of their use as spin probes and spin labels, the latter for their role in the photosynthetic mechanisms. Eaton and Eaton are the authors of a long series of papers in which various nitroxide ligands were found to have small exchange -1 interactions (typically of the order of lo-' cm ) with various metal ions, such as oxovanadium(IV), nickel(II),

copper(II),

silver(II),

with metal-radical distances ranging from

9

to

A176-182.

Similar results were obtained also by Soviet183-187

Austrian

authors188.

On

the

other

hand

in

15 and

bis( di-tert-butyl

nitroxide) cobalt(I1) bromide fairly strong exchange

between a

high spin cobalt(I1) and two nitroxide ligands was found on the

31

2: Transition-metal Ions

.

basis of EPR and magnetic susceptibility measurements189

The EPR spectra of frozen solutions o f Mn (ASQl4L2 (ASQ = 2 2-acetyl-1,4-benzosemiquinone) were interpreted as due to an &3 state originated by the interaction of two S=5/2 ions with four &% 190

radicals

.

0 0

Weak i r o n ( 111) semiqulnone exchange interactions of the order

of 0.1 cm’l

were

found in photosynthetic bacterial cells and

chromatophores of R. viridislgl. A review article dealing with the structure and stereochemistry of paramagnetic metal complexes with 192 o-quinones has appeared

.

Mn4(3,5-DBSQl8 semiquinone)

contains

(3,5-DBSQ=

3,5-di-tert-butyl-1,2-benz.o-

manganese(I1)

and

semiquinone

ligands.

Treating this compound with pyridine yields a manganese catecholate species, as confirmed by EPR spectralg3. Also chromium and vanadium complexes were characterized by EPR spectroscopy194,195 Triplet EPR spectra were observed when oxovanadiurn(1V) tetra phenyl

porhyrine was

Y-irradiated

77 K.

at

The

spectra were

attributed to spin-spin interactions between the oxovanadium(1V) 196 and ligand radical unpaired electrons

.

Electron Spin Resonance

32 Mixed

The

Valence.-

Creutz-Taube

complex

is

still

a

matter

of

controversy, since different

experimental techniques f a i l t o give

unequivocal

question

answers

to

the

whether

the

valencies

are

l o c a l i z e d o r d e l o c a l i z e d on t h e two r u t h e n i u m atoms. S i n g l e c r y s t a l EPR s p e c t r a y i e l d e d g =1.346, g = 2 . 7 9 9 , g = 2 . 4 8 7 , w i t h x o r t h o g o n a l

Y These d a t a ,

t o the pyrazine ligands. from s i n g l e conclude valence

c r y s t a l X-ray

as

that

species

yet were

[Rh (NHCOCF3)41+, rhodium( 1 1 1 )

ions

and

no

where

can

recognized

the

two

5+

be

by

d a t a were

fragment

and

199

used

reached!lg7.

EPR

formally

equivalentlg8,

c l u s t e r s c o n t a i n i n g t h e Tc2 A s e r i e s of

Mossbauer

conclusion

also

are

together with those obtained

"Ru

Mixed

spectroscopy rhodium( I 1 )

in

various

to

in and

binuclear

.

s e v e n mixed v a l e n c e b i n u c l e a r c o p p e r ( I 1 ) - c o p p e r ( 1 )

complexes o f m a c r o c y c l i c l i g a n d s of f o r m u l a :

+

But

BF4

42 Bu

were

studied.

ligands

were

Both

symmetric

employed.

At

(R =

room

RO,and temperature

asymmetric

(R

symmetric

+

R')

ligands

y i e l d e d delocalized s p e c i e s , while at l i q u i d n i t r o g e n temperature 200 l o c a l i z e d s p e c i e s were formed i n any c a s e

.

7i

s p e c i e s were r e p o r t e d t o form i n T i 0 , one o f 201,282 l 1 Magneli p h a s e s of oxygen d e f i c i e n t r u t i l e Ti2

A review a r t i c l e concerning

t h e p h y s i c a l p r o p e r t i e s of

several

8-type

o x i d e vanadium b r o n z e s a p p e a r e d 2 0 3 and s e v e r a l p a p e r s on t h e same

33

2: Transition-metal Ions

204-207 matter were published Both di- and tri-vanadium mixed valence species were observed in aqueous solutions of di- and

tri-substituted

derivatives of

[P W 0 16-. The V3 species is characterized by a hyperfine 2 18 6 2 splitting into 22 lines at pH 11.0 and 36 lines at pH 4.7. At low 208

temperature the valence is trapped on one single vanadium atom

.

Similarly in a series of fluoro- poly tungstates valence trapping was found2"

at low temperature and delocalization above 50 K. The

temperature dependence of the linewidth was used to estimate the thermal activation energy for electron hopping in a series of compounds belonging to the W 0 6 19' xw12040' As2W12062' AsH2w1802g structural types2''. The EPR spectra of [Cl W(p-C1) (p-SPh)2WCl 1 3

3

show a large anisotropy, g =2.343, g =1.896, g =1.752. They were 1 2 3 assigned on a qualitative MO scheme assuming equivalent metal ions.

-

Magnetic Materials.

The use of EPR spectroscopy in the study of

systems exhibiting extended interactions is still increasing. EPR experiments were usefully employed to investigate the mechanism of exchage, and

the spin

dimensionality of

the

system, and

the

transport phenomena in low dimensional conductors. In the linear antiferromagnet (C~N~H~)CUC~ 3 ( C6N2H9 2-amino-6-methyl-pyridinium) EPR spectra were used to estimate a

chain

very small J ' / J ratio, where J is the intrachain and J' is the interchain coupling constant211. EPR spectra of the alternating chain

compound

terpyridine; pyz

[Cu( terpy ) ( pyz =

1 ( C104 )

( terpy

=

2,2

I

:6 I ,2"-

pyrazine) were used to assign a square pyramidal 213

coordination to the copper(I1) ions

.

Ritter et al. reported214 the EPR spectra of the 1-dimensional [(CH3)3NH]CuC13.2H 0 from 4.2 K to room 2 temperature. In the high temperature region, the EPR data showed a

Heisenberg ferromagnet

rich angular dependence of the linewidths as the magnetic field is rotated away from the chain. The data were analyzed through a model for

spin

dynamics

and

extended

exchange

anisotropies

dimension, including also the antisymmetric exchange.

in

The

one spin

hamiltonian parameters obtained were: J=1.15 cm-l, DdiP=o -1 , Dexc=0.0460 cm-l, and d=0.0619 cm-'. An anomalous monotonic

cm

Electron Spin Resonance

34

b r o a d e n i n g o f t h e l i n e s i s o b s e r v e d as t h e t e m p e r a t u r e

i s lowered

below 4 K . A t h e o r e t i c a l t r e a t m e n t , u s i n g t h e Mori f o r m a l i s m , w a s u s e d t o

p r e d i c t dynamic s h i f t s o f t h e r e s o n a n c e f i e l d s and a n o m a l i e s i n t h e l i n e s h a p e s of broad l i n e s f o r i n s u l a t o r s w i t h dominating i s o t r o p i c exchange.

The

was

model

compared215

(CH3)4NMnC13, C s N i F 3 and MnF

with

experimental

on

data

2’

T h e o r e t i c a l a p p r o a c h e s were d e v e l o p e d t o c o r r e l a t e t h e f e a t u r e s o f EPR s p e c t r a w i t h t h e b e h a v i o u r o f

low d i m e n s i o n a l m a g n e t s .

p a r t i c u l a r some a n a l y s e s were d e v o t e d t o t h e s t u d y o f and

broadening

phenomena

2-dimensional

connected

Heisenberg

with

the

l i n e shape

presence

p a r a m a g n e t s 216-218.

Some

In

of

1-

models

or

were

c h e c k e d w i t h e x p e r i m e n t a l e v i d e n c e on ( C H ) NMnCl 219, on CsCoCl 3’ 3 3 4 2 20 , on the MnCl - g r a p h i t e CoC12.2NC5H5, ( C H 3 ) 3NHCoC13.2H20 2 222 i n t e r c a l a t i o n compound221, on CsMnBr , on Rb 2 Mn5Cd l-zc14 = 223 1.0; 0.8)

(x

.

The c r i t i c a l b e h a v i o u r

of

t h e EPR l i n e w i d t h s i n CdCr Se 2 4 and frequency. I n t h e r e g i o n

CdCr2S4 w a s d e t e r m i n e d a t X-

and Q-band

above t h e C u r i e p o i n t , a s t r o n g f i e l d dependence o f t h e l i n e w i d t h s is

observed,

even

when

the

s u s c e p t i b i l i t y is small.

field

variation

of

the

magnetic

The a u t h o r s s u g g e s t e d a n e x p l a n a t i o n 224

of

.

t h e phenomenon b a s e d on s p i n d i f f u s i o n e f f e c t s

EPR s p e c t r a o f TMMC ( t e t r a m e t h y l ammonium manganese( 11) t e t r a c h l o r i d e ) were magnetic

recorded

fields225.

s y s t e m doped w i t h line

shapes,

using

spin

far-infrared

dynamics were

c a d m i u m ( I 1 ) . The

dynamic

c o n s t a n t s were c r y s t a l s with

by

The

shifts,

measured different

at

X-

dopant

and

room fine

and

lasers

studied

temperature and

Q-band

and p u l s e d

in

frequency A

same

linewidths,

hyperfine

concentrations.

the

structure in

good

several

agreement

between t h e o r y and e x p e r i m e n t c o u l d n o t be f o u n d 2 2 6 . An a n a l o g o u s r e s e a r c h w a s performed

d o p i n g TMMC w i t h

copper(I1) to

study

the

e f f e c t o f m a g n e t i c i m p u r i t i e s on t h e h i g h t e m p e r a t u r e dynamics 227 The

EPR

spectra

quasi-two-dimensional temperatures.

The

of

quasi-one-dimensional

BaMnF4 were change

in

the

studied

over

critical

.

CsMnC13.2H 0 and 2 a wide r a n g e o f

broadening

of

the

2: Transition-metalIons

35

resonance line was found near the Nee1 temperature. A half-field 228 satellite line was observed in the spectra of CsMnC13.2H20

.

EPR

experiments

were

made

on

the

ideal

two-dimensional

Heisenberg ferromagnet K CuF at X- and K-band frequency in the 2 range 4.2-300 K. The variation of spectra with temperature, of linewidth and line shape with crystal orientation in the external magnetic field were considered. These phenomena seem to reflect the increasing importance of long wavelength ferromagnetic fluctuations

of spin with

decreasing temperat~re~~'. The concentration and

temperature dependence of the EPR transitions in the mixed twodimensional Heisenberg systems K Cu Mn F are reported. K2CuF 2 5 1-5 4 4 and K MnF4 are ferro- and antiferro-magnets respectively. The 2 effect of varying the amount of one metal ion on the exchange 230 mechanisms was studied

.

The

EPR

spectra

NH3(CH2)11NH3CuBr4

("=

of

the

eclipsed

layered

compounds

2,3,4) were studied in the range 113-296 K.

The anisotropies of the line widths seem to confirm the expected departure from two-dimensional behaviour. Comparisons were made 231 with the isomorphous chloride salts

.

The structural, magnetic and EPR properties of bis(I3-alaninium) tetra

bromo

Magnetization

cuprate(II),

(13-alaH)2CuBr4 ,

studies reveal

the

were

existence of

an

reported. Ising

type

anisotropy, with the easy axis normal to the layers of square 2planar CuBr4 anions. The EPR spectrum consists of a single exchange-narrowed line with g - 2 . 0 4 4

I1 -

and g =2.098. The linewidths

I

are strongly temperature dependent, phonon modulation of the spin 232 anisotropies being the principal source of line broadening

.

Single crystals of the copper( 11) derivative of DL- u -amino butyric acid were studied by EPR at 300 K and X-band frequencies. Only

one

exchange

narrowed

line

was

observed

for

the

two

inequivalent copper ions in the lattice. The angular variation of the linewidths shows a contribution from spin diffusion, as is characteristic of two dimensional magnetic systems. Anisotropic and antisymmetric exchange interactions were assumed to contribute to 233 the observed linewidths

.

36

Electron Spin Resonance

The linewidth behaviour of CoS2 having pyrite structure was investigated234 at X-band frequency from 4.2 up to 135 K through T = 122 K. EPR investigations were carried out on the impure Ising -e linear chain compound [(CH ) NH](CoMn)C13.2H20 at 1.8-300 K and 24 3 3 GHz. Only single lines with g= 2 f o r manganese(I1) are observed at 10-80 K. Below 10 K five lines, corresponding to the fine structure of manganese(II1, and two resonances attributed to the cobalt(I1) linear chain were observed. The possibility of a Co-Mn interaction 235 was also taken into consideration . Dichloro-

and

dibromo-oxo

bis(N,N,N',N'-tetramethylurea)

vanadium(1V) are two isomorphous exchange coupled systems. By means of EPR spectra at Q-band frequency in the range 4.2-300 K, the presence of a weak isotropic exchange interaction within a two dimensional

layer was

suggested. A

model

was

proposed236

to

reproduce the unusual fine structure observed at 120-170K. A

cluster model, developed for the EPR

antiferromagnetic compound, was

modified

to

spectra of take

into

a

pure

account

magnetic dilution effects, and was applied to Ga doped Cr203. It was suggested that the EPR of a doped crystal could be explained by the

absorption

within

configurations

arising

from

different

coupling energies, due to the presence of diamagnetic ions. It was possible

to estimate the coupling energies between nearest chromium(II1) neighbours and between next-nearest neighbours237

.

The

EPR

spectrum

of

tetramethylammonium

bis(maleonitri1e

dithiolato) nickelate(I1) (NEt Nimnt ) was observed only below 100 4 2 K. It consists of two sets of lines corresponding to the two magnetically non-equivalent

[Ni(mr~t)~]- units in the monoclinic

crystals. The angular behaviour o f the fine structure was simulated 238 on the basis of a weak interchain dipolar interaction

.

Conductors.-Magnetic

susceptibility

and

EPR

studies

of

highly

conductive CuL(TCNQI2 ( L = ethylenediamine, 2,2'-bipyridine; TCNQ= tetracyano quinodimethane) were reported. The g values increase with decreasing temperature. On the basis of g shift and static susceptibility, the local magnetic susceptibility contribution from

2: Tmnsition-metal Ions

37

copper(I1) ions and from TCNQ were individually evaluated. On this basis

some suggestions on

the nature of

the

conduction were

proposed239'240. The dielectric function, EPR, and dc conductivity were

studied

for

copper-iodo phthalocyanine, CuPcI,

compounds

obtained by iodine diffusion in polycrystalline j-CuPc at 387-493 K. The data were

one-dymensional carrier

the formation of quasi

interpreted assuming

crystals with

delocalization

structural

along

CuPc

disorder and

and

iodine

current 241 stacks

.

[CU(DMP)~]2(TCNQ)2 (DMP= 2,9-dimethyl-l,lO-phenanthroline) contains chains of

o-bonded

dianionic TCNQ

dimers,with

low

interdimer

overlap. The excited triplet EPR spectra of (TCNQ)2 were recorded and the fine structure parameters were accounted for by theoretical 242 calculations

.

The dynamic behaviour of the 3d electrons in the quasi-one dimensional conductor

NaxV205 were studied by EPR relaxation in

-

the range 19-303 K. The origin of the anisotropy and temperature dependence of the linewidth was considered and the correlation time of the fluctuating local field evaluated. On this basis a mechanism for the trasverse and longitudinal conduction 'with respect to the 243 chain was proposed

.

Phase transitions.

-

In the study of phase transitions in the solid

state by means of EPR spectroscopy, the number of paramagnetic ions used is still increasing. For instance, together with the widely studied manganese(I1) and chromium(II1) ions, several researches were performed on systems containing the vanadium ion in various oxidation states. The EPR spectra of

y-irradiated polycrystalline

(NH ) VO F showed the existence of trans [VO F 14- species iti the 4 3 2 4 2 4 solid. Evidence was obtained for the occurrence of the 1st-order phase transition at 418 K with a large coexistence region (418 273 K).

[V02F4]3-

The

transition may

units in the high

be

associated with

temperature phase.

-

the motion of The

hyperfine

constants are essentially temperature independent in the 440 -130 K range. However an abnormal increase in A

II

occurs at 77 K.

This

observation, in conjunction with an anomalous decrease of the Q-

38

Electron Spin Resonance

value of the cavity suggests phase change,

the possible

probably a s s o c i a t e d w i t h

occurrence of

the

onset

of

another

ordering of

e l e c t r i c d i p o l e s 2 4 4 . The p h a s e t r a n s i t i o n s a t 413 K and 264 K i n (NH H(S0 c r y s t a l s were s t u d i e d as t h r o u g h t h e EPR s p e c t r a o f 4 3 4 2 oxovanadium(1V) i o n s . The domain s t r u c t u r e i n t h e EPR s p e c t r a

s u g g e s t s t h a t t h e t r a n s i t i o n a t 413 K i s c h a r a c t e r i z e d by t h e l o s s o f t h e symmetry p r e s e n t a t t e m p e r a t u r e above 413 K . d i s t r i b u t i o r i o f t h e v a r i o u s i o n s i n t h e 413245 264 K i s proposed

The s p a t i a l

264 K r a n g e and below

.

The m o l e c u l a r o r i g i n f o r t h e f e r r o e l e c t r i c phase t r a n s i t i o n i n t r i s s a r c o s i n e c a l c i u m c h l o r i d e (TSSC) c r y s t a l s w a s i n v e s t i g a t e d by using

oxovanadium(1V)

probes.

The

paramagnetic

impurities

s u b s t i t u t e d f o r c a l c i u m ( I 1 ) i o n s i n t h e TSSC l a t t i c e e x h i b i t e d s i x s p e c t r a , corresponding t o the sarcosine

molecules.

The

interaction w i t h individual ligand

pattern

of

spectra

at

different

t e m p e r a t u r e s w a s i n t e r p r e t e d i n t e r m s of t h e r e o r i e n t a t i o n o f t h e 246 v a n a d y l moiety c a u s e d by l a t t i c e s t r a i n

.

Amorphous

and

polycrystalline

zinc

oxide-vanadium

(ZnO-EV 0 ) system were s t u d i e d a t v a r i o u s microwave 2 5

oxide

frequencies.

The v a r i o u s p h a s e s o b t a i n e d by d i f f e r e n t c o o l i n g r a t e s have v e r y dissimilar l i n e shapes.

The amorphous phase i s c h a r a c t e r i z e d by a

w e l l r e s o l v e d h y p e r f i n e s t r u c t u r e and t h e ZnV206 p h a s e has a v e r y asymmetric l i n e s h a p e . The l i n e s h a p e becomes more symmetric i n t h e low r a n g e o f f r e q u e n c i e s

247

.

The e f f e c t of an e x t e r n a l e l e c t r i c f i e l d on t h e domain volume in

the

f e r r o e l e c t r i c phase

of

oxovanadium(1V)

doped

s u l p h a t e s i n g l e c r y s t a l s was examined248. An a n a l o g o u s

triglycine study w a s

performed249 on chromium (111) doped RbH3(Seo3l2. From a t e m p e r a t u r e dependent EPR s t u d y o f m a n g a n e s e ( I 1 ) doped M(BF4)2*6H 0 ( M = Fe, Co, N i , Zn) new s t r u c t u r a l p h a s e t r a n s i c t i o n s 2 were d e t e c t e d 2 5 0 . For m a n g a n e s e ( I 1 ) doped c r y s t a l s o f Mg(C104)2. 6H 0 two p h a s e t r a n s i t i o n s a t 2

room t e m p e r a t u r e from

the

perfect

measurements251.

335 K and 3 2 4 K were d e t e c t e d .

the water-perchlorate P6 mc 3

symmetry

At

symmetry d e v i a t e s s l i g h t l y

proposed

from

previous

&-ray

The EPR s p e c t r a of manganese( 11) i n ZnTiF6- 6H20

2: Trculstion-metal hns

39

was studied in the 77-300 K temperature range at X-band frequency. At 173

f

2 K a structural phase transition was observed. The axial

symmetry of room temperature, with a single magnetic

site, was

destroyed in the low temperature phase

, where two crystallographic

inequivalent sites 252 identified

from

.

rotated

by

'8

the

axis

could

be

phase transitions between the melting point of RbMgBr and 3 77 K were revealed253 by the EPR spectra of the compound doped with TWO

Mn2+, Cr3+, and Gd3+. The EPR spectra of manganese(I1)

impurities in NaN3 Single

crystals were measured between 178 K and 300 K transition from the monoclinic

low temperature

trigonal high temperature R-phase. the

zero

field

splitting

The successive phase

investigated

by

ESR

and

study

the

to the

The temperature dependence of

parameters

allowed

ascertain the existence of a continous phase K254.

to

u-phase

transitions of differential

the

authors

to

transition at 292

CN(C H )J~ p C 1 4

scanning

were

calorimetry

of

manganese(I1) doped samples. The changes In the EPR spectra with I

temperature show the presence of six phases between 300 K and 133 K255 The phase

transition of chromium(II1) doped K S C ( M O O ~ ) ~was

studied and a structural distortion model of the high temperature phase was proposed, based on the motion of the oxygen atoms of the (MOO 12-

anions,

constituting

the environment

of

scandium(111)

ionsd56. EPR experiments were performed on the paramagnetic and magnetic ordered states of both crystallographic phases of the magnetic semiconductor GaCr4S8. A critical behaviour was observed typical of magnetic phase transitions, resulting in a shift and a broadening of the resonance line as well as a distortion of its shape.

The authors related these magnetization fluctuations257

features to

the presence

of

.

The progression rate for the structural transitions in NiTiF '6 6H 0 was measured under supercooling conditions by monitoring an 2

EPR line of nickel(I1) from the trigonal phase. Together with the main trigonal-monoclinic transformation at 136.3 & 0.5 K, a second

Electron Spin Resonance

40

one is detected at 126.0 2 0.5 K. This transition may be associated with

an

additional

distortion

of

the

[TiF612-

ion258.

The

broadening of the EPR line of NiC12-2H 0 confirmed the existence of 2 259 a crystallographic phase transition near 200 K

.

-

to Mineral Systems and Glasses.

Application

The use of EPR

spectroscopy is still increasing in new areas of research dealing with structural problems of mineral and glasses. For instance, the application of EPR to distinguish synthetic from natural gemstones has been reviewed260 and Russian authors reviewed the use of ENDOR 261 in mineralogy and geochemistry

.

The EPR spectra of paramagnetic ions or centres were used to the process of formation of granitic rock262, of of albite265, and of calcareous rocks263, of gypsum264,

determine

266 ultrapotassic rocks

.

The presence of manganese(I1) asbestos

fibers

identification

was

of

determined

various

types

and iron(II1) by of

EPR

in chrisolitic

which

ionic

allowed

the

inclusions in

the

Rhyolitic pumice from widely different geological flows and from a recent submarine vulcanic eruption were studied to evaluate the distribution and structural configuration of iron(II1) ions. Generally, the pumice samples show five different EPR spectra with variously contributing narrow resonances at g 268

signal at g =2.3 and at g ~ 2 . 0 The

system

different ability

3d

metal

experimental of

calcium

=

4.3, a broad

.

ions-montmorillonite

conditions269 ' 270. montmorillonite

In

to

was

studied

particular,

remove

in the

copper(II),

vanadium(1V) and manganese (11) from aqueous solutions was tested 271 by means of EPR

.

The kaolinite samples have Fe20g content varying from 0 to EPR

spectra

of

these

samples

allowed

the

2%.

quantitative

identification of two types of iron sites. The authors correlated the presence of the different type of sites to the degree of crystallinity of the mineral and to the presence of defects272. In kaolin samples Bomin et al. revealed, again with the help of EPR,

2: Tmnsition-metalIons

Mijssbauer

and

41

EXAFS

spectroscopies,

two

different

types

of

iron(II1) and they attributed one to a distribution of the isolated ion sites with variable geometry and the other to iron(II1) metal 273 ions arranged in a superparamagnetic phase

.

Ultrasonically modulated electron resonance (UMER) was used to study

S-state

Manganese(I1)

ions and

in

iron

substitutional (111)

in

sites

natural

of

single

minerals.

crystals

of

tremolite were used as examples. Combined EPR and UMER measurements allowed the authors to establish that the manganese(I1) enters preferentially the calcium(I1) sites rather than the magnesium(I1) sites. For the iron(II1) ion, it was not possible to state the site 274 of impurity ions

.

A comparative study of blue and green beryl crystals using EPR and optical absorption spectroscopies was performed by Blak

et

s.

The EPR spectra show that the iron(II1) ions in blue beryl occupy substitutional aluminum(II1) sites, while in green beryl they are 275 localized in structural channcels between two oxygen planes

.

The EPR spectrum of chromium(II1) ions in synthetic crystals of

i.solated

forsterite consists primarily of lines of ions

in

two

different

superhyperfine-split

sextets

positions. with

The

different

non

paramagnetic equivalent

intensities

were

assigned by ENDOR experiments to the interactions of chromium(II1) ions with adjacent aluminum(II1) ones. The g, D, A, and A ' tensors of the Cr-A1 pairs were determined at room temperature and were found quite similar to those of isolated chromium(II1) ions. 276 Calas et al. studied iron(II1) ions in cassiterite from various deposits through EPR spectroscopy.

Iron occupies four kinds of

sites, two of which are of substitutional type. Some samples show the presence of vanadium(1V) ions or of finely divided iron oxides

in

various

concentrations,

which

could

explain

the 277 ferromagnetism often encountered in natural cassiterites

.

weak

EPR spectroscopy is used to follow the behaviour of glagses under thermal or p r a y treatment. With this technique beryllium glass278 , manganese(I1) doped-arsenic selenium glasses279 , germanium oxide glass280, copper( 11) doped borate glasses281 ,

42

Electron Spin Resonance

silicate glasses282, and iron oxide doped borate glasses283 were studied. Reports on vitreous semiconductors have been published. Mom0

al.

et

studied the equimolar lead(I1) oxide-vanadium(V) oxide systems

in the 150-675 K temperature range, determining the presence of both amorphous and polycrystalline PbV206 as a consequence of the cooling rate. The presence of hyperfine structure was discussed on the

basis

of

the

phosphate-titanate

mobility amorphous

of

the

charge

semiconductors,

carriers284.

In

the

of

nature

variations and interrelations of the EPR parameters was connected with the structural and electronic characteristic of titanium(II1) 285 and with the kinetic processes of the change transfers

.

The EPR spectra of copper(I1) and vanadium(1V) were studied In binary MO-B203

glasses

(M =

Ba,

Sr, Pb, zn) and

in

ternary

PbO-ZnO-B 0 glasses. The experimenbtal data allowed the authors to 2 3 study the similarity and/or the immiscibility of various systems, the presence of different phase structures, and the mechanism of substitution of diamagnetic ions286. A similar study on vanadium doped A1203-P205-Si02 and A1 0 -P 0 -B 0 287 and on iron(II1) doped 2 3 2 5 2 3 phosphate glasses288 were also performed. Magnetic susceptibility data together with EPR useful

in determining the magnetic

systems,

expecially

when

metal

behaviour exchange

of

spectra were some

vitreous

interactions

were

operative. The systems examined were Ga S GeS2-MnS289, iron( 111) 2 3290 291 , CuO[19TeO2-Pb0J I and doped CaF -C a0-S i0

V 0 -[2B203-PbO]292. 2 5

The same technique were applied to the study 294.

of amorphous barium vanadate doped with transition

With laser irradiation, the low field EPR line of chromium(II1) ion in glasses was narrowed by 30% to 50%:

this behaviour was

attributed to the resonant excitation of a portion of chromium(II1) 295

from the ground to the excited state

.

EPR was measured on PbO-Si02 and Li 0-2B 0 glasses doped with 2 2 3 G H z and 2.15 GHz. THe linewidths of the

CuO at 9.77 GHz, 3.15

parallel component with

m

=

-3/2 and 2 = -%

were measured to get

information on the strain induced changes on EPR parameters296. The

2: TmnsitionmetalIons

effects

of

43

heat

treatment

on

lithium

borate

vitroceramics

[85(B 0 -Li 0) 15(Ti0 -BaO)I were monitored by the EPR spectra of 2 3 2 2 copper(I1) and gadolinium(II1) impurities. The observed changes in the coordination of copper(I1) and gadolinium(II1) are attributed to growth and symmetry changes of BaTiO microcrystallites in the 3297 vitreous matrix during the heat treatment

.

Linewidths and Relaxation Studlee.

-

to

from

establish

approximation,

the

deviation

generally

used

to

EPR spectroscopy was employed the

describe

Solomon-Bloembergen the

effect

of

a

paramagnetic centre on the nuclear spin-lattice relaxation time, in a

NMR

study

of

dopamine

in

the 298 manganese(I1) and iron(II1) cations

presence

of

copper(II),

.

Vanadyl

complexes

from

oil

shale

formation,

whose

hamiltonian parameters are consistent with porphyrin

spin

structure,

were characterized by their very different rotational correlation times

under

similar

temperature2”.

Russian authors for a function

of

conditions

Vanadyl and study

temperature,

of

solvent

viscosity

copper( 11) compounds were of EPR

pressure

linewidth variation and

paramagnetic

and

used

by

as a center

concentration300 ’ 301. The stability of the copper(I1) carnosine dimer in aqueous solutions at pH=7.2 was investigated by Fourier transform IR and EPR spectroscopy. Possible contributions to the linewidths of the EPR signals of the dimer were examined, revealing that the effect of the anisotropy of dipole-dipole interaction is of the same order of magnitude as that from 302 hyperfine interactions

the anisotropy of

the

Zeeman

and

.

The variable Lorentzian contribution to the lineshape of the low-field

feature

[Fe,S,(S,-_o-~ylyl)~]

of 2-

EPR

spectra

of

the

complex

(S2-o-xylyl = g-xylene-s,p’-dithiolate) in

different solvents was related to spin lattice relaxation times evaluated in the 6 K- room temperature range and an interpretation of these results is given in terms either of dominant Orbach mechanism or of a Raman process303. An Orbach relaxation process

Electron Spin Resonance

44

weaker than expected was obtained by the study of the EPR signal of horseradish peroxidase

compound

I

(HRP-I) supporting a model

involving a g=x free radical coupled to a S=1&v 304 interaction

a weak exchange

.

The ratio of probabilities of spin-phonon transition AM

S'

and A,Ms EPR

=

=

+ -

1

2 2 and the inverse relaxation time were aetermined by an

single

crystal

study

of chromium(1)

doped

in

cubic

ZnS

containing packing defects305, while an analysis of the strongly temperature dependent linewidth of EPR lines of chromium(II1) ions in powdered AgCr02 allowed the authors to propose a mechanism for 306 this dramatic change

.

Single crystal EPR

with M = Cr

spectra of (M30(RC00)6L3)X

H 0 2 or m i n e ligand, were used to evaluate both the isotropic and

and/or Fe, RCOO- is a carboxylic anion, X

NO3- or C1- and L

=

=

antisymmetric exchange interactions in trimeric exchange homo- and hetero- nuclear clusters and their effects on the EPR lineshape. The probabilities of relaxation transitions, supposed to be induced by the modulation of isotropic exchange interaction by the lattice vibration,

were

determined307.

One

phonon

processes

were

suggested308 to dominate the spin lattice relaxation in KNiF3 whose EPR spectra were recorded in the temperature range 253-530 K.

d1

Configuration.

-

coordination compounds of solution

and

titanium(II1)

polycrystalline

containing aliphatic

-

Tervalent Titanium

samples

alcohols were

absorption spectra309 ' 310.

A

ions

of

series of

labile

in alcohol/water

chloro

characterized

titanium(II1) by

EPR

and

Isolated titanium( 111) complexes were

identified during the preparation of a MgC12 supported high mileage catalyst

for propylene-polymeri zat ion3''

and

two

structurally

different centres were recognized during an investigation of the 312 state of titanium(II1) in Na Si03 slags 2 The presence of titanium( 111) paramagnetic Centers in Ti02

.

2: Tnansition-metal Ions

45

pigment was indicated and studied313'314 and EPR data showed that surface titanium(II1) centers are not required for suppression of hydrogen chemisorption seen on adding small amounts of K to Pt/TiO2 catalysts315. TiPO was prepared and the temperature dependence of 4

EPR

spectra and magnetic susceptibility examined 316 existence of metal-metal bonding

suggested

the

.

Tetravalent complexes

Vanadium, were

Niobium

actively

and

-

Tantalum

synthesized

and

Oxovanadium(1V)

EPR

was

used

characterize them. Distorted octahedral symmetry was observed

to 317

for VOL 2 (with HL = CH3C(C2H5):"HC(X)NH2, X = S, 0 , 2 = C1, Br, 2 2 and ESR parameters were given for oxovanadium(1V)

NOg, C104, %SO,)

complexes of salicylaldoxime and p - v a n i l l i n - ~ x i m e ~ ~Substituent ~. effects in oxovanadium (IV) R-diketonates were examined by studying 319 chloroform solutions EPR spectra

.

Japanese

authors

examined

the

stereo

configuration

of

vanadium(1V) complexes formed in the extraction from hydrochloric acid solutions with organophosphorus compounds320 and the EPR of vanadium(1V) species was used to characterize complexes of the type VOL

with

showing

L=

salicyidene

metal-metal

(benzoy1)hydrazone-type Schiff bases,

interacting

dimeric

forms321.

The

crystal

~ = 2-hydroxy-6structure was given for V 0 C1 ( ~ - H m h p ) (Hmhp 2 2 4 the first reported neutral dinuclear

methylpyridine),

oxovanadate(1V) complex containing

exclusively

neutral

bridging

ligands and showing an EPR signal at g = 1.9757 without hyperfine structure322. The divalent cations of the first transition row formed square pyramidal complexes with 20-tungsto-2-arsenate,

a

rigid tetradentate ligand, four oxygen atoms of which form the equatorial

plane

of

the

coordination

environment.

The

axial

position is occupied by a water molecule for m 2 + , Co2+, Ni2+, Cu2+, and Zn2+ ions and by the vanadyl oxygen for the vanadium(1V) complex. The EPR spectra of frozen aqueous solutions confirm this assumption showing g

It

and A

11

values sensibly lower than those of

other polyoxotungstic vanadium hexacoordinated

complexes323.

and

square

visible

spectra

suggested

a

distorted

ESR

pyramidal

Electron Sptn Resonance

46

structure

for

the

bimetallic

(Pt(S,CNR,)(Ph,PO),H)

complexes

=

(R

acetylacetonate,

while

exhibited

the

square

complex CHMe,

formed

Et)

with

corresponding planar

by

oxovanadium

copper

coordination

treating

and

and

analogue tetrahedric environment324. ESR data were

nickel

the

cobalt

reported for

oxovanadium(1V) sulphate complexes with some substituted thiourea l i g a n d ~and ~ ~ a~ polymeric structure was identified for some VOL type

complexes

with

H2L

2-hydroxy-1-naphthylidene326 o-phenylenediammine, or tetra-dentate Schiff bases Single

crystal

EPR

spectra

=

of

.

N,N,N',N'-tetramethylethylene

aqua-bis(malonato)oxovanadate(IV)

diaminidium

dihydrate,

whose

crystal structure indicates the presence of two-dimensional layers of anions in the crystal, with all the V=O bonds perpendicular to the planes, show a weak extended exchange interaction of magnitude comparable to the nuclear hyperfine splitting evaluated in the -4 -1. range A = 180-190 x 10 cm The isotropic exchange interaction,

II -J, was seen to be

temperature dependent varying from ferromagnetic

at 300 K to antiferromagnetic at 4.2 K. The possibility that the changes in J could be due to changes in the dipolar zero-field spitting with temperature was

taken into account, but

allowing

relatively large variation in vanadium-vanadium distance does not lead to a change in sign of J. It was suggested that there could be both

a ferromagnetic and

isotropic

exchange,

one

antiferromagnetic contribution

or

both

of

which

to

varies

the with

temperature327. Single crystal EPR spectra at Q-band frequency were reported

for

(VOC12(0PPh3)2)

and

interpreted

using

the

spin

hamil tonian parame t ers gxx=gyy=1.974(2), g =1.930(2), -4 -1 Axx=A --66(2) x 10 cm and AZz=-175(2) x cmMolecular YYorbital coefficients were derived, assuming for the compound a C * -42 symmetry and a b2 ground term, and compared with those of other

I".

0x0-vanadium( IV) complexes328. The preparation and structures of two other vanadium thiolate complexes (Me N)Na(VO(SCH CH S) )'2EtOH 4 2 2 2 and (Ph4P)2(V2(SCH CH S) , with EPR data for the monomeric 329 complex, were also reported

.

ESR

spectra

of

oxovanadium(1V)

complexes

of

biguanidine,

2: Tmnsition-metalIons

47

dibiguanidines, and o-methyl-1-aminourea are very similar to each other indicating330 the similarity of bonding in these complexes with spin hamiltonian parameters g .= 1.99, A = 1.94 and A

It

ern-'?

= 154 x

I = 54

cm-l

x

;

gll

Spectral parameters, their pH

dependence, and relative electron energy levels were calculated for oxovanadium( IV) cysteine complexes331. Three types of coordination environments are proposed for the complexes formed between vanadyl

ion and adenosine triphosphate at different pH values with the help 332 of optical, EPR and NMR spectroscopy

.

Several host materials were used333 to dope vanadyl ions with the aim of investigating the coordination environment and the spin hamiltonian parameters of

the paramagnetic centers

structural characteristics of the material under variations

in

the

EPR

values

of

the

and/or

study

paramagnetic

the

through

probe.

An

interstitial position was identified for V02+ in (NH4)3H(S04)2, while the EPR spectra of vanadyl doped KHC204 crystals reveal two different sites, one of which is interstitial, the other being 334 subsitutional

.

The values and directions of the spin hamiltonian parameters were experimentally obtained for the vanadyl ion in zinc selenate hexahydrate single crystals and compared with theoretical values previously obtained. The z axis is nearly along the shortest Zn-OH

2 bond direction335. The effect of the host lattices on the number of the vanadyl complexes formed and their orientation were discussed for V02+ doped Tutton salts, diamagnetic Mg(NH ) (SO ) '6H 0 and 4 2 4 2 2 the paramagnetic Co analogue336. ESR parameters of V02+ ions adsorbed on calcium phosphates were used by Japanese authors to analyze the surface structure of amorphous calcium phosphate337 and 338 other apatite materials

.

Vanadium(IV), chromium(II1) and chromium(V) ions were used by Russian authors as paramagnetic probes to study the structure of lithium

aluminosilicate

glasses339.

and

calcium-magnesium

aluminosilicate

The coordination environment of vanadium( IV) centres

formed during the preparation of

3-component

glasses based

on

vanadium, tellurium and alkali metal oxides were investigated by

Electron Spin Resonance

48

ESR spectra340. A characteristic glass EPR spectrum of V02+ ions in substitutional Zn2+ sites was observed341 in K SO -ZnS04 glasses at 2

288 K.

MO

coefficients

were

evaluated

from

spectral data at 300 K and 77 K for V02+

4

EPR

and

optical

ions doped in cesium

cadmium sulphate hexahydrate single crystals342.

ESR parameters of vanadium species in oxide type matrices were reviewed and a reassignement of the paramagnetic vanadium species in aluminophosphates from V

4+

to V02+ was proposed343.

environment and motion of V4+ ions in a-VOPO

4

Position,

and u-VOP04*2H20 were

studied and the relevant difference between ESR parameters of the paramagnetic centre in the two hosts was attributed to a possible 344 layered structure of the latter

.

The dispersion mode of copper(I1) and vanadyl(1V) ions adsorbed surfaces is determined by

on Si02 and A1203

the nature

of

the

surface and its degree of hydration. After a drastic dehydration of both surfaces, pairs of ions with coupled spins were observed by

ESR spectroscopy on the silica surface with a metal-metal distance of 3

for V02+ and 5.2

formation

of

the

for Cu2+. On the alumina surface the

dimeric

species

is

fixation of the cation on the surface

ESR

parameters

suggesting a d 2

of

V

doped

345

inhibited

by

the

strong

Si02 were

given,

.

a-cristobalite

346

2 type ground state for the vanadyl ion

,x -2

. x-ray

diffraction, ESR and IR spectroscopies were used to identify the final and

transient V02+

species

in the hydrogen

Te2V2O9 at 300-450 OC at atmospheric and

reduced

reduction of pressure.

The

magnetic interaction between paramagnetic vanadium(1V) centres in the reaction products were related to the structure of the reduced oxides347. spectra

Reduced vanadium centres in Ca NaMg2V3OI2 2

interpreted

as

arising

from

tetrahedrally

vanadium( IV) and oc tahedrally coordinated The

photoinduced

vanadium(1V)

reduction

defects

in

the

of

the

same

EPR

showed EPR coordinated

vanadium( I11 ) signal

compound

was

ions348.

intensity

of

reported

and

attributed to light-induced electron transport within the vanadium ~ublattice~~'. Mechanistic studies in non-aqueous media based on

EPR spectroscopy of vanadium( IV) species in the vanadium catalyzed

2: 7hsitionmetal Ions

49

oxidation of 2-dianisidine with tert-butylhydroperoxide, 350 a radical mechanism

indicated

.

Zeolites are used

as catalysts and

their activity can

either cation exchange or by

modified by

changing

be

the cation

binding site. EPR parameters can give a variety of structural information, but a more complete insight into the structure of these systems can be obtained by hyperfine splitting measurements obtained with ENDOR. The potentiality and the selective possibility of

these technique 351 Y-zeolite

illustrated

were

using

V02+

adsorbed

on

.

The oxovanadium(1V) ions have been used in EPR studies of metal ions binding in a variety of system of biologic&:

interest. The

ENDOR technique can provide information on hyperfine and quadrupole

tensor components of ligands, identifying those which are involved in metal

ion binding and giving additional information on the

structure of the complexes. With the aim of giving evidence of the potentiality

of

V02+-imidazolate

this

method,

complexes

spectra

ENDOR

were

compared

solution

of

with

those

of of

V02+-histidine and V02+-carnosine352. ENDOR .and ESEM (electron Spin 2+ echo modulation) results were reported for VO(H20)5 and in cross

VO(D20) 52+

linked poly(vinylaicoho1) and

an

accurate

comparison of structural information obtained from the analysis of the two different spectral data is given353 9 354. TRIPLE resonance study of VO(acac)2 is used to illustrate the analysis of the data 355 obtained with this useful technique

.

A single crystal EPR study of ( T ~ - C , H , C H ~ ) ~ N ~ Cdoped ~~ in the zirconium(1V) analogue yielded the spatial distribution of unpaired electron from the orientation dependence of hyperfine

interaction.

The

spin

hamiltonian

the

the

93Nb

parameters

are

g -1.9754, g =1.9396, gz=2.0018 , Ax=118.7 G, A =178.4 G and AZ=59 Y Y G. The g and A tensors are parallel and the large anisotropy of the hyperfine splitting tensor and the superhyperfine splitting due to interaction

with

35Cl

and

37Cl

is

discussed

using

MO

calculations356. The unpaired electron resides in a metal based molecular

orbital,

essentially

a

4dZ2

-

orbital

with

a

small

.

so

Electron Spin Resonance

contribution from a d 2 2 orbital. The same conclusions were 5 reached for [Nb(q -C5H5-) (PS2(OR)2)2] complexes357 with R = Me or

x-x

Et and also for the (q-C H ) Ta(SR)2 with R = Me or Ph in which the 5 5 2 unpaired electron resides mainly on the central metal ion358. IR, ESR and magnetic susceptibility data were given359 for

niobium( IV)

and tantalum(1V) complexes [(Me PCH ) MC1 H 1. 2 2 2 2 2 Quinquevalent 'Chromium, Molybdenum resonance

spectra

and

ENDOR

and

Tungsten.

investigation

-Electron spin of

mononuclear

pentacoordinated nitrido (tetra-p-tolylporphynato) chromium(V) and

nitrido(octaethy1-porphynato)

chromium(V) supported

conclusions of a strong spin localization in d

3

the

authors'

ground state of

the central ion360. The crystal structure of the former is also 361 available

.

The ethers

formation of were

darkness thermal

several

followed,

and on

using

chromium(V) different

complexes

starting

irradiation362; chromium(V)

degradation were

with

crown

compounds,

- species

and

in

their

studied

in X-ray irradiated alkaline 2d i c h r o m a t e ~while ~ ~ ~ X-irradiation of CrO doped Mg(NH4) (SO,); 4 6H 0 generates paramagnetic radical defects, NH;, and CrO) ions 2 364 in two inequivalent sites

.

Chromium(V) has been substituted for phosphorous as a magnetic probe of the local distortion produced by various substitutional ions in calcium phosphate apatites. Single crystal EPR spectra, recorded at 4.2 K, are in agreement with structural data, showing that while anion substitution tends to induce a phase transition, cation substitution induces larger distortions of 365 tetrahedron

the phosphate

.

The

d1

investigated

MOO -SiO

3 2 prolonged

molybdenum(V) using

EPR

centres

spectroscopy

were in

recognized surface

and/or

complexes

in

samples366, in sodium molybdate single crystals367 after thermal treatment at 53OoC, during the reduction of

molybdate by soil organic matter368, studying the sulphur dioxide adsorption on molybdenum- nickel/alumina catalysts369, and during

an electrochemical study on bis(peroxo)molybdenum(VI)

tetra-5-tolyl

2: TmnsitionJnetalIons

51

porphyrin where the reduction of the central atom does not affect 370 the nature of the metal-oxygen bond

.

Ti-Mo catalysts were actively studied by Russian authors, in particular

Mo-doped

Structurally

rutile 371-374.

different

d1

centres, molybdenum(V) and tungsten(V), were identified and used as magnetic

probes

to

investigate

the

structural

of

properties

adsorbing surfaces375 or soda borate and phosphate glasses376,377 EPR data for molybdenum(V) ions are actively used to analyze reaction mechanisms and/or kinetics and photochemical reduction of molybdenum(V1)

materials

as

in

phosphoric acid by hydrogen378,

the

of

reduction

12-molybdo-

the reaction of oxomolybdenum(V)

tetraphenyl porphyrin complex with

alcohols in the presence of

s~peroxide~~'. Intramolecular electron transfer from NCS ligand to

a metal centre can explain380 the formation of EPR detectable molybdenum(V) species in the sunlight-induced reversible reduction Of omodinuclear [Mo(O)~(NCS)(~~SO)~]~O. Intensity measurements of

molybdenum(V) signals permitted the evaluation of the lifetime of the copper(II)/molybdenum(V) valence isomer of CU(II)[MO(VI)(CN)~], obtained with a photochemical induced electron transfer381. ESR investigations of Moo2 samples interacting with atomic hydrogen while being irradiated gave information about the mechanism of in 382 heterogeneous photocatalysis on transition metal oxides

.

The coordination chemistry of molybdenum(V)

is enriched with

new examples and paramagnetic resonance becomes a routine procedure for characterizing the complexes. Compounds of the type MoOC1L2 '

(with L t B - d i k e t ~ n a t e ~ ~HL=8-quinolinol ~, L=diphenyldithio phosphinic

and 8-quinolinethiol

acid) and MoOC12L

or MoOL3

(

384

,

wi th

Lzdiphenyldi thio phosphinic acid385 ) were reported. Spence and coworkers reported solution spectra of molybdenum(V) monomeric species [Mo(hbma) J - and of electrochemically generated

[Moo (hae)I

-

and

mercaptoaniline,

LMo0(hbpd) 2]

-

4 N.N'-bis(2-hydroxybenzyl)-~-phenylene

whose

crystal

environment

for

(H3hbma=;-

( 2-hydroxybenzyl )-2-

.H hae=1,2-bis(2-hydroxymilino)ethane;

diammine). [Et4Nl [Mo(hbma),],

structure

indicates

the

ion,

metal

H4hbpd=

a

exhibits

distorted axial

octahedral

spectra,

with

Electron Spin Resonance

52

cm-

g =1.9790, g ~1.9455, AI=22 x

I

I\

. EPR

superhyperfine splitting show A

> II

1

,

cm-'

A -60 x 11-

with no

data of 0x0-molybdenum(V) complexes

Al with a well resolved superhyperfine I4N splitting,

indicating a substantial overlap of a nitrogen orbital with the 386 metal centreed orbital

.

Synthesis

and

CPPh3Me] poNC14]

crystal

were

structure

published

determination

together

with

IR

And

of EPR

characterization of the complex387. Similar characterization was made388

for two dimeric molybdenum(V) complexes, obtained under

.different conditions

by

reaction

of

M o O ( O H ) ~ with

acac

and

indicated as Mo 0 (acac)4 and [ M ~ ~ O ~ ( a c a c ) ~ ( H ~H20. O ) ~ lEPR was 2 3 used to study paramagnetic species formed during the decomposition of M O ( C O ) ~ in the hydrogen and sodium form of zeolite Y, provides

a #relativeEy well

defined

environment

for

that

molybdenum

cations. A complex mixture of paramagnetic species is produced and the observed spectra after activation of the samples depend on the experimental conditions. Accurate examination of the signals and the

interaction

of

oxygen,

paramagnetic centre were given Molybdenum(V),

ammonia

389

and

.

niobium(IV1

and

substitutional cation sites in Ge02

pyridine

with

tungsten(V) single

crystals

the

ions

in

have

been

studied at 22 K. The spectra show superhyperfine lines attributed to the coupling with the 73Ge isotope, that is to admixture of the non-occupied Ge 4s orbital into the outer d orbital of the doped cation, for the isotropic superhyperfine interaction, and of 390 orbitals for the anisotropic interaction

4p

.

d5

Configuration.

Tetravalent Cobalt.

-

Tervalent

Iron,

Ruthenium

4

Osmium,

Single crystal EPR spectra of six thiolate

adducts of (tetraphenylporphinato) iron(II1) were reported, and the principal directions of the g tensors were related to the nature of 391 the axial ligands

.

High-spin N3(CH3l2SO. g -2.16, 2-

low-spin

The

g =1.75. 3

equilibrium

low-spin

species

is

was

observed392

characterized by

in

hemin

gl=2.81,

Low spin behaviour was found for a potassium

2: Trensitionmetal Ions

53

cryptate salt of bis(4- methylimidazolato)(tetra

phenylporphinato)

iron( 1 ~ ) ~ ' ~ .EPR data of imidazole and imidazolate derivatives of natural iron porhyrins were used to separate the isotropic shifts into dipolar and contact

contribution^^'^.

EPR data were reported

also for iron(II1) complexes with 2-pyridine-carboxaldehyde dithio 395 396 carbazonate , 2-ace tylpyridinethiosemicarbazone , and 397 diacetyl monoxime thiosemicarbazone

.

Octahedral

coordination

with

low

symmetry

components

was

assigned to ruthenium(II1) complexes with diethyldithiocarbamate, 398 , and ethylxanthogenate, 8-mercaptoquinoline, 8-hydroxy-quinoline dithiophosphinate~~ on~ ~ the basis of

EPR

spectra, while

cis-

[(catechol)Ru(NH ) ] +gives an axial spectrum400 with g =1.889 and 3 4 II g -2.722.

1-

Osmium(II1) species are formed401 in single crystals of AgCl and AgBr. Low

spin

cobalt(1V)

ions

are

stabilized

elongated octahedron of oxygen atoms, as

in

a

strongly

shown by the EPR spectra

observed402 in Sr

0.gLal. gLiO. 5c00.' 4 ' 5

-

d7 Configuration. Tervalent

Nickel,

Bivalent Cobalt, Palladium

[(C2H5)4N] 2Ni ( S2C4N2)

&

Rhodium,

Platinum.

and

-

Iridium

Cobalt

and

doped

showed relevant nuclear quadrupole effects.

It was stated that such effects should become more evident when detailed

single

cobalt( I1 prepared

crystal

complexes403. by

vacuum

studies The

are

unusual

performed

*+

on

[C~(en)~]

decomposition

of

low

spin

species

was

[C~(en)~] 2+

(en=1,2-diaminoethane) on the surface of hectorite at 423-500 K. The EPR parameters are: g =1.964, g -2.579, ~,,=149,A - 1 7 4 ~ 1 0 - ~ II 11-1 cm showing that the unpaired electron is essentially localized in a z2 orbital

404

.

Bis(dimethylg1yoximato) cobalt(I1) complexes were

synthesized

in a Co-exchanged NaX zeolite and the species formed studied by EPR405. Detailed ENDOR studies and MO Htickel, INDO, and Xa

calculations, in the Extended

formalisms, were reported for "'-ethylene

Electron Spin Resonance

54

bis(acety1acetoneiminato)

cobalt(I1) 406'407.

In

particular

the

theoretical treatment explained the non-coaxiality of the g, cobalt hyperf ine and quadrupole tensors. The spin hamiltonian parameters of

several other 408-414

complexes

with

Schiff

base

ligands

were

reported The

cobalt(I1)

hemiporphyrazine

complexes,

doped

into

the

corresponding nickel(I1) derivative, show a spectrum much different from that of the porphyrin type complexes, with g,=3.560, g,=1.831, g =1.725, A1=250, A - 3 2 ~ 1 0 -cm-l, ~ resembling the'schiff ba:e type. 3 3If the compound is doped into the non-isomorphous zinc derivative, cobalt(I1) becomes high spin, as shown by normal porphyrin

type spectrum

is

the EPR spectra415.

observed

complex with tetra-2,3-pyridino-p0rphyrazine~'~.

for

the

A

cobalt(I1)

The EPR spectra of

mercapto- and donor and acceptors of cobalt(I1) porphyrins were 417,418 reported Single crystal EPR spectra of vitamin B 12r, substituted in a B12b single crystal yielded g =2.310, g =2.190, g =2.004, A1=22, 1 2 3 A2=27, A3=101x10-4 cm-', AIN=A N =15, A3N=18x10-4 cm-l which agree 2 with the unpaired electron in a predominantly cobalt z2 orbital. The spectra recorded under high oxygen pressure were those of the dioxygen adduct of vitamin B

From this the Co-01-O2 moieties at

low temperature was estimatid24;'9 to be bent with a bond angle of 2 2

lllq A novel species with an unpaired electron in the cobalt x -y

orbital was found to form at temperatures higher than 77 K when 420 y-rays

.

methyl cobalamins and coenzyme B12 was exposed to High-spin

low-spin

tris(2,2'-bipyridine)

equilibria

cobalt(I1)

were

complexes

studied421

in

in zeolite Y

and in 422 complexes with Schiff bases derived from 3-formylsalicylic acid

.

It is suggested that the condition

for spin equilibria in six

coordinate complexes is that the metal lies in a 4+2 tetragonal environment. Bivalent rhodium is formed in KC1 single crystals doped with K3Rh(CNI6 irradiated at 77 K with 2 MeV electrons423 and analogous results were obtained424 with K3Ir(CNl6.

Very highly anisotropic g

values, g = 3.88, g - 1.57, g - 1.26 were reported for RhCl (PCy ) 1 232 3 2

2: Tmnsition-metal Ions

55

(Cy= cyclohexyl), which by treatment with CO in the solid state 425 yields a paramagnetic carbonyl derivative

.

The crystal structure and single crystal EPR spectra of low-spin

complex

ethylene

dithiolato)

4,

[Ni ( S2C2( COOCH3) 2)

c( 'gH5

nickelate(III),

having

a

the

bis-(cis-1,2-dicarbometoxy

tetraphenylarsonium

nearly

ideal

4As1

square

planar

coordination, indicate426 S4 coordination and the presence of a substantial spin-spin exchange between the two magnetically non equivalent sites in the triclinic cell. The g values are g =2.063, g =2.151

and

Y

gz=1.986.

organometallic

The

nickel(II1)

preparation complexes

of

of

a

series

general

of

formula

Ni [C6H3(CH2NMe2)2-2 ,6]X2 (where X = C1, Br, I), in which the square pyramidal metal coordination sphere comprises two haloatoms, two nitrogen atoms and a direct N-C reported. EPR

spectra

in

u bond to an aryl function, was

toluene

glass

at

133

K,

showing

a

superhyperfine coupling with a single halo atom, indicate that the unpaired electron is probably localized in a unique metal-halogen 427 orbital

.

Several macrocyclic ligands have been show to form complexes with

nickel(II1)

consistent

with

cations low

whose

spin

EPR

spectra

tetragonally

are

in

distorted

general

octahedral

g =2.02 and gl=2.20. This is the case of the II cation, recognided as predominant species in frozen

complexes with LNiLClJ+

hydrocloric

solution

spectra

of

(L=a-rac-hexamethyl-l,4,8,11 hexaazacyclotetradecane)

X=C1,

.

EPR study of a series of uncharged complexes NiLX

A complete

with

CrJiLc11(~10~)~ 428

Br,

I

undeca-3,8-diene-2,10

and

L=anion

of

2' 3,9-dimethyl-4,8-diaza-

-dione dioxime, was reported. The anisotropy

of the magnetic parameters were readily observed in Q-band spectra of

the

nickel(II1)

complexes

magnetically

diluted

in

the

cobalt(II1) analogue. Single crystal data confirmed that the z axes of the g tensor is perpendicular to the molecular plane determined by

the macrocyclyc

experimental g

ZZ

ligand.

Despite

the large deviation of the

values from the free electron value

(2.020 and

2.026 for chlorine and bromine complexes respectively) the marked

Electron Spin Resonance

56

halogen

superhyperfine

splitting

indicate

that the 429 electron is in an orbital with mainly d 2 character

unpaired

.

2-

EPR spectra of nickel(II1) generated for irradiation of single crystals

of

two

square

rac-(5,7,7,12,14,14-hexamethyl-

planar

nickel(I1)

complexes,

1,4,8,11-tetraaza-cyclo-tetradeca-

and

4,11-diene)nickel(II)perchlorate

rac-(5,7,7,12,12,14-hexa

methyl-1,4,8,11-tetraazacyclo-tetradeca-4,14-diene)perchlorate show

that the unpaired electron is principally

located

in

molecular plane. The proximity of the z axes to Ni-OC10

a

NiN4

directions

suggests that nickel(II1) formation is accompanied by a lattice rearrangement leading to weakly coordinated perchlorate ions. The values of the energy difference between ground and excited states, much smaller than those observed in nickel(II1) macrocycles with axially coordinated chlorine ions, agree with this conclusion430. A single crystal of pseudooctahedral complexes of nickel(1V) with 3,14-dimethyl-4,7,10,13tetraaza-hexadeca-3,13-diene-2,15

ions dione

oxime, exibited EPR spectra after exposure to air for three days. The spin hamiltonian parameters g =2.046, g =2.152 and g =2.130

Y

were

attributed

to

the

formation of

nickel(II1)

paramagnetic

centres in a tetragonally elongated octahedral environment of the 431 nickel(1V) complex host lattice

.

detailed

A

study

of

-

(p-(dimethy1arnino)phenyl)dimethyl

ammonium-bis (maleonitriledithiolato)nickelate(III), (TMDP)Ni(mnt)

2

has been carried out by a variety of physical methods. Both the (TMDP)+ and Ni(mnt)2-

moieties are S=y systems, planar with highly

delocalized

electronic

themselves,

forming

structure. segregated

The

two

regular

ions

stack

stacks.

among

Magnetic

susceptibility measurements and solution, powder and single crystal EPR data in the temperature range 4.2-300 K give a detailed picture 432 of the magnetic phenomena

.

A

clarification was reported433 for the debated problem of the

correct

formulation

of

the

two

'forms',

brown-yellow

and

green-black, obtained by chlorination of saturated solution of nickel(II)(ethylenediammine)2C12

under smoothly different reaction

conditions.

form

The

brown-yellow

exibited

an

EPR

spectrum

2: Transition-metalIons

51

characteristic of a tetragonally distorted nickel(II1) ion and it

is clearly

[Ni(en)2C1JCl,

while

the

other

isomer

is

a

mixed

valence class I1 complex [Ni(II)(en),Ni(IV)(en)

ClgCl 2' 2 the bis(dipeptide) nickelate(II1)

The solution properties of

complex were studied and the unpaired electron appears to be in a d 2

2 orbital in a tetragonally compressed octahedral environment 434 values

.

15 -J! by ESR spectra showing g < g II

I

A reanalysis of the EPR data of nickel(II1) doped A1203 system

under uniaxial stress was carried out using the C. S. G.

Cousin

model of inner elasticity435; the formation and the stability of nickel(II1) centres on high surface area of Ni-MgO solid solution were investigated using EPR spectroscopy436, while EPR studies of X-irradiated nickel(I1) doped

NH4Cl

crystals reveal

reveal

the

formation of both nickel(1) and nickel(II1) centres whose 437 hamiltonian parameters are given

spin

.

The ESR features are useful probes of nickel(II1) centrer in hydrogenases as shown by the study of the nickel(II1) complexes of

-N-mercaptoacetylglycyl-L-hystidine the rhombic ESR pattern 438 chromophore of hydrogenases

having

and g-mercaptoacetyl-Gly-Gly-Gly similar to that of nickel(II1)

.

An

ENDOR

study was carried

out

for Ni3+

doped

in

gallium

phosphide showing that the paramagnetic ion is substitutional for gallium(II1)

and

that

the

unpaired

spin

distribution

has

a

pronounced tetrahedral symmetry along the (111) direction439. The characterization palladium in NaPdF presence

of

a

of

the

tripositive oxidation

by ESR spectroscopy at 8 K

4 significant

axial

state

of

demonstrate^^^'

the

distortion

in

palladium(II1) coordination polyhedron, with g

-

ll

the

low

spin

2.050 and g

-

I -

2.263 The synthesis of

the

macrocyclic

platinum(1V)

(1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6] icosane), (1,8-diaminoPt( dasarI4+

and

3,6,10,13,16,19-hexaaza their

chemical

and

complex

with

P t ( ~ e p ) ~ +and

bicyclo[6.6.6]icosane), electrochemical

behaviour

towards reduction were investigated with a view to seeking evidence for encapsulated platinum in lower oxidation states, particularly

Electron Spin Resonance

58

octahedral

platinum(II1) ,

d7

8

detected, and possibly the d 9

d

Configuration.

-

whose

spectra 441 platinum(I1) state EPR

are readily

.

Bivalent Copper and Silver.

-

The same comments

of the last report apply also to this case: the number of articles with EPR data on copper(I1) complexes is beyond any possibility of serious scrutiny, so that pruning must be much more drastic than for any other metal ion. I will mention here only those papers reporting detailed data, such as those on single crystal studies, and/or

those

which

are

about

matters

of

current

interest,

neglecting completely all those articles where the use of EPR is only intended to give confirmation of structural assignments. In the analysis of the EPR 63Cu(dtc)2,

spectra of frozen solutions of

bis(diethy1dithiocarbamato) copper(II),

it was found

that the copper nuclear quadrupole coupling constant increases 442 dramatically on axial ligation Accurate single crystal data of copper(I1) doped aqua tris (L-glutmato) cadmium(I1) monohydrate yielded the g tensor, copper hyperfine and nuclear quadrupole tensors, and nitrogen hyperfine tensor. The largest component of the

Cu-N

bond443.

interaction

in

Only the

two EPR

makes an angle of fluorine

atoms

spectra

of

E.

showed

14O with hyperfine

copper(I1)

doped

(NH ) ZnF4.2H20444. Data were also reported for copper doped 4 2 446 , cadmium diglycinate hydrate445, strontium tartrate trihydraLe cadmium oxalate t r i h ~ d r a t e ~An ~ ~ essentialy . z2 ground state was reported for copper doped (NH4)2Cd2(S04)3

448

.

Dejehet and Dubuyst studied single crystal EPR spectra of both pure and doped copper(I1)

complexes449-451. In particular in a

copper(I1) doped zinc complex with a crown ether the observed g values were: g =1.991, g =2.293, g =2.340. Since the zinc ion has

-

Y

-

tetrahedral geometry, the observed spectra were explained assuming that the copper ions enter a different site. The EPR spectra of 5,7,7,12,14,14- hexamethyl-

1,4,8,11- tetraaza

cyclotetradeca-

4,ll- diene copper( 11) iodide452 were reported. Single crystals of actyltetralone containing a small amount of copper(1.I) were studied

59

2: Transition-metal Ions

453 as a model for one of the complexation sites of tetracyclines

.

Chloroform/toluene solutions of bis(acety1acetonato) copper(I1) yield ENDOR spectra at temperatures higher than 77 K. The hyperfine 454 splitting due to the CH and CH3 groups were obtained

.

Unusual pseudo-tetrahedral copper( I1 ) species were shown455 to be present in 4A-zeolites exchanged with K+, Cs+,

and NH4+. EPR

spectra of [(CH ) N] CuBr4 showed a temperature dependence of the 4563 g,, direction

.

Several pseudo-tetrahedral copper(I1)

complexes were used as

models of the stereoelectronic properties of type I copper centres 457 Copper chromophores of varying in metallo-proteins

.

stereochemistry containing N and S donors have been studied in relation

to

copper

coordination

in

metallo-proteins.

Pseudo-tetrahedral complexes of bidentate !-substituted

B-amino

thiones, with substituents R=iso-butyl, p-tolyl, cyclohexyl, have small

458

values

All

.

Bis[4-(alkylmercaptomethyl)irnidazole~

copper(1I) dipercnlorate, chromophore N2S2, have both g

\I and gl values which are higher than observed in CuN2S2 chromophores where ~

S and

N

are

thioether sulfur and

respecti~ely~~’.Rhombic CuN S

amino-

or

pyridyl-nitrogen

spectra were observed in a

series of

chromophores, but no d e f i n i t e trend emerged in the g and A

Similar rhombic spectra were obtained also for .trigonal bipyramidal CuN S X chromophores (X= H 0, Br)461. 2 2 2 species were formed by copper(I1) salts

Five coordinate and

1-

and

2-formylquinoline thiosemicarbazone, as shown by EPR spectra462. An unusually small A

value was observed in fluid solutions of a

pentacoordinate complex of a thioether- benzimidazole ligand. It 463 was attributed to the difference in sign of A II and * The compound Cu(cyclam)(SC6F5)p (cyclam= 1,4,8,11-tetraza 0

cyclotetradecane) has axial Cu-S bonds 2.94 A long. The EPR spectra do not reveal any characteristic effect of the axial thiolates, a result which

may

be

relevant

to

the

interpretation

of

the

electronic stmcture of plastocyanin, where a similar Cu-S bond 464 distance is observed

.

EPR spectra and &-ray structure determination were reported for

60

Electron Spin Resonance

copper(I1)

bis(4,4',5,5'-~etrame~hyl-2,2'-biimidazole)

dinitrate

and for the analogous zinc complex doped with copper(I1).

The EPR

spectra of the pure copper complex and of the one doped into zinc are very similar to each other, showing that the structure is essentially preserved although che X-ray crystal structure of the 465 zinc compound is different from that of the copper

.

Several copper(I1) complexes with chloride, bromide, and iodide 466-469

donors were studied by EPR spectroscopy

pH dependent spectral properties of small peptide-copper(I1) complexes

were

adduc t s

Several

investigated470-476.

of

1,lO-phenanthroline and 2,2'-bipyridine with copper(I1) dipeptides were

shown

to

environments477. are

quenched

have

distorted

square

pyramidal

coordination

The EPR spectra of a copper( 11) peptide complex

by

the

addition

cyano-bridge is formed478.

.

K3 [Cu( glygly )Fe (CN)6] 6H20

EPR

of

Fe(CN)6

spectra at

(glygly=

3-

, showing

room

glycyglycine)

that

a

temperature of were

on

the

contrary reported. The authors interpreted the spectra on the basis

of isolated copper(I1) ions, although the magnetic data suggest a coupling with the low spin iron(lI1) species479 . Numerous copper(I1)

studies ions with

ace

concemed

various

D - c y ~ l o s e r i n e ~L-cys ~ ~ , cine482

with

the

interaction

amino-acids, such as

of

glycine480,

L - l y ~ i n e his ~ ~ tidine ~ and glutamic

acid484 and with Schiff bases containing amino-acids as part of the 485,486 1igand The

interaction of

copper(I1)

with

hurnic

acid was

studied

488,489.

eithei- with the real acid487 o r on models

The complex shown below, and copper( 11) complexes of saturated macrocyclic ligands

H

2: Transitionmetal Ions

61

were s t u d i e d as s u i t a b l e models f o r t h e m e t a l b i n d i n g s i t e s o f t h e 490,491 pH and solvent dependence of bleomyc i n an: i b i o t i c

.

copper( 11 )

binding

to

studies492.

The

frozen

was

nystatin

solution

found

by

spec t r a

EPR

of

spect~oscopic

the

copper*(11)

b a c i t i - a c i n A complex were u s e d t o p r o p o s e a b i n d i n g model i n which t h e m e t a l i o n c o o r d i n a t e s two n i t r o g e n and two oxygen atoms493. EPR specti%

wei-e

used

b a r b i t u r a t e complexes EPR and e l e c t r . o n

characterize

the

z e o l i t e s495-497.

indicated

to 494

assign

the

stereochemistry

of

copper

.

s p i n echo modulation

location

of

to

t e c h n i q u e s were used

copper(I1) ions

in

both

and

X

I n p a r t i c u l a r a new t r i g o n a l b i p y r a m i d a l

Y

site was

i n C a X z e o l i t e . The same t e c h n i q u e s wei-e a l s o u s e d fOi?

studying

the

effect

of

molecular

cage

size

on

the

motion

and

c o o P d i n a t i o n o f c o p p e r ( I 1 ) i n c r o s s - l i n k e d p o l y ( v i n y 1 a l c o h o l ) and p o l y ( e t h y 1 e n e o x i d e ) g e l s 4 9 8 . EPR s p e c t r a a l l o w e d M a r t i n i e t a l . t o study

the

different,

steps

in

into s i l i c a gels

the

with

imppegnation

varying pore

pi-ocess

sizes4”.

of Both

Cu(N0 ) .6H 0 2 3 2 mononuclear t e t r a c o o r d i n a t e d and p o l y n u c l e a r s p e c i e s were found G O be

formed

spectra

on

the

were

surface

to

used

of

polyamine

determine

c o p p e r ( I 1 ) s u r f a c e complexes on S i , or T i 0 2 A1203

501

.

indicated

presence

exchangers500.

stability Aerosil

constants

EPR of

hydrolized

300,

t e r n a r y c o p p e r ( I 1 ) complexes o f

An ENDOR seudy of r;he

the

6-A1203,

anion

of

protons

of

both

axially

and

e q u a t o r i a l l y c o o r d i n a t e d H 0 , and an e s t i m a t i o n o f t h e metal-oxygen 2 d i s t a n c e s w a s made502. The r o l e o f s a l t s , n e u t l a l l i g a n d s , and ionic

surfactants

in

rnicellar

concen2rations

on

the

rate

of

o x i d a t i o n o f a s c o r b i c a c i d c a t a l y z e d by c o p p e r ( I 1 ) i o n s w a s s t u d i e d by EPR503.

The n a t u r e of

t h e c o p p e r ( 1 1 ) - a q u o a n d -amine

complexes

which form on t h e s u r f a c e o f t h e l a y e r e d i n o r g a n i c i o n e x c h a n g e r 504 Z2(HPO4i2.H 0 w a s s t u d i e d u s i n g EPR 2 Aqueous s o l u t i o n s of AgN03 r e a c t w i t h c o l l a g e n y i e l d i n g an A g 2+

.

c o l l a g e n complex as shown by observed them

have

EPR

i n t h e EPR s p e c t r a of in

common a

ground

spectra505.

Several

s i l v e r ( I 1 ) doped xy

orbital

in

s i t e s wei‘e

Ca(OD)2:

agreement

a l l of with

a

t e t r a g o n a l e l o n g a t i o n a n a l o g o u s t o thaL s e e n ir, t h e c a s e of c o p p e r

62

Electron Spin Resonance 506

doping

.

EPR spectra of single crystals of AgNO -butanediniti-ile 3 X-irradiated at 77 K showed the presence of a -CN-Ag2+-NC and of a

N O ~ ~ - - A ~centres. +

The

Ag

2+

cen-cre has

g =2.050, 1

g =2.302, A =21.8, A =22.0, A =34.9x10-~ cm-'; 3 1 507~ . splitting was also resolved

Univalent Nickel, Palladium

g =2.053, 2 nitrogen hyperfine

Platinum.- z-ray irradiation of a

single crystal of nickel doped

of Ni+

creates two kinds

SrF2

centres having respectively pure

tetragonal symmetry and

orthorhombic distortion. The EPR

spectTa show

splitting due to

the

interaction with

a

slight

superhyperfine

four equivalent fluorine

nuclei508. An orthorhombic spectrum with superhyperfine splitting was observed also for Ni+ centres produced X-ray Ni'

by

room temperature

irradiation of nickel doped BaF2 crystals509 510. Luminescent

centres were induced in Ni2+ doped ZnS and ZnSe. Photosensitive

the signals revealed 511 recombination processes .

EPR

photoionization

and

radiative

Coordinatively unsaturated nickel(1) ions on silica and alumina surfaces were obtained by photoreduction of nickel(I1) ions in a hydrogen

atmosphere

at

77 K.

The

EPR

technique

was

used

to

investigate the complexation of these tons with CO, H2, N2, 02,

H20, and

NH3.

Adsorption

of

C2H2

and

C2H4

resulted

in

the

disappeaeance of the nickel (I) signals, probably due to catalytic activity of the ion on the oligomarization ~eaction~'~' 513. Similar studies wer'e performed for coo?dination unsaturated palladium(1) on 514 alumina surfaces . Phosphine complexes of 515-516 systerns and the

nickel(1) effect

of

in

Ziegler-type the

catalytic

nickel-naphthenate

alkyl-aluminium sesquichloride-diethyl ether catalytic system on the

oligomerization

of

e-butadier~e~ were ~~

investigated

by

Russian authors. Nickel(1) and cobalt(1) species obtained by reducing NiL(PF6)2 and CoL(BPh ) .4H 0 ( L = 4 I ,4 "-diphenyl-2 : 6,2": 6",2 I 4 2

: 6 ' ,2""-quinque

2: Transition-metal Ions

pyi'idine)

63

with

electrolysis

cyclic

were

voltammetry

characterized

by

and

controlled and

EPR 518

with

potential the

same

technique the geometric configuration of a not previously reported olefin-nickel(1) derivative, obtained

during

study of nickel-triphenyl phosphine system 519 acrylonitrile was investigated . The

EPR

spectra of

M(Ph2C2S2)21 with M

=

the

one-electron

an in

electrochemical the

presence

of

reduction products

of

Ni, Pd, Pt, indicate that several species

with slightly differing g values are formed in this process The EPH technique was

used

to

520

.

investigate the condition of

preparation and the stability under different conditions of the one electron reduction products

of

bis(dithiooxa1ato)-nickelate(II),

-palladate( 11) and -platinare( 11) complexes521. Platinum( i) spectra were recognized after redox 52 2 A1203 and Ti0 surfaces . 2 4

d

3

2

=

treatment of p1a.tinum supported on

3/2

Tungsten. -

Configuration. - Tervalent Chromium, Molybdenum

Electron paramagnetic resonance is almost always observed using fixed

frequency

in

a

appropriate pairs of non levels

I -

whose

field-swepc

spectrometers.

crossing levels-

When

so called

S=1,

'repelling

;pacing vary non-linearly with the magnetic field,

give rise to pairs of asymmetric transitions which coalesce at a particular angle from the crystal symmetry axis. These transitions -referred to as

'looping transltions' - have been

observed for

chromiurn(II1) ions in ruby single crystals and are used to analyze the transition probabilities. A method for point-by-point numerical 523 simulation is given . Alexandrite (A12-&CrLBe04)

is a broadband

solid state laser,

but the analysis of the lasing and optical properties of

this

material is complicated by the two substitutional possibilities of chroiiiium(II1) and alurninum(II1) ions in the host A1 Be0 lattice 2 4 having different symmetry. The linewidths and the intensities of EPR lines detected for two single crystals with different levels of

Electron Spin Resonance

64

doping were used to chsracterize the distribution of chromium( 111) within the two sites an6 the contribution of each substitutional 524 centre to the overall optical properties of the crystals . The effect of a change in the chromium concentration on the shape and on the intensity of the EPR in silica

and

line of chr.omiun(II1) in

was

Russi an

investigated

authors. Chromium(II1) was used as a paramagnetic probe in a study

of

elastic

inteyaction

CsA1(S04)2.12H20528

of

point

defects

in

crystal

of

and RbIn(S04)2.12H 20 alum was used as a host

lattice to measure the temperature dependence of the zero field splitting parameter of 53Cr3t, for which hyperfine parameters were 529 obtained by E N D O R measurements

.

I9F E N D O R was also reported in a electTon paramagnetic study of chromium(II1) impurities in K MgFq and K ZnF

crystals eventually

2

doped with sodium

and

lithium ions530-‘32.

Redox

properties of

chromium(II1) ions doped in NH Y zeolites and the Teactivity of the 4

macerial with respect to

the adsorption of NO were 533 . with the use of EPR spectroscopy It

is widely

accepted

that

for

investigated

Co(Ni)-Mo(W)/y-A1203

(Si02)

hydTodesulphuration catalysts the catalytic activity resides in the molybdenum(II1) or tungsten(Il1) ions at the surface of MoS2 or WS phases, while the natule of

2 the p=omoter ac-cion of cobalt and

nickel is still not well undeistood. EPR, used as a sensitive and selective method, correlation

allowed

between

the

the

conclusion

intensity

of

that the

there

exists

signals

of

a the

molybdenum(II1) or tungsen(II1) ions interacting with the promotor. ions on the surface of the metal

sulphides crystallites and

?he

hydrodesulphuration reaction rate of thiophene, used as catalytic 534 test

.

Paramagnecic

diene

monomeric

complexes

of

molybdenum(lI1)

[MoX (diene)(q-C5H5)] (X = C1, Br, I) were 2 synthesized and their electronic struc cures are discussed on the 95 and basis of the analysis of the hyperfine ( Mo, 9 7 M 0 ) formulated

as

siiperhyperfine coupling (35’37C1, 535 and THF solution EPR spectra

79’81Br)

.

obsei-ved in the a-MeTHF

2: Transition-metal ions Divalent

65

-

Vanadium.

The

spin

hamiltonian

parmeters

for

v a n a d i u m ( I 1 ) c e n t e r s i n CdBr2 a n d CdCl

h o s t l a t t i c e s i n d i c a t e _D 2 536 " symmetry f o r t h e p a r m a g n e t i c i o n s r e p l a c i n g d i v a l e n t cadmium

.

Q u a d r i v a l e n t Manganese a n d Rhenium.

- The

t h e r m a l l y u n s t a b l e MnLX

w i t h L = 5,10,15,20-tetraphenylporphynato, X

(

spectroscopic manganese(1V)

ion,

show

toluene-chloroform splitting crystal

a re

pL-operties

at

>

0.6

ID1

is

structure

highly

glasses

parameter,

consistent

also

a

with

anisotropic

12

For

reported537.

the A

2 NCO), w h i c h

3' high-spin

d3

spectra

in

ESR

a

with

K

cm-'.

N

=

large

zero

field

deriva-iive

NCO

theoretical

study

rhe of

h y p e i - f i n e s t r u c t u ~ eo f t h e EPR s p e c t r a o f m a n g a n e s e ( 1 V ) i o n s i n a strong trigonal

crystal

dependence o f t h e An

estimate

?henium(IV) 539 made

transition the

doped

in

to

arbit;.ary

of the expression obtained f o r the angular

-%

of

a generalization

f i e l d gave

half-integei- spin S ) 3 / 2

538

splitring

.

of

(NHql2PtCl6

the

by

4

state

ground

A2

trigonal

of

was

distortion

.

d

5

Configuration. - T e r v a l e n t I r o n . - Dirneric-monomeric

and d i f f e r e n t l y

S=3/2

ground

c o o r d i n a t e d monomeric

state

were

recognized

Fe(II1)

by

equilibi-ia

species,

solution

EPR

showing a spectra

in

various solvents of a dimeric Fe(II1) complex, (Ph P)2Fe [(S2C2(COOCH3)2]4, which c r y s t a l s t r u c t u T e i s a l s o r e p o r t e d 540

.

EPR s p e c t r a r e c o r d e d a t 10 K ,

for t h r e e

'basket handle'

Fe(II1)

p o r p h y i - i n c o m p l e x e s c o n f i r m e d a quantum m i x t u r e o f S = 5 / 2 a n d S=3/2 spin

scates

strongly

c h a i n and r e a d i l y

dependent

upon

the

nature

i n t e r p r e t e d w i t h Maltempo's

symmetry a p p r ~ x i r n a t i o n ~The ~ ~ . same t h e o r e z i c a l determine

the

ground

state

for c h l o r o

of

model

the in

b?idging the axial

model was u s e d

phthalocyanine

to

i i . o n ( 111),

o b t a i n i n g a good f i t f o r t h e m a g n e t i c moment t e m p e r a c u r e d e p e n d e n c e 4 A,, g r o u n d s t a t e w i t h a

a n d powder EPR s p e c t r a f o r a p r e d o m i n a n t l y 542 35% o f 6A mixed i n 1

.

L

66 d

7

Electron Spin Resonance

Configuration.

trigonal

-

Bivalent Cobalt. - The EPR spectra of several

octahedral

information

on

cobalt(I1)

the

corrrplexes were

n-bonding

anisotropic

used

to

obtain

interactions

of

nonlinearly ligating ligands. The pattern of g values is strongly influenced by the diffei-ence between the two n -bonding parameters of individual ligands in the Angular Overlap Model. Data on COO 6 543 and CoN6 chromophore were reported and discussed .

EPR spectra of single crystals of cobalt(I1)-carboxypeptidase A (CoCPA) and copper(I1) carboxipeptidase A

(CuCPA) were obtained.

The g tensors in CoCPA and CuCPA are substantially parallel to each other. The assignement of the observed features to different sites

o r different molecules 544 obscure

in

the

monoclinic

structure

is

rather

.

The

structure

of

cobalt(I1)

in

a-LiIO

was

studied

by

radiofrequency discrete saturation (RFDS) in the X-band at 4.2 K i n several orientation of the ci-ystals in the magnetic field. The EPR and RFDS spectra are consistent with the hypothesis that cobalt(I1) substitute

lithium(1)

in

the

lattice

and

the

excess

charge

compensation is realized by the nearest lithium vacancy along the c 545,546 axis 2t The Co(H20l6 complexes in the lattices of ZnSiF6.6H20, La Mg (N03)12.24H20, and La Zn (N03)12.24H 0 under the effect of 2 3 2 3 2 isotr’opic and axial compression was studied at 4.2 K. The EPR spectra showed that the chromophore distortion are different for 547 different lattices

.

Coo-MgO solid solutions with COO concentrations in the range 0.25 -15.5 mol% were investigated by ESR spectroscopy at 4.2-77 K. The distribution of cobalt(I1) among different sites with distorted octahedral and tetrahedral geometries was estimated. The hypotesis of cobalt( 11) couplings with the paramagnetic neighbours was 548 considered .

also

67

2: Transition-metal Ions 5

2

5/2

=

- The e f f e c t s o f i n f r a r e d i l l u m i n a t i o n on t h e

U n i v a l e n t Chromium. ESR o f

c r y s t a l s a t 77 K

c e n t e r s doped i n ZnSe

Cr(1)

w i t h a t e n t a t i v e model o f t h e e n e r g y l e v e l s 5 4 9 .

a s e r i e s of

spectra of

ESR

n i t i > o x y l compounds

of

of

the

is

chromium(1)

f o r t h e mechanism o f 550

p r e s e n t e d t o g e t h e r w i t h a model

is r e p o r t e d

Parameters

transfer

of

.

s p i n d e n s i t y f r o m C r ( 1 ) t o 14N(NO)

Manganese. - The ESR s p e c t r u m o f M n ( I 1 ) doped i n S2F2 c r y s t a l s was m e a s u r e d a n d , i n a d d i t i o n t o t h e w e l l known s i g n a l d u e

Bivalent

.to c u b i c M n ( I I ) , a new s i t e h a v i n g a t r i g o n a l symmetry w a s r e v e a l e d

was

which

suggested

to

consist

of

an

vacancy

F

next

a

to

s u b s t i t u t i o n a l M n ( I I ) 5 5 1 . ENDOR m e a s u r e m e n t s w e r e made on manganese doped

CsCaF3

to

investigate

the

unpaired

spin

densities

on

the

n e a r e s t neighbour P i o n s i n r e l a t i o n t o t h e h y p e r f i n e i n t e r a c t i o n s with t h e second 2 s , 2p0 a n d 2p

The

ESR

neighbour

spectra

f l u o r i t e s CaF2, S i F line

the

Mn(I1)

doped

superionic

.

conductivity

and BaF2 were used t o i d e n t i f y t h r e e d i f f e r e n t

Mn( I1

mechanisms553.

doped

crystals

of

( M = N i , Zn) w e r e s t u d i e d , and t h e f i e l d d e p e n d e n c e of

t h e l i n e w i d t h s were chromium(II1)

of

2

broadening

K MSe04.6H 0 2 2

F n u c l e i . The u n p a i r e d s p i n f r a c t i o n o f

o r b i t a l s on F wer'e o b t a i n e d from Lhe s p e c t r a 552

d i s c u s s e d 5 5 4 f 555

and Manganese(I1)

.

ions

The in

spatial distribution spinel

lattices

and

of the

e f f e c t of l o c a l s u r r o u n d i n g s on s p e c t r o s c o p i c c h a r a c t e r i s t i c s w a s studied556,557 . V a r i o u s t y p e s of m a n g a n e s e ( I 1 ) doped A1203 s y s t e m s

were s L u d i e d by means o f EPR s p e c t r o s c o p y i n o r d e r t o m o n i t o r e f f e c t o f y - i r r a d i a t i o n on t h e s t a b i l i t y o f Mn( 11)

to

follow

processes

of

the

paramagnetic

ion

and

c o p r e c i p i c a t i o n 5609561.

s t o i c h i o m e t r i c ammonium-B-alurninate

,the

In

non

c r y s t a l s t h e s i t e occupancy o f

was d e t e r m i n e d and

t r e a t m e n t s up t o 8OOOC were a l s o c o n s i d e r e d

effects

the 562

of

'ihermal

.

The EPR o f m a n g a n e s e ( 1 I ) s u b s t i t u t i n g for t h e z i n c ( I 1 ) i o n s i n ZnP

w e r e u s e d t o o b - c a i n s t r u c t u r a l i n f o r m a t i o n 5 6 3 . I n p l a t e l e t s of 2 Cs2Zn3S4 w i t h a b o u t 4% of t h e z i n c r e p l a c e d by manganese( 11) , two

Electron Spit1 Resonance

68

non equivalent centres olr isolated manganese ions were observed in addition to a broad signal with g close to 2 which the authors assigned

to

clusters

of

interconnected

MriS

4

units564.

The

occurrence of different, manganese centres in single crystals of ZnSe and ZnS, as well as in ZnS powder was reinvestigated. A new axial manganese(I1) centre in ZnS was observed and the formation of Mn(I1)-Mn(I1)

pai1.s detected for both the systems. F o r the first

time EPR results of manganese centers in thin film structures of 565 ZnS and ZnSe were presented . Accurate

value

of

the

main

spin

hami1:onian

parameters

of

rnanganese(I1) in Tutzon salts were obtained by using the .technique

of zero-field EPR. Anomalies in the sign of the 2nd-order axial fine structure were shown from single-crystal measui-ements to be due to orthogonal transformations of the axis system566. The EPR spectra of rnanganese(II), vanadyl(II), copper(I1) and &-i,ay induced centre

in Rb2C204

.

H 0 single crystals were studied at 9.45 G H z

2 and 290 K. The ions appear to enter the lattice as interstitial sites. The doublet separations of forbidden hyperfine transitions 567 were studied in rnanganese(I1) doped c r y s t a l s . 568 The effects of pressure on single crystals of Na ZnC14.3H20 2 and CsCdCl 569 doped with small amounts of manganese(I1) were 3 studied. The mechanism of reduction from manganese(1V) were elucidated by following the variation of EPR spectra of Mn-doped 570 SrTi03 at different temperatures . The manganese(I1) ion was used

as a probe to study the conformational changes in Zn(en) (NO ) 3 3 2 and Cd(en)3(N03)2 complexes. A model was proposed to correlate the variation

of

the

magnetic

tensors

with

temperature 571 . conformations of ethylenediamine (en) chelate ring

to

the

The effect of UV irradiation of alkaline solutions of Mn(acac)

3 and Mn(acac) OCOCF3 was studied by examining the EPR spectra at 77 2

K which show the presence of manganese(I1) ions and free organic radical molecules

coming from the oxidation of solvent and 572 . Spin hamiltonian parameters, observed at

frequency for manganese(I1)

doped Zn(pyrazine)2X2

ligand Q-band

(X= C1, Br),

allowed the authors to propose a polymeric structure with pyrazine

2: Transition-metal fons

69

bridges f o r the diamagnetic complexes573. An analogous study was performed on similar cadmium(I1) and zinc(I1) derivatives at X- and 574 . Q-band frequencies Exposure of manganese( 111) porphyrin solutions to y-rays at 77

K gave the manganese(I1) derivatives which showed EPR spectra very similar

to

those

of

the

chemically

prepared

manganese(I1)

compounds. In addition features at g close to 2.0 whose intensities 575

grew on melting and refreezing, were observed The

reversible

of

binding

carbon

.

monoxide

by

MnX2(PR3)

(R =PhMe2, PhEt2, n-Pr3; X=Cl, Br, I ) in the solid state and in 3 solution was studied with the help of EPR spectroscopy. In THF the spectra

indicate

[MnX2(THF),(C0)(PR3)], pseudotetrahedral

that

the

CO

adduct

is

pseudooctahedral is

whereas

it in the solid state 576 [MnX2(CO)(PR3)] . EPR measurements

of

manganese(I1) ions in aqueous rnanganese(dodecylsu1phate) (Mn(DS) ) 2

solution with or without 0.05%

of poly(E-vinylpyrrolidenc) (PVP)

were carried out at 303 K. From the quantitatively analysis of the data the authors concluded that the Mn(DS)2 molecules form a nearly micelle-like

577

aggregate

.

The

mechanism

of

rnanganese(I1)

ion

binding in soil-organics and pure organic compounds were compared by using EPR spectroscopy. The observation that the paramagnetic ion in lower than cubic symmetry produces a very broad signal, was used to determine the degree of metal complexation by organics578

.

Electron Spin Resonance

70

References 1 2 3 4 5 6 7 8 9 10

11

12 13 14 15 16 17

18 19 20 21 22 23 24 25 26 27 28 29 30 31

32 33 34 35 36

H.A. Buckniaster, Magn. Reson. Rev., 1983, 8, 283. J.R. Wasson, Anal. Chem., 1984, 56, 129R. N. Niccolai, E. Tiezzi, and G. Valensin, Chem. Rev., 1982, 82, 359. L. Rue, Coord. Chem. Rev., 1983, 50, 73. I. Bertini, and C. Luchinat, in 'Metal Jons in Biological Systems', ed. Sigel, Marcel Dekker, New York, 1983, p . 59. J.T. Spence, Coord. Chem. Rev., 1983, 59, 48. L.C. Dickinson, and M.C.R. Symons, Chem. SOC. Rev., 1983, 12, 387. E.B. Boyar, and S.D. Robinson, Coord. Chem. Rev., 1983, 50, 109. R. Brarnley, and S.Y. Strach, Chem. Rev., 1983, 83, 49. S.G. Lakeev, Yu.A. Lyul'kin, and L.S. Lyubchenko, Sovrem. Metody YAMR EPR Khirn. Tverd. Tela (Mater Vses. Koord. Soveshch.1 3 r d , 1982, 124. Yu.A. Lyul'kin, S.G. Lakeev, and L.S. Lyubchenko, Zh. Fiz. Khim., 1983, 57, 751. L.S. Lyubchenko, Yu.A. Lyul'kin, and S.G. Lakeev, Zh. Fiz. Khim., 1983, 57, 2558. G. Ablart, J. Pescia, S. Clement, and J.P. Renard, Solid. State Commun., 1983, 45, 1027. T.C.L. Wang, and A.G. Marshall, Anal. Chem., 1983, 55, 2348. A. Schweiger, J. Magn. Reson., 1983, 2 , 286. R. Zimmerrnann, J. Magn. Reson., 1982, 49, 425. T. Herrling, and U. Ewert, Ger. (East) DD 200, 936. S. Schults, and E.M. Gullikson, Rev. Sci. Instrum., 1983, 3 , 1383. S.I. Weissman, Annu. Rev. Phys. Chem., 1982, 33, 301. A.I. Vistnes, and L.R. Dalton, J. M a p . Reson., 1983, 54, 78. J . R . Morton, K.F. Preston, and S . J . Strach, Rev. Sci. Instrum., 1981, 52, 1358. J.R. Morton, and K.F. Preston, J. Magn. Reson., 1983, 52, 457. U. Netzelmann, H. Lerchner, J . Pelzl, and M.W. Sigrist, J. Phys. Colloq., 1983, ( C 6 ) , 221. B.G. Berulava, R.I. Mirianashvili, and D.M. Daraseliya, Izv. Akad. Nauk SSSR, Ser. Fiz., 1983, 47, 2314. P. De Groot, P. Janssen, F. Herlach, G. De Vos, and J. Witters, Int. J. Infrared Millimiter Waves, 1984, 5 , 135. F.S. Irnamutdinov, and A.Kh. Khasanov, Fiz. Tverd. Tela (Leningrad), 1983, 25, 3708. M. Bonori, G. Franconi, P. Galuppi, and C.A. Tiberio, J. Magn. Reson., 1982, 50, 349. A.I. Vistnes, D.I. Worrnald, S. Isachsen, and D. Schrnalbein, Rev. Sci. Instrum., 1984, 55, 527. M. Giordano, F. Momo, and A. Sotglu, J. Phys. E , 1983, 2 , 774. J.R. Anderson, and C. Mailer, Rev. Sci. Instrum., 1982, 53, 1727. S.A. Jacobs, G.W. Kramer, R.E. Santini, and D.W. Margerum, Anal. Chirn. Acta, 1984, 157, 117. M. Kaise, K. Fujii, and K. Someno, Bunseki Kagaku, 1983, 32, T1E. S. Basu, K.A. Mc Lauchlan, and G.R. Sealy, J. Phys. E, 1983, 16,767. R. Schultz, G. Hurst, T.E. Thieret, and R.W. Kreilich, J. Magn. Reson., 1983, 53, 303. J.D. Lipscomb, and R.W. Salo, Cornput. Enhanced Spectosc., 1983, 1, 11. F. Momo, G.A. Ranieri, and A. Sotgiu, Cornput. Enhanced Spectrosc., 1983, 1, 79.

2: Transition-metal Ions

71

37 R.N. Bagchi, T.L. Henderson, and F.L. Walter, Chem. Biomed. Environ. Instrum., 1982, 3,175. 38 J.C. Ireland, J.A. Willet, and A.M. Bobst, J. Biochem. Biophys. Methods, 1983, 8, 49. 39 C.P.Jr. Poole, and H.A. Farach, Appl. Spectrosc. Rev., 1983, 19, 167. 40 M. Rudin, J. Forrer, and A. Schweiger, J. Magn. Reson., 1983, 3 , 447. 41 A. Schweiger, M. Rudin, J. Forrer, and H.H. Guenthard, J. Magn. Reson., 1982, 50, 86. 42 A. Schweiger, M. Rudin, and H.H. Guenthard, Chem. Phys. Lett., 1983, 95, 285. 43 B.M. Odintsov, N.K. Danilov, I.Kh. Khusainov, and R.G. Yakhin, Deposited Doc., 1982 VINITI 2041. 44 A.F. Melkopf, F.G. Kuiper, J. Smidt, and T.A. Tiggelman, Rev. Sci. Instrum., 1983, 54, 695. 45 S.A. Kazanskii, Zh. Eksp. Teor. Fiz., 1983, 84, 1202. A6 S.A. Kazanskii, Opt. Spektrosk., 1983, 5 , 304. 47 K. Ohno, J. Magn. Reson., 1982, 50, 145. 48 Yu.V. Rakitin, and R.D. Kasumov, Deposited Doc., 1981, VINITI 4017. 49 Yu.V. Rakitin, R.D. Kasumov, G.M. Larin, and V.T. Kalinnikov, Deposited Doc., 1981, VINITI 5070. 50 S.K. Hoffmann, and J.J. Goslar, Solid State Chem., 1982, 44,343. 51 K. Spartalian, and C.J. Carrano, J. Chem. Phys., 1983, 78, 4811. 52 S. Larsson, B.O. ROOS, and P.E.M. Siegbahn, Chem. Phys. Lett., 1983, 96, 436. 53 H. Nakatsuji, Y. Onishi, J. Ushio, and T. Yonezawa, Inorg. Chem., 1983, 22, 1623. 54 P.G. Perkins, and F.A. Schultz, Inorg. Chem., 1983, 22, 1133. 437. 55 J. Weber, Stud. Phys. Theor. Chern., 1982, Helv. Chim. Acta, 1982, 3,2486. 56 C. Daul, and J. Weber, _______ 57 A. Goursot, H. Chermette, and C. Daul, Inorg. Chem., 1984, 23, 305. 58 A. BenciEi, and D. Gatteschi, J. Am. Chem. SOC., 1983, 105, 5535. 59 L. Noodlernan, and E.J. Baerends, J. Am. Chem. SOC., 1984. 106,2316. 60 N.M. Atherton, and .J.F. Shackleton, Chem. Phys. Lett., 1984, 103, 302. 61 M.G. Zhao, J. Xu, and G. Bai, Sci. Sin., Ser. A. (Engl. Ed. ) , 1982, 25, 1066. 62 M.G. Zhao, and Y.F. Zhang, IEEE Trans. Magn., 1983, MAG-19, 1972. 63 B.G. Vekhter, Fiz. Tverd. Tela (Leningrad), 1983, 25, 3128. 64 A.A. Bugai, and V.S. Vikhnin, J z v . Akad. Nauk SSSR, Ser. Fiz., 1983, 47, 2322. 65 A.A. Bugai, V.S. Vikhnin, and V.E. Kustov, Fiz. Tverd. Tela (Leningrad), 1983, 25, 1466. 66 B.G. Veckter, Fiz. Tverd. Tela (Leningrad), 1983, 25, 1537. 67 D. Reinen, and C. Friebel, Inorg. Chem., 1984, 23, 791. 68 W. Henke, S . Kremer, and D. Reinen, Inorg. Chem., 1983, 22, 2858. 69 C.J. Simmons, K. Seff, F. Clifford, and B.J. Hathaway, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1983, B, 1360. 70 G.F. Kokoszka, J. Baranowski, C. Goldstein, J. Orsini, A.D. Mighell, V.L. Himes, and A.R. Siedle, J. Am. Chem. SOC., 1983, 105, 5627. 71 M.V. Eremin, T.A. Ivanova, Yu.V. Yablokov, and R.M. Gumerov, Pis'ma & Eksp. Teor. Fiz., 1983, 37, 226. 72 C. Friebel, J. Pebler, F. Steffens, M. Weber, and D. Reinen, J. Solid. State Chem., 1983, 46, 253. 73 C.P. Keijzers, T. Jansen, E. De Boer, G. Van Kalkeren, and J.S. Wood, J-

u,

72

1:'iectron Spin Resonance

Magn. Reson., 1983, 52, 211. 74 C.P. Keijzers, R.K. McMullan, J.S. Wood, G. Van Kalkeren, R. Srinivasan, and E. De Boer, Inorg. Chem., 1982, 21, 4275. 75 C.P. Keijzers, G. Van Kalkeren, E. De Boer, and J.S. Wood, Mol. Phys., 1983, 2,1187. 76 G. Van Valkeren, C.P. Keijzers, R. Srinivasan, E. De Boer, and J.S. Wood, Mol. Phys., 1983, 5 , 1. 77 G. Van Valkeren, R. Srinivasan, C.P. Keijzers, J.S. Wood, and E. De Boer, Solid State Commun., 1982, 44, 1285. 78 L.M.H. Paulissen, and C.P. Keijzers, J. Mol. Struct., 1984, 113, 267. 79 D.K. De, R.S. Rubins, and T.D. Black, Phys. Rev. 8: Condens. Matter, 1984, 29, 71. 80 A.M. Ziatdinov, R.L. Davidovich, V.Ya. Shevchenko, and Yu.V. Yablokov, Koord. Khim., 1983, 2, 1644. 81 A.M. Ziatdinov, V.Ya. Shevchenko, and Yu.V. Yablokov, Koord. Khiw., 1983, 9, 39. 82 M.D. Joesten, F.D. Srygley, and P.G. Lenhert, Inorg. Chem., 1983, 22, 1254. 83 M.D. Joesten, and J.H. Venable, Inorg. Chem., 1983, 12, 1733. 84 M.V. Rajasekharan, R. Bucher, E. Deiss, L. Zoller, A.K. Salzer, E. Moser, J. Weber, and J.H. Ammeter, J. Am. Chem. SOC., 1983, 105,7516. 8 5 R. Ray, Physica B+C (Amsterdam) 1983, 115, 247. 86 V.P. Desai, E. Koenig, and B. Kanellakopulos, J. Chem. Phys., 1983, 3, 6299. 87 H. Bill, and Y.R. Sekhar, Phys. Rev. B: Condens. Matter, 1983, 28, 2352. 88 H. Bill, and Y.R. Sekhar, J. Phys. C, 1983, g ,L889. 89 V.D. Lipatov, Fiz. Tverd. Tela (Leningrad), 1983, 25, 1386. 90 A. Schweige, and Hs.H. Guenthard, Cheni. Phys., 1982, 70,1. 91 E.A. Harris, and J . N . 'Tucker,Phys. Staxus Solidi B, 1983, 117,301. 92 V.G. Grachev, S.S. Ishchenko, A.A. Klirnov, and S.M. Okulov, Fiz. Tverd. Tela (Leningrad), 1983, 25, 1882. 93 R.C. Stevenson, J. Magn. Reson., 1984, 57, 24. 94 J. Uhrin, and M. Rakos, Czech. J. Phys., 1983, B,87. 95 S.K. Misra, and S. Subramanian, J. Phys. C, 1982, 15, 7199. 96 J. Barak, A. Raizman, and J.T. Suss, J. Magn. Reson., 1983, 53, 23. 97 M.P. Byrn, and C.E. Strousse, J. Magn. Reson., 1903, 53, 32. 98 S.K. Hoffmann, and L.S. Szczepaniak, J. Magn. Reson., 1983, 52, 182. 99 S. Brumby, Magn. Reson. Rev., 1983, 8 , 1. 100 R.S. De Biasi, and J.A.M. Mendonca, Cornput. Phys. Commun., 1982, 28, 69. 101 R.S. De Biasi, and J.A.M. Mendonca, J. Magn. Reson., 1983, 53, 462. 102 P.H. Rieger, J. Magn. Reson., 1982, 2 , 485. 103 S. Lee, D.F. Ames, and J.M. Putnam, J. Magn. Reson., 1982, g , 312. 104 R.P. Bonomo, and F. Riggi, Chem. Phys. Lett., 1982, 93,99. 105 A . Bals, and J. Kliava, .J.Magn. Reson., 1983, 53, 243. 106 E. Buluggiu, 2 . Naturforsh., A: Phys. Phys. Chem., Kosmophys., 1983, *, 1320. 107 E. Buluggiu, J. Phys. Chem.Solids, 1982, 43, 997. 108 A. Bencini, and D. Gatteschi, Mol. Phys., 1982, 47, 161. 109 P. Freund, J. Phys. C, 1983, E, 5039. 110 S.S. Eaton, K.M. More, B.M. Sawant, and G.R. Eaton, J. Am. Chem. S O C . , 1983, 105, 6560. 111 L.B.Jr. Knight, R.W. Woodward, R.J. Van Zee, and W.Jr. Weltner, J. Chem. 1983, 79,5820. 112 J.A. Howard, R. Sutcliffe, J.S. Tse, and B. Mile, Chem. Phys. Lett., 1983,

m.,

73

2: Transition-metal Ions

94, 561. 113 J.A. Howard, R. Sutcliffe, and B. Mile, J. Phys. Chem., 1983, 87, 2268. 114 C.A. Baumann, R.J. Van Zee, S.V. Bhat, and W.Jr. Weltner, J. Chem. Phys., 1983, 78, 190. 115 J.A. Howard, K.F. Preston, R. Sutcliffe, and B. Mile, J. Phys. Chem., 1983, 87, 536. 116 J.A. Howard, R. Sutcliffe, and B. Mile, J. Chem. S O C . , Chem. Commun., 1983, 1449. 117 C.A. Baumann, R.J. Van Zee, and W.Jr. Weltner, J. Chem. Phys., 1983, 79, 5272. 118 C.A. Baumann, R.J. Van Zee, and W.Jr. Weltner, J. Chem. Phys., 1984, 88, 1815. 119 Yu.V. Rakitin, and V.I. Nefedov, Zh. Neorg. Khim., 1984, 2 , 510. 120 J.G.M. Van der Linden, M.L.H. Paulissen, and J.E.J. Schmitz, J. Am. Chem. __ SOC., 1983, 105, 1903. 121 T. Beringhelli, F. Morazzoni, and D. Strurnolo, J. Organomet. Chem., 1982, 236, 109. 122 A.W. Maverick, J.S. Najdzionek, D. MacKenzie, and D.G. Nocera, J. Am. Chem. SOC., 1983, 105, 1878. 123 P.N. Lindsay, B.M. Peake, B.H. Robinson, J. Simpson, U. Honrath, H. Vahrenkamp, and A.M. Bond, Organornetallics, 1984, 3 , 413. 124 D. Gatteschi, ,J. Mol. Catal., 1984, 23, 145. 125 S.K. Hoffrnann, and W.E. Hatfield, J. Magn. Reson., 1983, 53, 341. 126 S.K. Hoffmann, Chem. Phys. Lett., 1983, 98, 329. 127 B.L. Ramakrishna, and P.T. Manoharan, Chem. Phys. Lett., 1983, 97,98. 128 W.E. Marsh, K.C. Patel, W.E. Hatfield, and D . J . Hodgson, Inorg. Chern., 1983, 22, 511. 129 L. Banci, A. Bencini, and D. Gatteschi, J. Am. Chem. SOC., 1983, 105, 761. 130 V.K. Voronkova, L.V. Mosina, Yu.V. Yablokov, G.S. Matuzenko, and M.A. Yampol'skaya, Sovrem. Metody YaMR EPR Khirn. Tverd. Tela, IMater. Vses. Koord. Sovesh.1, 3rd 1982, 179. 131 V.K. Voronkova, M.V. Eremin, L.V. Mosina, and Yu.V. Yablokov, Mol. Phys., 1983, 3 , 379. 132 F. Cariati, G. Micera, A. Scozzafava, G. Minghetti, and G. Banditelli, Inorg. Chem., 1982, 21, 3843. 133 S. Sikorav, I. Bkouche-Waksman, and 0. Kahn, Inorg. Chem., 1984, 3 , 490. 134 K. Kawano, R. Nakata, and M. Sumita, J. Phys. SOC. Japan, 1983, 52, 4356. 135 H. Yokoi, and M. Chikira, J. Chem. SOC., Chem. Commun., 1982, 1125. 136 H. Yokoi, and M. Chikira, Chem. Lett., 1982, 1433. r 137 H. Yokoi, and A. Hanaki, Chem. Lett., 1983, 1319. 138 S. Konishi, M. Hoshino, and M. Imarnura, .T. Phys. Chem., 1982, E, 4888. 139 A. Skorobogaty, R. Lancashire, T . D . Smith, J.R. Pilbrow, and G.R. Sinclair, J. Chem. SOC., Faraday Trans. 2, 1983, 79, 1123. 140 M. Julve, M. Verdaguer, M.F. Charlot, 0. Kahn, and R. Claude, Inorg. Chim. Acta, 1984, 82, 5. 1357. 141 0. Cozar, and V. Znarnirovschi, Czech. J. Phys., 1983, 142 0. Cozar, and I. Ardelean, Stud. Univ. Babes-Bolyai, ISer.1 Phys., 1982, 27, 41. 143 0. Cozar, I. Ardelean, and G. Ilonca, Solid State Commun., 1982, 44,809. and J. Mrozinski, 144 M. Bukowska-Strzyzewska, J. Skoweranda, E. Heyduk, Inorg. Chim. Acta, 1983, 73,207. 145 L. Antolini, L. Menabue, P. Prampolini, and M. Saladini, J. Chem. SOC., Dalton Trans., 1982, 2109.

m,

Electron Spin Resonance

74

146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175

176 177 178 179

I. Bkouce-Waksman, S. Sikorav, and 0. Kahn, J. Cristallogr. Spectrosc. Res. 1983, 13,303. F. Cariatl, L. Erre, G. Micera, L. Menabue, M. Saladini, and P: Prampolini, Inorg. Chim. Acta, 1982, 63, 85. A. Mederos, F.G. Manrique, and A. Medina, An. Quim., Ser. B 1981, 12, 206. A.N. Glebov, Yu.1. Sal'nikov, and V.V. Ustyak, Zh. Neorg. Khim., 1983, 28, 2691. M. Melnik, Polyhedron, 1982, 1,679. M. Melnik, and J. Mrozinski, Acta Chim. Acad. Sci. Hung., 1982, 111. J. Mrozinski, and M. Bukowska J. Mrozinski, and E. Heyduk, M. Melnik, and J. Mrozinski, Finn. Chem. Lett., 1983, 81. M. Melnik, and J. Mrozinski, Finn. Chem. Lett., 1983, 15. P. Arrizabalaga, P. Castan, and P. Sharrock, Polyhedron, 1983, 2, 323. V.M. Potapov, E.G. Pekshueva, and A.V. Garbar, Zh. Obshch. Khim., 1983, 53, 1620. R.C. Van Landschoot, J.A.M. Van Hest, and J. Reedijk, Inorg. Chim. Acta, 1983, 72, 89. P.K. Mascharak, G.C. Papaefthymiou, W.H. Armstrong, S. F'oner, R.B. Frankel, and R.H. Holm, Inorg. Chem., 1983, 2 , 2851. G.D. Friesen, J.W. McDonald, W.E. Newton, W.B. Euler, and B.M. Hoffmann, Inorg. Chem., 1983, 2 , 2202. J.W. McDonald, G.D. Friesen, W.E. Newton, A. Mueller, W. Hellmann, U. Schimanski, A. Trautwein, and U. Bender, Inorg. Chim. Acta, 1983, 76,L297. P. Beardwood, and J.F. Gibson, J. Chem. soc., Dalton Trans., 1983, 737. P.Beardwood, J.F. Gibson, C.E. Johnson, and J.D. Rush, J. Chem. SOC., Dalton Trans., 1982, 2015. D.W. Stephan, G.C. Papaefthymiou, R.B. Frankel, and R.H. Holm, Inorg. Chem., 1983, 22, 1550. O.F. Gataullin, M.M. Zaripov, and Yu.M. Ryzhmanov, Fiz. Tverd. Tela (Leningrad), 1983, 25, 3304. J.M. Griinshaw, and M.G. Read, J. Phys. C, 1983, 16, 7053. Y. Tonomo, T. Kato, and Y. Tanokura, J. Phys. SOC. Japan, 1984, 53, 773. 1754. M.V. Eremin, Fiz. Tverd. Tela (Leningrad), 1983, P. Meriaudeau, B. Clerjaud, and M. Che, J. Phys. Chem., 1983, 87. 3872. A.W. Clauss, S.R. Wilson, R.M. Buchanan, C.G. Pierpont, and D.N. Hendrickson, Inorg. Chem., 1983, 2 , 628. Yu.V. Rakitin, N.V. Nemtsev, A.A. Pasynskii, V.T. Kalinnikov, and G.M. Larin, Koord. Khim., 1983, 2, 616. D.E. Bo,+ster, P. Guetlich, W.E. Hatfield, S. Kremer, E.W. Mueller, and K. Wieghardt, K. Inorg. Chem., 1983, 3, 1725. M.Heming, G. Lehmann, H. Mosebach, and E. Siegel, Solid State Commun., 1982, 44, 543. L. Borer, L. Thalken, J.H. Zhang, and W.M. Reiff, Inorg. Chem., 1983, 22, 3174. L. Borer, L. Thalken, C. Ceccarelli, M. Glick, J.H. Zhang, and W.M. Reiff, Inorg. Chem., 1983, 22, 1719. R. Damoder, K.M. More, G.R. Eaton, and S.S. Eaton, Inorg. Chem., 1983, 22, 2836. S. Hafid, G.R. Eaton, and S.S. Eaton, J. M a p . Reson., 1983, 51, 470. R. Damoder, K.M. More, G.R. Eaton, and S.S. Eaton, J. Am. Chem. SOC., 1983, 105, 2147. R. Damoder, K.M. More, G.R. Eaton, and S.S. Eaton, Inorg. Chem., 1983, 2 ,

z,

2: Transition-metalIons

15

3738. 180 P.H. Smith, G.R. Eaton, and S.S. Eaton, J. Am. Chem. S O C . , 1984, 106,1986. 181 S.S. Eaton, P.M. Boymel, B.M. Sawant, J.K. More, and G.R. Eaton, J. Magn. Reson., 1984, 56, 183. 182 K.M. More, G.R. Eaton, and S.S. Eaton, Inorg. Chem., 1983, 22, 934. 183 P.M. Solozhenkin, F.A. Shvengler, N.I. Kopitsya, and A.V. Ivanov, Dokl. Akad. Nmik SSSR, 1983, 269, 881. 184 O.M. Petrukhin, V.Yu. Nad, and L.B. Volodarskii, Koord. Khim., 1983, 9 , 298. 185 L.L. Makarshin, and V.M. Berdnikov, Teor. Eksp. Khim., 1983, 19, 193. 186 A.A. Medzhidiv, and I.A. Timakov, Koord. Khim., 1982, 8, 1043. 187 V.A. Gaevoi, N.N. Kalibabchuk, N.N. and V.S. Kuts, Teor. Eksp. Khim., 1983, 19, 594. 188 K.E. Schwarzhans, and A. Stuefer, Monatsh. Chem., 1983, 114, 137. 189 C. Benelli, D. Gatteschi, and C. Zanchini, Inorg. Chem., 1984, 23, 798. 190 P. Mathur, and G.C. Dismukes, J. Am. Chem. SOC., 1983, 105, 7093. and K. Sauer, Biochim. Biophys. 19i G.C. Dismukes, H.A. Frank, R . Friesner, Acta, 1984, 764, 253. 192 M.I. Kabachnik, N.N. Bubnov, S.P. Solodovnikov, and A.I. Prokofi'ev, Khim., 1984, 53, 487. 193 M.W. Linch, D.N. Hendrickson, B.J. Fitzgerald, and C.G. Pierpont, J. Am. Chern. S O C . , 1984, 106,2041. 194 M.E. Cass, D.L. Green, R.M. Buchanan, and C.G. Pierpont, J. Am. Chem. SOC., 1983, 105,2680. 195 R.M. Buchanan, J. Claflin, and C.G. Pierpont, Inorp,. Chem., 1983, 2 , 2552. 196 M. Hoshino, S. Konishi, M. Imamura, S. Watanabe, and Y. Hama, Chem. Phys. Lett., 1983, 102, 259. 197 U. Fuerholz, H.B. Buergi, F.E. Wagner, A. Stebler, J.H. Ammeter, E. Krausz, R.J.H. Clark, M.J. Stead, M.J. and A. Ludi, J. Am. Chem. SOC., 1984, 106, 121. 198 A.M. Dennis, R.A. Howard, D. Lancon, K.M. Kadish, and J.L. Bear, J. Chem. SOC., Chem. Commun., 1982, 399. 199 Yu.V. Rakitin, S.V. Kryuchkov, A.I. Aleksandrov, A.F. Kuzina, N.V. Nemtsev, B.G. Ershov, and V.I. Spitsyn, Dokl. Akad. Nauk SSSR, 1983, 269, 1123. 200 H.C. Long, and D.N. Hendrickson, J. Am. Chem. SOC., 1983, 105, 1513. 201 S.A. Fairhurst, A.D. Inglis, Y. Le Page, J.R. Morton, and K.F. Preston, Chem. Phys. Lett., 1983, 95, 444. 202 S.A. Fairhurst, A.D. Inglis, Y.Le Page, J.R. Morton, and K.F. Preston, JMagn. reson., 1983, 54, 300. 203 V.K. Kaputstkin, Osob. Elektron. Str. Svoistva Tverdofaznykh Soedin. Titana Vanadiya, 1982, 57. 204 C. Sanchez, F. Babonneau, R. Morineau, J. Livage, and J. Eullot, Philos. Mag., [Parti B, 1983, 47, 279. and Y. 205 F. Babonneau, C. Sanchez, J. Livage, J.P. Launay, M. Daoudi, Jeannin, Nouv. J. Chime, 1982, 6, 353. 206 J. Van der Berg, A.J. Van Dillen, J.W. Geus, and M.C. Stolk, Ber. Bunsenpes. Phys. Chem., 1983, 87, 120. 207 V.S. Grunin, I.B. Petrina, and Z.N. Zonn, Phys. Status Solidi B, 1983, g , 545. 208 S.P. Harmalker, M.A. Leparulo, and M.T. Pope, J. Am. Chem. SOC., 1983, 105, 4286. 209 C. Sanchez, J. Livage, P. Doppelt, 9. Chauveau, and J. Lefebvre, J. Chem. SOC., Dalton Trans., 1982, 2439.

76

Electrori Spirt Resotlance

210 C. Sanchez, J. Livage, J.P. Launay, and M. Fournier, J. Am. Chem. SOC., 1983, 105, 6817. 211 J.M. Ball, P.M. Boorman, K..J. Moynihan, V.D. Patel, J.F. Richardson, D. Collison, and F.E. Mabbs, J. Chem. S O C . , Dalton Trans., 1983, 2479. 212 U. Geiser, and R.D. Willett, J. Appl. Phys., 1984, 55, 2407. 213 E. Coronado, M. Drillon, and D. Beltran, Inorg. Chirn. Acta, 1984, g ,13. 214 M.B. Ritter, J.E. Drumheller, T.M. Kite, L.O. Snively, and K. Emerson, Phys. Rev. B: Condens. Matter, 1983, 28, 4949. 215 H. Benner, M. Brodehl, H. Seitz, and J. Wiese, J. Phys. C, 1983, 16,6011. 216 H. Okamoto. and H. Mori, Phys. Lett. A, 1983, m, 255. 217 Waldner, F. J. Mag,?. Magn. , . ~ a - e.~, 1983, 31-34, 1203. 218 A.G. Anders, and S.V. Volotskii, J. Magn. Magn. Mater., 1983, 31-34, 1169. 219 Y. Natsume, F. Noda, F. Sasagawa, and H. Kanazawa, J. Phys. SOC. Japan, 1983, 52, 1427. 220 Y. Ajiro, K. Adachi, and M. Mekata, J. Magn. Uagn. Mater., 1983, 31-34,1141. 221 K. Koga, and M. Suzuki, J. Phys. SOC. Japan, 1984, 53, 786. 222 H. Kalt, E. Siegel, N. Pauli, H. Mosebach, J. Wiese, and A. Edgar, JPhys. C, 1983, 16, 6427. 223 G.A. Petrakovskii, L.S. Yernelyanova, and V.V. Velichko, Solid State Commun., 1983, g ,647. 224 V.N. Berzhanskii, V.I. Ivanov, and A.V. Lazuta, Solid. State Commun., 1982, 44, 771. 225 P. De Groot, P. Janssen, F. Herlach, G. De Vos and J. Witters, J- Magn. Maan. Mater., 1983, 31-34, 637. 226 N. Shah, G. Kernmerer, and J.S. Karra, Phys. Rev. B: Condens. Matter, 1983, 27, 5360. 227 E. Siegel, J. Ibruegger, and A. Lagendijk, J. Phys. C, 1982, _1.5, 4583. 228 A.G. Anders, and S.V. Volotskii, Fiz. Nizk. Temp. (Kiev), 1982, 8, 963, 229 I. Yamada, I. Norishita, and T. Tokuyarna, Physica B+C (Amsterdam), 1983, 115, 179. 230 I. Yamada, S. Nagano, and S. Shimoda, Physica B+C (Amsterdam), 1983, E, 47. 231 T.M. Kite, and J.E. Drurnheller, J. Magn. Reson., 1983, 54, 253. 232 R.D. Willett, R.J. Wong. and M. Numata, Inorg. Chern., 1983, 22, 3189. 233 R. Calvo, and M.A. Mesa, Phys. Rev. B: Condens. Matter, 1983, 28, 1244. 234 T. Miyaday, T. Sekiguchi, and K. Manabe, Ferrites, Proc. ICF 3 r d , 1980, 926. 235 I. Matsubara, K. Iio, and K. Nagata, J . Phys. SOC. Japan, 1982, 51, 3071. 236 B. Gahan, and F.E. Mabbs, J. Chern. S O C . , Ualton Trans., 1983, 1695. 237 K. Draeger, Z. Naturforsch., A: Phys., Phys. Chem., Kosmophys., 1983, 1223. 238 B.L. Rarnakrishna, and P.T. Manoharan, J. Phys. Colloq., 1983, 1405. 239 T-,Seto, M. Inoue, M.B. Inoue, and D. Nakamura, Bull. Chern. SOC. Japan, 1983, 56, 1903. 240 M.B. Inoue, and M. Inoue, Mol. Cryst. L i q . Cryst., 1983, 3,183. 241 E.G. Sharoyan, and H.A. Samuelyan, Phys. Status Solidi A,1982, 73, K213. 242 W.E. Hatfield, S.K.Hoffmann, P.J. Corvan, P. Singh, C.N. Sethulekshmi, and R.M. Metzger, J. Phys. Colloq., 1983, 1377. 243 M. Onoda, and H. Nagasawa, J. Phys. SOC. Japan, 1983, 52, 2231. 244 U.R.K. Rao, K.S. Venkateswarlu, B.R. Wani, M.D. Sastry, A.G.I. Dalvi, and B.D. Joshi, Mol. Phys., 1982, 47, 637. 245 M. Fujimoto, and B.V. Sinha, Ferroelectrics, 1983, 5 , 227. ~

z,

2: Trunsition-tnetul Iorrs

77

246 M. Fujimoto, Ferroelectrics, 1983, 47, 177. 247 F. Morno, A. Sotgiu, M. Bettinelli, A. Montenero, and E. Baiocchi, J. Mater. Sci., 1983, 18, 1993. 248 K. Furukawa, K. Yonekura, and T. Kawano, Kagoshima Daigaku Rigakubu Kiyo, Sugaku, Butsurigaku Kagaku, 1982, 43. 249 S. Jerzak, S. Waplak, and L.A. Shuvalov, Phys. Status Solidi A, 1982, 72, 783. 250 A.K. Jain, and M. Geoffroy, J. Pnys. Chem. Solids, 1983, 44, 535. 251 A.K. Jain, and G.C. Upreti, J. Phys. Chern. Solids, 1983, 44, 549. 252 G. Jayaram, and G.S. Sastry, Chem. Phys. Lett., 1983, 97, 431. 253 K.H. Kirklin, and G.L. McPherson, J . Phys. C, 1983, E, 6539. 254 W.W. Schmidt, and A. Wieser, Z. Phys. B: Condens. Matter, 1983, 52, 237. 255 M. Machida, M. Suhara, S. Aono, and T. Kobayashi, Phase Transitions, 1983, 3, 227. 256 M.B. Zapart, W. Zapart, J. Stankowski, and A.I. Zviagin, Physica B+C (Amsterdam), 1982, 114, 201. 257 P. Huguet, G. Alquie, M. Potel, and M.J. Sergent, Phys. Lett., 1983, 44, 393. 258 R . L . Lichti, J. Chem. Phys., 1983, 78,7323. 259 K.R. Juraitis, J . B . Domiciano, and W. Sano, J. Phys. Chern. Solids, 1983, 44, 531. 260 G.J. Troup, and D.R. Hutton, J. Gernrnol., 1983, 18,421. 261 L.V. Bershov, A.B. Brik, and I.D. Ryabov, Mineral Zh., 1982, 4, 3. 262 M.M. Manuilova, A.M. Danilevich, and A.D. Kirikov, Sov. Geol., 1983, 77. 263 V.A. Timescov, V.F. Krutikov, and N . G . Bogdanov, Sov. Geol., 1983, 93. 264 K.S.V. Nambi, PACT (Rixensart, Relg.), 1982, S , 314. 265 V.I. Ivchenko, V.V. Pavshukov, and Yu.N. Sergeev, Geokhimiya, 1983, 626. 266 L . I . Panina, M.Ya. Shcherbakova, and V.E. Istornin, Geol. Geofiz., 1983, 112. 267 P. Sharrock, Geochirn. Cosmochirn. Acta, 1982, 46, 1311. 268 N. Burriesci, N. Giordano, F. Cariati, M. Petrera, and J.C. Bart, J. Bull. Mineral., 1983, 106, 571. 269 M. Patel, Indian J. Technol., 1983, 21, 173. 270 K. Dyrek, Z. Klapyta, and Z. Sojka, Clays Clay Miner., 1983, 31, 223. 271 P. Monsef-Mirzai, and W.R. McWhinnie, Inorg. Chim. Acta, 1983, 73,41. 272 M.M. Mestdagh, A.J. Herbillon, L. Rodrique, and P.G. Rouxhet, Bull. Mineral., 1982, 2 ,457. 273 D. Bonnin, S. Muller, and G. Calas, Bull. Mineral., 1982, 105,467. 274 D.G. McGavin, R.A. Palmer, W.C. Tennant, and S.D. Devine, Phys. Chern. Miner., 1982, 8, 200. 275 A.R. Blak, S. Isotani, and S. Watanabe, Phys. Chem. Miner., 1982, 2, 161. 276 L.V. Bershov, J.M. Gaite, S.S. Hafner, and H. Rager, Phys. Chem. Miner., 1983, 2, 95. 277 G. Cals, and J.F. Cottrant, Bull. Mineral., 1.982,105, 598. 278 L.A. Blaginina, A.F. Zatsepin, 1.A. Dmitriev. and S.L. Votyakov, Fiz. Khim. Stekla, 1983, 2, 443. 279 K. Wang, W. Wang, and D. Shen, J. Non-Cryst..Solids, 1983, 297. Phys., 1983 54, 5394. 280 G. Kordas, R.A. Weeks, and D.L. Kinser, J.:)pl. 281 H. Hosono, H. Kawazoe, and T. Kanazawa, J.;m-Cryst. Solids. 1983, 55, 3. 282 G. Kordas, B. Carnara, and H.J. Oel, J. Non-Cvyst. Solids, 19t32, 50, 79. 283 N. Peteanu, and A. Nicula, Proc. Int. Conf. ;'Amorphous Semicond."-82, 1982, 162. 284 F. Momo, A. Sotgiu, E. Baiocchi, M. Bettine.'li arid A. Montenero, J. Mater.

78

Electron Spin Resonance

Sci., 1982, 17,3221. 285 V.M. Nagiev, Izv. Akad. Nauk 'Az. SSR, Ser. Fiz.-Tekn. Mat Nauk, 1982, 3, 93. 286 L.D. Bogomolova, and V.A. Yachkin, J. l\lon-Cryst.Solids, 1983, 58, 165. Roshchina, J. Non-Cryst. 287 L.D. Bogomolova, T.K. Pavlushkina, and A.V. Solids, 1983, 58, 99. 288 M. Vaivada, J. Klava, Z. Konstants, J. Purans, and J. Troks, Fiz. Khim. Stekla, 1984, lo, 53. 289 S. Barnier, M. Guittard, M. Winternberger, and J. Flahaut, J. Non-Cryst. Solids, 1983, 56, 319. 290 N. Iwarnoto, Y. Makino, and S. Kasahara, J. Non-Cryst. Solids, 1983, 55, 113. 291 I. Ardelean, G. Ilonca, and M. Peteanu, J. Non-Cryst. Solids, 1982, 51, 389. 292 0. Cozar, I. Ardelean, and G. Ilonca, Mater. Chern., 1982, I , 755. 293 L.D. Bogomolovs, M.P. Glasova, O.E. Dubatovko, S.I. Reiman, and S.N. Spasibkina, J. Non-Cryst. Solids, 1983, 58, 71. 294 L.D. Bogomolova, and S.N. Spasibkina, Physica B+C (Amsterdam), 1983, 117-118, 998. 295 F. Gan, H. Liu, and W. Fu, Glastech. Ber., 1.983, 3 , 1011. 296 F. Momo, A. Sotgiu, M. Bettinelli, and A. Montenegro, Phys. Status Solidi A, 1984, 81, K27. 297 S. Simon, an? A. Nicula, Phys. Status, Solidi A , 1984, g , K1. 298 L. Burlamacchi, A. Lai, M. Monduzzi, and G. Saba, J. Magn. Reson., 1983, 55, 39. 299 B.B. Garrett, and W.M.Jr. Gulick, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1733. 300 A.I. Filippov, and N.B. Yunusov, Zh. Fiz. Khirn., 1982, 56, 2900. 301 A.I. FIlippov, and N.B. Yunusov, Zh. Fiz. Khim., 1983, 57, 2061. 302 C.E. Brown, D. Vidrine, R.L. Julian, and W. Froncis, J. Chern. SOC., Dalton Trans., 1982, 2371. 303 P. Beardwood, J.F. Gibson, P. Bertrand, and J.P. Gayda, Biochirn. Biophys. Acta, 1983, 742, 425. 304 J . T . Colvin, R. Rutter, H.J.Stapleton, and L.P. Hager, Biophys. -J., 1983, 41, 105. 305 V.Ya. Bratus, A.A. Bugai, M.F. Bulanyi, and B.D. Shanina, Fiz. Tverd. Tela (Leningrad), 1982, 24, 2648. 306 M. Krygowska-Doniec, J. Kwiatkowska, M. Stojik, and V. Spiric, Acta Phys. Pol. A, 1983, E , 143. 307 B.S. Tsukerblat, M.I. Belinski, B.Ya. Kuyavskava, and V.E. Fainzil'berg, Chem. Phys. Lett., 1983, 3 , 149. 291. 308 M.T. Causa, and M.C.G. Passeggi, Phys. Lett. A, 1983, 309 S.Ya. Kuchrnii, A.V. Korzhak, and A.I. Kryukov, Teor. Eksp. Khim., 1983, 19, 590. 310 G. Plesch, Inorg. Chirn. Acta, 1983, 11,117. 311 J.C.W. Chien, J.C. Wu, and C.I. Kuo, .J. Polyrn. Sci., Polym. Chern. Ed., 1982, 20, 2019. 312 N. Iwamoto, Y. Makino, and H. Hidaka, Trans. JWRI, 1982, 11,47. 313 V.I. Zarko, G.M. Kozub, N.P. Srnirnova, and A. Chuiko, Ukr. Khirn. Zh., 1983, 49, 1037, 314 V.S. Zarko, L.N. Ganyuk, G.M. Kozub, and A.A. Chuiko, Dopov. Akad. Nauk UKR. RSR, Ser. B: Geol. Khirn. Biol. Nauk, 1983, 48. 315 B.H. Chen, and J.M. White, J. Phys. Chem., 1983, g ,1327.

w,

2: Transition-metalIons

79

316 N. Kinoura, F. Muto, and M. Koizuni, J.Solid State Chem., 1982, 45, 252. 317 S. Chandra, Acta Chem. Hung., 1983, 112, 253 J. Chem., Sect. A, 1982, 318 I. Rani, K.B. Pandeya, and R.P. Singh, =an 502. 319 B.B. Adeleke, and K.S. Patel, J. Coord. Chem., 1982, 11,201. 320 T. Sato, and T. Nakamura, Denki Kagaku Oyobi Kogyo Butsuri Kagaku, 1982, 50, 812. 204. 321 R . L . Dutta, and M.M. Hossain, Indian J. Chem., Sect. A, 1983, g , 322 F.A. Cotton, G.E. Lewis, and G.N. Mott, 1nor.g. Chem., 1983, 22, 378. 323 F. Lefebvre, M. Leyrie, G . Herve, C. Sanchez, and J. Livage, Inorg. Chim. Acta, 1983, 73, 173. 324 J.R. Allan, and G.H.W. Milburn, J. Chem. Res., Synop., 1983, 215. 729. 325 M.B. Adi, and A.S.R. Murty,J. Indian Chem. SOC., 1983, 326 K.B. Pandeya, 0 . Prakash, and R.P. Singh, J. Indian Chem. S O C . , 1983, 531. 327 D . Colison, B. Gahan, and F.E. Mabbs, J. Chem. S O C . , Dalton Trans., 1983, 1705. 328 B. Gahan, and F.E. Mabbs, J. Chem. S O C . , Dalton Trans., 1983, 1713. 329 R.W.Iggins, J.C. Huffman, and G. Christou, J. Chem. SOC., Chem. Cummun., 1983, 1313. Indian J. Pure Appl. Phys., 1983, 21, 130. 330 A. Syamal, 331 Y. Xu, X . Li, L. Yu, C. Liu, J. Lu, and W.C. Lin, Jiegou Huaxue, 1982, 1, 55. 332 H. Sakurai, T . Goda, S. Shimomura, and T. Yoshimura, Nucleic Acid Symp. Ser., 1982, g ,253. 333 J. Minge, S . Waplak, and L. Szczepanska, Acta Phys. Pol. A, 1983, @, 151. 334 S . Radhakrishna, and M. Salagram, Solid State Cornmun., 1983, 47, 77. 335 V.K. Jain, V.P. Seth, and S.K. Yadav, Phys. Status Solidi B, 1983, 115, K113. 336 B. Jayaram, and J. Sobhanadri, Cryst. Lattice Defects Amorphous Mater., 1983, lo, 47. 337 T. Oniki, and M. Oyamada, Calcif. Tissue Int., 1983, 35, 477. 338 T . Oniki, and Y. Doi, Calcif. Tissue Int., 1983, 35, 538. 339 R . S . Abdrakhmanov, E.M. Konoval, and I.V. Shishkin, Fiz. Khim. Stekla, 1983, 9 , 403. 340 Z.A. Ibragimov, and Ch.0. Kadzhar, Izv. Akad. Nauk Az. SSR, Ser. Fiz.-Tekh. Mat. Nauk, 1983, 4, 79. 341 V.P. Seth, V.K. Jain, and R.K. Malhotra, J. Non-Cryst. Solids, 1983, 57, 199. 342 S.V.J. Lakshman, and T.V. Krishna Rao, Solid. State Commun., 1984, 49, 567. 343 N. Narayana, and L. Kevan, J. Phys. C, 1983, g,L863. 344 D. Ballutaud, E . Bordes, and P. Courtine, Stud. Inorg. Chem., 1983, 3 (Solid State Chemistry), 531. 345 0. Cozar, V. Znamirovschi, and M. Gridan, Rev. Roum. Phys., 1982, 27, 389. 346 R.S. De Biasi, J. Magn. Magn. Mater., 1983, 31-34, 653. 347 F . Cariati, L. Erre, G. Micera, and J.C.J. Bart, Z. Anorg. Allg. Chem., 1983, 496, 159. 348 G. Overluizen, and R. Metselaar, J. Phys. C, 1982, 15,4869. 349 G. Overluizen, and R. Metselaar, J. Phys. C, 1983, I&, 355. 350 M. Otto, J . Stach, and R. Kirmse, Anal. Chim. Acta, 1983, 147,277. 351 H. Van Willigen, and T.K. Chandrashekar, J. Am. Chem. SOC., 1983, 105, 4232. 352 C.F. Mulks, B. Kirste, and H. Van Willigen, J. Am. Chem. SOC., 1982, 104,

a,

so,

so,

80

353 354 355 356 357 358 359

360

361 362 363 364 365 366 367 368 369 370

371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386

387 388

ElcJctronSpin Resonunce

5906. H. Van Willigen, J. Phys. Chem., 1983, 87, 3366. L. Kevan, J. Phys. Chern., 1984, 88, 327. B. Kirste, and H. Van Willigen, J. Magn. Reson., 1982, 49, 530. J.L. Petersen, and J.W.Jr. Egan, Inorg. Chern., 1983, 22, 3571. J. Sala-Pala, J.L. Migot, J.E. Guerchais, L. Le Gall, and F. Grosjean, JOrganomet. Chern., 1983, 248, 299. A.H. Al-Mowali, Inorg. Chim. Acta, 1983, 77,L51. M.L.Jr. Luetkens, W.L. Elcesser, J.C. Huffman, and A.P. Sattelberger, JChem. SOC., Chern. Commun., 1983, 1072. J.W. Buchler, C. Dreher, K.L. Lay, A. Raap, and K. Gersonde, Inorg. Chem., 1983, 22, 879. J.T. Groves, T. Takahashi, and W.M. Butler, Inorg. Chern., 1983, 22, 884. M. Mitewa, P. Russev, P.R. Bonichev, K. Kabassanov, and A. Malinovski, Inorg. Chim. Acta, 1983, 2 , 179. A.A. Alybakov, and N. Toichiev, Izv. Akad. Nauk Kirg. SSR, 1982, 18. S. Radhakrishna, and M. Salagram, Spectrochirn. Acta, Part A, 1982, E, 1337. J.H. Pifer, S. Ziernski, and M. Greenblatt, J. Chem. Phys., 1983, 211, 7038. K.Kh. Razikov, E.S. Sventsitskii, and V.N. Vorob’ev, Adsorbts. Adsorbenty 1981, 2, 42. V.S. Grunin, I.B. Patrina, and Z.N. Zonn, Tverd. Tela (Leningrad), 1982, 2 , 2859. B.A. Goodman, and M.V. Cheshire, Nature (London), 1982, 299, 618. A.R. Gonzales,-Elipe, and J. Snria, J. Catal., 1983, 83, 235. K.M. Kadish, D. Chang, T. Malinski, and H. Ledon, Inorg. Chem., 1983, 22, 3490. E.G. Isrnailov, and A.M. Musaev, React. Kinet. Catal. Lett., 1982, 23_, 181. E.G. Ismailov, and F.B. Kasurnov,. React. Kinet. Catal. Lett., 1983, 22, 95. F.S. Gadzhieva, and V.F. Anufrienko, Zh. Strukt. Khirn., 1982, 23, 43. F.S. Gadzhieva, G.K. Boreskov, and V.F. Anufrienko, Dokl. Akad. Nauk SSSR, 1982, 265, 638. B.G. Silbernagel, J. Magn. Magn. Mater., 1983, 31-34, 885. S. Simon, and A. Nicula, J. Non-Cryst. Solids, 1983, 57, 23. J. Klava, and A. Bais, Fiz. Khim. Stekla, 1984, lo, 47. N. Mizuno, K,Katamura, Y. Yoneda, and M. Misono, J. Catal., 1983, 83, 384. T. Irnamura, K. Hasegawa, T. Tanaka, W. Nakajirna, and M. Fujimoto, Bull. Chern. SOC. Japan, 1984, 5 T. Shibahara, H. Kuroya, S. Ooi, and Y. Mori, Inorg. Chirn. Acta, 1983, 76, L315. H. Hennig, A. Rehorek, D. Rehorek, P. Thoirnas, and D. Baezhold, Inorg. L11. Chim. Acta, 1983, E. Serwicka, and R.N. Schindler, Z. Phys. Chem. (Wiensbaden), 1982, 133, 175. I.N. Marov, M.K. Pupkova, and V.K. Belyaeva, Zh. Neorg. Khim., 1983, 28, 382. I.W. Boyd, and A.G. Wedd, Aust. J. Chern., 1984, 37, 293. G.M. Larin, and T.A. Mastryukova, Koord. Khim., 1983, 2, 221. O.A. Rajan, J.T. Spence, C. Lernan, M. Minelli, M. Sato, J.H. Enemark, P.M.H. Kronock, and K. Sulger, Inorg. Cheni., 1983, 22, 3065. J. Schmitte, C. Friebel, F. Weller, and K. Dehnicke, Z. Anorg. Allg. Chem., 1982, 495, 148. J. Sobczak, and J. Ziolkowski, J. Transition Met. Chern. (Weinheirn,Ger.), ~~

m.

z,

2: Transition-metal Ions

81

1983, 8, 333. 389 S . Abdo, and R.F. Howe, J. Phys. Chem., 1983, 87, 1722. 390 D.P. Madacsi, M. Stapelbroek, R.B. Bossoli, and O.R. Gilliarn, J. Chern. 1982, 3803. 391 M.P. Byrn, B.A. Katz, N.L. Keder, K.R. Levan, C.J. Magurani, K.M. Miller, J.W. Pritt, and C.E. Strouse, J. Am. Chern. SOC., 1983, 105, 4916. 392 S . Neya, and I. Morishirna, J. Am. Chem. SOC., 1982, 104, 5658. 393 R. Quinn, C.E. Strouse, and J.S. Valentine, Inorg. Chem., 1983, 22, 3934. 394 V.P. Chacko, and G.N. La Mar, J. Am. Chem. SOC., 1982, 104,7002. 395 R . Raina, and T.S. Srivastava, Inorg. Chirn. Acta, 1984, 91, 137. 701. 396 R . Raina, and T.S. Srivastava, Indian J. Chern., Sect. A, 1983, g , 490. 397 R. Raina, and T.S. Srivastava, Indian J. Chem., Sect. A, 1982, g , 398 S.A. Luchkina, and T.P. Sergeeva, Zh. Neorg. Khirn., 1983, 28, 2580. 399 R.N. Mukherjee, and V.S. Vijaya, Indian J. Chem., Sect. A, 1982, 21A, 427. 400 R.B. Salmonsen, A . Abelleira, M.J. Clarke, and S.D. Pell, Inorg. Chem., 1984, 3,385. 401 R.S. Eachus, and M.T. Olm, Radiat. Eff., 1983, 73,69. 402 8. Buffat, G. Demazeau, M. Pouchard, J.M. Dance, and F. Hagenmuller, JSolid State Chem., 1983, 50, 33. 403 E . P . Duliba, E.G. Seebauer, and R.L. Belford, J. Magn. Reson., 1982, 2, 507. 404 R.A. Schoonheydt, and J. Pelgrims, J. Chem. S O C . , Faraday Trans. 2, 1983, 79, 1169. 405 C.J. Winscom, W. Lubitz, H. Diegruber, and R. Moeseler, Stud. Surf. Sci. Catal., 1982, 12, 15. 406 M . Rudin, A. Schweiger, and H.H. Guenthard, Mol. Phys., 1982, 47, 171. 407 M. Rudin,A. Schweiger, and H.H. Guenthard, Mol. Phys., 1982, 47, 1027. 408 W. Kanda, H. Osaka, and S. Kida, Bull. Chem. SOC. Japan, 1983, 56, 3268. 409 G.E. Jackson, and L . G . Sco-tt,S. Afr. J. Chern., 1983, 36, 120. 410 E.G. Jaeger, and K. Mueller, Z. Anorg. Allg. Chem., 1983, 501, 40. 411 R. Kirmse, and L. Beyer, 2 . Anorg. Allg. Chem., 1983, 498, 185. 412 R. Kirmse, and L. Bsyer, Z. Chern., 1982, 22, 272. 413 M. Hoshino, S . Konishi. M. Nakajima, and M. Imarnura, Bull. Chem. SOC. Japan, i963, 56, 1233. 414 K. Kasuga, T. Nagahara, A. Tsuge, K. Sogabe, and Y. Yarnarnoto, Bull. Chem. SOC. Japan, 1983, 56, 95. 415 D , Attanasio, I. Collamati, and E. Cervone, inorg. Chem., 1983, 22, 3281. 416 T.D. Smith, J. Livorness, H. Taylor, J.R. Pilbrow, and G.R. Sinclair, JChem. SOC., Dalton Trans., 1983, 1391. 417 M. Iwaizumi, Y. Ohba, H. Iida, and M. Hirayama, Inorg. Chim. Acta, 1984, 82, 47. 418 P . Doppelt, and R. Weiss, Nouv. J. Chim., 1983, I , 341. 1983, 105, 419 E . Soerin, A. Schweiger, and H.H. Guenthard, J. Am. Chem. SE., 4277. 420 D.N.R. Rao, and M.C.R. Symons, J. Chem. S O C . , Faraday Trans. 1, 1983, 2 , 269. 421 K. Mizuno, and J.H. Lunsford, Inorg. Chern., 1983, 22, 3484. 422 J. Zarembowitch, and 0. Kahn, Inorg. Chem., 1984, 23, 589. 423 N.V. Vugman, and W.O. Franco, J. Chem. Phys., 1983, 78, 2099. 424 N.V. Vugman, and N.M. Pinhal, Mol. Phys., 1983, 9, 13i5. 425 G. Valentini, G. Braca, G. Sbrana, and A. Colligiani, Inorg. Chim. Acta, 1983, 69, 221. 426 K.K. Brown, T.J. Bergendahl, J.S. Wood, and J.H. Waters, Inorg. Chim. Acta

w.,

z,

Electron Spin Resonance

82

1983, E, 79. 427 D.M. Grove, G. Van Kote, R,. Zoet, N.W. Murrall, and A.J. Welch, J. Am. Chem. SOC., 1983, 105,1379. 428 L.J. Kirschenbaum, and R.I. Haines, Inorg. Chim. Acta, 1983, 76,L127. 429 J.M. Bemtgen, H.R. Gimpert, and A. Von Zelewsky, Inorg. Chem., 1983, 22,

3576. 430 A . McAuley, J.R. Morton, and K.F. Preston, J. Am. Chem. SOC., 1982, 104, 7561. 431 A. McAuley, and K.F. Preston, Inorg. Chem., 1983, 22, 2111. 432 B.L. Ramakrishna, and P.T. Manoharan, Inorg. Chem., 1983, 11, 2113. 433 D.A. Cooper, S.J. Higgins, and W. Levason, J. Chem. SOC., Dalton Trans., 1983, 2131. 434 S.A. Jacobs, and D.W. Margerum, Inorg. Chem., 1984, 23, 1195. 435 A . S . Abhvani, C.A. Bates, and R.S. Wardlaw, J. Phys. C, 1983, 16,4209. 436 D. Cordischi, V. Indovina, S. Febbraro, and M. Occhiuzzi, Congr. Naz. Chim. Inorg. [Atti[ 16th, 1983, 226. 437 B. Rambabu, C. Ramasastry, and B.V.R. Chowdari, Phys. Status Solidi B, 1983, 118,381. 438 Y. Sugiura, J. Kuwahara, and T. Suzuki, Biochem. Biophys. Res. Commun., 1983, 115,878. 439 Y. Ueda, J.R. Niklas, J.M. Spaeth, U. Kaufmann, and J. Schneider, Solid 127. State Commun., 1983, 5, 440 A. Tressaud, S. Khairoun, J.M. Dance, J. Grannec, G. Demazeau, and P. Hagenmuller, C. R. Seances Acad, Sci., Ser. 2, 1982, 295, 183. 441 H.A. Boucher, G.A. Lawrance, P.A. Lay, A.M. Sargeson, A.M. Bond, D.F. Sangster, and J.C. Sullivan, J. Am. Chem. SOC., 1983, 105,4652. 442 D.L. Liczwek, R. L. Belford, J.R. Pilbrow, and J. S. Hyde, ,J. Phys. Chem., 1983, 87, 2509. 443 R.P. Bonomo, J.R. Pilbrow, and G.R. Sinclair, J. Chem. SOC., Dalton Trans., 1983, 489. 444 E. Balasivasubramanian, M. Seshasayee, and P.T. Manoharan, Mol. Phys., 1983, 50, 763. 445 J. Goslar, L. Szczepaniak, and A. Dezor, Acta Phys. Pol. A, 1983, E , 671. 446 M. Korkmaz, and B. Aktas, J. Phys. Chem. Solids, 1983, 44,651. 447 M. Korkmaz, and B. Aktas, J. Phys. Chem. Solids, 1984, 5, 259. 448 V.C. Mouli, and G.S. Sastry, J. Mol. Struct., 1982, E , 163. 449 F. Dejehet, R. Debuyst, F. Mullie, J.M. Arietta, G. Germain, and M. Van Meerssche, J. Chim. Phys. Phys.-Chim. Biol., 1983, 80, 355. 450 F. Dejehet, R. Debuyst, and M. Vandenbroucke, J. Chim. Phys. Phys.-Chim. Biol., 1982, 3,597. 451 F. Dejehet, R. Debuyst, M. Spirlet, J.P. Declerq, and M. Van Meersche, JChim. Phys. Phys. Chim. Biol., 1983, 819. 452 U. Abram, J. Stach, R. Kirmse, and W. Dietzsch, 2 . Chem., 1982, 11, 271. 453 M. Geoffroy, A. Jain, A. Celalyan, and G. Bernardinelli, Z. Naturforsch., B: Anorg. Chem., Org. Chem., 1983, 388, 830. 454 B. Kirste, and H. Van Willigen, J. Phys. Chem., 1983, 87, 781. 455 M. Narayana, and L. Kevan, J. Phys. C, 1983, 16,361. 456 D.R. De, J. Chem. Phys., 1983, 3,535. 457 J.R. Dorfman, R.D. Berenian, and M.H. Whangbo, _Copper Coord. Chem.: Biochem. Inorg. Perspect., 1983, 75. 458 P. Beardwood, and J.F. Gibson, J. Chem. SOC., Chem. Commun., 1983, 1099. SOC., Dalton Trans., 1983, 459 N. Aoi, G. Matsubayashi, and T. Tanaka, J.Chem. 1059.

so,

2: Transition-metalIons

83

460 D.E. Nikles, M . J . Powers, and F.L. Urbach, Inorg. Chem., 1983, 22, 3210. 461 P.J.M.W.L. Birker, E.F. Godefroi,, J , Helder, and J. Reedijk, J. Am. Chem. Soc., 1982, 104,7556. 462 M. Mohan, and Manmohan,Synth. React. Inorg. Met.-Org. Chem., 1982, 12, 761. 463 A.W. Addison, P.J. Burke, K. Henrick, T.N. Rao, and E. Sinn, Inorg. Chem., 1983, 22, 3645. 464 A.W. Addison, and E. Sinn, Inorg. Chem., 1983, 22, 1225. 465 E.E. Bernarducci, P.K. Bharadwaj, R.A. Lalancette, K. Krogh-Jespersen, J.A. Potenza, and H.J. Schugar, Inorg. Chem., 1983, 2 , 3911. 466 P.M. Solozhenkin, A.V. Ivanov, N.J. Kopitsya, and F.A. Shvengler, Dokl. Akad. Nauk SSSR, 1982, 266, 137. 467 P.M. Solozhenkin, A.V. Ivanov, N.I. Kopitsya, and F.A. Shvengler, Dokl. Akad. Nauk SSSR, 1982, 266. 918. 468 P.M. Solozhenkin, N.I. Kopitsya, F.A. Shvengler, and A.V. Ivanov, Dokl. Akad. Nauk. SSSR, 1982, 265, 382. 469 P.M. Solozhenkin, N.I. Kopitsya, F.A. Shvengler, and A.V. Ivanov, Dokl. Akad. Nauk SSSR, 1983, 268, 1159. 470 H. Kozlowski, M. Bezer, L.D. Pettit, M. Bataille, and B. Hecquet, J. Inorg. Biochem., 1983, 18,231. 471 J . P . Laussac, R. Haran, and B. Sarkar, Biochem. J., 1983, 209, 533. 472 S.V. Deshpande, R.K. Sharma, and T.S. Srivastava, Inorg. Chim. Acta, 1983, 78, 13. 473 W.S. Kittl, and B.M. Rode, J. Chem. SOC., Dalton Trans., 1983, 409. 474 L. Menabue, P. Prampolini, M. Saladini, and P. Morini, Inorg. Chim. Acta, 1983, E, 157. 475 G. Formicka-Kozlowska, D. Konopinska, H. Kozlowski, and B. Decock-Le Reverend, Inorg. Chim. Acta, 1983, 78,L47. 476 E.R. Werner, and B.M. Rode, Inorg. Chim. Acta, 1984, '31. 217. 477 S.V. Deshpande, and T.S. Srivastava, Inorg. Chim. Acta, 1983, 78, 75. 478 J.M. Anast, and D.W. Margerum, Inorg. Chem., 1982, 21, 3494. 479 T.S. Srivastava, and S.V. Deshpande, Inorg. Chim. Acta, 1983, 78, 37. 480 R . Boettcher, H. Metz, and W. W i n d s c h , * J . Mol. Struct., 1982, 83, 31. 481 P. O'Brien, Inorg. Chim. Acta, 1983, 3,L37. 482 B.N. Misra, S.D. Sharma, and S.K. Gupta, Proc. Natl. Acad. Sci., India, Sect. A, 1982, 52, 168. 483 L . F . Blank, C. Huxtable, and P. O'Brien,,1982, 65, L159. 484 P. Cocetta, S. Deiana, L. Erre, G. Micera, and P. Piu, J. Coord. Chem., 1983, 12, 213. 485 L.G. MacDonald, D.H. Brown, and W.E. Smith, Inorg. Chim. Acta, 1982, 3 , 213. 3021. 486 M.R. Wagner, and F.A. Walker, Inorp. Chem., 1983, 487 S.A. Boyd, L.E. Sommers, D.W. Nelson, and D.X. West, Soil Sci. SOC. Am. J., 1983, 47, 43. 488 F. Cariati, L. Erre, G. Micera, A. Panzanelli, and P. Piu, Thermochim. Acta, 1983, S,1. 489 F. Cariati, L. Erre, G. Micera, A. Panzanelli, G. Ciani, and A. Sironi, Inorg. Chim. Acta, 1983, 80, 57. 490 J.P. Henichart, R. Houssin, J.L. Bernier, afid J.P. Catteau, J. Chem. SOC., Chem. Commun., 1982, 1295. 491 K. Miyoshi, H. Tanaka, E. Kimura, S. Tsuboyama, S. Murata, H. Shimizu, and K. Ishizu, Inorg. Chim. Acta, 1983, 78, 23. 492 E.V. Komarov, O.V. Travkin, G.G. Ivanov, M.A. Mikhailova, M.A. Shneider, and S.V. Amelin, Antibiotiki (Moscow), 1983, 28, 437.

z,

84

Electroti Spin Resonance

493 E.G. Seebauer, E.P. Duliba, D.A. Scogin, R.B. Gennis, and R.L. Belford, JAm. Chem. SOC., 1983, 105, 4926. 494 A. Pezeshk, F.T. Greenaway, and J.R.J. Sorenson, Inorg. Chim. Acta, 1983, 80, 191. 495 T. Ichikawa, and L. Kevan, J. Am. Chem. SOC., 1983, 105,402. 496 M. Narayana, and L. Kevan, J. Chem. Phys., 1983, 78, 3573. 497 T. Ichikawa, and L. Kevan, J. Phys. Chem., 1983, 87, 4433. 498 D. Suryanarayana, P.A. Narayana, and L. Kevan, Inorg. Chem., 1983, 22, 474. 499 G. Martini, M.F. Ottaviani, and L. Burlamacchi, 2. Naturforsch. A: Phys., & € 3 723. Phys. Chem., Kosmophys., 1983,, 500 A.I. Kokorin, V.V. Berentsveig, V.D. Kopylova, and E.L. Frurnkina, Kinet. Katal., 1983, 24, 181. 501 H. Motschi, Naturwissenschaften, 1983, 2,519. 502 M. Rudin, and H. Motschi, J. Colloid Interface Sci., 1984, 98,385. 503 C. Fabre, and C. Lapinte, Nouv. J. Chim., 1983, 3 , 123. 504 A. Clearfield, and L.R. Quayle, Inorg. Chem., 1982, 21, 4197. 505 E.I. Berus, S.P. Gabuda, and V.A. Nadolinnyi, Biofizika, 1983, 28, 337. 506 F. Holuj, J. Magn. Reson., 1983, 51, 37. 507 Y. Kurita, and H. Ohigashi, Bull. Chem. SOC. Japan, 1983, 56, 3722. 508 P.J. Alonso,J. Casas Gonzales, H.W. Den Hartog, and R. Alcala, Phys. Rev. B: Condens. Matter, 1983, 27, 2722. 509 P.J. Alonso, J. Casas Gonzales, H.W. Den Hartog, and R. Alcala, J. Phys. C, 1983, Is, 3593. 510 P.J. Alonso,J. Casas Gonzales, R. Alcala, and H.W. Den Hartog, Radiat. Eff.,1983, 73,215. 511 G. Roussos, J. Nagel, and H.J. Schulz, Z. Phys. B: Condens. Matter, 1983, 5 3 , 95. 512 V.B. Kazanskii, I.V. Elev, and B.N. Shelimov, J. Mol. Catal., 1983, 21, 265. 513 I.V. Elev, A.N. Pershin, B.N. Shelimov, and V.B. Kazanskii. Kinet. Katal., 1982, 23, 936. 514 V.E. Shubin, V.A. Shvets, G. A. Savel'eva, N.M. Popova, and V.B. Kazanskii, Kinet. Katal., 1982, 23, 1153. 515 V.V. Saraev, F.K. Shmidt, G.M. Larin, and E.N. Sedykh, Koord. Khim., 1983, 9 , 1400. 516 V.V. Saraev, B. Ri, F.K. Shmidt, and G.M. Larin, Koord. Khim., 1982, 8, 1485. 517 N.M. Yasinskaya, V.N. Starukhin, B.S. Turov, I.P. Gol'berg, and F.P. Chernyakovskii, Prom-st. Sint. Kauch., 1982, 22. 518 E.C. Constable, J. Lewis, and M. Schroeder, Polyhedron, 1982, I, 311. 519 G. Bontempelli, F. Magno, D.S. Daniele, and G. Sciavon, J. Electroanal. Chem. Interfacial Electrochem., 1983, 159, 117. 520 G . A . Bowmaker, P.D.W. Boyd, and G.K. Campbell, Inorg. Chem., 1983, 22, 1208. 521 G.A. Bowmaker, P.D.W. Boyd, and G.K. Campbell, Inorg. Chem., 1982, 1, 3565. 522 T. Huizinga,and R. Prins, J. Phys. Chem., 1983, 87, 173. 523 J.R. Pilbrow, G.R. Sinclair, D.R. Hutton, and G.J. Troup, J. Magn. Reson,, 1983, 52, 386. 524 C.E. Forbes, J. Chem. Phys., 1983, 79,2590. 525 E.P. Nikolova, Yu.N. Savvin, and B.L. Tirnan, Zh. Prikl. Spektrosk., 1983, 38, 1008. 526 R.M. Martirosyan, M.O. Manvelyan, and G.A. Mnatsakanyan, Fiz. Tverd. Tela

2: Transitiotz-metal Ions

85

(Leningrad) 1983, 25, 1564. 527 I.P. Beletskii, Adsorbens. Adsorbenty, 1981, 2, 39. 528 V.V. Zuryanov, and N.Z. Lyakhov, Sovrem. Metody YaMR EPR Khim. Tverd. Tela, IMater. Vses. Koord. Soveshch.1 3rd, 1982, 209. 529 A. Manoogian, and B.W. Chan, Phys. Status Solidi B, 1983, 118,K31. 530 H. Takeuchi, M. Arakawa, H. Aoki, T. Yosida, and K. Horai, J. Phys. SOC. Japan, 1982, 51, 3166. 531 H. Takeuchi, and M. Arakawa, J. Phys. SOC. Japan, 1983, 52, 279. 532 H. Takeuchi, and M. Arakawa, J. Phys. SOC. Japan, 1984, 53, 376. 533 B. Wichterlova, 2 . Tvaruzkova, and J. Novakova, J. Chem. Soc.,Faraday Trans. 1 1983, 79,1573. 534 A.J.A. Konings, A. Valster, V.H.J. De Beer, and R. Prins, J. Catal., 1982, 76, 466. 535 J.L. Davidson, K. Davidson, and W.E. Lindsell, J. Chem. SOC., Chem. Commun., 1983, 452. 536 H. Yugami, H. Nakagawa, T. Yamada, and H. Matsumoto, Fukui Daigaku Kogakubu Kenkyu Hokoku, 1983, 31, 41. 537 M.J. Camenzind, F.J. Hollander, and C.L. Hill, Inorg. Chem., 1983, 3, 3776. 538 0.S. Torosyan, Phys. Status Solidi B, 1983, 119, K101. 449. 539 J.W. Tucker, Phys. Lett. A, 1982, 92J, 540 M.G. Kanatzidis, and D. Coucouvanis, Inorg. Chern., 1984, 23, 403. 541 P. Bertrand, F.X. Theodule, J.P. Gayda, J. Mispelter, and M. Momenteau, Chem. Phys. Lett., 1983, 102, 442. 542 B.J. Kennedy, G. Brain, and K.S. Murray, Inorg. Chim. Acta, 1984, 81, L 2 9 . 543 A. Bencini, C. Benelli, D. Gatteschi, and C. Zanchini, Inorg. Chem., 1983, 22, 2123. 544 L.C. Dickinson, and J.C.W. Chien, < J . Am. Chem. SOC., 1983, 105,6481. 545 D.M. Daraseliya, and D.L. Dzhaparidze, Phys. Status Solidi B, 1983, 119, K57. 546 A.A. Mirzakhanyan, A.K. Petrosyan, and S.G. Maloyan, Izv. Akad. Nauk Arm. SSR, Fiz., 1983, 18,315. 547 V.N. Vasyukov, S.N. Lukin, and G.A. Tsintsadze, Fiz. Tekh. Vys. Davlenii, 1982, 8, 32. 548 K. Dyrek, and Z. Sojka, J. Chem. SOC., Faraday Trans. 1, 1982, 78,3177. 549 I.A. Gorn, and V.D. Chernyi, Tr. Mosk. Energ. Inst., 1981, 512, 11. 550 V.I. Murav’ev, Zh. Strukt. Khim., 1982, G , 166. 551 R. Alcala, P.J. Alonso, and R. Cases, J. Phys. C, 1983, Is, 4693. 552 H. Aoki, M. Arakawa, and T. Yoshida, J. Phys. SOC. Japan, 1983, 52, 2216. 553 J. Shinar, and V. Jaccarino, Phys. Rev. B: Condens. Matter, 1983, 13, 4034. 554 R . K . Malhotra, V.P. Seth, and V.K. Jain, Can. J. Phys., 1983, 61, 1359. 555 V.K. Jain, and S.K. Yadav, Phys. Status Solidi B, 1982, 114,K131. 556 T.A. Bazilevskaya, V.T. Gritsyna, N.V. Gritsenko, and V.A. Kobyakov, Zh. Prikl. Spectrosk., 1983, 39, 98. 557 V.T. Gritsyna, and V.A. Kobyakov, Deposited Doc., 1982, VINITI 2822. 558 M. Villedieu, N. Devismes, and A.M. De Goer, Radiat. Eff., 1983, 3 , 153. 559 N.A. Kulagin, A.L. Apanasenko, and N.A. Kazakov, Zh. Prikl. Spektrosk., 1983, 38, 988. 560 L. Abello, S.J. Schwerdtfeger, and C.F. Schwerdtfeger, Solid State Commun., 1982, 44,497. 561 L.K. Kurmanguzhina, I.N. Marov, G.A. Evtikova, and A.A. Fakeev, Zh. Neorg. Khim., 1983, 3 , 61. 562 P. Colornban, and D. Vivien, Phys. Status Solidi A, 1983, 76, 565.

Electron Spin Resonance

86

563 B. Frick, and D. Siebert, Ber. Bunsen-Ges. Phys. Chem., 1983, 87, 558. 564 M. Heming, and G. Lehmann, Z. Naturforsch., A: Phys., Phys. Chem., 149. Kosmophys., 1983, 565 J. Kreissl, and W. Gehlhoff, Phys. Status Solidi A, 1984, 81, 701. 566 S.J. Strach, and R. Bramley, J. Magn. Reson., 1984, 56, 10. 567 V.K. Jain, V.P. Seth, and R.K. Malhotra, J. Chem. Phys., 1984, 80, 1373. 568 S.N. Lukin, O.P. Teslya, and G.A. Tsintsadze, Fiz. Tverd. Tela (Leningrad), 1983, 25, 1075. 569 E.A. Petrakovskaya, V.V. Velichko, I.M. Krygin, S.B. Petrov, and L.G. Falaleeva, Fiz. Tverd. Tela (Leningrad), 1983, 25, 862. 570 K.W. Blazey, J.M. Cabrera, and K.A. Mueller, Solid State Commun., 1983, 45, 903. 571 S. Sastry, K.V. Lingam, and M. Rao, J. Mol. Phys., 1983, 50, 453. 572 A . Kryukov, Z.A. Tkachenko, V.K. Bukhtiyarov, and E.E. Kriss, Teor. Eksp. Khim., 1983, 19, 197. 573 R.B. Birdy, and M. Goodgame, J. Chem. SOC., Dalton Trans., 1983, 1469. 574 R.B. Birdy, and M. Goodgame, J. Chem. SOC., Dalton Trans., 1982, 1429. 575 R. Rao, M.C.R. Symons, and A. Harriman, J. Chem. S O C . , Faraday Trans. 1, 1982, 78, 3393. 576 C.A. McAuliffe, D.S. Barratt, C.G. Benson. A. Hosseiny, M.G. Little, and K. Minten, J. Organomet. Chem., 1983, 258, 35. 577 M. Aizawa, T. Komatsu, and T. Nakagawa, Bull. Chem. SOC. Japan, 1982, 55, 3434. 578 M.B. McBride, Soil Sci. S O C . Am. J., 1982, 46, 1137.

m,

3 Inorganic and Organometall c Radicals BY M. C. R . SYMONS

The g e n e r a l o r g a n i s a t i o n f o r t h i s C h a p t e r f o l l o w s t h a t u s e d i n Volume 8.l I b e g i n by d i s c u s s i n g ESR s t u d i e s of s y s t e m s which c a n b e c l a s s i f i e d under a r a n g e of h e a d i n g s from 'Trapped and S o l v a t e d E l e c t r o n s ' t h r o u g h A B , A B 2 , A B 3 , A B 4 , AB5 and A B 6 s p e c i e s .

This

l e a d s t o a s e c t i o n on r a d i c a l s which a r e n o t r e a d i l y c l a s s i f i e d i n I t i s i m p o r t a n t t o r e a l i s e t h a t t h i s method o f t h i s way. c l a s s i f y i n g s t r u c t u r e s i s a r b i t r a r y and most s e c t i o n s i n c l u d e more complex s p e c i e s which are s t r u c t u r a l l y s i m i l a r t o *ABB r a d i c a l s . I n p a r t i c u l a r , I have chosen t o i n c l u d e under t h e h e a d i n g "Monatomic R a d i c a l s " a r a n g e of s t u d i e s o n atom c l u s t e r s and a l s o on atom-ligand complexes which are n o t s o r e a d i l y c l a s s i f i a b l e e l s e w h e r e . I n s e v e r a l o t h e r s e c t i o n s , t h e ' l i g a n d s ' B i n *AB,-

are p o l y a t o m i c r a t h e r t h a n monatomic. S e c t i o n [ 9 1 on ' R a d i c a l s i n I n o r g a n i c M a t e r i a l s ' i s a l s o somewhat a r b i t r a r i l y s e l e c t e d s i n c e s e v e r a l of t h e r a d i c a l c e n t r e s d e t e c t e d t h e r e i n c o u l d have been l i s t e d p r e v i o u s l y and many of t h e r a d i c a l s i n S e c t i o n s [2-81 a r e a c t u a l l y formed i n s u c h m a t e r i a l s . I n S e c t i o n [ 9 1 t h e emphasis i s o n p a r a m a g n e t i c c e n t r e s t h a t a r e n o t c l e a r l y r e l a t e d t o i s o l a t e d r a d i c a l s of s p e c i f i c s t r u c t u r e , a l t h o u g h one always a t t e m p t s t o d i s c o v e r s i m p l e r a d i c a l a n a l o g u e s f o r e a s e of r e p r e s e n t a t i o n . S e c t i o n [ l o 1 u n d e r l i n e s t h e need t o c o n s i d e r t h e e f f e c t of t h e environment on t h e ESR p a r a m e t e r s f o r s i m p l e r a d i c a l s . T h i s c a n b e q u i t e minor b u t , under c e r t a i n c i r c u m s t a n c e s , it may d o m i n a t e t h e form of t h e s p e c t r u m , a s i n t h e c a s e of s o l v a t e d s u p e r o x i d e i o n s . There have been so few p u r e l y i n o r g a n i c s t u d i e s of mechanism i n which ESR s p e c t r o s c o p y p l a y s an i m p o r t a n t r6le t h a t I have o m i t t e d t h i s as a s p e c i f i c t o p i c . However, t h e u s e o f s p i n - t r a p s t o p r o b e mechanism r e m a i n s a much-used t e c h n i q u e and some examples are g i v e n i n S e c t i o n [ l l ] . S e c t i o n [ 1 2 1 c o u l d a s w e l l b e c o v e r e d i n t h e s e c t i o n on t r a n s i t i o n - m e t a l complexes b u t h a s t r a d i t i o n a l l y been c o v e r e d herein. However, a s e c t i o n on complexes h a v i n g t h e i r SOMO l a r g e l y c o n f i n e d t o t h e l i g a n d s h a s been o m i t t e d t h i s y e a r , s i n c e it i s 87

[ F o r references see p . 1 3 4

88

Electron Spin Resonance

adequately covered elsewhere. The Review e n d s w i t h a b r i e f a c c o u n t o f s t u d i e s o f r a d i c a l s i n t h e gas-phase

I n a g e n e r a l s e n s e , t h i s is a blossoming

1131.

f i e l d b u t t h e p u r e l y ESR a s p e c t s r e m a i n l i m i t e d a n d s p a c e d o e s n o t p e r m i t a major e x t e n s io n t o cover o t h e r s p ectr oscopi c t echni ques.

-

1.1 Books and Reviews.

From o u r p o i n t o f v i e w , by f a r t h e m o s t

i m p o r t a n t p u b l i c a t i o n i s t h e c o m p r e h e n s i v e book by W e l t n e r . This i s p r i m a r i l y concerned w it h t h e magnetic p r o p e r t i e s of s m a l l m o l e c u l e s a n d i o n s o f t h e t y p e which B i l l W e l t n e r a n d h i s c o w o r k e r s h a v e made v e r y much t h e i r own.

It s t a r t s with a l u c i d explanation

o f b a s i c ESR t h e o r y , w i t h p a r t i c u l a r l y u s e f u l s e c t i o n s on randomly o r i e n t e d r a d i c a l s and t h e d i f f i c u l t i e s which a r i s e f o r r a d i c a l s h a v i n g n o n - a l i g n e d g- and A - t e n s o r s .

A key a s p e c t of t h e

book i s t h e e x t e n s i v e T a b l e s g i v i n g ESR p a r a m e t e r s and d e r i v e d spin-densities

f o r a w i d e r a n g e o f s m a l l m o l e c u l e s and i o n s .

T h e s e i n c l u d e 'C MS and MM'

diatomic species. VO,

and

2~

d i a t o m i c s p e c i e s s u c h a s MH, M C , Mhal, M O ,

r a d i c a l s ( M i s any m e t a l a t o m ) a n d more common n o n - m e t a l

MoN and M-M'

Then AB s p e c 2 e s w i t h S = l , a r e covered.

3/2r

t r i a t o m i c a n d t e t r a a t o m i c r a d i c a l s s u c h as Mhal', MHa13, C H 3 , C F 3 , N O 3 ,

N03'-,

e t c . , such a s

0 2 ,

Similar treatment is given t o Mn03 a n d FeF3.

MM2'

,

CH2,

TiF2,

The book a l s o i n c l u d e s

a somewhat less e x t e n s i v e b u t c o m p a r a t i v e t r e a t m e n t o f s m a l l r a d i c a l s i n t h e gas-phase and t h e r e i s a g a i n a most u s e f u l t a b l e of d a t a . A s o u t l i n e d i n Volumes 7 a n d 8 , t h e r e h a s b e e n e x t e n s i v e r e c e n t

i n t e r e s t i n the study of t r a n s i e n t s a t very short i n t e r v a l s a f t e r t h e i r "birth".3

One t e c h n i q u e w h i c h , by i t s v e r y n a t u r e , c a n o n l y

b e c o n c e r n e d w i t h s h o r t t i m e i n t e r v a l s i s t h a t known a s 'muon s p i n rotation'.

From t h e ESR c h e m i s t s ' p o i n t of v i e w , t h i s t e c h n i q u e

i s e x c e l l e n t f o r t h e s t u d y o f t r a n s i e n t r a d i c a l s formed by t h e e f f e c t i v e a d d i t i o n o f muonium a t o m s (Mu*) t o u n s a t u r a t e d compounds. Muonium atoms behave l i k e a v e r y l i g h t i s o t o p e o f h y d r o g e n (E. one-ninth of t h e mass).

They a l s o h a v e I = f a n d a m a g n e t i c moment

ca. 3x t h a t o f h y d r o g e n .

Many a s p e c t s o f t h e c h e m i s t r y o f t h e s e

n o v e l t r a n s i e n t s p e c i e s have b e e n r e v i e w e d by Walker i n a book which s h o u l d be o f c o n s i d e r a b l e i n t e r e s t t o ESR s p e c t r o s c o p i s t s . The r e p o r t of t h e 7 t h . Yamada C o n f e r e n c e a t Shimoda g i v e s a f a r more d e t a i l e d c o v e r a g e of t h e whole f i e l d o f Muon P h y s i c s a n d Chemistry.

89

3: Inorgurric and Organoriietallic Radicals

Two more specific articles about triplet-states which this author has found to be useful include Stevenson's treatment of triplet state E S R spectra' and a discussion of the use of halffield (AMs=2) transitions for determining spin-separation in triplet species. 1 . 2 Techniques. - A useful extension of Bray's method of rapid freezing for trapping transient species which cannot be directly detected in the liquid-phase has been described Two important general methods for preparing matrix isolated radical cations have been developed over the past few years. The most powerful for small inorganic cations, exploited particularly by Knight and his coworkers (see Sections 4-7 and Table 1 1 , involves photoionization of the parent neutral molecules in a rare-gas stream during, or just prior to, deposition onto a cold

.'

Table 1:

Some Radical Cations prepared by Photoionization in Rare-Gas Matrices CO',

C~OZ',

N2+,

H20+, NH3+, CHI,',

H2CO'.

finger. This method gives extremely well-resolved E S R spectra for small species (cf. Fig. 1) but, apparently, larger cations give far less satisfactory spectra. Fortunately, the alternative technique, based on radiolysis after matrix isolation of the parent molecules, generally works well for medium sized cations. This technique depends for its efficiency on a series of reactions such as (a)-(d) for the most favoured matrix, CFC13.

- -

CFC13 CFC13.+ + eCFC13 + e[CFCl3-1 :FCl2 + C1CFC13*+ + CFC13 CFC13 + CFC13.' CFC13*' + S CFC13 + S -'

.... .... ....

....

(a) (b) (c)

(d)

After ejection (a) the electron is efficiently scavenged by solvent molecules (b) whilst the 'hole' is mobile via electron exchange (c) until it reacts with a solute molecule, S, (a). Complications which arise in the use of this technique are outlined in Section 1 0 . These methods are especially useful to ESR spectroscopists since the electron-capture centres make insignificant contributions to the spectra. The fate of the electrons in the photoionization

Electron Spin Resonance

90

4

M

1 3380

I

3400

I

I

I

3420

3440

3460

Fiqure 1 ESR spectrum of 12C170+isolated i n neon matrix a t 4 K. The M denotes methyl (CH3) impurity lines. The ccmputersimulated second-order powder spectrum for 1 2 C 1 7 0 + is shown a t the bottcm. ge denotes the magnetic f i e l d corresponding t o the f r e e spin g- value. [Fran Ref. 513 e x p e r i m e n t s i s unknown and r e m a i n s a problem of c o n s i d e r a b l e i n t e r e s t . F o r t h e work u s i n g h a l i d e m a t r i c e s , t h e s p e c t r a f o r t h e e l e c t r o n - c a p t u r e c e n t r e s are b r o a d a t ca. 7 7 K and h e n c e d o n o t interfere seriously. 2 Trapped and S o l v a t e d E l e c t r o n s 2.1 Electrons i n Solvents.

-

Another s t u d y of t h e p r o t o n h y p e r f i n e

c o u p l i n g f o r e l e c t r o n s weakly t r a p p e d i n s u g a r c r y s t a l s h a s b e e n d e ~ c r i b e d . ~A f t e r i r r a d i a t i o n a t 3 K , a n e l e c t r o n c e n t r e i n t r e h a l o s e showing c o u p l i n g t o two p r o t o n s was d e t e c t e d .

The

r e s u l t s s u g g e s t t h a t o n e i f from o n e of t h e C-OH g r o u p s o f t h e s u g a r and t h e o t h e r i s from a water m o l e c u l e .

D i s t a n c e s deduced

from t h e a n i s o t r o p i c c o u p l i n g c o n s t a n t s l e a d t o t h e model shown i n Fig. 2. This r e s u l t confirms a previous conclusion t h a t e l e c t r o n s r e q u i r e a minimum o f 2 OH g r o u p s f o r weak l o c a l i s a t i o n . They a r e f a r more s t r o n g l y t r a p p e d by s i x OH g r o u p s i n i r r a d i a t e d aqueous a l k a l i - m e t a l h y d r o x i d e g l a s s e s . l o R e s u l t s of a n

91

3: Inorganic and Organometallic Radicals

CH

\

Fiqure 2 We1 f o r

ei

in trekdose.

e l e c t r o n s p i n - e c h o s t u d y show t h a t s i x OH g r o u p s from s u r r o u n d i n g w a t e r molecules d e f i n e t h e c a v i t y containing t h e e l e c t r o n , but t h a t

a v a r i a b l e number o f n e x t n e a r e s t n e i g h b o u r c a t i o n s a l s o c o n t r i b u t e t o t h e s p e c t r a . However, t h e s p i n - d e n s i t y o n t h e s e c a t i o n s i s t i n y ( 5 0 0 nm) either CH4, H+ or H2 depending on both the cation structure and the matrixloo. Cyclohexane cation radical eliminates H2 under visible light to yield cyclohexene cation radical, but c-CGH12'. is thermally stable to 1 4 2 K39. Thermal elimination of a proton from CH3 from the methyl-substituted butane cation radicals takes place in CFC12CF2C1 to yield primary alkyl radicalslOO. Radiolysis of a number of simple alkanes and alkenes in Xe matrices at 4.2 K gave trapped H (or D) atoms with a comparable yield of the corresponding organic radical derived by scission of a C-H(D) bond following energy transfer from the matrix. Isotope effects of ca. 2 were apparent in the scission process. At 4 5 K Htr is detrapped to undergo reactions with solute molecules, e . g . attack on CH3CD3 to give k,/kD 91°1. An account has appeared of the 'alternation effect' in the y i e Z d s of R - in radiolysis of linear alkanes at 77 K (with higher G(R.) from 'even' alkanes),which is related to the observed prominence or even preponderance of chain end radicals from alkanes with an odd number of C atoms, and the preponderance of 'penultimate' or 'internal' radicals, x-%eHCH3 and %%%CH2bHCH2%%% from alkanes with even numbers of C atomslo2. Both alternation effects are related to crystal structural effects, which also show alternation in this serieslo3. I

-

4.2 Saturated Systems Containing Heteroatoms. - Radiolysis of alkylsulphenyl and alkylthiosulphenyl chlorides in glassy alkane matrices yields thiyl and perthiyl radicals respectivelylo4, although the

RSCl

+ e-

RSSCl

+ e-

3

RS-

+ C1-

RSS.

+ C1-

former yield no e.s.r. absorption. Photolysis of concentrated solutions of mercaptans in alkanes yield optical and e.s.r. bands attributed to R S i (H)R, which is photolysed ultimately to RSS. 04. The motional effects of n-alkanes ( n = 1 2 to 2 4 ) in their urea adducts have been investigated by converting the alkane to its peroxyl derivative following radiolysis in air

4: Organic Radicals in Solids

and determining the temperature variation of the e.s.r. spectra of R 0 2 - in the temperature range 100-200 K when characteristic motional effects are apparentLo5. While it has long been known that photo-oxidation of EtOH at 77 K by uranyl ion yields MeeHOHlo6, it is now reported that at high ( > 1.0 mol dm-3) oxidant concentrations, the principal process becomes electron-transfer to yield Me-; addition of water to EtOH also increase the relative yield of Me-l o 7 . Single crystal studies of carbohydrates continue to reveal interesting features, thus the trapping site of the electron at 3 K in x-irradiated rhamnose is situated between three hydroxyl groups, two on the carbohydrate and one on a water moleculeloB. et- decays on visible light photolysis by cleavage of an O-H bond and the liberated H atom abstracts an H atom from >C(H)OH to yield >t-OH. It is concluded that the trapping site p r e - e x i s t s in the lattice and no reorientation of dipoles is necessary to stabilise the electron108. X-irradiation of trehalose at 3 K yields a generally similar picture although the electron is now trapped in an intermolecular site formed by only two OH groups, one on the carbohydrate and one on waterlog. The electron decays as in rhamnose to yield an alkoxyl radical (at 0-4’) and an H atom, which attacks a trehalose molecule at C-3 to give a hydroxyalkyl radical. (All three radicals are present at 3 K.)lo9. The dominant radical in single crystals of anhydrous a-D-glucopyranose after x-irradiation at 77 K is the C-6 primary hydroxyalkyl radical which reorients slightly on warming to ca. 200 K and is converted to a C - 2 primary hydroxyalkyl radical on warming to ca. 300 Kilo. Annealing the sample at 300 K results in several radical products, one being a C-2 secondary hydroxyalkyl radicalllO. Reactions of H-atoms produced photolytically in acid glasses with myo-inositol yields three radicals, two displaying proton couplings and one as a non-specific singlet. The two former correspond to H-atom loss from all carbon positionsll’. Both these signals are converted to a quartet on annealing l. which is attributed to a species of structure O=(!!-CN-CH2. radicals are trapped more efficiently than the 5-yl radical136. An analogous type of study has been performed on polycrystalline samples of a series of pyrimidine nucleosides (both as free acids and alkali metal salts)137. Two types of radical were omnipresent, i.e. the 5-yl radical and radical-C-5’H2,formed by transformation of an original H-loss radical at C - 5 ’ . Other sugar-centred radicals were identified in certain n u c l e ~ s i d e s ’ ~ ~ . The n-cation radicals of 5-(hydroxymethyl)uracil and 5-(hydroxymethyl)cytosine have been produced radiolytically in LiCl(12 mol dm-3)-D20 glasses and by photoionisation in NaC104(8 mol ~ . K ~ ) - D ~ glasses O at 77 K. As the temperature is raised, the spectrum is transformed, probably as a result of a change in the state of protonation at a nitrogen atom and the subsequent adoption of a preferred conformation of the -CH20H group v i a hydrogen-b~nding’~~. A similar radiolytic approach was used in preparation of n-cation radicals of a number of 5-halopyrimidines, for which analysis of the spectra included nuclear quadrupolar terms for C1, Br and I in addition to the hyperfine and g - t e n s ~ r s ’ ~ ~Warming . led to secondary

155

156

Electron Spin Resonance

radicals in several cases, thus 5-fluorocytosine cation radical undergoes deprotonation of its exocyclic N-atom13'. A combined e.s.r.-ENDOR examination of x-irradiated xanthosine dihydrate single crystals revealed the presence of four radicals; at 65 K are found (i) the a-cation of the xanthine base deprotonated at N-3, (ii) the *-anion of the base protonated at 0-6, (iii) the (sugar) C-5' H-loss radical. On warming above 250 K, the fourth species appeared, o i z . the C-8 H-atom adduct140. X-irradiated single crystals of 6-methylmercaptopurine riboside feature three species at 20 K, o i z . a trapped H-atom, a secondary alkoxyl radical (at 0-3') and a centre with doublet splittings of 2.1 f 0.3 mT and 0.4 to 1.2 mT and principal g-values of 2.0019, 2.0077 and 2.0429 assigned to another secondary alkoxyl radical (at O-2')141. Further studies of 1:l cocrystals have appeared; those acid yield three species of 9-ethyladenine:5,5-di.ethylbarbituric on radiolysis at 295 K namely the babital 5-yl radical formed by l o s s of Et, and the H-atom adducts formed at C-2 and C-8 of g-eth~ladenine'~~.Those of adenosine:5-bromouracil yield, on radiolysis at 12 K, the bromouracil n-cation and the adenine n-anion. These ions decay above 40 K, reaching undetectable levels at 170 K. Above 200 K H-atom adducts are formed v i a both H-addition and protonation of the anions, and hydrogen-abstraction radicals (from C-3' and C-4') stabilised on the sugar residue are found above 200 K (but may be present, although undetected, at lower temperature^)'^^. The complex cytidine:salicyclic acid yields at 77 K the phenoxyl radical formed by oxidation of the salicyclic acid, but no cytidine-based products. Following decay of the phenoxyl at room temperature, four radicals were apparent, including the 5-yl and 6-yl radicals and an anisotropic doublet attributed to a C-1' or C-2' centred species of structure RkHCOR' 44. The concentration of protein in aqueous solutions of DNA influences the course of photolysis at 77 K: 1% of protein gave anionic and cationic pathways, thus purine bases are ionised and the electron transported to pyrimidines with the formation of anion-radicals and ultimately H-atom adducts of thymine. However, 11-12% protein gives mainly the anionic pathway145.

4: Organic Radicals in Solids

6

157

Radicals at Surfaces and in Clathrates

Brey has given a historical review of the applications of magnetic resonance in catalytic research146. The prolonged discussion over the oxidation state of adsorbed perylene on activated alumina may be nearing its conclusion judging from the effects of added F- ions which decrease its donor properties (and hence radical yields with 1,3,5-trinitrobenzene or tetracyanoethylene) but increase its acceptor power (and accordingly the radical yields with donors such as 9,lO-dimethylanthracene or triphenylamine, and notably, perylene, implying donor characteristics for the latter to produce perylene" 1 I,7. This conclusion is reinforced by observations on the reducing power of A1203 towards iodine, which is reduced by adsorbed 1,3,5-trinitrobenzene but enhanced by perylene or tri~henylaminel~~. E.s.r. studies of adsorption from aqueous solution of the anionic nitroxide spin probes 3-carboxy-2,2,5,5-tetramethyl-l-pyrrolidinyloxyl and 4-hydroxy-2,2,6,6-tetramethyl-l-piperidino-oxyl dihydrogen phosphate reveal that both are adsorbed rapidly onto the high surface area alumina and boehmite whereas only the organophosphate is adsorbed onto gibbsite. A loss in rotational motion accompanied adsorption, and the nature of the active sites was explored by competition with anions such as C1- and C104-148. y-irradiation at 77 K of CHI,encapsulated in zeolite 3A yields M e - which is stable in air to ca. 195 K 1 4 9 ; this parellels the interesting observation of weak, narrow-line Me. signals in certain naturally occurring lepispheric cherts (flints) which increase after heating, and are attributed to trapping in molecular-sized holes150. E.s.r. spectra of deoxycholic acid channel-type inclusion compounds doped with 2,2,5,5-tetramethyl-3carbimadopyrrolinyl-l-oxyl and 2,2,6,6-tetramethylpiperidinyl-loxyl-4-01 were recorded as a function of temperature, radical concentration and under U.V. irradiation. While at 300 K the probes are immobile and isotropically dispersed, molecular reorientation occurs at 323-343 K and is complete by 393-413 K with Ea c a . 24-32 kJ mol". Clustering is observed for radical concentrations C Q . 0.2 mol d m - 3 1 5 1 . Cyclohexyl and ally1 radicals are detected when cyclohexane and 2,5-dimethylhex-3-ene are deposited onto the uncontaminated, continuously renewed surface of NaCl evaporated onto the cold surface of a rotating cryostat at 7 7 K. Evidently

158

Electron Spin Resonance

the electrostatic field at the ionic surface can cause C-H scission, perhaps v i a charged intermediates’ . Adsorption of various linear alkenes on the zeolite H-mordenite at 195 K yields alkenic and allylic radicals, but at 293 K a rapid transformation occurs to give a signal identical for a l l the alkenes and featuring seven components with a binomial This was not assigned, but the intensity di~tribution’~~. observation demonstrates major isomerisation and oligomerisation processes occurring within the mordenite; also rapid proton exchange occurs between the radicals and the -OH groups of the zeolite153. The identification of solid-state defects on calcined H-ZSM-5 powder is discussed in terms of positive holes on oxygen atoms between the tetrahedral structure of Si02-Al203, and a charge-transfer mechanism is responsible for production of cation radicals from alkenes and alkynes. Intramolecular rearrangement of the 3,3-dimethyl-l-butene radical cation leads to 2,3-dimethyl2-butene radical cation, while the 2-butyne radical cation is aromatised to hexamethylbenzene radical cation at high temperatures1 5 4

.

7

Radicals in Semiconductors

Readers of CA Selects will appreciate the rapid increase in activity in this area, and only a brief selection of papers can be mentioned here. A useful general review is given by Undoped polyacetylene (CH)3: continues to attract Wudl’ detailed study, thus nuclear relaxation time (TI) measurements v e r s u s frequency and e.s.r. linewidth in the temperature 4.2 to 300 K are comprehensively explained in terms of highly-onedimensional diffusive spins which can be trapped at impurities or defects, in particular those associated with contamination by oxygenlS6. The spin density is delocalised over 10 to 17 CH units and all properties are consistent with the soliton-bondalternation defect picture, provided the trapping effect is taken into account’5 6 . Interestingly, the diffusion coefficient of the spins decreases as the temperature is lowered, in agreement with other T1 data‘57 but not earlier e.s.r. linewidth datalS8. A novel analysis of e.s.r. measurementson (CH) has been demonstrated by analysis of the conventional lineshape in time domain. Qyantitative results for the hyperfine coupling constant, the on chain diffusion rate, and the off-chain hopping rate were

.

4: Organic Radicals in Solids

159

extracted by non-linear curve fitting to the time-domain signals; these again confirm the soliton modell59. ENDOR measurements at 77 K on stretched films of undoped cis-rich (CH)x reveals anisotropy of the hyperfine coupling, signifying n-electron character and consistent with the assumption of the bond alternation kink in the n-electron This supports system in undoped (CHIX160 (see also ref. 161). earlier conclusions based on e.s.r. data, when anisotropy was found both in the g-tensor and the linewidth162. Pristine 9 8 % 13C-enriched cis-(CH)x has also been studied by ENDOR, and spectra are characterised by two lH and 13C hyperfine tensors. The relative magnitudes and symmetry of the tensor elements establish that the spin resides in a delocalised n-orbital width’63. Electron-spin-echo constrained to a well of 100 spectroscopy was applied to undoped cis/trans and pure trans-(CHIX to reveal three types of defect, v i z . a localised defect, a distributed defect (trapped soliton) and a highly mobile defect (soliton) 4. Photo-e.s.r. experiments on trans-(CH)X at 10 K enable an upper limit to be set for the quantum efficiency of photoimplying the photogeneration of unpaired spins of 2 x induced states to be spinless and to be associated with charged solitons’ 5 . Films of (CH)r doped with Fe(C104)3 become highly electronically conducting ( > 5 0 0 A - 1 crn-l) and gold-coloured; the gtensor is as for undoped (CHIx but the lineshape is Dysonian with an A:B ratio of 8 : l consistent with metallic electrons 6 . Electron-acceptor doping of poly (p-phenylene sulphide) affords new signals and g-tensors (isotropic value ca. 2 . 0 0 7 ) suggestive of a centre R-g’-R, an assignment supported from studies with the oligomeric model compound Ph-S-C6H4-S-Ph1 7. Optical excitation at 4 . 2 K of the phenazine component in the crystalline complex diacety1ene:phenazine yields a stable, triplet biradical caused by electron-transfer from the diacetylene to the phenazine and protonation of the phenazine. The species can be bleached optically at 4 . 2 K and thermally at 2‘ > 90 K, and the diacetylene units polymerise in the dark as T exceeds 2 0 0 K 1 6 8 . E.s.r. examination on the x-irradiated organic conductors, bis(tetramethyltetraselenofulvalene)hexafluorophosphate,

a

160

Electron Spin Resonance

[ P F ~-I and TMTSF :TCNQ (tetracyanoquinodimethane) (TE~T~F. reveals that a weak disorder extends the metallic phases to low temperatures 16q. The role of e.s .r. in many papers is supportive of n.m.r. or other techniques, thus in the salts (fluoroanthenyl)2 + * [XF6]- (X = P, As, Sb) relaxation processes from T I n.m.r. relaxation and pulsed e.s.r. point to three temperature-dependent proton relaxation processes, v i z . (i) mobile paramagnetic species (above 160 K), (ii) fluorine nuclei (40-160 K) and (iii) fixed paramagnetic centres (below 4 0 K) l 7 O , l 7 l . Substituting As or Sb for P leaves the hightemperature properties unchanged but results in higher hindering potentials for the reorientational motion of the anions and in a shift of the phase transitions to lower temperatures170. The e.s.r. signals (both as regards intensity and linewidth) are nearly constant above Tc and decrease below Tc with nearly identical activation energies of ca. 0.12 eVI7O. E.s.r. and d.c. conductivity measurements on the related salt (naphthalene" [ASF6]- indicate metallic properties above 240 K with the narrowest e.s.r.-linewidth ( A E = 0.25 pT) found in solids. PP Below 240 K the material becomes semiconducting, while below 110 K a structural phase transition takes place leading to an enhancement of the linewidth by an order of rnagnit~de'~~.E.s.r. lineshapes at 9.4 and 0.0332 GHz have also been determined as a function of the orientation of the magnetic field at 295 K for single crystals of bis[l,2-bis(2-methoxyethoxy)/ethane] sodium biphenylide; excellent agreement is found between the experimental results and those calculated on the basis of spin diffusion173 using the approach of Richards and Salamon'74. Schmidt has reviewed briefly the dynamic properties of excitons in molecular crystals as evinced by electron-spin-echo spectroscopy' 5 . An interesting model of a one-dimensional organic ferromagnet has been devised in the structure ( 2 ) which yields an 8-line spectrum at 4.2 K attributable to a nonet'76.

161

4: Organic Radicals in Solids References

21. 22.

T. J. Kemp in 'Electron Spin Resonance', ed. P. B. Ayscough (Specialist Periodical Reports), The Royal Society of Chemistry, London, 1983, Vol. 8, p.214 Ya. I. Azhipa, 'Metodiku-Biologicheskie Asperkty Primeneniya Metoda Elektronnogo Paramagnitnogo Resonansa', Nauka, MOSCOW, 1983 Yu. N. Molin, 'Spin Polarisation and Magnetic Effects in Radical Reactions', Elsevier, New York, 1983 A . Carrington, A . Hudson and A , D. McLachlan, 'Introduction to Magnetic Resonance', 2nd edition, Chapman and Hall, New York, 1983 J. R. Wasson, A n a l . Chem., 1984, 56, 129R 0. Ya. Grinberg, A. A. Dubinskii and Ya. S. Lebedev, Russ. Chern. Rev., 1983, 2, 850 Yu. D. Tsvetkov, Russ. Chem. Rev., 1983, 52, 866 T.-S. Lin, Chem. Rev., 1984, 84, 1 R. Bramley and S. J. Strach, Chem. Rev., 1983, 83, 49 J. W. Wells, Map. Reson. Rev., 1983, S , 117 F. Williams and E. D. Sprague, Acc. Chem. Res., 1982, 15,408 T. Shida, E. Haselbach and T. Bally, Acc. Chem. Res., 1984, 17,180 L. S. Lyubchenko, Yu. A . Lyul'kin and S. G. Lakeev, ~ S S J. . Phys. Chem., 1983, 57, 1542 P. H. Rieger, J . Magn. Reson., 1982, 50, 485 J. c. Evans and P. H. Morgan, J . Magn. Reson., 1983, 52, 529 S. Brumby, J . Map. Reson., 1980, 40,397 S. S. Eaton, K. M. More, B. M. Sawant and G. R. Eaton, J . Am. Chern. Soc., 1983, 105,6560 J. R . Byberg, N. Bjerre, A. Lund and P. 0. Samskog, J . Chem. Phys., 1983, 3, 5413 J. R. Morton and K. F. Preston, J . Magn. Reson., 1983, 52, 457 S. Fairhurst, R . S. Pilkington and L. H. Sutcliffe, J . Chem. SOC., Faraday Trans. I , 1983, 2, 439 Y. I~D,J . Chem. Phys., 1983, 2, 2650 P. J. O'Malley and G. T. Babcock, J . Am. Chem. SOC., 1984, 106,

23. 24.

817 Yu. N. Molin and 0. A . Anisimov, Radiat. Phys. Chem., 1983, 21, 77 E. Hankiewicz and J. Kroh, Radiochem. Radioanal. Lett., 1982, 55,

25. 26.

87 J. Hllttermann, UZtramicroscopy, 1982, M. Akaboshi, M. Noda, K. Kawai, H. Ma?+

1.

2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Origins Life, 1982, 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

12, 395

10,7 and K. Kawamoto,

T. J. Kemp in 'Electron Spin Resonance', ed. P. B. Ayscough (Specialist Periodical Reports) The Royal Society of Chemistry, London, 1982, Vol. 7, p.252 Lon B. Knight, Jr., J. Steadman, D. Feller and E. R. Davidson, J . Am. Chern. Soc.,1984, 106,3700 M. C. R. Symons, T. Chen and C. Glidewell, J . Chem. SOC., Chem. C o m n . , 1983, 326 K. Toriyama, K. Nunome and M. Iwasaki, J . Phys. Chern., 1981, 85, 2149 M. Iwasaki, K. Toriyama and K. Nunome, J . Am. Chem. SOC., 1981, 103, 3591 K. Toriyama, K. Nunome and M. Iwasaki, J . Chem. Phys.,1982, 77, 5891 K. Nunome, K. Toriyama and M. Iwasaki, J . Chem. Phys., 1983, 2, 2499 M. Iwasaki, K. TOriyama and K. Nunome, J . Chem. SOC., Chem. Commun., 1983, 202 K. Ushida, T. Shida, I. Iwasaki, K. Toriyama and K. Nunome, J . Am. Chern. SOC., 1983, 105, 5496 K. Toriyama, K. Nunome and M. Iwasaki, J . Chern. SOC., Chem. C o m n . , 1984, 143

162 37* 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72.

Electron Spin Resonance K. Toriyama, K. Nunome and M. Iwasaki, J . Chem. Soc.,

Chem. C o m n . ,

1983I 1346 K. Ohta, H. Nakatsuji, H. Kubodera and T. Shida, Chem. Phys., 1983, 76, - 271 M. Tabata and A . Lund, Chem. Phys., 1983, 75,379 M. Iwasaki, K. Toriyama and K. Nunome, Radiat. Phys. Chem., 1983, 21, 147 T. Shida and Y. Takemura, Radiat. Phys. Chem., 1983, 21, 157 M. C. R. Symons and M. M. Maguire, J . Chem. SOC., Faraday Trans. I , 1982, 2, 2547 A . Hasegawa, T. Wakabayashi, M. Hayashi and M. C. R. Symons, J . Chem. SOC., Faraday Trans. I , 1983, 2, 941 B. W. Walther and F. Williams, J . Chem. Phys., 1983, 79, 3167 S. P. Maj, M. C. R. Symons and P. M. R. Trousson, J . Chem. Soc., Chem. C o m n . , 1984, 561 M. Brustolon, A . L . Maniero and C. Corvaja, J . Magn. Reson., 1983, 51, 73 R. Knopp and A . Milller, Moz. Phys., 1983, 50, 369 V. A . Onischuk and S. Z. Shul'ga, Proc. Tihany. Symp. Radiat. Chem., 1982, (Publ. 1983), 5, 1037 W. A . Bernhard, T. Ly Horning and K. R. Mercer, J . Phys. Chem., 1984, 88, 1317 L . B. Knight, Jr. and J. Steadman, J . Chem. Phys., 1984, 80, 1018 L. D. Snow, J. T. Wang and F. Williams, Chem. Phys. L e t t . , 1983, 100, 193 M. C. R. Symons and B. W. Wren, Tetrahedron L e t t . , 1983, 24, 2315 M. C. R . Symons and B. W. Wren, J . Chem. Soc., Perkin Trans. 2, 1984, 511 T. Clark, J . Chem. Soc., Chem. C o m n . , 1984, 666 W. J. Bouma, D. Poppinger, S. Saebg4, J. K. Macleod and L . Radom, Chem. Phys. L e t t . , 1984, 104,198 L. D. Snow and F. Williams, Chem. Phys. Lett.,1983, 100, 198 M. C. R. Symons and P. J. Boon, Chem. Phys. L e t t . , 1982, 89, 516 M. C. R. Symons and P . J. Boon, Chem. Phys. L e t t . , 1983, 100,203 G. W. Eastland, D. N. R. Rao, J. Rideout, M. C . R. Symons and A . Hasegawa, J . Chem. Research (S), 1983, 258 D. Becker, K. Plante and M. D. Sevilla, J . Phys. Chem., 1983, 87, 1648 M. Iwasaki, H. Muto, K. Toriyama and K. Nunome, Chem. Phys. L e t t . , 1984, 105, 586 L. D. Snow and F . Williams, Faraday Discuss. Chem. Soc., 1984, 78, paper 78/2 H. Muto, K. Toriyama, K. Nunome and I. Iwasaki, Chem. Phys. L e t t . , 1984, 105,592 L . D. Snow and F. Williams, J . Chem. Soc., Chem. C o m n . , 1983, 1090 H. Chandra and M. C. R. Symons, J . Chem. Soc., Chem. Comn.,1983, 29 H. Chandra and M. C. R. Symons, J . Chem. Soc., Chem. C o m n . , 1984, 1044 B. C. Gilbert, P. A . Kelsall, M. D. Sexton, G. D. G. McConnachie and M. C. R. Symons, J . Chem. SOC., Perkin Trans. 2, 1984, 629 M. B. Khusidman, V. P. Vyatkin, N. V. Grigor'eva, and S. L . Dobychin, J . Appl. Chem. U.S.S.R. (Engl. TransZ.), 1983, 56, 214 M. C. R. Symons, J . Chem. SOC., Chem. C o m n . , 1982, 869 B. W. Walther, F. Williams, W. Lau and J. K. Kochi, OrganometaZZics, 1983, 2, 688 H. C. Box and E. E. Budzinski, J . Chem. Phys., 1983, 79, 4142 M. Tabak, A . Alonso and 0. R. Nascimento, J . Chem. Phys., 1983, 79, 1176 -

163

4: Organic Radicals in Solids 73. 74. 75. 76. 77.

H. S h i e l d s , T. d e Lyon, F. Chiu and P . J. Hamrick, Jr. , J . Chem. Phys., 1982, 77, 4333 M. K i r a , H . Nakazawa and H . S a k u r a i , J . Am. Chem. Soc., 1983,

105,

6983 K. Toriyama, K. Nunome and M . Iwasaki, Chem. Phys. Lett.,1984, 86 M. Tabata and A. Lund, 2. Naturforsch., A : Phys., Phys. Chem., 687 Kosmophys., 1983, T. S h i d a , Y . Egawa, H . Kubodera and T . Kato, J. Chem. Phys., 1980, 5963 J. Rideout and M. C. R. Symons, J.Chem. Research (S), 1984, 268 K. Ushida and T. Shida, Chem. Phys. L e t t . , 1984, 200 M. Iwasaki, K. Toriyama and K . Nunome, J . Chem. soc., Chem. c o r n . , 1983, 320 P . M a j . A . Hasegawa and M. C . R . Symons, J . Chem. soc., Faraday Trans. I , 1983, 2, 1931 M. Tabata and A . Lund, 2 . Naturforsch., A: Phys., Phys. Chem., 428 Kosmophys., 1983, Q. B. Broxterman and H. Hogeveen, Tetrahedron L e t t . , 1983, 639 D . N . R. Rao and M. C. R. Symons, J . Chem. soc., Perkin Trans. 2, 1983, 135 M. S h i o t a n i , Y . Nagata, M . T a s a k i , J. Sohma and T . S h i d a , J . Phys. Chem., 1983, 3, 1170 M. S h i o t a n i , H. Kawazoe and J . Sohma, J . Phys. Chem., 1984, 2220 A . Hasegawa and M. C . R. Symons, J . Chem. SOC., Faraday Trans. I , 1983, 2, 1565 B . W . Walther, F. Williams and D . M . Lemal, J . Am. Chem. SOC., 1984, 106, 548 M . T. J o n e s and E . d e Boer, M o ~ . Phys., 1982, 47, 487 P.-O. Samskog and L. D. K i s p e r t , J . Chem. Phys., 1983, 78, 2129 P.-O. Samskog, L . D. K i s p e r t and D. P. Murray, Chem. Phys. L e t t . , 1983, 106 P.-0. Samskog and L. D. K i s p e r t , J . Phys. Chem., 1984, 1385 M. C. R. Symons and P. M. R. Trousson, Radiat. Phys. Chem., 1984, 231 127 G . Maier, H . P . Reisenauer, B . Rohde and K . Dehnicke, Chem. Ber., 1983, 732 P . Rangunathan and S. K. Sur,J. Phys. Chem., 1983, 87, 3383 J. H . B . Chenier, J. A . Howard, B. M i l e and R . S u t c l i f f e , J . Am. Chem. SOC., 1983, 788 J. A . Howard, R . S u t c l i f f e and B . M i l e , J . Phys. Chem., 1984, 171 T. Chen, F. G r a t and H s . H . Gilnthard, Chem. Phys., 1983, 165 R . Krzyminiewski, A . M. Hafez, J . P i e t r z a k and A . Szyczewski, J . Magn. Reson., 1983, 51, 308 K . Nunome, K . Toriyama and M. Iwasaki, Chem. Phys. L e t t . , 1984, 414 H . Muto, K. Toriyama, K. Nunome and M. Iwasaki, Radiat. Phys. Chem., 1982, 201 K . Toriyama, M. Iwasaki and M. Fukaya, J . Chem. SOC., Chem. C o m n . , 1982, 1293 M. Fukaya, K. Toriyama, M. Iwasaki, T . Ichikawa and N . Ohta, Radiat. Phys. Chem., 1983, 3, 463 Yu. V. Razskazovsky and M. Y a . Mel'nikov, Radiochem. Radioanal. L e t t . , 1982, 54, 339 D. Suryanarayana, W . C h a m u l l t r a t and L . Kevan, J . Phys. Chem., 1982, 4822 D. Greatorex, R. J . H i l l , T. J . Kemp and T. J. S t o n e , J . Chem. Soc., Faraday Trans. I , 1972, 68, 2059 T. Harazono, S. S a t 0 and H . Fukutomi, B u l l . Chem. Soc. Jpn., 1984, 57, 768 P.-0. Samskog and L. D . K i s p e r t , J . Chem. Phys., 1983, 635 P.-0. Samskog, L . D . K i s p e r t and A . Lund, J . Chem. Phys., 1983, 78, 5790

107,

73,

78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91.

108,

*,

24,

88,

-

101,

92. 93. 94. 95. 96.

88,

116,

105,

97. 98. 99. 100. 101.

75,

105,

19,

102. 103. 104. 105.

86,

106. 107. 108. 109.

88,

2,

-

Electron Spin Resonance

164 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151.

P. Madden and W. A. Bernhard, J. P h y s . Chem., 1 9 8 2 , 86, 4033 Krieger and J. Htittermann, I n t . J. R a d i a t . B i o l . , 1983, 44, 207 Mach and K . Vacek, Coil. C z e c h . Chem. C o m n . , 1 9 8 3 , 48, 203 St8sser and U. Prt)sch, 2. Chem.,1983, 2 3 , 382 C. R. Symons and P. M. R. Trousson, R a z a t . P h y s . C h e m . , 1984,23,127 A . T. Tegowski and D. W. Pratt, J . Am. Chem. soc., 1 9 8 4 , 6 4 G. B. Sergeev, A . V. Pukhovskii and V. V. Smirnov, RUSS. J. P h y s . Chem., 1983, 57, 589 M. Fukaya, B. Eda and M. Iwasaki, R a d i a t . Phys. Chem.,1983, 375 S. Emori, J. P. Colpa and J. K. 5. Wan, Chem. Phys. L e t t . , 1 9 8 3 , 9 8 , 142

K. J. K. R. M.

106,

2,

-

G. G, Lazarev, Ya. S. Lebedev, A . L. Prokof'ev and R. R. Rakhimov, Chem. Phys. L e t t . , 1 9 8 3 , 95, 262 N. M. Shishlov, R. A. Sadykov, V. A . Mazunov and A . A . Panasenko, Khim. V y s . Energ., 1 9 8 2 , 16,523 N. M. Shishlov, V. N. Korobeinikova, V. A . Mazunov, A. A . Panasenko and R. A . Sadykov, Khim. V y s . Energ., 1 9 8 3 , 17,179 w. R. Bowman and M. C. R. Symons, J. Chem. R e s . ( S ) , 1 9 8 4 , 162 M. C. R. Symons, S. P. Maj, D. E. Pratt and L. Portwocd, J. Chem. SOC., P e r k i n T r a n s . 2, 1 9 8 3 , 191 T. K. Polovitsyna and V. S. Gurman, Khim. V y s . Energ.,1982, 5,6 6 Y. Takegami, S . Imamura, F. Masuda and Y. Watanabe, B u l l . Chem. SOC. J p n . , 1969, 2,1876 v. v. Akhlynin, L. I. Korshunov and V. D. Shatrov, Khim. v y s . Energ., 1 9 8 2 , 16,530 c. L. .%villar S. Swarts and M. n. Sevilla, J. Am. O i l . Chem. SOC., 1 9 8 3 , 3, 950 V. M. Syutkin and V. A . Tolkatchev, R a d i a t . Phys. Chem., 1 9 8 2 , 20, 28 1 M. Mahdavi and M. Dole, J. P h y s . Chem., 1983, 87, 5430 Y. Lion, G. Denis, M. M. Mossoba and P. R i e s z , I n t . J. R a d i a t . B i o Z . , 1983, 43, 7 1 H. Theisen and E. Sagstren, J. Chem. Phys., 1 9 8 3 , 78, 2254 D. N. R. Rao, M. C . R. Symons and J. M. Stephenson,J. Chem. soc., P e r k i n T r a n s . 2, 1 9 8 3 , 727 M. D. Sevilla, C. L. Sevilla and s. Swarts, R a d i a t . Phys. Chem., 1 9 8 2 , 20, 141 F. 2. Khalaf and I. Miyagawa, J. Chem. Phys., 1983, 78, 5886 J. Hflttermann, U l t r a m i c r o s c o p y , 1982, 10,25 R. A. Spelletta and W. A . Bernhard, R a d i a t . Res., 1 9 8 2 , 89, 11 Z.-Y. Zhang, M. Kuwubara and G. Yoshi, R a d i a t . R e s . , 1 9 8 3 , 9 3 , 213 M. D. Sevilla and M. McGlashen, J. P h y s . Chem., 1 9 8 3 , 87, 6 3 4 M. D. Sevilla, S. Swarts, H. Riederer and J. HUttermann, J. P h y s . Chem., 1984, 88, 1601 W. H. Nelson and D. M. Close,J. Chem. P h y s . , 1 9 8 3 , 2, 3240 H. Kim and C. Alexander, Jr., J. Chem. Phys., 1 9 8 2 , 77,4879 E. Sagstuen and D. M. Close, J. Chem. Phys., 1 9 8 3 , 2, 117 L. Kar and W. A . Bernhard, R a d i a t . R e s . , 1 9 8 3 , 93, 232 D. M. Close and E. Sagstuen, J. Chem. Phys., 1 9 8 3 , 2, 5292 2. P. Gribova, V. M. Zhil'tsova, 0. A . Azizova and K. E. Kruglyakova, R a d i o b i o l o g i y a , 1 9 8 2 , 22, 7 3 9 W. S. Brey, A.C.S. Symp. S e r . , 1 9 8 3 , 222, 375 B. D. Flockhart, I. M. Sesay and R. C. Pink, J . Chem. SOC., Fmaday T r a n s . I , 1 9 8 3 , 2,1009 M. B. McBride, C l u y s and C l a y Minerals, 1 9 8 2 , 30, 438 A . E. Lemire and H. D. Gesser, J . Chem. SOC., Chem. C o r n . , 1 9 8 3 , 1175 D. R. Griffiths, G. V. Robins, N. J. Seeley, H. Chandra, D. A . C. McNeil and M. C. R. Symons, Nature ( L o n d o n ) , 1 9 8 2 , 300, 435 E. Meirovitch, J. Phys. Chem., 1 9 8 2 , 86, 5237

165

4: Organic Radicals in Solids 152. 153. 154. 155. 156.

H . Dahmane, B . M i l e , H . M o r r i s , J . A . Howard and R . S u t c l i f f e ,

J . Chem. Soc., Chem. C o m n . , 1983, 1068 A . V. Kucherov and A . A . S l i n k i n , K i n e t . KataZ., S. Shih, J . Cataz., 1983, 2, 390 F. Wudl, Acc. Chem. Res., 1984, 17,227

158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172.

175. 176.

1172

27,

61

K . K u m e , K. Mizuno, K . Mizoguchi, K . Nomura, J . Tanaka, M. Tanaka, H . Fujimoto and H . Shirakawa, J . Map. Magn. Mater., 1983, 31-34,

1151 B. R. Weinberger, F. Ehrenfreund, A . Pron, A . J . Heeger and A . G. MacDiarmid, J . Chem. Phys., 1980, 72, 4749 J . Tang, C. P. L i n , M. K. Bowman, J . R . N o r r i s , J . I s o y a and H . Shirakawa, Phys. Rev. Ser. B, 1983, 28, 2845 S. Kuroda and H . Shirakawa, S o l i d S t a t e C o m n . , 1982, 43, 591 S. Kuroda, M. Tokwnoto, N. K i n o s h i t a , T. I s h i g u r o and H . Shirakawa, J . Map. Magn. Mater., 1983, 31-34, 1149 S. Kuroda, M. Tolumoto, N . K i n o s h i t a and H . Shirakawa, J . Phys. SOC. Jpn., 1982, 51, 693 T. Thomann, L. R. Dalton, Y. Tomkiewicz, N . S . S h i r e n , and T. C . C l a r k e , Phys. Rev. L e t t . , 1983, 50, 533 M. Mehring, W. M u l l e r and G . Wegner, S o l i d S t a t e C o m n . , 1983, 45, 1075 J. D. Flood, E . Ehrenfreund, A . J . Heeger and A . G . MacDiarmid, S o l i d S t a t e C o m n . , 1982, 44, 1055 W . R. Jackson, I . D . Rae, M. G. Wong, M. F. Semmelhack and J . N . G a r c i a , J . Chem. SOC., Chem. C o m n . , 1982, 1359 L. D. K i s p e r t , L. A . F i l e s , J . E . FrOmer, L. W . S h a c k l e t t e and R . R . Chance, J . Chem. Phys., 1983, 78, 4858 C. Bubeck, T. H . Nguyen Xuan, H . S i x l , B. Tieke and D . B l o o r ,

Ber. Bunsenges. Phys. Chem., 1983, 87, 1149 L. For6 and F. Benneu, S o l i d S t a t e C o m n . , 1982, 44, 623 P. M. Richards and M. B. Solamon, Phys. Rev. Ser. B, 1974,

9,

32

H6ptner , M . Mehring, J . V . von SchUtz, H . C . Wolf , B. S. Morra, V. Enklmann and G . Wegner, Chem. Phys.,1982, 253 E. Muller, J . V . von Schfltz and H . C . Wolf, Mol. C r y s t . L i g . C r y s t . , W.

1983,

173. 174.

23,

M. N e c h t s t e i n , F. Devrew, F. Genoud, M . Gugliemi and K . Holczer,

Phys. Rev. Ser. B, 1983, 157.

1982,

2,

93,

407

R . Murugesan and E . de Boer, Chem. Phys. L e t t . , 1983, 95, 301 W. HBptner, M. Mehring, J . V . von SchUtz, H. C. Wolf, B. S . Morra, V . Enklmann and G. Wegrev, Mol. Cryst. Lig. C r y s t . , 1983, 93, 395 J. Schmidt, Springer Ser. SoZid-State Scz., 1983, 49, 56 Y. T e k i , T. Takui, K. Itoh, H . Iwamura and K . Kobayashi, J . Am. Chem. SOC., 1983, 3722

105,

5 Organic Radicals in Solution BY B. J. TABNER

1 Introductim

The r a n g e of l i t e r a t u r e c o v e r e d i n t h i s C h a p t e r i s t h e same a s t h a t f o r t h e c o r r e s p o n d i n g C h a p t e r i n Volume 8.’ I have found i t c o n v e n i e n t t o r e t a i n t h e same g e n e r a l review a r e a s f o r t h i s r e p o r t a s l a s t time d e s p i t e t h e f a c t t h a t t h e r e a r e v e r y few r e p o r t s o f s u l p h u r - c e n t r e d r a d i c a l s on t h i s o c c a s i o n . The c o n t i n u i n g w i d e use o f e . s . r . s p e c t r o s c o p y i s u n d o u b t e d l y due t o t h e important r o l e of r a d i c a l s a s i n t e r m e d i a t e s i n r e a c t i o n s a n a t h e w e a l t h o f s t r u c t u r a l and k i n e t i c i n f o r m a t i o n a b o u t them t h a t becomes a v a i l a b l e w i t h t h e a i d o f t h e t e c h n i q u e . There has b e e n a s i g n i f i c a n t c h a n g e i n i n t e r e s t i n two a r e a s . F i r s t , t h e r e h a s been a d e c l i n e i n t h e number of p a p e r s d e a l i n g w i t h s p e c t r o s c o p i c a s p e c t s of n i t r o x i d e s , a l t h o u g h t h e s p i n - t r a p p i n g t e c h n i q u e c o n t i n u e s t o be w i d e l y u s e d a s o n e means o f s t u d y i n g s h o r t - l i v e d radicals. S e c o n d l y , d u r i n g t h e l a s t 1 8 months t h e r e h a s b e e n a t r e m e n d o u s i n t e r e s t i n r a d i c a l - c a t i o n s g e n e r a t e d by y - i r r a d i a t i o n i n F r e o n and r e l a t e d m a t r i c e s . A p a r t from t h e s e two a r e a s t h e number o f p a p e r s a v a i l a b l e f o r r e p o r t r e m a i n s much t h e same a s f o r Volume 8 . A number of r e v i e w a r t i c l e s h a v e a p p e a r e d c o v e r i n g a r e a s r e l e v a n t t o t h i s C h a p t e r which t h e i n t e r e s t e d r e a d e r m i g h t f i n d u s e f u l . *-5 Computers h a v e been w i d e l y u s e d f o r many y e a r s a s a n a i d t o i n t e r p r e t a t i o n , d t h e s u c c e s s f u l simulation of experimental s p e c t r a , and a u s e f u l review h a s now a p p e a r e d which s u m m a r i s e s t h e r e c e n t l i t e r a t u r e r e l e v a n t t o t h e s i m u l a t i o n of e.s. r. ~ p e c t r a . ~ An improved method f o r s p e c t r u m i n t e r p r e t a t i o n , b a s e d on c o r r e l a t i o n m e t h o d s , h a s been d e ~ c r i b e d ,a ~s h a s d e c o n v o l u t i o n a n a l y s i s u s i n g t h e F o u r i e r method.8 A number o f p a p e r s h a v e a p p e a r e d d e s c r i b i n g t h e i n t e r f a c i n g of v a r i o u s commercially a v a i l a b l e m i c r o p r o c e s s o r system^^"^ one o f which employs t h e r e a d i l y a v a i l a b l e Apple I1 P l u s m i c r o c ~ m p u t e r . ’ ~ 166

[For references see p . 215

5: Organic Radicals in Solution

167

Two e x c e l l e n t r e v i e w s d e s e r v e m e n t i o n a t t h i s s t a g e . O n e , by Weissman, d e a l s w i t h t h o s e v a r i o u s m e t h o d s i n which time e v o l u t i o n of t r a n s i e n t magnetization is observed. T h i s complements n i c e l y t h e review, by P o o l e and F a r a c h , d e a l i n g w i t h s h o r t time domain and d o u b l e r e s o n a n c e t e c h n i q u e s . T h i s l a t t e r review i n c l u d e s d e s c r i p t i o n s o f t h e s p e c t r o m e t e r systems r e q u i r e d a n d c o v e r s timer e s o l v e d e . s . r . , s a t u r a t i o n t r a n s f e r , ENDOR and T R I P L E r e s o n a n c e , a n d c h e m i c a l l y i n d u c e d dynamic e l e c t r o n p o l a r i z a t i o n . This l a s t t e c h n i q u e ( C I D E P ) i s o n e which h a s r e c e i v e d some a t t e n t i o n r e c e n t l y . McLauchlan h a s d e s c r i b e d a method f o r s e p a r a t i n g o v e r l a p p i n g e . s . r . s p e c t r a of f r e e r a d i c a l s o b s e r v e d d u r i n g f l a s h i r r a d i a t i o n . 1 6 The method i n v o l v e s a s t u d y of t h e d i f f e r e n t t e m p o r a l v a r i a t i o n s o f t h e two r a d i c a l s b u t t h e a u t h o r s warn t h a t i t i s p o s s i b l e t o m i s i n t e r p r e t results i f t h e f u l l temporal b e h a v i o u r of t h e system i s n o t i n v e s t i g a t e d . A study of t h e r a d i c a l s d e r i v e d from d i a z a n a p h t h a l e n e s by f l a s h p h o t o l y s i s e . s . r . i l l u s t r a t e s t h e p o t e n t i a l of C I D E P i n u n d e r s t a n d i n g t h e s h o r t term b e h a v i o u r of r a d i ~ a 1 s . l ~T h e r e i s a l s o a r e p o r t t h a t C I D E P e n h a n c e d ENDOR h a s p o t e n t i a l l y g r e a t e r s e n s i t i v i t y t h a n convent i o n a l ENDOR a s t h e c h a n g e s i n s p i n l e v e l p o p u l a t i o n s i n t h e f o r m e r c a n be much g r e a t e r . 1 8 There a r e s e v e r a l o t h e r r e p o r t s o f C I D E P which t h e i n t e r e s t e d r e a d e r might l i k e t o p u r ~ u e . ' ~ ' ~ ' F i n a l l y t h e r e i s a n i n t e r e s t i n g r e p o r t by Anisimov o f a n o p t i c a l l y d e t e c t e d e . s . r . s t u d y o f r a d i c a l - i o n p a i r s formed i n s o l u t i o n u n d e r i o n i s i n g i r r a d i a t i o n . 2 2 The method i s h i g h l y s e n s i t i v e and a l l o w s t h e s t u d y of r e a c t i o n s on t h e n s time s c a l e .

'

7 Carbon-centred Radicals

-.-

3.1 I n t h i s , t h e f i r s t p a r t of my r e p o r t c o v e r i n g a l k y l r a d i c a l s , I s h a l l f i r s t d i s c u s s t h o s e p a p e r s con-

c e r n e d w i t h ' s i m p l e ' a l k y l r a d i c a l s and t h e n t h o s e c o n c e r n e d w i t h 'cyclic' alkyl radicals. As i n p r e v i o u s r e p o r t s t h e p a p e r s r e p o r t i n g r e c e n t r e s u l t s i n t h i s a r e a cover a wide range of i n t e r e s t s i n c l u d i n g t h e d e t e r m i n a t i o n of t h e s t r u c t u r e of t h e s e r a d i c a l s a s w e l l a s t h e mechanisms and k i n e t i c s o f t h e i r r e a c t i o n s . The c o n f o r m a t i o n s o f a r a n g e of a l k y l r a d i c a l s h a v e been i n vestigated. F o r e x a m p l e , a number of p r i m a r y r a d i c a l s XCH2CH2' ( X = CMe3, CH2CMe3, and CPh3) h a v e been p r e p a r e d by p h o t o l y s i s o f t h e appropriate diacyl peroxide i n a toluene/cyclopropane mixture.23 The e . s . r . s p e c t r u m o f t h e CMe3CH2CH2* r a d i c a l shows no l i n e w i d t h

Electron Spin Resonance

168

v a r i a t i o n a n d h a s a a(B-HI v a l u e s m a l l e r t h a n t h a t o f t h e e t h y l radical exhibits linewidth r a d i c a l . I n c o n t r a s t t h e Me3CCH2CH2CH2' a l t e r n a t i o n s i m i l a r t o t h a t observed f o r t h e n-butyl r a d i c a l . The Ph3CCH2CH2' r a d i c a l h a s s i g n i f i c a n t l y b r o a d e n e d l i n e s w i t h M B = 0 . It a p p e a r s t h a t i n t h e Me3CCH2CH2' r a d i c a l t h e l a r g e s i z e of t h e CMe3 g r o u p r e s u l t s i n s t a b i l i z a t i o n o f c o n f o r m a t i o n s i n which t h i s g r o u p i s e c l i p s e d by t h e p - o r b i t a l o f t h e u n p a i r e d e l e c t r o n , b u t t h e unambiguous a s s i g n m e n t o f t h e c o n f o r m a t i o n a l p r e f e r e n c e s i s n o t y e t p o s s i b l e f o r t h e Me3CCH2CH2CH2' and Ph3CCH2CH2' r a d i c a l s . E t h y l r a d i c a l s s u b s t i t u t e d with groups c o n t a i n i n g C , S i , and S a r e s t e r i c a l l y h i n d e r e d . 2 4 The Et3SiCH2CHX ( X = SiMe3 o r C02H) r a d i c a l s h a v e n o n - e q u i v a l e n t B-proton s p l i t t i n g c o n s t a n t s s u g g e s t i n g a n e a r l y e c l i p s e d c o n f o r m a t i o n i n which t h e two $-CH bonds o f t h e CH2 g r o u p h a v e d i f f e r e n t a n g l e s w i t h r e s p e c t t o t h e d i r e c t i o n of t h e 2pz o r b i t a l c o n t a i n i n g t h e unpaired e l e c t r o n . S e l e c t i v e l i n e - b r o a d e n i n g i n t h e s p e c t r a o f t h e s e r a d i c a l s below 233 K i n d i c a t e s r e s t r i c t e d r o t a t i o n a b o u t t h e Ca-C bond. The B f a v o u r e d a d d i t i o n o f t h e E t 3 S i ' ( o r MeS') r a d i c a l t o CH3CH=CHC02H ( g i v i n g o n l y t h e CH3CH(X)tHC02H r a d i c a l ) c a n be a t t r i b u t e d t o t h e i n f l u e n c e o f t h e e l e c t r o n e g a t i v e C02H s u b s t i t u e n t a t t h e a-position. The s p e c t r u m o f t h e CH(SMe)2t(SMe)C02H r a d i c a l c a n be i n t e r p r e t e d i n terms o f a c o n f o r m a t i o n i n which t h e B-proton i s c l o s e t o t h e n o d a l p l a n e , and t h e two SMe g r o u p s i n t h e B - p o s i t i o n are close t o the g-orbital. O f r e l a t e d i n t e r e s t i s t h e 3-methyl3-phenylbut-1-yl r a d i c a l ( 'CH2CH2CMe2Ph) .25 A t 156 K t h i s e x i s t s i n b o t h a ( p r e d o m i n a n t ) & u c h e c o n f o r m a t i o n and a trans c o n f o r m a t i o n , i n d i c a t i n g t h a t r o t a t i o n a b o u t t h e C -C bond i s s l o w on B Y t h e e.s.r. timescale a t t h i s temperature. The o b s e r v e d s p l i t t i n g c o n s t a n t s i n t h e d i s u b s t i t u t e d e t h y l r a d i c a l s 'CH2CHR1R2 [ R 1 = C02Et, R2 = C02Et o r C ( O ) S E t l , p r e p a r e d by hydrogen-atom a b s t r a c t i o n (by ButO'), i n d i c a t e t h a t they t o o exist i n p r e f e r r e d conformat i on s .26 The i n t e r p r e t a t i o n of t h e h y p e r f i n e c o u p l i n g t o 14N, 19F, a n d y-13C i n t h e s p e c t r a of ( C F 3 S ) 3 C ' , (CF3SI2CH, and (CF3S)tH2 indicates t h a t these radicals are planar with a nearly f u l l y stagg e r e d c ~ n f o r m a t i o n . ~The ~ well r e s o l v e d h y p e r f i n e s t r u c t u r e o b s e r v e d i n a s i m i l a r r a n g e o f r a d i c a l s XkH2, XCHMe, and XkMe2 ( X = R02C) i n d i c a t e s t h a t t h e s e r a d i c a l s a r e a l s o p l a n a r w i t h a The MeeHC02Me r a d i c a l e x i s t s substantial barrier t o rotation.28 a s two i s o m e r s , o n e ( a s s i g n e d t o t h e r a d i c a l w i t h t h e Me g r o u p LA% t o t h e c a r b o n y l o x y g e n ) h a s a(a-H) 2 . 0 4 8 ,

a(B-H)

2 . 4 7 6 , and a(&-HI

5: Organic Radicals in Solution

169

0.159 mT, and t h e o t h e r ( a s s i g n e d t o t h e trans s t r u c t u r e ) h a s a(a-H) 2 . 0 3 0 , a ( 6 - H ) 2 . 4 9 2 , and a ( y - H ) 0.128 mT. The Ea f o r A n o t h e r example o f e x c h a n g e between t h e i s o m e r s i s 4722 k J mol". t h e u s e o f l o n g - r a n g e h y p e r f i n e c o u p l i n g t o o b t a i n i n f o r m a t i o n on p r e f e r r e d c o n f o r m a t i o n s i s i l l u s t r a t e d by a s t u d y of c o u p l i n g t o t h e a c e t y l p r o t o n s i n some a - a c e t o x y r a d i c a l s . 2 9 The 6-CH p r o t o n c o u p l i n g s o b s e r v e d i n kR1R20C(0)Me r a d i c a l s p r o v e t o be s e n s i t i v e probes of t h e i r s t e r e o c h e m i s t r y , t h e v a r i a t i o n s i n a(6-H) a r i s i n g from c h a n g e s i n r a d i c a l c o n f o r m a t i o n . T h r e e p a p e r s r e p o r t h a l o g e n a t e d r a d i c a l s p r o d u c e d by Y-radiolysis. The e . s . r . s p e c t r u m o f t h e MeCF2' r a d i c a l , p r o d u c e d by r a d i o l y s i s o f MeCF2C1 i n CD30D a t 77 K , i n d i c a t e s h y p e r f i n e coupling t o only one of t h e methyl protons.30 The i n t e r n a l r o t a t i o n of t h e methyl group i s t h e r e f o r e g r e a t l y hindered and t h e r a d i c a l a p p e a r s t o h a v e p y r a m i d a l g e o m e t r y a t Ca. Another f l u o r i n a t e d r a d i c a l , (-CF2CF2)2CF', i s formed d u r i n g t h e r a d i o l y s i s o f p o l y t e t r a f l u o r o e t h e n e a t low t e m p e r a t ~ r e . ~ ' The a - , 6-, and Y - c o u p l i n g s t o 19F h a v e been i n t e r p r e t e d i n terms o f t h e b a s i c h e l i c a l c o n f o r m a t i o n o f t h e polymer. The r a d i o l y s i s o f C B r 4 i n C D 3 0 D l e a d s t o a n e . s . r . s p e c t r u m w i t h c o n t r i b u t i o n s from t h r e e s p e c i e s L e . , 'CBr3, D O D 2 C ' , and t h e CBr4 r a d i ~ a l - a n i o n . ~ ~ The p h o t o c h e m i c a l f o r m a t i o n o f r a d i c a l s f o r e . s . r . s t u d y continues t o a t t r a c t attention. The f o r m a t i o n o f a - h y d r o x y a l k y l r a d i c a l s by p h o t o l y s i s o f k e t o n e s h a s been t h e a c t i v e s u b j e c t o f r e s e a r c h in t h e p a s t . A f r e s h r e p o r t h a s now b e e n made o f t h e s e r a d i c a l s , p r e p a r e d by p h o t o l y s i s , i n a p r o t i c s o l v e n t s ( s u c h a s m e t h y l c y c l o h e x a n e ) .33 The h y p e r f i n e s p l i t t i n g c o n s t a n t s o f t h e 1-hydroxyethyl and 1-hydroxy-1-methylethyl r a d i c a l s have been m e a s u r e d o v e r a w i d e t e m p e r a t u r e r a n g e . The m a g n i t u d e o f t h e h y d r o x y l i c p r o t o n s p l i t t i n g c o n s t a n t i s i n f l u e n c e d by hydrogenb o n d i n g a t h i g h p r e c u r s o r c o n c e n t r a t i o n s and t h e v a l u e o f da/d3: f o r t h i s p r o t o n i s f o u n d t o b e much g r e a t e r t h a n f o r t h e a - and $-protons. U.V. i r r a d i a t i o n o f 2 - t - b u t y l p e r o x y e t h a n o l (Me3COOCH2CH20H) i n b e n z e n e g a v e a n e . s . r . s p e c t r u m w i t h a(H) 0.07 a n d a ( 2 H ) 1 . 7 4 mT.34 This analysis is not consistent with the e x p e c t e d r a d i c a l ( 1 , R = H I , b u t i n d i c a t e s t h a t t h e hydroxymethyl r a d i c a l ('CH20H) h a s b e e n formed by f r a g m e n t a t i o n o f i t . I n t h e c a s e o f ( 1 , R = H I f a s t i n t r a m o l e c u l a r e x c h a n g e would l e a d t o t h e same f r a g m e n t a t i o n p r o d u c t . However, when R = Me two f r a g m e n t a t i o n p r o d u c t s ( 2 , R = Me) a n d ( 3 ) a r e p o s s i b l e . R a d i c a l (3) h a s t h e same e . s . r . s p e c t r u m a s t h e r a d i c a l d e r i v e d from M ~ ~ C O O C H Z C H ~ O H ,

Electron Spin Resonance

170

b u t a s e c o n d s p e c t r u m w i t h a(H) 1 . 5 3 and a ( 3 H ) 2 . 2 3 mT i s now a l s o p r e s e n t and can be a s s i g n e d t o r a d i c a l ( 2 , R = Me).

p; ko H

\

H

R

H

H

R

T h e r e h a v e been s e v e r a l p r e v i o u s r e p o r t s of t h e c l e a v a g e o f c a r b o n - m e t a l bonds by b i m o l e c u l a r h o m o l y t i c s u b s t i t u t i o n . However a n e x a m p l e o f t h e p h o t o l y t i c u n i m o l e c u l a r c l e a v a g e o f s u c h a bond i n some o r g a n o m e r c u r y compounds, t o g i v e B - a l k o x y a l k y l r a d i c a l s , h a s now b e e n r e p o r t e d ( E q u a t i o n The v a l u e s o f a ( B - H I i n t h e e.s.r. s p e c t r a of t h e s e r a d i c a l s g i v e u s e f u l i n f o r m a t i o n concerning t h e i r p r e f e r r e d c o n f o r m a t i o n , which a p p e a r s t o b e d e t e r m i n e d by a b a l a n c e of h y p e r c o n j u g a t i v e a n d s t e r i c i n t e r a c t i o n s between t h e B - s u b s t i t u e n t s and t h e r a d i c a l c e n t r e . I n c o n t r a s t t o t h e s e s t u d i e s two r a d i c a l s a r e formed d u r i n g t h e p h o t o l y s i s o f t h e e s t e r , [(Me2(X)N)2, X = C02Mel, i n t h e p r e s e n c e of t r i m e t h y l aluminium i n toluene.36 One of t h e s e , h a s a(B-H) 2 . 1 0 8 , a(&-H)0 . 2 1 2 , a n d a(27Al) 0.049 mT and h a s been a s s i g n e d t o t h e c o o r d i n a t e d s p e c i e s ( 4 1 , a n d t h e o t h e r r a d i c a l w i t h a(B-H) 2 . 1 3 a n d a ( 6 - H ) 0.125 mT h a s I t h a s a l s o been r e p o r t e d t h a t been a s s i g n e d t o r a d i c a l ( 5 ) . y - r a d i o l y s i s o f Et2S0 a t 77 K p r o d u c e s a n a d d u c t between E t ' and S 0 ~ t . 3 7 Upon p h o t o l y s i s t h e a d d u c t t r a n s f o r m s t o g i v e t h e E t ' radical. [R1R2C(OR4)CHR312Hg

.

Me2C-C

@O-AlMe 'OMe

+ 3

+

2 R1R2C(OR4)iHR3

Me

t-

Hg

(1)

0

'

C@

OMe

The r e a c t i o n o f t h e h y d r o x y l r a d i c a l w i t h a wide v a r i e t y o f m o l e c u l e s h a s proved a p o p u l a r means o f p r e p a r i n g r a d i c a l s f o r many y e a r s and s e v e r a l f u r t h e r e x a m p l e s e m p l o y i n g t h i s method I n t h e f i r s t of t h e s e r e p o r t s t h e Ti(III)-H202 have been r e p o r t e d . c o u p l e h a s been u s e d i n c o n j u n c t i o n w i t h t h e s p i n - t r a p p i n g technique t o i n v e s t i g a t e t h e decarboxylation of methionine

5: Organic Radicals in Solution

171

-

[MeSCH2CH2CH(C02-)NH3+l.38 I n t h e pH r a n g e 1 . 5 9 two r a d i c a l s , tH2SCH2CH2CH( C02’) NH3+ and bH( SMe ) C H 2 C H ( C02-) N H 3 + a r e o b s e r v e d . However, i n t h e p r e s e n c e o f MeN02 a t pH 9 . 5 t h r e e s p e c i e s , MeNO2-, MeSCH2NO2-, and 02NCH2CH2N02- a r e d e t e c t e d and i n t h e pH r a n g e 2.5 4 . 5 , i n t h e p r e s e n c e of ButNO, t h e s p e c t r u m o f MeSCH2CH2CH(NH3+)N(b)But [a(N) 1 . 4 5 5 , a(@-N)0 . 2 9 0 , a(B-H) 0 . 1 4 5 , a n d a ( 2 , y - H ) 0 . 0 3 5 mT1 i s o b s e r v e d . T h e s e r e s u l t s h a v e been i n t e r p r e t e d i n terms o f a s e q u e n c e of r e a c t i o n s i n v o l v i n g o x i d a t i v e d e c a r b o x y l a t i o n when t h e pH i s r a i s e d t o a.2. I n a second study e d t a h a s b e e n added t o t h e T i ( I I I ) - H 2 0 2 c o u p l e t o o b t a i n i n f o r -

-

m a t i o n on t h e n a t u r e o f t h e r a d i c a l s formed from e t h a n o l . 3 9 The p r o p o r t i o n o f ‘CHMeOH t o ‘CH2CH20H v a r i e s w i t h [ e d t a l s u g g e s t i n g a r a d i c a l - t e r m i n a t i o n pathway a d d i t i o n a l t o t h e u s u a l b i m o l e c u l a r termination. It i s suggested t h a t t h e r e i s a one-electron oxidat i o n o f t h e ‘CHMeOH r a d i c a l by a T i ( 1 V ) - e d t a complex. Pulse r a d i o l y s i s h a s a l s o been u s e d t o g e n e r a t e t h e ‘OH r a d i c a l and h e n c e t o s t u d y i t s r e a c t i o n w i t h sodium v i n y l s ~ l p h o n a t e . ~The ~ spectrum o f t h e * O H a d d u c t , HOCH2tHS03-, La(2,B-H) 2 . 3 7 3 , a ( 6 - H ) 2 . 1 5 5 , and ( Y - H ) 0 . 0 3 4 mT1 i s accompanied by t h a t o f t h e H ’ a d d u c t , CH3tHS03-. However t h e h y p e r f i n e s p l i t t i n g from t h e y - p r o t o n i n t h e f o r m e r r a d i c a l d i s a p p e a r s a t pH 1 2 d u e t o r a p i d b a s e - c a t a l y s e d exchange. Sodium v i n y l s u l p h o n a t e a l s o p r o v e s a n e f f e c t i v e t r a p f o r C 0 2 - , SO3-, and SO4-. A p a r t i c u l a r r a d i c a l c a n be removed from any r e a c t i o n s y s t e m by a w i d e v a r i e t y o f p r o c e s s e s , s u c h a s i s o m e r i z a t i o n , d i m e r i z a t i o n , d i s p r o p o r t i o n a t i o n , r e a r r a n g e m e n t , and r i n g - o p e n i n g , many o f w h i c h l e n d t h e m s e l v e s t o k i n e t i c s t u d i e s . F u r t h e r e x a m p l e s o f many s u c h r e a c t i o n s h a v e been r e p o r t e d r e c e n t l y . The r a t e c o n s t a n t f o r t h e i s o m e r i z a t i o n o f t h e 2,2-dimethyl-3-buten-l-y1 r a d i c a l C(61, a ( 2 H ) 2 . 1 5 0 , a ( Y - H ) 0 . 2 8 5 , and a ( 6 - H ) 0 . 2 8 5 mT1 t o t h e 1 , l dimethyl-3-buten-1-y1 r a d i c a l “81, a ( d , ~ - H ) 2.304 a n d a ( 2 , p - H ) 1 . 7 2 2 mT1 h a s been d e t e r m i n e d from 1 2 8 t o 1 7 2 K Clogk = 1 2 . 5 ( 2 7 . 7 / 2 . 3 R T ) kJ m 0 1 ” I . ~ ~ I t i s p r o p o s e d t h a t t h e r e a c t i o n p r o c e e d s y h t h e 2,2-dimethylcyclopropylcarbinyl r a d i c a l ( 7 1 , b u t t h i s l a t t e r r a d i c a l i s n o t observed d u r i n g t h e c o u r s e of t h e reaction. This is an extremely rapid rearrangement r e a c t i o n

-

(k = 4 . 3 x l o 7 s - l a t 298 K). The k i n e t i c s of a n o t h e r i s o m e r i z a t i o n r e a c t i o n ( t h e PhC(SPh2)6H2 r a d i c a l t o t h e Ph6(SPh)C(SPh)H2 r a d i c a l ) h a v e a l s o been d e t e r m i n e d . 4 2 T h i s i s o m e r i z a t i o n , i n v o l v i n g i n t r a m o l e c u l a r m i g r a t i o n o f PhS, h a s k = 8 x 1 0 1 o e x p (-8800IRT) s-’. A k i n e t i c s t u d y h a s a l s o b e e n made o f r i n g - o p e n i n g

Electron Spin Resonance

172

i n c y c l o p r o p y l m e t h y l r a d i c a l s [ ( 9 ) , X = H o r Me, and Y H , OH, o r OSiR3 ( R = a l k y l ) ] t o g i v e b u t - 3 - e n y l r a d i c a l s The r a t e c o n s t a n t f o r t h i s r e a c t i o n i s i n t h e r a n g e 107-108 s - l a t 298 K ( v a r y i n g w i t h s u b s t i t u e n t ) w i t h Ea i n t h e r a n g e 25-31 k J mol”.

=x’ (9)

*x (10)

Two e x a m p l e s o f t h e d e t e r m i n a t i o n o f a b s o l u t e r a t e s o f d i m e r i z a t i o n of r a d i c a l s by e . s . r . s p e c t r o s c o p y h a v e b e e n r e p o r t e d . The f i r s t o f t h e s e i n v o l v e s some c a p t o - d a t i v e s u b s t i t u t e d r a d i c a l s (Me3COEHCN, Me3CSCHCN, a n d Me0EHCO2Me) .44 These r a d i c a l s a r e formed by hydrogen-atom a b s t r a c t i o n from t h e p a r e n t m o l e c u l e by ButO*. The v a l u e s of t h e r a t e c o n s t a n t s f o r d i m e r i z a t i o n o f t h e s e r a d i c a l s ( i n t h e r a n g e l o 8 - l o 9 1 mol” s - ’ ) s u p p o r t s d i f f u s i o n c o n t r o l . The e f f e c t o f d i f f u s i o n on t h e d i m e r i z a t i o n o f i - p r o p y l o l r a d i c a l s CMe2e(0H) 1 h a s a l s o b e e n s t u d i e d . 4 5 When t h e r e a c t i o n i s s t u d i e d i n 2 - b u t a n o l o r t e t r a h y d r o f u r a n some s o l v e n t d e r i v e d r a d i c a l s a r e a l s o d e t e c t e d and a t t e m p e r a t u r e s below 183 K t h e Me2(0H)C60 r a d i c a l Ca(6H) 0.096 mT1 i s a l s o o b s e r v e d . I n t h i s i n t e r e s t i n g s t u d y it i s found t h a t , i n a d d i t i o n t o t h e combination r e a c t i o n ( t o g i v e p i n a c o l ) , d i s p r o p o r t i o n a t i o n ( E q u a t i o n s 2 and 31, w h i c h i s more s i g n i f i c a n t a t l o w e r t e m p e r a t u r e s , a l s o o c c u r s a n d t h a t t h e r e l a t i v e importance of combination t o d i s p r o p o r t i o n a t i o n i s d e p e n d e n t upon s o l v e n t v i s c o s i t y . The a d d i t i o n o f E t 3 S i ’ r a d i c a l s t o v a r i o u s u n s a t u r a t e d compounds (R’ R 2 C = C R 3 R 4 ) h a s b e e n The r a t e o f t h e s e shown t o be a r e m a r k a b l y f a c i l e p r o c e s s . 4 6 r e a c t i o n s i s e n h a n c e d by e l e c t r o n - w i t h d r a w i n g g r o u p s l o c a t e d n e a r t h e new r a d i c a l c e n t r e , t h e i r p r e - e x p o n e n t i a l f a c t o r s i n d i c a t i n g a ‘ l o o s e ’ t r a n s i t i o n s t a t e . T h e s e r e a c t i o n s h a v e Ea v a l u e s o f o n l y a few kJ mo1-l. E.s.r. s p e c t r o s c o p y h a s a l s o b e e n employed t o s t u d y t h e k i n e t i c s of t h e r e a c t i o n of t h e methyl r a d i c a l w i t h methanol The k i n e t i c c u r v e s f o r t h i s o v e r t h e t e m p e r a t u r e r a n g e 20-105 K.47 r e a c t i o n a r e non-exponential and t h e r e a c t i o n a p p e a r s t o occur i n two s t a g e s t h e f i r s t o f which i s i n d e p e n d e n t o f t e m p e r a t u r e .

-

173

5: Organic Radicals in Solution

2 He2COH

Me2CHOH

+

CH2=C(Me)OH

(2)

Me2CHOH

+

(MeI2C=O

(3)

-

There is now a growing i n t e r e s t i n a p p l i c a t i o n s of t h e timer e s o l v e d t e c h n i q u e . Although c o n v e n t i o n a l e . s . r . r e v e a l s v a l u a b l e i n f o r m a t i o n on t h e s t r u c t u r e of r a d i c a l s p e c i e s , t h e time-resolved t e c h n i q u e g e n e r a l l y has poorer r e s o l u t i o n b u t i t i s , of c o u r s e , us l i f e t i m e ) i d e a l l y s u i t e d t o t h e s t u d y of t r a n s i e n t (h, s p e c i e s . Two r e p o r t s may be of p a r t i c u l a r i n t e r e s t t o some r e a d e r s a s they d e s c r i b e i n some d e t a i l t h e c o n s t r u c t i o n of time-resolved e.s. r. s p e c t r o m e t e r s t o g e t h e r w i t h some t y p i c a l a p p l i c a t i o n ~ . ~ ~ t ~ ~ The t e c h n i q u e h a s been employed t o study t h e fundamental r e a c t i o n of t h e a d d i t i o n o f t h e hydrogen atom t o v i n y l monomers.50 The r a t e c o n s t a n t f o r t h i s a d d i t i o n r e a c t i o n f a l l s i n t o two c l a s s e s dependi n g upon t h e n a t u r e of t h e v i n y l compound. For example, a c r y l i c a c i d , m a l e i c a c i d , and a c r y l o n i t r i l e can a l l be regarded a s having t h e r e a c t i v e polymethine s t r u c t u r e (k ca. 2 x lo9 1 mol'l s") whereas i n methyl m e t h a c r y l a t e and v i n y l a c e t a t e t h i s s t r u c t u r e i s d i s t u r b e d by t h e s u b s t i t u e n t (k m. 5 x lo8 1 mol" s"). The t e c h n i q u e h a s a l s o been used t o s t u d y recombination of some cyanos u b s t i t u t e d a l k y l r a d i c a l s i n t h e t e m p e r a t u r e range 223-323 K.51 The presence of a r a d i c a l s t a b i l i z i n g s u b s t i t u e n t , such a s CN, does n o t appear t o reduce t h e v a l u e of t h e r a t e c o n s t a n t f o r recombin a t i o n s i g n i f i c a n t l y below t h a t f o r t h e d i f f u s i o n - c o n t r o l l e d l i m i t . T h e v a l u e of La f o r recombination o f 'CH2CN is 18.9 kJ mol" and t h a t f o r Me2(CN)C' i s 19.9 kJ mol". The 'CH2CH=CHCN r a d i c a l can e x i s t a s t h e 9yn la(H-1 1.242, a(H-1 1.315, a(2-H) 0.378, a(3-HI 1.503, and a(N) 0.189 mT1 and t h e anti la(H-1 1.220, a(H-1 1.305, a(2-HI 0.379, a(3-H) 1.418, and a(N) 0.226 mT1 isomers f o r which E, f o r recombination has v a l u e s of 10.8 and 12.8 kJ mol" r e s p e c t i v e l y . The time-resolved study of t h e CH2CO2r a d i c a l h a s been b r i e f l y d e s c r i b e d . 5 2 J u s t a s with non-cyclic r a d i c a l s , common methods of f o r m a t i o n of c y c l i c a l k y l r a d i c a l s i n v o l v e a d d i t i o n and a b s t r a c t i o n r e a c t i o n s . T h i s s e c t i o n of t h e r e p o r t f e a t u r e s some f u r t h e r examples of t h e s e r e a c t i o n s . A b s t r a c t i o n of a hydrogen atom by B u t O * [formed by p h o t o l y s i s of ( B u ~ O ) ~p]r o v i d e s a s o u r c e of r a d i c a l s i n some t e t r a h y d r o f u r a n derivative^^^ and some c y c l i c f l u o r o a c e t a l s. 54 Hydrogen-a tom a b s t r a c t i o n from t e t r a h y d r o f u r a n

Electron Spin Resonance

174

i t s e l f o c c u r s p r e f e r e n t i a l l y a t t h e 2 - p o s i t i o n t o g i v e ( 1 1 ) . The r a d i c a l s formed from s p e c i f i c a l l y d e u t e r i a t e d t e t r a h y d r o f u r a n h a v e now a l l o w e d a c o m p l e t e a s s i g n m e n t o f t h e s p l i t t i n g c o n s t a n t s t o be made.53 The a ( 2 H ) 0 . 1 7 9 mT s p l i t t i n g c a n now be a s s i g n e d t o t h e Y - C H 2 0 p r o t o n s and t h e 0 . 0 7 4 mT s p l i t t i n g t o t h e Y-CH2 p r o t o n s . A b s t r a c t i o n from e i t h e r o f t h e c a r b o n a t o m s a t o t h e oxygen atom occurs i n 2-methyltetrahydrofuran. I n t h e c a s e of c y c l i c f l u o r o a c e t a l s a b s t r a c t i o n a l s o o c c u r s a t two a l t e r n a t i v e s i t e s t o g i v e r a d i c a l s (12) and ( 1 3 ) . 5 4 Radical ( 1 2 ) h a s h y p e r f i n e c o u p l i n g t o o n l y two y - p r o t o n s [a(y-H) 0.071 mT1 a n d i t a p p e a r s t h a t s u b s t i t u t i n g a f l u o r i n e atom f o r a h y d r o g e n atom i n t h e C H 3 g r o u p h a s a s i g n i f i c a n t e f f e c t upon t h e p r e f e r r e d c o n f o r m a t i o n . The e . s . r . s p e c t r u m o f r a d i c a l ( 1 3 ) c a n be e x p l a i n e d i n terms o f r a p i d e q u i l i b r i a t i o n between two e n a n t i o m e r s . Hydrogen-atom a b s t r a c t i o n f r o m some s u l p h u r - c o n t a i n i n g a c e t a l s g i v e s r a d i c a l "141, X = 0 o r S, n = 1 , and R = However when R = Me o r when n = 2 o r 3 t h e s e l e c t i v i t y f o r s i t e of a t t a c k i s reduced.

b,

n "Yo

1 ' 1 O

x

0

The s t u d y of t h e e . s . r . s p e c t r a of t h e r a d i c a l s formed by hydrogen-atom a b s t r a c t i o n from c a r b o h y d r a t e s by 'OH h a s now b e e n e x t e n d e d t o i n c l u d e some f u r a n o s e compounds. 56 The s p e c t r u m o b t a i n e d from e x p e r i m e n t s w i t h D - x y l o s e i n d i c a t e s a m i x t u r e o f s e v e r a l r a d i c a l s a s a r e s u l t o f hydrogen-atom a b s t r a c t i o n from d i f ferent positions. S i m i l a r r e s u l t s were o b t a i n e d f r o m e x p e r i m e n t s i n v o l v i n g s u c r o s e a g a i n w i t h an a p p a r e n t l a c k of p r e f e r e n t i a l a t t a c k . T h i s i s , however, i n s t r i k i n g c o n t r a s t t o t h e s e l e c t i v e a t t a c k a t C(5')-H o b s e r v e d f o r t h e f u r a n o s e r i n g , d e s p i t e t h e h i g h r e a c t i v i t y o f t h e 'OH r a d i c a l . It i s proposed t h a t t h e enhanced r e a c t i v i t y a t t h i s position is stereoelectronic i n origin. A novel r e p o r t h a s a p p e a r e d c o n c e r n i n g r a d i c a l s p r o d u c e d by i r r a d i a t i o n w i t h u l t r a s o u n d f r o m a-D-glucose a n d a - l a ~ t o s e . ~The ~ radicals h a v e been o b s e r v e d e m p l o y i n g t h e s p i n - t r a p p i n g t e c h n i q u e a n d f u r t h e r i n t e r e s t i n g r e s u l t s may be f o r t h c o m i n g .

5: Organic Radicals in Solution

175

U n l i k e a l k o x y r a d i c a l s , which g e n e r a l l y a b s t r a c t a h y d r o g e n a t o m , a l k a n e t h i y l r a d i c a l s t e n d t o a b s t r a c t o r a d d t o d o u b l e bonds. A n i c e i l l u s t r a t i o n of t h i s f e a t u r e o c c u r s i n t h e r e a c t i o n o f m e t h a n e t h i y l r a d i c a l s ( M e S ' ) , formed by p h o t o l y s i s o f (MeS12, w i t h 2 , 3 - d i h y d r o f ~ r a n . ~R ~ a d i c a l ( 1 5 ) i s formed by a b s t r a c t i o n o v e r t h e t e m p e r a t u r e r a n g e 253-293 K a n d h a s a(H) 1 . 3 3 5 , 0 . 2 0 5 , a n d 1 . 3 1 5 , a n d a ( 2 H ) 3 . 6 2 5 mT. R a d i c a l ( 1 6 ) i s formed by a d d i t i o n o v e r t h e t e m p e r a t u r e r a n g e 153-233 K a n d h a s a ( a - H ) 1 . 4 5 , a(B-H) 1 . 5 6 2 , a n d a ( y - H ) 0.385 and 0 . 1 0 0 mT. ( T h e a l t e r n a t i v e r a d i c a l , formed by a d d i t i o n t o t h e 2 - p o s i t i o n , was n o t o b s e r v e d ) . Both m e t h a n e t h i y l a n d B u t O ' r a d i c a l s o n l y a b s t r a c t from 2 , 3 - d i h y d r o p y r a n t o g i v e r a d i c a l (17) b u t , s u r p r i s i n g l y , w i t h 2,3-dihydro-1,4-dioxan t h e o n l y r a d i c a l o b s e r v e d from r e a c t i o n w i t h ButO' i s formed by a d d i t i o n (18). A n o t h e r example o f a r a d i c a l a d d i t i o n o r a b s t r a c t i o n r e a c t i o n o c c u r s between B u t O ' and 1,4-disilacyclohexa-2,5d i e n e i n c y c l ~ p r o p a n e . ~A ~t 1 6 8 K r a d i c a l ( 1 9 ) i s f o r m e d by a d d i t i o n b u t a t 238 K a n a b s t r a c t i o n r e a c t i o n o c c u r s t o form a s i l y l r a d i c a l ( 2 0 ) which u n d e r g o e s r e a r r a n g e m e n t t o g i v e a s i l a c y c l o p r o p - 2 - y l r a d i c a l ( 2 1 1, a(3H) 2 . 0 5 mT1.

I n t h o s e compounds which c o n t a i n more t h a n o n e t y p e of m u l t i p l e bond a l t e r n a t i v e r e a c t i o n p a t h w a y s f o r r a d i c a l a d d i t i o n c a n l e a d t o a v a r i e t y o f d i f f e r e n t r a d i c a l s . An e x a m p l e o f t h i s i s t h e r e c e n t l y r e p o r t e d a d d i t i o n o f Group IVB m e t a l - c e n t r e d r a d i c a l s t o m a l e i c a n h y d r i d e a n d some r e l a t e d compounds.60 A t temperatures below 300 K r a d i c a l [ ( 2 2 ) , R = S i P h 3 o r CePh3, XR' = 0, S, o r N H I was f o r m e d , b u t r a d i c a l C(231, R = S i P h 3 o r CePh3, XR1 = 0, S, o r N H I was formed a t t e m p e r a t u r e s a b o v e 330 K. The r e a c t i o n pathway

Electron Spin Resonance

176

a p p e a r s t o be k i n e t i c a l l y c o n t r o l l e d a t l o w e r t e m p e r a t u r e s and thermodynamically c o n t r o l l e d a t higher temperatures. Radical [ ( 2 2 ) , XR1 = 01 h a s a l s o b e e n p r e p a r e d p h o t o l y t i c a l l y from m a l e i c a n h y d r i d e i n a flow system.61 The t e m p e r a t u r e d e p e n d e n c e o f t h e

a(a-H) a n d a(f3-H) s p l i t t i n g s i n t h i s r a d i c a l h a s been i n t e r p r e t e d i n terms o f h y d r o g e n b o n d i n g b e t w e e n s o l v e n t m o l e c u l e s a n d d i f f e r ent sites within t h e r a d i c a l . n OMR3

I

(22)

(23)

Some r a t h e r u n u s u a l aminocarboxy r a d i c a l s h a v e b e e n p r e p a r e d i n c h l o r o f o r m by c l e a v a g e o f t h e c o r r e s p o n d i n g d i m e r ~ . ~The ~ , ~ ~

3,5,5-trimethyl-2-oxopiperazin-3-yl r a d i c a l h a s a(p-CH3) 1 . 1 7 3 , a ( B - N ) 0 . 5 1 0 , a(y-H) 0 . 3 7 1 , a(y-N) 0 . 2 5 8 , a n d a ( 6 - H ) 0.126 mT.62 T h i s r a d i c a l can be p r e p a r e d from b o t h t h e mesp- (Ea 116 k J mol-'1 a n d fi- (E, 117 k J mol") dimers. Likewise t h e 3,5,5-trimethyl2-oxomorpholin-3-yl r a d i c a l h a s a ( B - C H 3 > 1 . 1 9 , a(k3-N) 0 . 5 7 , a ( y - H > 0 . 3 7 , a n d a ( 2 , 6 - H ) 0 . 0 4 mT, w i t h Ea f o r t h e bond c l e a v a g e o f t h e c o r r e s p o n d i n g d i m e r 1 1 5 kJ mol" .63 A n o t h e r s t u d y w i t h a k i n e t i c a s p e c t i n v o l v e s t h e d e c a y of t h e 2 - c y c l o h e x a n o n y l r a d i c a l [a(a-H) 1.80 and a(B-H) 2 . 3 5 a n d 4.37 mT1 i n a d a ~ n a n t a n e . ~The ~ decay is The ' f a s t ' d e c a y I s b i - e x p o n e n t i a l w i t h E a ' s o f 8 and 1 4 k J mol". t h o u g h t t o be d u e t o r a d i c a l - r a d i c a l r e c o m b i n a t i o n , a n d t h e ' s l o w ' d e c a y d u e t o hydrogen-atom a b s t r a c t i o n from t h e h o s t . One o f t h e more i n t e r e s t i n g a s p e c t s o f c y c l i c a l k y l r a d i c a l s

i s t h a t many of them u n d e r g o r i n g - o p e n i n g r e a c t i o n s . Such a n e x a m p l e i s found i n b i c y c l o [ n . l . O l a l k - 2 - y l r a d i c a l s formed by hydrogen-atom a b s t r a c t i o n ( w i t h ButO' 1 from t h e c o r r e s p o n d i n g hese r a d i c a l s , (241, b i c y c l o [ n . l . O l a l k a n e s i n c y ~ l o p r o p a n e . ~T ~ g i v e e i t h e r (25) o r (26) i f n = 1 o r 2 but only (26) i f n = 3 t o 6. The a b s o l u t e v a l u e s o f a(B-H) a n d t h e p o s i t i v e s i g n o f da(B-H)/dX f o r t h e cyclobutenyl- and cyclopentenyl-methyl r a d i c a l s i n d i c a t e t h a t t h e s e r a d i c a l s p r e f e r a b i s e c t e d conformation, whereas cyclohexenyl-, cycloheptenyl-, and cyclooctenyl-methyl r a d i c a l s p r e f e r an e c l i p s e d conformation. Ring-opening r e a c t i o n s a r e a l s o o b s e r v e d These with s i l y l s u b s t i t u t e d cyclopropylmethyl radicals.66

177

5 : Organic Radicals in Solution

r a d i c a l s a r e a g a i n p r e p a r e d by hydrogen-atom a b s t r a c t i o n by ButO' from t h e p a r e n t m o l e c u l e . For e x a m p l e , t h e s i l y l b i c y c l o a l k y l r a d i c a l (27) g i v e s [ ( 2 8 ) , a(B-H) 0 . 6 1 and a ( 1 8 H ) 0.035 mT1 r a t h e r than (29).

.

OSiM

iMe3

SiMeg

iMe3 (28)

SiMeg

T h e r e a r e r e l a t i v e l y few r e p o r t s o f b i c y c l i c a l k y l r a d i c a l s with a bridgehead r a d i c a l s i t e . An i n t e r e s t i n g e x a m p l e , t h e b i cyclo[l.l.llpent-l-y1 r a d i c a l , p r e p a r e d by hydrogen-atom a b s t r a c t i o n from t h e p a r e n t m o l e c u l e by ButOD, h a s now b e e n r e p o r t e d . 6 7 T h i s r a d i c a l h a s a(H) 6.96 and a ( 6 H ) 0 . 1 2 mT. The f o r m e r s p l i t t i n g c o n s t a n t h a s been a s s i g n e d t o t h e y - p r o t o n and i t s m a g n i t u d e i s a t t r i b u t e d t o t h r o u g h - s p a c e a n d through-bond i n t e r a c t i o n s s u p p o r t i n g one a n o t h e r . The s p e c t r u m of t h e 2,3-dimethylbicyclol2.2.11hept-2-en-7-yl r a d i c a l h a s been i n t e r p r e t e d i n terms of a(H) 0 . 9 9 2 , a(2H) 0 . 1 7 3 and 0 . 1 6 1 , and a ( 6 H ) 0.140 mT.68 These v a l u e s i n d i c a t e t h a t t h e u n p a i r e d e l e c t r o n i s d e l o c a l i z e d t o some e x t e n t i n t o t h e .rr-system of t h e v i n y l e n e b r i d g e t h u s i n d u c i n g a n i n c r e a s e i n t h e c a r b a n i o n c h a r a c t e r o f t h e t e r v a l e n t c a r b o n atom. Evidence f o r t h e f o r m a t i o n o f a m u l t i c y c l i c r a d i c a l h a s been o b t a i n e d d u r i n g t h e c o u r s e o f t h e c y c l o a d d i t i o n r e a c t i o n between x a n t h e n e t h i o n e a n d pheny l a l l e n e 9. T h r e e p a p e r s h a v e been p u b l i s h e d d e a l i n g w i t h t h e ' t r a p p i n g ' of t h e p h e n y l r a d i c a l w i t h v a r i o u s t r a p p i n g a g e n t s . P h e n y l r a d i c a l s a r e r e a d i l y p r o d u c e d by t h e t h e r m a l d e c o m p o s i t i o n o f b e n z o y l p e r o x i d e ( E q u a t i o n s 4 and 5 ) . The p h e n y l r a d i c a l , b u t n o t t h e b e n z o y l o x y r a d i c a l , is s u c c e s s f u l l y t r a p p e d by PhNO and B U ~ N O , ~ OI n c o n t r a s t o n l y t h e b e n z o y l o x y r a d i c a l is t r a p p e d by N-t-butyl-a-phenylnitrone Ca(N> 1 . 3 0 7 a n d a(13-H)0.144 mT1. F l u o r a n i l a l s o t r a p s t h e p h e n y l r a d i c a l , formed d u r i n g t h e t h e r m a l d e c o m p o s i t i o n of b e n z o y l p e r o x i d e [ a ( 2 F ) 1.36-1.70 a n d a ( 2 F )

.

Electron Spin Resonance

178

mT v a r y i n g w i t h s o l v e n t ] .71 However t h e e . s . r. s p e c t r a o f t h e r a d i c a l s t r a p p e d by f l u o r a n i l i n m e t h y l - s u b s t i t u t e d b e n z e n e s contain an a d d i t i o n a l small a(2H) s p l i t t i n g i n d i c a t i n g t h a t t h e p h e n y l r a d i c a l h a s a b s t r a c t e d a h y d r o g e n atom from t h e m e t h y l g r o u p o f t h e s o l v e n t and t h a t i t i s t h e r e s u l t i n g s e c o n d a r y r a d i c a l s which a r e t r a p p e d . The p h e n y l r a d i c a l i s a l s o t r a p p e d by n i t r o s o durene d u r i n g i t s p h o t o l y s i s i n benzene a g a i n i n d i c a t i n g t h a t i t i s a s o l v e n t d e r i v e d r a d i c a l t h a t h a s been trapped.72 0.38-0.60

Two d i f f e r e n t r e s e a r c h g r o u p s h a v e r e p o r t e d t h e r e s u l t s o f t h e i r i n v e s t i g a t i o n s of imidoyl r a d i c a l s . 7 3 f 7 4 One s o u r c e of t h e s e r a d i c a l s i s hydrogen-atom a b s t r a c t i o n , by ButO', from a n a p p r o p r i a t e s i l a n e ( i n c y c l o p r o p a n e ) t h e r e s u l t i n g R3Si* r a d i c a l t h e n a d d i n g t o a n a l k y l i s o c y a n a t e ( E q u a t i o n 6).73 These i m i d o y l r a d i c a l s g e n e r a l l y h a v e a(N) u , 0 . 0 8 mT b u t a ( y - H ) v a r i e s w i t h t h e n a t u r e of R1 and R 2 . The e . s . r . p a r a m e t e r s o b s e r v e d f o r t h e s e r a d i c a l s a p p e a r t o e s t a b l i s h them a s a - r a d i c a l s , w i t h t h e r a t e o f a d d i t i o n o f R13Si' t o t h e a l k y l i s o c y a n a t e s c o n t r o l l e d m a i n l y by steric factors. The s e c o n d i n v e s t i g a t i o n i n v o l v e s t h e s t u d y of Re=NBut r a d i c a l s formed by hydrogen-atom a b s t r a c t i o n by ButO' from t h e p a r e n t i n ~ i n e . Again ~ ~ t h e o b s e r v e d e . s . r . p a r a m e t e r s c a n be r a t i o n a l i z e d i f t h e r a d i c a l s have a a - e l e c t r o n i c c o n f i g u r a t i o n . Rl3Si*

+

R2N=C=0

--+

R2N=tOSiRl3

(6)

F i n a l l y i n t h i s s e c t i o n I h a v e c o l l e c t e d t o g e t h e r two rep o r t s , one i n v o l v i n g v i n y l r a d i c a l s t h e o t h e r a c y l r a d i c a l s . A new s e r i e s o f v i n y l r a d i c a l s h a s b e e n formed by r e a c t i o n o f SO3' w i t h a l k y n e s i n f l o w s y s t e m e x p e r i m e n t s which a l l o w t h e d e t e c t i o n o f r a d i c a l s w i t h l i f e t i m e s i n t h e r a n g e 5-100 m s . 7 5 S t e r i c considera t i o n s support t h e assignment of t h e observed s p e c t r a t o t h e (-03S)CH=tC(OH)R1R2 r a d i c a l r a t h e r t h a n t h e 'CH=C(S03')C(OH)R1 R2 r a d i c a l . The f a c t t h a t t h e s e r a d i c a l s a r e o b s e r v e d s u g g e s t s t h a t t h e SO3' i o n s t a b i l i z e s t h e r e s u l t i n g v i n y l r a d i c a l . A s e r i e s o f r i n g - s u b s t i t u t e d c y c l o p r o p y l a c y l r a d i c a l s a l s o p r e p a r e d by hydrogen-atom a b s t r a c t i o n ( b y But0' ) from t h e p a r e n t a l d e h y d e s h a v e been r e p o r t e d . 7 6 Two p a r e n t r a d i c a l s (C3H5e=O) a r e o b s e r v e d o n e

5: Organic Radicals in Solution

l

179

o f which h a s a n e . s . r . s p e c t r u m i n t e r p r e t e d i n terms o f a(B-H) 0 . 0 5 a n d a(2,Y-HI 0 . 0 9 5 and 0 . 0 6 mT ( a s s i g n e d t o t h e c i a - c o n f o r m a t i o n ) a n d t h e o t h e r h a s been i n t e r p r e t e d i n terms o f a(6-H) 1 . 8 2 and a ( 2 , y - H I 0 . 0 2 mT ( a s s i g n e d t o t h e t r a g g - c o n f o r m a t i o n ) . The a c t i v a t i o n e n e r g y f o r e x c h a n g e b e t w e e n t h e two i s o m e r s i s 17.5 kJ mol". S i m i l a r r e s u l t s h a v e been o b s e r v e d f o r some o x i r a n y l and a z i r i d i n y l - a c y l r a d i c a l s .

3.3 D-.Several i n t e r e s t i n g new a l l y 1 and p r o p y n y l r a d i c a l s h a v e been r e p o r t e d . Ally1 radicals generally e x i s t a s sy1? and anti i s o m e r s and s u c h i s t h e c a s e i n t h e 1-cyano-1-methoxyallyl r a d i c a l s t u d i e d over t h e t e m p e r a t u r e range 193-277 K and formed by hydrogen-atom a b s t r a c t i o n , by Bu3Sn* r a d i c a l s , from t h e p a r e n t m o l e c u l e . 7 7 The e . s . r . s p e c t r u m o f t h e gyn-1-cyano-anti-1-methoxyallyl r a d i c a l h a s a(H) 1 . 0 9 3 , 1 . 0 0 6 , a n d 0 . 3 3 8 , a ( 3 H ) 0 . 2 0 8 , and a(N) 0.211 mT and t h e U - 1 - c y a n o s y 1 ? - l - m e t h o x y a l l y l r a d i c a l h a s a(H) 1 . 0 7 0 , 1 . 0 3 0 , and 0 . 3 1 1 , a ( 3 H ) 0 . 1 7 3 , and a(N) 0 . 2 4 2 mT. The r o t a t i o n a l b a r r i e r s i n t h e two i s o m e r s h a v e a l s o been e v a l u a t e d ( E a 2 5 and 26 k J mol" respecti v e l y ) . d - A l k y l - s u b s t i t u t e d a m i n o a l l y l ( R 2 N k H C H = C H 2 ) a n d aminop r o p y n y l r a d i c a l s ( R 2 N t H C Z C H ) a r e of i n t e r e s t p a r t i c u l a r l y a s t h e l a t t e r a r e t h o u g h t t o b e i n t e r m e d i a t e s i n enzyme d e a c t i v a t i o n by a c e t y l e n i c a m i n e ~ . The ~ ~ hyperf i n e s p l i t t i n g c o n s t a n t s of t h e s e r a d i c a l s , a g a i n p r e p a r e d by hydrogen-atom a b s t r a c t i o n f r o m t h e parent molecules, i n d i c a t e considerable spin-density d e l o c a l i z a t i o n on t h e N R 2 g r o u p . Below 2 6 0 K t h e two NH2 p r o t o n s i n t h e arninopropynyl r a d i c a l h a v e d i f f e r e n t s p l i t t i n g c o n s t a n t s i n d i c a t i n g r e s t r i c t e d r o t a t i o n a b o u t t h e C-N bond b u t t h e s e p r o t o n s become m a g n e t i c a l l y e q u i v a l e n t a t 300 K . 7 9 I t h a s been p o s s i b l e t o d e t e r m i n e t h e b a r r i e r t o r o t a t i o n a b o u t t h i s bond (44% k J mol") from t h e t e m p e r a t u r e d e p e n d e n c e o f t h e l i n e w i d t h s i n t h e e . s . r . spectrum.78 A c a r e f u l study of t h e 13C s p l i t t i n g c o n s t a n t s i n t h e prop-2-ynyl r a d i c a l ( . C H 2 C Z C H ) i s o f i n t e r e s t b e c a u s e t h e y r e v e a l u s e f u l d i r e c t i n f o r m a t i o n on s p i n - d e n s i t y d i s t r i b u t i o n . 8 0 The r a d i c a l h a s a ( a - 1 3 C ) 3 . 3 9 , a(B-13C) 1 . 8 1 , a(y-13C) 2 . 2 9 , a ( 2 , a - H ) The r e l a t i v e l y s m a l l a ( a - 1 3 C ) v a l u e 1 . 8 9 , a n d a ( y - H ) 1 . 2 7 mT. i n d i c a t e s t h a t t h e r a d i c a l a d o p t s a I'r-delocalized s t r u c t u r e . An i n t e r e s t i n g p a p e r r e p o r t s t h e e . s . r . s p e c t r a o f t h e r a d i c a l s formed by hydrogen-atom a b s t r a c t i o n from u n s a t u r a t e d f a t t y a c i d e s t e r s . 8 1 A b s t r a c t i o n from t h e two a l l y l i c s i t e s i n m e t h y l e l a i d a t e g i v e s two ( s p e c t r o s c o p i c a l l y i n d i s t i n g u i s h a b l e ) transoid

Electron Spin Resonance

180

r a d i c a l s ( 3 0 1 , w h e r e a s m e t h y l o l e a t e g i v e s t h e crisoid r a d i c a l ( 3 1 ) w h i c h , upon p r o l o n g e d p h o t o l y s i s a t 330 K , c o n v e r t s t o t h e transoid t y p e r a d i c a l ( 3 0 ) . Hydrogen-atom a b s t r a c t i o n from m e t h y l l i n o l e a t e g i v e s a r a d i c a l of t y p e ( 3 2 ) and from methyl h e n i c o s a - 8 , l l - d i y n o a t e g i v e s a r a d i c a l o f t y p e ( 3 3 ) . I n many c a s e s s e c o n d a r y r a d i c a l s were a l s o o b s e r v e d p r o d u c e d by hydrogen-atom a b s t r a c t i o n f r o m a c h a i n m e t h y l e n e g r o u p and from t h e c o n c e n t r a t i o n s o f t h e v a r i o u s r a d i c a l s p r o d u c e d i t was p o s s i b l e t o e s t i m a t e t h e r e l a t i v e r a t e s o f hydrogen-atom a b s t r a c t i o n ( b y ButO') a t 293 K from t h e d i f f e r e n t s i t e s : - s e c o n d a r y : p r o p y n y l i c : a l l y l i c : b i s a l l y l i c = 1 : 1 8 : 36 : 116.

R2

1

'R

A.

Hydrogen-atom

a b s t r a c t i o n from f u r a n and r e l a t e d compounds by These s t u d i e s h a v e now b e e n e x t e n d e d t o i n c l u d e 2 , 3 - d i h y d r o f u r a n and some 2 - s u b s t i t u t e d furans.82 I n t h e c a s e of 2,3-dihydrofuran a b s t r a c t i o n o c c u r s a t t h e 4 - p o s i t i o n b u t w i t h 2-methyl- and 2 - a c e t y l - f u r a n s p e c t r a c o r r e s p o n d i n g t o 'OH a d d i t i o n a t b o t h t h e 2- and 5- p o s i t i o n s a r e o b s e r v e d . The b i c y c l i c r a d i c a l s b i c y c l o [ 3 . 1 . 1 l h e p t e n y l a n d b i c y c l o [ 4 . 1 . 0 l h e p t e n y l c a n be g e n e r a t e d by X - i r r a d i a t i o n i n a d a m a n t a n e . 83 The l a t t e r r a d i c a l , however, r e a r r a n g e s t o g i v e t h e f o r m e r which i s r e a s o n a b l y s t a b l e up t o m . 3 7 3 K . The c y c l o p e n t a d i e n y l s y s t e m i s a l w a y s o f i n t e r e s t a s i t a p p e a r s t o be t h e s m a l l e s t n e u t r a l a n n u l e n e t o w h i c h n - e l e c t r o n t h e o r y c a n be a p p l i e d . A s t u d y of t h e e.s.r s p e c t r a of 'OH h a s been r e p o r t e d i n t h e p a s t .

3 C - l a b e l l e d pentamethylcyclopentadienyl r a d i c a l s g i v e s a ( ~ z - ' ~ C ) 0 . 2 6 8 a n d a ( 6 - 1 3 C ) 0.355 mT and l e a d s t o t h e c o n c l u s i o n t h a t t h e Me5C5' r a d i c a l i s p l a n a r . 8 4 The r a t h e r l e s s known C5F5' r a d i c a l , g e n e r a t e d by p h o t o l y s i s of C5F5C1, h a s a ( 5 F ) 1.60 mT ( a t 1 7 0 K) a n d a(13C) 0.21 mT ( i n 3 - m e t h y l p e n t a n e ) .85 The s p e c t r a l l i n e w i d t h o f t h i s r a d i c a l e x h i b i t s pronounced t e m p e r a t u r e and MF d e p e n d e n c e a t t r i b u t e d t o i n c o m p l e t e a v e r a g i n g o f t h e a n i s o t r o p i c components by t h e molecular re-orientation process. A w i d e r a n g e o f c y c l o h e x a d i e n y l r a d i c a l s , g e n e r a t e d by a v a r i e t y of experimental t e c h n i q u e s , have been r e p o r t e d . For e x a m p l e , S c h u l e r e t al. h a v e r e p o r t e d a number of c a r b o x y - a n d

5: Organic Radicals in Solution

181

h y d r o x y - c y c l ~ h e x a d i e n y l s . ~The ~ ~ ~ c~y c l o h e x a d i e n y l - I , 2 , 4 , 5 t e t r a c a r b o x y l a t e r a d i c a l , formed by hydrogen-atom a d d i t i o n t o p y r o m e l l i t i c a c i d , h a s 13C h y p e r f i n e s p l i t t i n g c o n s t a n t s s i m i l a r t o t h o s e o f t h e ' O H a d d u c t e x c e p t a t t h e C-3 and C-6 p o s i t i o n s . 8 6 The d i f f e r e n c e i n magnitude of a t t h e s e p o s i t i o n s i n t h e s e two r a d i c a l s i s a t t r i b u t e d t o a change i n t h e i n t e r a c t i o n between t h e 0- and 71-systems. The e f f e c t o f i o n i c s t r e n g t h on l i n e s h a p e i n t h e s p e c t r a of t h e ' O H a d d u c t of C6H(CO2-l5 l e a d s t o t h e c o n c l u s i o n t h a t bf v a r i e s w i t h i o n i c s t r e n g t h b u t k r d o e s One i n t e r e s t i n g f e a t u r e of cyclohexadienyl r a d i c a l s i s t h e l a r g e s p l i t t i n g c o n s t a n t s a s s o c i a t e d with t h e methylene protons. This e f f e c t h a s been i n v e s t i g a t e d by S a k u r a i st aa. i n some Me3Sis u b s t i t u t e d radical^.^^,^^ I n m o n o - s u b s t i t u t e d d e r i v a t i v e s a ( 2 H ) v a l u e s of u.4 . 8 mT a r e o b s e r v e d d e c r e a s i n g somewhat i n d i - s u b s t i t u t e d d e r i v a t i v e s . 8 8 The v a r i a t i o n o f a( 6-HI w i t h s u b s t i t u e n t and t e m p e r a t u r e i n d i c a t e s a s i g n i f i c a n t out-of-plane deformation of t h e c a r b o n framework. A t t e m p e r a t u r e s a b o v e 333 K t h e C6H5(SiMe3)2 r a d i c a l i s u n s t a b l e and e l i m i n a t i o n o f a s u b s t i t u e n t o c c u r s . 8 9 The r e s u l t i n g *SiMe3 r a d i c a l i s t r a p p e d by ( M e 3 C I 2 C = C H 2 t o g i v e (Me3CI2kCH2SiMe3 w i t h a(I3-H) 1.56 and a ( 1 8 H ) 0.035 mT. The r a t e of d i m e r i z a t i o n o f a number o f I - a l k y l - 4 - p h e n y l p y r i d i n y l r a d i c a l s ( a l k y l = Me, E t , o r Me2CH1 h a s been s t u d i e d . 9 0 T h e s e k i n e t i c s t u d i e s r e v e a l a two s t e p r e a c t i o n t h e f i r s t ' f a s t ' s t e p b e i n g a t t r i b u t e d t o t h e f o r m a t i o n o f a 4 , 4 ' - d i m e r (Ea 2 8 kJ rnol") a n d t h e second ' s l o w ' s t e p t o a r e a r r a n g e m e n t o f t h i s i n t e r m e d i a t e d i m e r t o e i t h e r t h e 2 , 2 ' - o r t h e 2 , 6 ' - d i m e r (Ea The s p l i t t i n g c o n s t a n t o f t h e a c e t y l p r o t o n s i n 47-56 kJ mol"). t h e r e l a t e d 1 -methyl-4-acetylpyridinyl r a d i c a l i n c r e a s e s w i t h s o l v e n t p o l a r i t y . 9 1 The s e n s i t i v i t y o f t h i s s p l i t t i n g c o n s t a n t t o t h e n a t u r e of t h e s o l v e n t i s due t o t h e l o c a l i z a t i o n of t h e s o l v e n t - s o l u t e i n t e r a c t i o n on t h e oxygen o f t h e c a r b o n y l g r o u p . Some r a t h e r n o v e l . r r - r a d i c a l s h a v e been r e p o r t e d . T r i v i n y l m e t h y l r a d i c a l s ( 3 4 ) h a v e been g e n e r a t e d by hydrogen-atom a b s t r a c t ~ i o n ( b y B u t O ' ) from t r i v i n y l m e t h a n e i n c y c l ~ p r o p a n e . ~Only 0 . 7 4 , and a(H-1 c o n f o r m e r ( 3 4 ) i s o b s e r v e d a(13-H) 0 . 3 3 , a(H-1 0.70 mT. Bromine-atom a b s t r a c t i o n f r o m l-bromohepta-2,6-dien-4-yne g i v e s t h e hepta-2,6-dien-4-ynyl r a d i c a l ( 3 5 ) . The two p o s s i b l e c o n f o r m e r s o f t h i s l a t t e r r a d i c a l were f o u n d t o be s p e c t r o s c o p i cally indistinguishable. Some s u b s t i t u t e d c y c l i c n i t r o g e n - and s u l p h u r - c o n t a i n i n g r a d i c a l s [ l l 2 , 4 , 6 - t h i a t r i a z i n y l s ( 3 6 1 1 h a v e been r e p o r t ~ A - These ~~ r a d i c a l s have a(N) m . 0 . 3 6 and a ( 2 N ) m . 0 . 5 3

182

Electron Spin Resonance

mT and a r e s u r p r i s i n g l y s t a b l e .

S e v e r a l r e p o r t s o f t h e b e n z y l , and o f b e n z y l r e l a t e d , r a d i c a l s h a v e a p p e a r e d . One o f t h e s e i n v o l v e s t h e s t u d y o f t h e e q u i l i b r i u m b e t w e e n d i b e n z y l and b e n z y l r a d i c a l s u n d e r p r e s s u r e i n t o l u e n e a t 878 K . 9 4 E.s.r. s p e c t r o s c o p y h a s been employed t o d e t e r m i n e t h e r a d i c a l c o n c e n t r a t i o n and hence t h e e q u i l i b r i u m c o n s t a n t f o r d i s s o c i a t i o n . The r a t e c o n s t a n t f o r r e c o m b i n a t i o n a t 8 7 8 K h a s b e e n d e t e r m i n e d a s 0 . 7 7 x lo1' 1 mol" s-'. S i m i l a r l y i t h a s been p o s s i b l e t o s t u d y t h e t h e r m o l y s i s of b o t h USXI and DL-2,3-dimethoxy-2,3-diphenylsuccinonitrile t o a-cyano-a-methoxybenzyl r a d i c a l s e g 5 The d i s s o c i a t i o n o f d i b e n z y l a m i n o n i t r e n e , (PhCH2NI2, t o g i v e b e n z y l r a d i c a l s h a s a l s o been s t u d i e d . 9 6 L i v i n g s t o n e t al. h a v e a l s o i n v e s t i g a t e d t h e p y r o l y s i s of b e n z y l e t h e r o v e r t h e temp e r a t u r e r a n g e 725-773 K u n d e r p r e s s u r e . 9 7 The e . s . r . s p e c t r u m o f b e n z y l , r a t h e r t h a n PhEHOCH2Ph, i s o b s e r v e d b u t t h e s p e c t r u m o f t h e l a t t e r r a d i c a l [a(a-H) 1 . 4 9 4 , a ( 2 , y - H I 0 . 1 3 5 , a ( m - H ) 0.156 a n d 0 . 1 6 1 , and a(H) 0 . 4 5 2 , 0 . 5 0 0 , and 0 . 5 7 2 mT ( t h e s e l a t t e r t h r e e v a l u e s h a v e , a s y e t , t o b e a l l o c a t e d t o t h e p- a n d p - p r o t o n s ) ] i s o b s e r v e d d u r i n g t h e p h o t o l y s i s of ( B u t 0 I 2 i n b e n z y l e t h e r . The Q- a n d m-protons a r e i n e q u i v a l e n t i n d i c a t i n g t h a t t h e phenyl r i n g i s not r o t a t i n g rapidly. A c a r e f u l s t u d y of t h e i n f l u e n c e o f a w i d e r a n g e of m- and g - s u b s t i t u e n t s o n t h e a - p r o t o n h y p e r f i n e c o u p l i n g i n b e n z y l h a s a l s o been ~ n d e r t a k e n . ~S i~n c e t h e a - p r o t o n s p l i t t i n g constant is related t o the spin-density a t the benzylic p o s i t i o n t h e s e results p r e s e n t an approach t o determining a s c a l e of 0,' s u b s t i t u e n t c o n s t a n t s , which a r e i m p o r t a n t i n d e t e r m i n i n g radical stabilization versus polar f a c t o r s i n radical reactions.

I n s i m p l e s u b s t i t u t e d b e n z y l r a d i c a l s , XC6H4eH(CH2F), t h e v a l u e of a(a-H) i s c l o s e t o t h a t i n b e n z y l i t s e l f . 9 9 The e . s . r . s p e c t r a o f t h e s e r a d i c a l s i n d i c a t e t h a t r o t a t i o n a b o u t t h e C-CH2F bond i s hindered ( t h e value of a(B-19F) is s o l v e n t dependent) with e q u i l i b r i a t i o n between a number o f p o s s i b l e r o t a m e r s . The b e n z y l r e l a t e d r a d i c a l s Pht(0Y)X (X and Y = S i P h 3 , GePh3, SnBu3, SnMe3, o r S i M e 3 ) , a r e p r e p a r e d by t h e r e a c t i o n of p h o t o l y t i c a l l y o r t h e r m a l l y g e n e r a t e d MR13 r a d i c a l s w i t h PhC(0)MR23. l o o The s p e c t r a

5 : Organic Radicals in Solution

183

of t h e s e r a d i c a l s a l l show s i m i l a r f e a t u r e s w i t h , a s e x p e c t e d , a(p-H) > ~ ( Q - H ) >> a(m-H), a n d w i t h c o u p l i n g t o t h e 2 9 S i , 7 k e , a n d There i s a g a i n h i n d e r e d 17'' 19Sn n u c l e i where a p p r o p r i a t e . r o t a t i o n of t h e phenyl r i n g , with t h e magnitude of t h e s p l i t t i n g c o n s t a n t s from t h e r i n g r e f l e c t i n g i t s a v e r a g e d e v i a t i o n from t h e p l a n e o f t h e sp' h y b r i d o r b i t a l . Two v e r y d i f f e r e n t methods o f p r e p a r i n g s u b s t i t u t e d 1 , l - d i -

p h e n y l e t h y l r a d i c a l s h a v e been d e s c r i b e d . S m i t h e t al. h a v e p r e p a r e d Ph2CCMe3 [ ~ ( Q - H ) 0 . 2 6 , a(m-H) 0 . 1 3 , a(p-H) 0 . 2 7 , and a ( 9 H ) 0 . 0 2 4 mT1 by t h e a d d i t i o n of t h e c o r r e s p o n d i n g a n i o n t o tetramethylethene dibromide. lo' A c o m p a r i s o n of t h e s p l i t t i n g c o n s t a n t s f o r t h i s r a d i c a l w i t h t h o s e f o r 'CPh3 s u g g e s t s t h a t t h e r a d i c a l h a s a ' p r o p e l l e r ' c o n f o r m a t i o n w i t h t h e CMe3 g r o u p h a v i n g a s i m i l a r i n f l u e n c e t o a Ph g r o u p . P e d u l l i e t al. h a v e p r e p a r e d r e l a t e d r a d i c a l s Ph2eCH2X ( X = MR,) by t h e a d d i t i o n o f ' M R , t o 1 ,1 - d i p h e n y l e t h e n e . l o 2 A l l t h e s e r a d i c a l s p r e f e r t h e c o n f o r m a t i o n corresponding t o t h e l e a s t s t e r i c crowding around t h e a-carbon. The r i n g p r o t o n s p l i t t i n g c o n s t a n t s a r e a l m o s t i n d e p e n d e n t o f t h e n a t u r e of X. Neither t h e deformation of t h e molecular s k e l e t o n due t o s t e r i c e f f e c t s o r t h e d i f f e r e n t e l e c t r o n e g a t i v i t y of t h e subs t i t u e n t s s a t i s f a c t o r i l y accounts f o r t h e experimental data i n these l a t t e r radicals. A d d i t i o n o f ' M R , t o &- o r t r a n s - s t i l b e n e p r o d u c e s a r a d i c a l whose e . s . r . s p e c t r u m i s i n d e p e n d e n t o f t h e n a t u r e o f t h e i s o m e r . I o 3 D e v i a t i o n s from t h e e c l i p s e d c o n f o r m a t i o n a r e a g a i n e x p l i c a b l e i n terms o f s t e r i c c r o w d i n g . An i n t e r e s t i n g p a p e r , which r e v e a l s i m p o r t a n t i n f o r m a t i o n on s y n t h e t i c r o u t e s , r e p o r t s t h e e . s . r . s p e c t r a ( t o g e t h e r w i t h ENDOR and TRIPLE) of 3 C - l a b e l l e d and C1- and F - s u b s t i t u t e d p h e n a l e n y l r a d i ~ a 1 s . l ' ~ The c a r e f u l i n t e r p r e t a t i o n of t h e s p e c t r a of t h e s e r a d i c a l s , p r e p a r e d by a v a r i e t y o f a l t e r n a t i v e r o u t e s , i n d i c a t e s t h a t one of t h e s e r o u t e s i n v o l v e s a p r e v i o u s l y u n s u s p e c t e d rearrangement r e a c t i o n . The p r o t o n s p l i t t i n g c o n s t a n t s i n a t e t r a methoxy- and i n a d i e t h o x y - p h e n a l e n y l r a d i c a l r e v e a l t h a t t h e s u b s t i t u e n t h a s l i t t l e i n f l u e n c e on t h e i r v a l u e s . l o 5 Some l a r g e d e l o c a l i z e d c a r b o n r a d i c a l s h a v e been c h a r a c t e r i z e d from t h e i r e. s. r. s p e c t r a . The 1,8-dihydroxy-9-anthron-lO-yl r a d i c a l ( 3 7 ) c a n be p r e p a r e d a t t e m p e r a t u r e s a b o v e 3 7 3 K f r o m t h e The e . s . r . s p e c t r u m can be i n t e r p r e t e d i n parent molecule.lo6 terms o f a ( 4 H ) 0 . 4 3 3 , a ( 2 H ) 0 . 1 0 , a(H) 1 . 0 4 , and a ( H - O H ) 0 . 0 4 mT. When t h e r a d i c a l i s t r a p p e d w i t h 2,4,6-tri-t-butylnitrosobenzene t h e s p e c t r u m o f t h e r e s u l t i n g r a d i c a l h a s a( N ) 1 . 1 2 5 , a(m-H) 0 . 1 8 5 ,

Electron Spin Resonance

184

and a(H) 0.100 mT. These l a t t e r v a l u e s s u g g e s t t h a t t h e spectrum p r e v i o u s l y a t t r i b u t e d t o t h i s t r a p p e d r a d i c a l may i n f a c t be t h a t o f C6 H2 ( CMe3 N( 61 H An unusua 1 s u b s t i t u t e d benz 0- 1 5-c rown-5 e t h e r ( 3 8 ) h a s been p r e p a r e d and t h e i n f l u e n c e of g u e s t i o n s (u, Cu2+) on i t s e . s . r . s p e c t r u m i n v e s t i g a t e d . l o 7 No g r e a t c h a n g e i n t h e magnitude of t h e proton s p l i t t i n g c o n s t a n t i s observed i n t h e p r e s e n c e of t h e g u e s t i o n b u t t h e p o s i t i o n o f t h e d i m e r - r a d i e a l equilibrium changes q u i t e s i g n i f i c a n t l y . F i n a l l y e v i d e n c e h a s been p r e s e n t e d t h a t t h e o b s e r v a t i o n o f t h e ‘CPh3 r a d i c a l d u r i n g t h e

.

r e a c t i o n o f l i t h i u m d i m e t h y l c u p r a t e w i t h Ph3CC1 ( i n E t 2 0 o v e r t h e t e m p e r a t u r e r a n g e 268-273 K ) s u g g e s t s t h e p o s s i b i l i t y o f a s i n g l e e l e c t r o n t r a n s f e r mechanism f o r t h i s r e a c t i o n . l o 8

(38)

The r e a c t i o n s o f o r g a n o m e t a l l i c r a d i c a l s w i t h p - d i k e t o n e s c o n t i n u e s t o p r o v e of i n t e r e s t . P a r t of t h i s i n t e r e s t l i e s i n t h e o b s e r v a t i o n t h a t t h e r e s u l t i n g d e r i v a t i v e s can e x i s t a s e i t h e r a monodentate ( s t a t i c or f l u x i o n a l ) l i g a n d (39) o r a s a b i d e n t a t e l i g a n d (40). An example o f t h i s s i t u a t i o n i s f o u n d i n t h e o r g a n o t i n d e r i v a t i v e s of 3,6-di-t-butyl-l,2-benzoquinone. l o g When X = C13Sn o r RC12Sn t h e e . s . r . s p e c t r u m a t low t e m p e r a t u r e r e v e a l s h y p e r f i n e c o u p l i n g t o a s i n g l e c h l o r i n e atom t y p i c a l o f r a d i c a l (40). P o s i t i o n a l e x c h a n g e o c c u r s a t h i g h e r t e m p e r a t u r e s . However, when X = R2C1Sn r a d i c a l ( 3 9 ) i s formed w i t h m i g r a t i o n between t h e two p o s s i b l e s i t e s . I t a p p e a r s t h e r e f o r e t h a t t h e i n t r o d u c t i o n o f e l e c t r o n e g a t i v e s u b s t i t u e n t s f i r s t reduces t h e f l u x i o n a l i t y of t h e r a d i c a l and e v e n t u a l l y l e a d s t o b i d e n t a t e s t r u c t u r e s . T h i s conc l u s i o n i s c o n f i r m e d by a v e r y s i m i l a r s t u d y of SnC12R c o m p l e x e s When X = M ( C O ) , R ( R = q 5 - c y c l o when R = n - c a r b o r a n - 9 - y l . 1 1 0 p e n t a d i e n y l , M = F e , Mo, o r W > r a d i c a l ( 3 9 ) i s o b s e r v e d b u t t h i s e l i m i n a t e s a CO g r o u p t o g i v e r a d i c a l [ ( 4 0 ) , X = M ( C O ) n - l R l . l l l R a d i c a l (40) i s a l s o o b s e r v e d when X = S i F 3 and a g a i n t h e r e i s h i n d e r e d r o t a t i o n of t h e SiF3 group.l12 A s i m i l a r range of r a d i c a l s i s formed when M(CO15 ( M = Mn o r Re) r e a c t s w i t h N,N1-di-t-butyl-l,4-diaza-l,3-butadiene t o g i v e r a d i c a l [(41),

5: Organic Radicals in Solution

185

X = M(C0)41.113

I t i s i n t e r e s t i n g t o n o t e t h a t i n some c a s e s upon r e a c t i o n w i t h Group VB m e t a l c o m p l e x e s a r a d i c a l w i t h a ( 4 H ) 0 . 2 7 4 , a ( 2 N ) 0 . 8 6 2 , and a ( 1 8 H ) 0.011 mT i s o b s e r v e d . This spectrum is a t t r i b u t e d t o t h e J,JI-di-t-butylpyrazinium radical-cation but it i s n o t c l e a r how i t i s formed. O r g a n o m e t a l l i c r a d i c a l s a l s o add t o p - d i k e t o n e s t o form s t a b l e a d d u c t r a d i c a l s . With 2 , 6 - d i methoxy-p-benzoquinone two p o s s i b l e i s o m e r r a d i c a l s c a n b e f o r m e d . The s p e c t r a o f b o t h i s o m e r i c a d d u c t s a r e o b s e r v e d upon a d d i t i o n of 'SnBu3 and 'PbPh3, b u t o n l y t h a t of t h e l e s s h i n d e r e d i s o m e r upon a d d i t i o n o f ' S i P h 3 and 'GePh3. But

A v a r i e d range of n i t r o g e n - c e n t r e d r a d i c a l s h a s been s t u d i e d d u r i n g t h e l a s t 18 m o n t h s . Where p o s s i b l e I h a v e c o l l e c t e d t o g e t h e r t h o s e p a p e r s which a p p e a r t o h a v e a r e l a t e d i n t e r e s t . Simple aminyl r a d i c a l s r e p o r t e d cover a range o f i n t e r e s t s . The e . s . r . s p e c t r a of some d i a r y l a m i n y l s h a v e been i n t e r p r e t e d . ' 1 5 T h e s e r a d i c a l s a p p e a r t o be more p e r s i s t e n t when t h e r i n g s u b s t i t u e n t s i n one r i n g a r e e l e c t r o n d o n o r s , and i n t h e o t h e r a r e e l e c t r o n acceptors. I t i s n o t i c e a b l e t h a t d o n o r g r o u p s d e l o c a l i z e more s p i n - d e n s i t y t h a n a c c e p t o r g r o u p s i n t h e s e r a d i c a l s . F o r example i n t h e 3,5-di-t-butylphenyl r i n g ~ ( Q - H ) i s 0.55 a n d a(p-H) i s 0 . 7 6 mT w h i l e i n t h e 2 ' , 6 ' - d i n i t r o p h e n y l r i n g a ( m - H ) i s 0 . 1 1 and a(p-H) i s 0 . 1 3 mT. E v i d e n c e f o r t h e f o r m a t i o n of a n a m i n y l r a d i c a l by N-0 bond c l e a v a g e i n XC6H4N(Ac>OC(0)CMe3 h a s b e e n s o u g h t w i t h t h e a i d o f N-t-butyl-a-phenylnitrone a s a s p i n - t r a p . ' l 6 One a d d u c t h a s a n e . s . r . specbrum i n t e r p r e t e d i n terms of a(N) 1 . 4 4 , ~ ( B - H ) 0 . 3 9 1 , and a(B-N) 0 . 1 5 4 mT and h a s been a s s i g n e d t o t h e t r a p p e d XC6H4hAc r a d i c a l t h u s a p p e a r i n g t o s u p p o r t a mechanism i n v o l v i n g h e t e r o l y s i s o f t h e N - 0 bond. I n a n o t h e r r e a c t i o n of a n a m i n y l r a d i c a l Ph2N' h a s been f o u n d t o r e a c t w i t h organomagnesium com-

p o u n d s , RMgX, t o g i v e Ph2NMgX and R ' . ' 1 7 These l a t t e r r a d i c a l s were t r a p p e d i n t h e r e a c t i o n s y s t e m by 2 , 4 , 6 - t r i b r o m o n i t r o s o -

Electron Spin Resonance

186

benzene. A d e t a i l e d s t u d y o f t h e r e a c t i o n s o f a number o f d i s i l y l a t e d aminyl r a d i c a l s h a s shown t h a t t h e y a r e much more I n cyclopropane r e a c t i v e than t h e i r carbon analogues. l 1 (Me3SiI2N* r a d i c a l s r e a c t w i t h E t O H t o g i v e MekHOH, a n d i n ' p u r e ' c y c l o p r o p a n e t h e y a b s t r a c t a h y d r o g e n atom t o g i v e cyclo-C3H5'. T h e r e a r e many o t h e r n o v e l r e a c t i o n s o f t h e s e r a d i c a l s . With e t h e n e a d d i t i o n o c c u r s t o g i v e (Me3SiI2NCH2CH2* [a(2,a-H) 2 . 1 9 , a ( 2 , B - H ) 3 . 1 2 , a n d a(N) 0 . 2 5 mT1 a n d w i t h ButN=C a d d i t i o n g i v e s B u ~ N = ~ N ( S ~[ aM ( N~) ~1 ). 1~5 and ~ ( c x - ~ ~ 9.52 C ) mT1. However, i n f l u i d s o l u t i o n , t h e (Me3SiI2N' r a d i c a l h a s n o t s o f a r been d e t e c t e d i n t h e s e systems. Novel c y c l i c t h i o a m i n y l r a d i c a l s c a n be formed by r e a c t i o n o f N2S4 w i t h s t r a i n e d a l k e n e s i n FCC13.119 Two t y p e s o f r a d i c a l s [ ( 4 2 ) , R 2 = R4 = H; R1 and R3 c a r b o n a t o m s o f a n o r b o r n a n e r i n g ] a n d [ ( 4 3 ) , R1 = R2 a r y l , a l k y l , a m i d e , o r e s t e r g r o u p s ] h a v e been studied. T h e s e r a d i c a l s c o u l d p r o v e u s e f u l model compounds f o r s t u d i e s of r o t a t i o n a l r e o r i e n t a t i o n i n both l i q u i d and f r o z e n media. R a d i c a l [ ( 4 3 ) , R 1 = R2 CF I h a s a ( 1 4 N ) 1 . 1 1 8 and a ( 6 1 9 F ) 0.072 mT i n FCC13 a t 2 3 8 K w i t h a ( 3 3 S 1 0 . 4 1 9 and 1 . 5 6 0 rnT i n C C 1 4 a t 245 K o b s e r v e d a s s a t e l l i t e l i n e s i n n a t u r a l a b u n d a n c e . l 2 O The e x p e r i m e n t a l s p e c t r a show t h a t s u b s t i t u e n t s h a v e l i t t l e e f f e c t a n d t h e m a g n i t u d e of t h e h y p e r f i n e s p l i t t i n g c o n s t a n t s a n d I N D O c a l c u l a t i o n s b o t h s u p p o r t t h e c o n c l u s i o n t h a t t h e r e i s no a p p r e c i a b l e d e l o c a l i z a t i o n of t h e unpaired spin-density i n t h e s e r a d i c a l s . The f o r m a t i o n o f (CF3SI2N' r a d i c a l s [a(N) 1.320 and a ( 1 9 F ) . 0 . 1 9 5 mT3 f r o m (CF3SI2NN(SCF3l2 by h o m o l y s i s i s r e v e r s i b l e . 1 2 ' The r a t e o f t h e h o m o l y s i s r e a c t i o n h a s been s t u d i e d , h o w e v e r , by s c a v enging t h e thioaminyl r a d i c a l s with s t a b l e galvinoxyl r a d i c a l s [ E a ( c l e a v a g e ) 77 k J mol" 1 . 0

The f i r s t r e p o r t o f t h e g e n e r a t i o n o f L i - a l k y l - A - ( a l k y l t h i o ) a m i n y l r a d i c a l s , R1 ( b y hydrogen-atom a b s t r a c t i o n from t h e p a r e n t a m i n e s ) h a s b e e n made.122 The i n t e r p r e t a t i o n o f t h e i r s p e c t r a i n d i c a t e s t h a t b o t h a(N) a n d i l ( k 3 - H ) a r e r e l a t i v e l y Amidinyl r a d i c a l s i n s e n s i t i v e t o t h e n a t u r e of R1 and R 2 .

NsR2,

5: Organic Radicals in Solution

187

( A r S i C ( A r ) = N S A r ) form a n o t h e r n o v e l c l a s s of p e r s i s t e n t n i t r o g e n c e n t r e d r a d i ~ a 1 s . l ~These ~ r a d i c a l s h a v e e.s. r . s p e c t r a c o n s i s t i n g o f a s i m p l e 1:2:3:2:1 q u i n t e t [ a ( 2 N ) B . 0 . 6 mT1. Earlier i n t h i s r e p o r t I d e s c r i b e d t h e r e a c t i o n of &,I’-di-t-butyl1 , 4 - d i a z a - 1 , 3 - b u t a d i e n e w i t h M ( C O I 5 t o form r a d i c a l ( 4 1 ) . l 1 3 T h i s b u t a d i e n e h a s a l s o been f o u n d t o r e a c t w i t h ‘ S i P h 3 t o g i v e t h e c o n f o r m e r s o f t h e a p p r o p r i a t e a d d u c t s [ButN(X)?HCH=NButl a n d w i t h ‘GePh3 t o g i v e b o t h t h e cis and trans c o n f o r m e r s . 1 2 4 The e . s . r . s p e c t r a o f a number o f c y c l i c h y d r a z y l r a d i c a l s h a v e been r e p o r t e d . These i n c l u d e some s u b s t i t u t e d v e r d a z y l r a d i c a l s ( 4 4 ) formed by hydrogen-atom a b s t r a c t i o n from t h e p a r e n t m o l e c u l e . 1 2 5 The e . s . r . s p e c t r u m o f [ ( 4 4 ) , X = S and R = H I , i n t o l u e n e a t 220 K , h a s been i n t e r p r e t e d i n terms o f a ( 2 - , 4 - N ) 0 . 6 3 7 , a ( l - , 5 - N ) 0 . 5 4 5 , a ( 3 H ) 0 . 5 4 5 , and a ( 6 H ) 0 . 0 5 3 mT. In t h i s radical t h e v a l u e of a(l-,5-N) i s c o n s i d e r a b l y l a r g e r and t h a t of a(3H) s i g n i f i c a n t l y s m a l l e r t h a n i n most known v e r d a z y l r a d i c a l s l e a d i n g t o t h e conclusion t h a t t h e r e i s a non-planar arrangement a t t h e tricoordinate nitrogen. The s p e c t r a of two u n u s u a l five-membered r i n g r a d i c a l s h a v e a l s o been i n t e r ~ r e t e d . ’ ~ The ~ , ~ s~p l~i t t i n g constants for the substituted tetrazolyl radicals (45) indicate t h a t t h e y a r e n e a r l y p l a n a r w i t h t h e s p i n - d e n s i t y on N-I and N-4 a l m o s t i n d e p e n d e n t of t h e s u b s t i t u e n t . 1 2 6 I t h a s been e s t a b l i s h e d t h a t 4-phenyl-4H-1,2,4-triazoline-3,5-dione behaves a s an e f f i c i e n t s c a v e n g e r f o r a v a r i e t y o f r a d i c a l s f o r m i n g r a d i c a l s ( 4 6 ) by a d d i t i o n t o t h e N = N bond.127 The e . s . r . s p e c t r a of t h e s e r a d i c a l s h a v e been i n t e r p r e t e d a n d h a v e a ( 1 - N ) a.0 . 7 5 , a ( 2 - N ) m. 0 . 6 , a n d a ( 4 - N ) m. 0 . 1 4 mT, w i t h h y p e r f i n e c o u p l i n g t o t h e a d d e d g r o u p a l s o observed where a p p r o p r i a t e . Experiments with s p e c i f i c a l l y l a b e l l e d d e r i v a t i v e s may b e r e q u i r e d b e f o r e a n a b s o l u t e a s s i g n m e n t of t h e s p l i t t i n g c o n s t a n t s t o N-1 a n d N-2 can be c o n f i r m e d . The e . s . r s p e c t r u m of a n o n - c y c l i c h y d r a z y l , R 1 2 N h C O R 2 , h a s b e e n r e c o r d e d i n t o l u e n e o v e r t h e t e m p e r a t u r e r a n g e 183-333 K.128 The two R1 g r o u p s ( R 1 = d i - t - b u t y l p h e n y l ) a r e i n e q u i v a l e n t due t o r e s t r i c t e d r o t a t i o n a b o u t t h e N-N bond ( A H * 2 5 . 3 kJ mol” 1 .

X e M Me , ,‘N! ,

I

R2‘N-

I

N ‘Y RN

N’

I

I

”y

N’

R1

188

Electron Spin Resonance 0

(46)

The r a n g e of new r e p o r t s o f i m i n y l s i n c l u d e some q u i t e u n u s u a l radicals. The Me2C=N* r a d i c a l [ a ( N ) 0 . 9 9 and a ( 6 H ) 0 . 1 3 8 mT1 c a n be p r e p a r e d by h e a t i n g M e 2 C ( H ) N 3 and (But0NI2 i n b e n z e n e and i t s e . s . r . s p e c t r u m h a s been r e p o r t e d p r e v i o u s l y . I t i s now r e p o r t e d t h a t i n t h e p r e s e n c e o f ButNO, a s a s p i n - t r a p , t h e o v e r l a p p i n g s p e c t r u m o f t h r e e r a d i c a l s i s o b t a i n e d . 1 2 9 The i n d i v i d u a l s p e c t r u m h a v e been a s s i g n e d t o B u ~ O N ( ~ ) B UBut2N0. ~, , and Me2C(N3)N(6)But b u t no s p e c t r u m a t t r i b u t a b l e t o t h e t r a p p e d Me2C=N' r a d i c a l c o u l d be d e t e c t e d . One o f t h e more u n u s u a l s t u d i e s i n v o l v e s t h e p r e p a r a t i o n o f M C H = N ' r a d i c a l s ( M = Cu, Ag, o r Au) e m p l o y i n g t h e r o t a t i n g c r y o s t a t t e c h n i q u e . I3O I n some r e s p e c t s t h e s e r a d i c a l s a r e a n a l o g o u s t o t h e i s o e l e c t r o n i c o r g a n o m e t a l l i c v i n y l s , M C H = e H , and t h e y a p p e a r t o e x i s t i n two i s o m e r i c f o r m s a l t h o u g h t h e a b s e n c e of r e s o l v e d I 4 N - h y p e r f i n e c o u p l i n g when M = Cu o r Au makes unambiguous i d e n t i f i c a t i o n of t h e isomers r a t h e r d i f f i c u l t . The c y c l i c 2,2,6,6-tetramethylcyclohexyliminyl r a d i c a l h a s b e e n p r e p a r e d by p h o t o l y s i s o f ( B u ~ O )w~i t h d i p h e n y l m e t h y l o x i m e e t h e r i n t o l u e n e

[a(N) 1 . 0 4 and a ( 6 H ) 0 . 0 6 mT1 .13' The e q u a t o r i a l m e t h y l g r o u p s a r e responsible f o r t h e observed proton hyperfine coupling. This r a d i c a l i s r e l a t i v e l y u n s t a b l e however and d e c a y s a ringopening r e a c t i o n a t higher temperatures. F i n a l l y i n t h i s s e c t i o n I have g a t h e r e d t o g e t h e r t h o s e p a p e r s d e a l i n g w i t h amidyls and t r i a z e n y l s . A range of four- and f i v e membered r i n g s u l p h o n a m i d y l s s u c h a s ( 4 7 ) a n d ( 4 8 ) h a v e been g e n e r a t e d i n CH2C12/FCC13 by p h o t o l y s i s o f t h e c o r r e s p o n d i n g s u l t a m s and c h a r a c t e r i z e d from t h e i r e . s . r . s p e c t r a . 132 From t h e o b s e r v e d v a l u e s of t h e I 4 N - s p l i t t i n g c o n s t a n t i t i s deduced t h a t t h e unpaired e l e c t r o n o c c u p i e s an o r b i t a l o f r - c h a r a c t e r . The magnit u d e and t e m p e r a t u r e d e p e n d e n c e o f t h e e . s . r . p a r a m e t e r s o f t h e r e l a t e d a c y c l i c a m i d y l s RhS02Me and RhC(0)OEt i n c y c l o p r o p a n e a l s o i n d i c a t e a n - e l e c t r o n i c g r o u n d s t a t e . 1 3 3 N e a r l y a l l of t h e s e l a t t e r r a d i c a l s decay (by bimolecular s e l f - r e a c t i o n ) a t r a t e s c l o s e t o the diffusion-controlled l i m i t . The e . s . r . s p e c t r a o f 1,3-

5: Organic Radicals in Solutiori

189

d i a l k y l t r i a z e n y l r a d i c a l s R N N N R h a v e s p l i t t i n g c o n s t a n t s o f a( N ) 1 . 1 5 a n d a ( 2 N ) B. 0.40 mT c o n s i s t e n t w i t h a a - r a d i c a l w i t h t h e s i n g l y o c c u p i e d m o l e c u l a r o r b i t a l i n t h e N N N p l a n e . 1 3 4 However t h e c y c l i c t r i a z e n y l [ ( 4 9 ) , R = Me1 h a s a(2-N) 1 . 3 0 , a(3-N) 0 . 2 6 , a n d a(31P>0 . 7 3 mT a t 219 K , b u t a t h i g h e r t e m p e r a t u r e s a(1-N) a n d a(3-N) become e q u i v a l e n t 10.14 a n d a(31P) 1 . 2 3 mT1 d u e t o r a p i d e x c h a n g e a s a r e s u l t of i n t e r c o n v e r s i o n . The e . s . r . p a r a m e t e r s f o r t h e s e r a d i c a l s i n d i c a t e t h a t , a s f o r ( 4 7 ) and (481, t h e unpaired e l e c t r o n o c c u p i e s an o r b i t a l of n - c h a r a c t e r , w i t h t h e phosphorous and t h e t h r e e n i t r o g e n atoms probably c o p l a n a r .

u.

There i s a n a t u r a l i n t e r e s t i n t o c o p h e r o l s because of t h e i r i m p o r t a n c e i n b i o l o g i c a l systems a n d t h i s h a s been r e f l e c t e d by t h e p u b l i c a t i o n o f s e v e r a l p a p e r s c o n c e r n e d w i t h r a d i c a l s d e r i v e d from t o c o p h e r o l and some of i t s model compounds. 35-139 Typically the h y p e r f i n e s p l i t t i n g c o n s t a n t s r e p o r t e d by t h e v a r i o u s r e s e a r c h g r o u p s f o r ( 5 0 ) a r e a(2H) 0.139-0.155 mT and t h r e e a ( 3 H ) v a l u e s w i t h r a n g e s 0.602-0.635, 0.441-0.472, and 0.075-0.106 mT a l t h o u g h t h e r e i s a l s o e v i d e n c e of s i g n i f i c a n t s o l v e n t d e p e n d e n c e . ' 35 Two research groups have independently assigned t h e s e s p l i t t i n g constants. N i k i e t al. have i n t e r p r e t e d t h e e . s . r . s p e c t r a o f a-, B-, and y - t o c o p h e r o l s a n d c o n c l u d e d t h a t t h e l a r g e s t m e t h y l s p l i t t i n g c o n s t a n t i s a s s o c i a t e d w i t h t h e Me g r o u p a t C-5, and t h e s m a l l e s t w i t h t h e Me g r o u p a t C-8.136 Matsuo et a l . h a v e employed d e u t e r i a t i o n a s a n a i d t o a s s i g n m e n t a n d h a v e drawn t h e same conc l u s i o n . 137 (An a s s i g n m e n t b a s e d s o l e l y upon I N D O c a l c u l a t i o n s , however, c o n f l i c t s w i t h t h e s e c o n c l u s i o n s ) It a p p e a r s t h a t both conformational and p o l a r f a c t o r s i n f l u e n c e t h e a n t i o x i d a n t a c t i v i t y of a-tocopherol

and d e t a i l e d r e s u l t s o f s u p p o r t i n g k i n e t i c

s t u d i e s a r e p r o m i s e d i n t h e n e a r f u t u r e . 1 3 8 The v i t a m i n K1 chromanoxyl r a d i c a l ( 5 1 ) i s a l s o of i n t e r e s t i n b i o l o g i c a l s y s t e m s . T h i s r a d i c a l h a s a ( 3 H ) 0 . 8 0 6 , a(8-H) = a ( l 0 - H ) = a ( 2 H ) 0 . 1 6 8 , and a(7-HI = a(9-HI 0 . 0 4 2 mT.' 40 P r o v i s i o n a l a s s i g n m e n t s h a v e been

Electron Spin Resonance

190

made w i t h t h e a i d o f McLachlan MO c a l c u l a t i o n s b u t t h e a c c i d e n t a l e q u i v a l e n c e o f s e v e r a l of t h e s p l i t t i n g c o n s t a n t s makes a b s o l u t e a ssi gnm en t s d i f f i c u l t

.

R

A l a r g e number o f t h e r e p o r t e d o x y g e n - c e n t r e d r a d i c a l s c a n be c o n s i d e r e d t o be 4 - s u b s t i t u t e d 2,6-di-t-butylphenoxyls. These r a d i c a l s a r e g e n e r a l l y formed by removal o f t h e h y d r o g e n atom from t h e OH g r o u p of t h e c o r r e s p o n d i n g p h e n o l . The e . s . r . s p e c t r u m of o n e s u c h a r a d i c a l , w i t h a crown e t h e r d e r i v a t i v e a s t h e s u b s t i t u e n t ( 5 2 1 , h a s been i n v e s t i g a t e d i n o r d e r t o o b t a i n s t r u c t u r a l i n f o r m a t i o n upon c o m p l e x a t i o n w i t h v a r i o u s a l k a l i and a l k a l i n e e a r t h ions.14’ The s p e c t r a i n d i c a t e t h e p r e s e n c e of t h r e e p o s s i b l e s p e c i e s , LL, t h e monomer complex a n d two d i m e r c o m p l e x e s ( o n e w i t h t h e oxygen a t o m s o f t h e crown e t h e r n e a r l y c o i n c i d e n t a n d t h e o t h e r w i t h them e s s e n t i a l l y e v e n l y s t a g g e r e d ) . When t h e s u b s t i t u e n t i s i t s e l f s t e r i c a l l y crowded t h e s p i n - d e n s i t y c a n be l o c a l i z e d i n t h e phenoxy r i n g . Such i s t h e c a s e when t h e s u b s t i t u e n t i s a n a n t h r a c e n e d e r i v a t i v e when c o u p l i n g i s o n l y o b s e r v e d t o t h e two p p r o t o n s o f t h e phenoxy g r o u p . 42 S e v e r a l s t u d i e s h a v e examined phenoxyls with a h e t e r o c y c l i c group a t t a c h e d t o t h e 4-position t h r o u g h a CH2 b r i d g e . 1 4 3 - 1 4 5 F o r e x a m p l e , when t h e s u b s t i t u e n t i s ( 5 3 ) t h e s p e c t r u m can be i n t e r p r e t e d i n terms o f a(m-H) 0 . 1 9 , a(N) S i m i l a r s t u d i e s have i n c l u d e d sub0 . 1 3 , and a ( 2 H ) 1 . 1 5 mT.143 s t i t u e n t s s u c h a s -CH2NMe2144 and ( 5 4 ) .145 I n b o t h of t h e s e l a t t e r

r a d i c a l s t h e s p e c t r a i n d i c a t e t h a t t h e r e i s nitrogen-atom inversion. The s p e c t r u m of a 6 - s u b s t i t u t e d 2 , 4 - d i - t - b u t y l p h e n o x y l h a s a l s o been r e p o r t e d . ’ 4 6

5 : Organic Radicals in Solution

191

The s i l v e r o x i d e o x i d a t i o n of 4-methoxy-3-t-butylphenol

in

C C 1 4 r e s u l t s i n a s p e c t r u m due t o a m i x t u r e o f two p h e n o x y l

r a d i c a l s a s s i g n e d t o t h a t d e r i v e d from t h e p h e n o l a n d t o t h a t f r o m i t s C-C c o u p l e d d i m e r . 1 4 7 F u r t h e r o x i d a t i o n e v e n t u a l l y r e s u l t s i n a s p e c t r u m c o n s i s t e n t w i t h t h e f o r m a t i o n o f a phenoxyphenoxyl (55). The s i m i l a r o x i d a t i o n o f b i p h e n y l - 2 , 2 ' - d i o l s p r o d u c e s a monoarylo x y l r a d i c a l w i t h t h e u n p a i r e d e l e c t r o n s h a r e d e q u a l l y between b o t h r i n g s . 1 4 8 The s p e c t r u m of t h e r a d i c a l d e r i v e d from t h e d i o l (56) i s temperature dependent possibly a s a r e s u l t of an e q u i l i b r i u m between t h e l e f t - h a n d a n d r i g h t - h a n d t w i s t e d c o n f o r m a t i o n s . A f u r t h e r r e p o r t o f p h e n o x y l r a d i c a l s p r e p a r e d from b i p h e n y 1 - 2 , 2 ' d i o l s ( t o g e t h e r w i t h t h o s e p r e p a r e d from b i p h e n y l - 4 , 4 1 - d i o l s ) , by hydrogen-atom a b s t r a c t i o n w i t h ButOO' i n n o n - p o l a r s o l v e n t s , a l s o confirms a n equal unpaired e l e c t r o n d i s t r i b u t i o n i n both rings."19 It i s s u g g e s t e d t h a t t h i s i s a c h i e v e d U a t a u t o m e r i c e q u i l i b r i u m . S i m i l a r p h e n o x y l r a d i c a l s c a n be g e n e r a t e d from 2 , 2 ' - and 4 , 4 ' a l k y l i d e n e b i s p h e r ~ o l s . ~F~o~r example [ ( 5 7 ) , R' = B u t , R2 = R3 = H h a s a(3-,5-H) 0 . 1 7 5 a n d ~ ( 4 - H I 0.925 mT1. The s t a b i l i t y o f t h e s e r a d i c a l s a p p e a r s t o be i n d e p e n d e n t o f t h e n a t u r e of R2 a n d R3 and they subsequently transform t o give galvinoxyl type r a d i c a l s . These r e a c t i o n s p r o v i d e i n t e r e s t i n g m o d e l s f o r t h e t r a n s f o r m a t i o n s of p h e n o l i c a n t i o x i d a n t s .

R1

R1

192

Electron Spin Resonance

RO

(56)

The r a d i c a l s d e r i v e d from ( + ) c a t e c h i n a r e o f i n t e r e s t a s t h e s e compounds a r e w i d e l y d i s t r i b u t e d t h r o u g h o u t t h e p l a n t kingdom. A l k a l i n e a i r o x i d a t i o n of ( + ) c a t e c h i n l e a d s t o a primary r a d i c a l ( 5 8 ) , 1 5 1 Above pH 1 2 . 8 , however, s e c o n d a r y r a d i c a l s a r e o b s e r v e d A t pH > 13.6 a due t o hydroxylation a t p o s i t i o n s 2 ' and 6'. f u r t h e r s p e c t r u m i s o b s e r v e d which h a s been a s s i g n e d t o r a d i c a l [ ( 5 9 ) , w i t h a p p r o p r i a t e h y d r o x y l a t i o n a t p o s i t i o n s 2 ' a n d 6'3 d e r i v e d from t h e s e s e c o n d a r y r a d i c a l s . S p e c t r a which c o u l d be a s s o c i a t e d w i t h t h e d e g r a d a t i o n of ( + ) c a t e c h i n t o s i m p l e r p h e n o x y l s were n o t o b s e r v e d however. 0-

I

OH (59)

I n a most i n t e r e s t i n g d e v e l o p m e n t R a k o c z i and C u n t h a r d o z o n o l y s i s of e t h e n e a n d t h e s u b s e q u e n t t r a p p i n g o f t h e r a d i c a l s formed i n a n argon m a t r i x f o r e.s.r. study.'52 The r a d i c a l s d e t e c t e d depend t o some e x t e n t on r e a c t o r r e s i d e n c e times a n d upon t h e p r e s e n c e o f oxygen i n t h e i n i t i a l r e a c t i o n m i x t u r e . I n t h e a b s e n c e o f oxygen * C H 3 and H ' r a d i c a l s c a n be d e t e c t e d t o g e t h e r w i t h some e v i d e n c e I n t h e p r e s e n c e of oxygen n e i t h e r ' C H 3 f o r t h e formation of 'C2H5. o r H' a r e d e t e c t e d . The f u l l r e a c t i o n scheme i s u n d o u b t e d l y complex b u t i n c l u d e s t h e f o r m a t i o n o f 'C2H5 by hydrogen-atom a d d i t i o n t o e t h e n e a n d t h e f o r m a t i o n of p e r o x y r a d i c a l s . The comp l e x i t y of t h e s e r e a c t i o n s i s f u r t h e r i l l u s t r a t e d by a s p i n - t r a p s t u d y o f t h e o z o n o l y s i s o f t e t r a m e t h y l e t h e n e . 153 T h e s e l a t t e r experiments have resulted i n t h e conclusion t h a t a h y d r o t r i o x i d e , ROOOH, a c t s a s a r a d i c a l p r e c u r s o r . The two m a j o r s p e c i e s o b s e r v e d describe a m i c r o r e a c t o r f o r s t u d y i n g t h e gas-phase

5: Organic Radicals in Solution

193

with N-t-butyl-a-phenylnitrone

a r e a t t r i b u t e d t o t h e t r a p p i n g of

RC(0)O’ and ROO’ r a d i c a l s . The Co2+ i n d u c e d d e c o m p o s i t i o n o f d i - t - b u t y l h y d r o p e r o x i d e g i v e s b o t h a l k o x y a n d peroxy r a d i c a l s ( E q u a t i o n s 7 a n d 8).15‘ T h e s e r a d i c a l s h a v e b o t h been t r a p p e d by I-t - b u t y l - a - p h e n y l n i t r o n e t o give e.s.r. d i s t i n g u i s h a b l e products. I n some r e s p e c t s , however, methyl-4-durylnitrone a c t s a s a b e t t e r s p i n - t r a p g i v i n g a(N) 1.280 and a(H) 0.461 mT when t r a p p i n g Me3COO’ and a(N) 1.410 Alkylperoxy r a d i c a l s can a n d a(H) 0.747 mT when t r a p p i n g Me3CO’. a c t a s chain propagating s p e c i e s (Equation 9). A k i n e t i c study of t h e i r r e a c t i o n s i s t h u s a l w a y s o f v a l u e and e . s . r . s p e c t r o s c o p y h a s now been u s e d t o o b t a i n s u c h i n f o r m a t i o n when AH i s a n a r y l a m i r ~ e . ’ ~The ~ a c t i v a t i o n e n e r g y l i e s i n t h e r a n g e 5-I< k J mol” f o r t h o s e arnines s o f a r s t u d i e d . Me3COOH

+

Co2+

--+

Me3COOH

+

Co3+

+ Me3COO’

ROO

+

AH

Me3CO’

+

ROOH

+ +

OH’

H+

+

+

Co3+

(7)

+

Co2+

(8)

A’

(9)

I n t h i s s e c t i o n of t h e r e p o r t , d e a l i n g w i t h n i t r o x i d e s , I have c o l l e c t e d t o g e t h e r t h o s e p a p e r s where t h e main i n t e r e s t i s i n t h e n i t r o x i d e i t s e l f r a t h e r t h a n i n t h e i d e n t i f i c a t i o n of p r i m a r y r a d i c a l s i n s p i n trapping experiments. I have t r i e d , where p o s s i b l e , t o d e a l w i t h papers of t h e l a t t e r t y p e i n t h e s e c t i o n a p p r o p r i a t e t o t h e primary r a d i c a l . I n t h e p a s t t h e r e h a s been some i n t e r e s t i n t h e e . s . r . s p e c t r a of n i t r o x i d e s c o n t a i n i n g h a l o g e n s u b s t i t u e n t s ( s u c h a s CF3N(d)Xl. R e s t r i c t e d r o t a t i o n o f t h e CF3 g r o u p i s o f t e n o b s e r v e d i n t h e s e r a d i c a l s a n d c a n be a t t r i b u t e d t o n o n - p l a n a r i t y a t t h e r a d i c a l centre. A f u r t h e r example o f r e s t r i c t e d r o t a t i o n h a s now been r e p o r t e d i n CF3N(b)OCMe3 i n which t h e t h r e e f l u o r i n e a t o m s become i n e q u i v a l e n t a t low t e m p e r a t u r e [ a t 104 K a(19F) 1.997, 0.229, and 0.089 mT1, b u t a r e e q u i v a l e n t a t h i g h e r t e m p e r a t u r e s [ a t 267 K 8 ( 3 1 9 F ) 0.606 The a c t i v a t i o n e n e r g y f o r t h i s h i n d e r e d r o t a t i o n , 5 9 t 4 k J m o l ” , h a s been d e t e r m i n e d by c o m p u t e r s i m u l a t i o n of t h e s p e c t r a i n t h e i n t e r m e d i a t e t e m p e r a t u r e r e g i o n .

Electron Spin Resonance

194

Halogen s u b s t i t u t e d n i t r o x i d e s , formed a s r e a c t i o n p r o d u c t s , h a v e a l s o been s t u d i e d . ' 57 f 1 58 N i t r o x i d e s o f t h e t y p e ArN(6)CF2CXY ( X and Y F, C 1 , o r B r ) h a v e been i n v e s t i g a t e d u s i n g 2,4,6t r i c h l o r o n i t r o s o b e n z e n e a s a s p i n - t r a p . 57 Two n i t r o x i d e s a r e formed. A t low t e m p e r a t u r e s ArN(b)CF2CXYR i s f o u n d t o p r e d o m i n a t e b u t a t h i g h t e m p e r a t u r e ArN(b)CXYCF2R p r e d o m i n a t e s . A large h y p e r f i n e coupling t o 79/81Br h a s been r e p o r t e d i n Me3C6(")CHCH(Br)N(b)CMe3 C a(N) 1 . 1 1 , a(B-79Br) 3 . 4 2 and a ( p a l B r ) 3.71 mT1. T h i s r a d i c a l i s formed d u r i n g t h e p h o t o l y s i s o f Br2 i n b e n z e n e i n t h e p r e s e n c e of g l y o x a l b i s ( t - b u t y l n i t r o n e ) .'58 I t h a s now been f i r m l y e s t a b l i s h e d t h a t t h e I 4 N h y p e r f i n e s p l i t t i n g constant i n simple nitoxides increases with s o l v e n t polarity. The l i n e a r r e l a t i o n s h i p between b o t h a(N) a n d a(B-H) and s o l v e n t p o l a r i t y found i n n i t r o x i d e s of t h e g e n e r a l t y p e XCH(Ph)N(b)CMe3 may a l l o w s p l i t t i n g c o n s t a n t s t o be p r e d i c t e d i n other solvents.'59 However, t h e r e l a t i o n s h i p between t h e s e p a r a m e t e r s i s n o t always s t r a i g h t f o r w a r d . F o r e x a m p l e , when X = RCHOH t h e OH g r o u p o f t h e r a d i c a l c a n a c t a s a h y d r o g e n d o n o r i n a p r o t i c s o l v e n t s , t h u s i n f l u e n c i n g t h e v a l u e o f a( B - H I I 6 O A n o t h e r a s p e c t o f t h e s t u d y o f s o l v e n t e f f e c t s i s t h e s t u d y of r o t a t i o n a l c o r r e l a t i o n times i n r a d i c a l s s u c h a s ( B u ~ ) , N ( ~ ) . ' ~ ' One m i g h t e x p e c t t h e n i t r o x i d e g r o u p t o h a v e e i t h e r a p l a n a r o r p y r a m i d a l g e o m e t r y . The t e m p e r a t u r e d e p e n d e n c e of t h e s p e c t r u m o f Me3CN(b)H i s t h e r e f o r e of i n t e r e s t . 1 6 , Both v a r i a t i o n o f a(N) mT K'l and a n d a(cc-H) w i t h t e m p e r a t u r e ( d a ( a - H ) / d I - 8 . 4 x da(N)/dlT 2 . 3 x mT K-') and t h e i r a b s o l u t e v a l u e s a r e c o n s i s t e n t with an i n c r e a s i n g amplitude of t h e out-of-plane bending with temperature. The p r e f e r r e d c o n f o r m a t i o n i n n i t r o x i d e s formed by t h e a d d i t i o n o f o r g a n o m e t a l l i c g r o u p s a p p e a r s t o depend upon s e v e r a l factor^.^^^-'^^ I n a r a n g e of n i t r o x i d e s o f g e n e r a l f o r m u l a RN(b)CH2X t h e f3-SiMe3 e x e r t s a s t r o n g i n f l u e n c e on t h e

.

p r e f e r r e d c o n f o r m a t i o n . 163 G e n e r a l l y i n r a d i c a l s o f t h i s t y p e a( N ) a n d a(B-HI b o t h i n c r e a s e w i t h s o l v e n t p o l a r i t y . T h i s h a s a l s o been f o u n d t o be t h e c a s e when R = Ph and X = H o r Me. However, when R = Ph and X SiMe3 a(N) i n c r e a s e s b u t a(B-H) d e c r e a s e s i n d i c a t i n g t h a t t h e s o l v e n t h a s a s i g n i f i c a n t i n f l u e n c e on t h e c o n f o r m a t i o n a l preference. The n i t o x i d e s R C H ( X ) N ( b ) H a d o p t a s i n g l e c o n f o r m a t i o n when X = MeS o r BUS, b u t when X = E t 3 S i o r Ph3Ge t h e r e l a t i v e p o p u l a t i o n s o f t h e p o s s i b l e r o t a m e r s i s i n f l u e n c e d by t h e I n d i a r y 1 n i t r o x i d e s r o t a t i o n of t h e a r y l r i n g s c a n be

5: Organic Radicals in Solution

195

quite significant. R a d i c a l s o f t h e t y p e PhCH(SiMe3)N(b)CMe3 a r e c h a r a c t e r i z e d by a l a r g e a(B-H) and a s i g n i f i c a n t a ( 2 9 S i ) o f sa. 1 . 3 mT a s a r e s u l t o f U-IT d e l o c a l i z a t i o n between t h e s e m i - o c c u p i e d IT*-orbital and t h e C-Si u - o r b i t a l . 1 6 6 The m a g n i t u d e of t h e s p l i t t i n g c o n s t a n t s i n naphth-1-yl p h e n y l n i t r o x i d e s i n d i c a t e s t h a t t h e n a p h t h y l r i n g i s t w i s t e d o u t o f c o n j u g a t i o n w i t h t h e NO g r o u p t o a g r e a t e r e x t e n t t h a n t h e phenyl ring.167 The s e a r c h f o r s p i n - t r a p s s u i t a b l e f o r p a r t i c u l a r a p p l i c a t i o n s continues t o a t t r a c t attention. This i s nicely i l l u s t r a t e d in a p a p e r by J a n z e n e t i n which s p i n - t r a p s of d i f f e r i n g s o l u b i l i t y h a v e been u s e d t o p r o b e r e a c t i o n s o c c u r r i n g i n t h e a q u e o u s p h a s e a n d i n t h e i n t e r i o r of some m i c e l l e s . 1 6 8 Sodium 2 - s u l p h o n a t o p h e n y l ( t - b u t y l l n i t r o n e h a s been u s e d t o p r o b e r a d i c a l e v e n t s i n t h e a q u e o u s p h a s e o f t h e s e s y s t e m s , and 4-dodecyloxyphenyl(t-butyl)nitrone t o detect radicals i n the micellar interior. The well known t r a p , 4-t-butyl-a-phenylnitrone, a p p e a r s t o t r a p r a d i c a l s i n A n o t h e r well known s p i n - t r a p , ButNO, h a s b e e n u s e d t o both phases. d e t e c t t h e r a d i c a l s formed d u r i n g t h e y - i r r a d i a t i o n of u r a c i l i n a q u e o u s s o l u t i o n . 1 6 9 The e . s . r . s p e c t r a o b t a i n e d d u r i n g t h e s e e x p e r i m e n t s i n d i c a t e t h a t t h e r a d i c a l s a r e p r o d u c e d by ‘OH a d d i t i o n t o e i t h e r C-6 o r C-5. Also f o r r e a c t i o n s i n aqueous s o l u t i o n 4 - n i t r o s o p y r i d i n e 1 - o x i d e a p p e a r s t o be a h i g h l y e f f i c i e n t t r a p f o r b o t h oxygen- and c a r b o n - c e n t r e d r a d i c a l s . 170 However, n o t a l l r e a c t i o n s of r a d i c a l s w i t h s p i n - t r a p s l e a d t o n i t r o x i d e s . For e x a m p l e , t h e r e a c t i o n of 2,4,6-tri-t-butylnitrosobenzene w i t h o r g a n o a l u m i n i u m compounds i n t h e p r e s e n c e o f s t r o n g L e w i s b a s e s g i v e s t h e a p p r o p r i a t e m e t h y l - o r e t h y l - n i t r o x i d e w i t h Me3A1, E t 3 A l , and E t A 1 C 1 2 , b u t t h e t - b u t y l - a n i l i n o r a d i c a l Ca(N) 1.01 and a(u-H) 0 . 1 8 mT1 w i t h But3A1.I7’ An u n u s u a l f e a t u r e o f t h e s e r e s u l t s i s t h e f o r m a t i o n o f a f u r t h e r r a d i c a l w i t h a(N) 0 . 5 3 , a ( 2 H ) 0 . 5 9 , and a ( m - H ) 0 . 1 3 mT upon r e a c t i o n w i t h E t A 1 C 1 2 which h a s been a s s i g n e d t o a phenoxyamino r a d i c a l [ A r O ( k ) E t l . A l t h o u g h most n i t r o x i d e s a r e formed by a n i n t e r m o l e c u l a r addition reaction an interesting intramolecular addition reaction o c c u r s b e t w e e n n i t r o b e n z a l d e h y d e a n d d i - t - b u t y l p e r o x y l a t e . 1 7 2 The r e a c t i o n i s t h o u g h t t o p r o c e e d y h a c y c l i c a c y l o x y r a d i c a l (60) t o

a.

g i v e t h e a r y l a m i n o x y l r a d i c a l ( 6 1 ) w i t h a(N) 1 . 3 7 5 , a(3-HI 0 . 2 8 2 , a(4-HI 0.095,- a ( 5 - H I 0 . 3 0 1 , a n d a ( 6 - H ) 0 . 0 9 5 mT1. Examples o f i n t e r m o l e c u l a r r e a c t i o n s a r e , however, a b u n d a n t a l t h o u g h s o m e t i m e s i n v o l v i n g r a t h e r complex mechanisms s u c h a s t h a t o b s e r v e d f o r t h e r e a c t i o n between 5,7-di-t-butyl-3,3-dimethyl-3&-indole 1 - o x i d e and

Electron Spin Resonance

196

Crignard reagents.173 A f u r t h e r example i s t h e r e a c t i o n between t h y m i n e a n d tetramethylpiperidin-1-oxyl b r o m i d e t o g i v e t h y m i n e g l y c o l and t h e w e l l known 2,2,6,6-tetramethylpiperidin-l-oxyl. 174 Not a l l r e a c t i o n s , of c o u r s e , p r o c e e d t o c o m p l e t i o n a n d e . s . r . s p e c t r o s c o p y c a n be employed t o m o n i t o r e q u i l i b r i u m c o n c e n t r a t i o n s p r o v i d i n g t h e s p e c t r a of t h e r a d i c a l s i n v o l v e d a r e r e a s o n a b l y s i m p l e s o t h a t t h e y c a n be d i s t i n g u i s h e d from one a n o t h e r . A n i c e example of t h i s a p p l i c a t i o n i s t h e s t u d y o f t h e h y d r o g e n e x c h a n g e r e a c t i o n between PhC(0) N ( B u t ) O H and 2,2,6,6-tetramethyl-4-piperid o n e & - 0 x y 1 . ~ 7 ~ The e s t i m a t e d 0-H bond d i s s o c i a t i o n e n e r g i e s f o r N - t - b u t y l h y d r o x a m i c a c i d s a s a r e s u l t of t h e s e m e a s u r e m e n t s i s u. 330 kJ mol”

S o l v e n t and s u b s t i t u e n t e f f e c t s i n c y c l i c n i t r o x i d e s h a v e again attracted attention. T h e d e p e n d e n c e o f a(N) upon t h e n a t u r e of t h e s o l v e n t i n some 4 - s u b s t i t u t e d 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e - 1 - o x y l s h a s been i n ~ e s t i g a t e d la ~s ~h a s t h e i n f l u e n c e of t h e 4 - s ~ b s t i t u e n t . ~I n~ ~t h e l a t t e r c a s e a r e a s o n a b l e c o r r e l a t i o n between t h e e l e c t r o n e g a t i v i t y o f t h e s u b s t i t u e n t and a(N) h a s been established. The pKa v a l u e s f o r a number of i m i d a z o l i n e and i m i d a z o l i d i n e n i t r o x i d e s h a v e been d e t e r m i n e d from a s t u d y o f t h e i n f l u e n c e of pH on a(N) i n t h e p r o t o n a t e d a n d n o n - p r o t o n a t e d s t a t e s . 1 7 8 The s p e c t r a o f t h e s e n i t r o x i d e s a r e q u i t e s e n s i t i v e t o pH t h u s p r o v i d i n g p o s s i b l e pH p r o b e s p a r t i c u l a r l y i n b i o l o g i c a l s y s t e m s . C o u p l i n g t o 1 5 N h a s been o b s e r v e d i n some c l o s e l y r e l a t e d r a d i c a l s . 179 The i m p o r t a n c e o f crown e t h e r s a s c a t i o n c o m p l e x i n g a g e n t s h a s b e e n t h e s u b j e c t o f v e r y c o n s i d e r a b l e i n t e r e s t f o r some y e a r s . In some r e c e n t l y s y n t h e s i s e d s p i n l a b e l l e d crown e t h e r s i n which t h e N ( 6 ) g r o u p i s p o s i t i o n e d n e a r t h e ‘ c a v i t y ’ of t h e crown e t h e r t h e a n t i c i p a t e d i n f l u e n c e of c a t i o n c o m p l e x a t i o n upon a( N) i s , s u r p r i s i n g l y , a b s e n t ( a l t h o u g h t h e r e i s s e p a r a t e e v i d e n c e o f complexation)

.’’’

5: Organic Radicals in Solution

197

To c o n c l u d e t h i s s e c t i o n t h e r e a r e two i n t e r e s t i n g p a p e r s conc e r n i n g some i m i n o x y l r a d i c a l s . The Pb(OAcI4 o x i d a t i o n o f a r y l a z o a l d e h y d e o x i m e s y i e l d s t h e c o r r e s p o n d i n g irninoxyl r a d i c a l s

CR1N:NC(R2):N(6>l.181 The e . s . r . s p e c t r a of t h e s e r a d i c a l s h a v e a ( 1 - N ) ixa. 1 . 1 5 , a ( 2 - N ) m . 0 , and a ( 4 - N ) m. 1 . 9 5 mT. These r a d i c a l s , t h e r e f o r e , a p p e a r t o e x i s t i n a c o n f o r m a t i o n i n which N-1 i s a d j a c e n t t o t h e oxygen atom w i t h a s i g n i f i c a n t t h r o u g h s p a c e interaction. The c y c l i z a t i o n r e a c t i o n s o f some i m i n o x y l s h a v e a l s o been s t u d i e d . The i m i n o x y l [ ( 6 2 ) , X N l cyclizes t o give the c o r r e s p o n d i n g i n d a z o l e , b u t no c y c l i z a t i o n r e a c t i o n c o u l d be d e t e c t e d f o r t h e c o r r e s p o n d i n g i m i n y l r a d i c a l . 182 The i m i n o x y l [ ( 6 2 ) , X = C H , R = Me3 a l s o c y c l i z e s t o g i v e a r a d i c a l w i t h a(N) ca. 2 . 0 0 , a ( 4 H ) 0 . 0 9 , and a(H) 0 . 5 2 5 mT. The m a g n i t u d e o f a(N) s u g g e s t s t h a t a b i c y c l i c n i t r o x i d e ( 6 4 ) h a s been formed & r a d i c a l (63).

Compared w i t h o t h e r n e u t r a l r a d i c a l s v e r y few r e p o r t s h a v e been made of s u l p h u r - c e n t r e d r a d i c a l s . One r e p o r t c o n c e r n s t h e measurement o f d e c a y r a t e s o f t h i y l r a d i c a l s (p-RC6H4S’) i n t h e p r e s e n c e o f s p i n - t r a p s . ’ 83 The r a t e c o n s t a n t f o r r e a c t i o n w i t h e a c h s p i n - t r a p (PhNO, C6Me5N0, and ButNO) d e p e n d s upon t h e p - s u b s t i t u e n t b u t d e c r e a s e s i n t h e s e q u e n c e CgMegNO > PhNO > ButNO. 2 - M e t h y l - 2 - n i t r o s o p r o p a n e h a s a l s o been u s e d t o t r a p ROC(S>S* r a d i ~ a 1 s . l ~ ~ e.s.r. s p e c t r a of t h e r e s u l t i n g n i t r o x i d e s a r e The v i r t u a l l y i n d e p e n d e n t o f t h e n a t u r e of R. Thiuram d i s u l p h i d e s a r e a c c e l e r a t o r s i n t h e v u l c a n i z a t i o n of r u b b e r and t h e i r t h e r m a l d i s s o c i a t i o n i n t o d i t h i o c a r b a m a t e t h i o r a d i c a l s , R2NC(S)S’, i s t h e r e f o r e o f i m p o r t a n c e . 185 The s p e c t r a of t h e s e r a d i c a l s c o n s i s t s of a s i n g l e b r o a d a b s o r p t i o n , b u t t h e i n t e n s i t y v a r i a t i o n o f t h i s a b s o r p t i o n w i t h t e m p e r a t u r e e n a b l e s AHo t o be d e t e r m i n e d (98-1 17 kJ mol”). The k i n e t i c s o f t h i s s y s t e m i n d i c a t e t h a t t h e r a t e

198

Electron Spin Resonance

c o n s t a n t f o r r a d i c a l recombination i s c l o s e t o t h e d i f f u s i o n controlled l i m i t . The e . s . r . s p e c t r a o f c o m p l e x e s o f s i m p l e t h i y l r a d i c a l s w i t h A 1 B r 3 and GaC13 h a v e been r e p o r t e d . 1 8 6 I n a s t u d y of a d i f f e r e n t n a t u r e t h e a d d i t i o n a n d a b s t r a c t i o n r e a c t i o n s o f SO2-, SO3', and SO4- t o w a r d s o r g a n i c s u b s t r a t e s h a v e been i n v e s t i g a t e d Only SO4' a b s t r a c t s w i t h t h e a i d of f l o w s y s t e m t e c h n i q u e s . 1 8 7 from s a t u r a t e d compounds, s u c h a s MeOH ( g i v i n g ' C H z O H ) , b u t b o t h SO3& and SO4' add t o u n s a t u r a t e d compounds s u c h a s CH2=CMeCOOH [ g i v i n g *CH2C(Me)(S03')COOH and (S04-)CH2C(Me)COOH r e s p e c t i v e l y ] . N e i t h e r SO3' o r SO4- r e a c t w i t h a r o m a t i c n i t r o compounds b u t SO2' t r a n s f e r s a n e l e c t r o n t o g i v e , f o r e x a m p l e , (PhN02)'.

The r e c e n t d i s c o v e r y t h a t s t a b l e r a d i c a l - c a t i o n s c a n be r e a d i l y formed i n s o l v e n t s s u c h a s FCC13 h a s c r e a t e d t r e m e n d o u s interest i n these species. I n t h i s s e c t i o n of t h e r e p o r t I s h a l l f i r s t d e a l w i t h r a d i c a l - c a t i o n s p r e p a r e d i n t h i s way b e f o r e d i s c u s s i n g t h o s e p r e p a r e d by t h e more t r a d i t i o n a l methods s u c h a s chemical o r electrochemical generation. The r a n g e o f r a d i c a l c a t i o n s p r o d u c e d b y r a d i o l y s i s h a s become v e r y e x t e n s i v e a n d I h a v e t r i e d w h e r e p o s s i b l e t o g r o u p t o g e t h e r t h o s e p a p e r s which a p p e a r t o h a v e a common i n t e r e s t . I s h a l l s t a r t with a look a t t h e r a d i c a l - c a t i o n s of t h e a l k a n e s which were u n d e t e c t e d b e f o r e t h e merits o f t h i s a p p r o a c h were f u l l y a p p r e c i a t e d . The most r e c e n t r e s u l t s h a v e b e e n o b t a i n e d i n SFg, C F g , and CFC12CF2C1 which a p p e a r t o s t a b i l i z e t h e s e s p e c i e s . lg8 Coupling t o t h e out-of-plane p r o t o n s i s r e l a t i v e l y s m a l l compared t o t h a t t o two e n d p r o t o n s and most o f t h e s e s p e c t r a a r e d o m i n a t e d by a 1 : 2 : 1 t r i p l e t w i t h s m a l l a d d i t i o n a l s p l i t t i n g s only o c c a s i o n a l l y observed. S u r p r i s i n g l y t h e secondary r a d i c a l s formed upon warming a p p e a r t o depend upon t h e m a t r i x . 1 8 8 , 1 8 9 The r a d i c a l - c a t i o n s c o n v e r t t o a l k y l r a d i c a l s by d e p r o t o n a t i o n i n SFg a n d CFC12CF2C1 b u t c o n v e r t t o o l e f i n i c n r a d i c a l - c a t i o n s by H2 o r CH4 e l i m i n a t i o n i n FCC13. Compared w i t h t h e r a d i c a l - c a t i o n s of l i n e a r alkanes t h e unpaired e l e c t r o n i n branched alkane r a d i c a l c a t i o n s i s r a t h e r more c o n f i n e d t o o n e o f t h e C-C b o n d s , t h u s maxim i s i n g t h e number o f h y p e r c o n j u g a t i v e m e t h y l g r o u p s . l g O I n a l l of t h e s e b r a n c h e d r a d i c a l - c a t i o n s c o u p l i n g i s a g a i n o b s e r v e d t o two e n d p r o t o n s p l u s c o u p l i n g , of a p p r o x i m a t e l y t h e same m a g n i t u d e , t o

5: Organic Radicals in Solution

199

one p r o t o n o f e a c h m e t h y l g r o u p . It i s a n a t u r a l e x t e n s i o n of t h e study of a l k a n e r a d i c a l c a t i o n s t o c o v e r t h o s e of c y c l o a l k a n e s . A t 7 7 K t h e s p e c t r u m o f t h e c y c l o p r o p a n e r a d i c a l - c a t i o n , p r e p a r e d by r a d i o l y s i s i n FCC13, c o n s i s t s of a s i n g l e l i n e . l g l However, t h e s p e c t r u m o b s e r v e d a t 4 K h a s a ( 4 H ) 1 . 2 5 a n d a ( 2 H ) 2.10 mT i n d i c a t i n g a s p i n - d e n s i t y o f 0 . 5 on C-2 and C-3.1g2 T h i s would s u g g e s t J a h n - T e l l e r a v e r a g i n g of t h e p o s i t i v e and n e g a t i v e h y p e r f i n e c o u p l i n g s a t h i g h e r t e m p e r a t u r e [ i . e . , (-12.5x2+21.0)/31. The s t a t i c d i s t o r t i o n o b s e r v e d f o r t h e cyclopropane radical-cation a t 4 K i s a l s o observed f o r t h e c y c l o b u t a n e r a d i c a l - c a t i o n a t 4 K [ a ( 2 H ) 4.9 and a ( 2 H ) 1 . 4 mT1 . l g 3 A t low t e m p e r a t u r e s t h e c y c l o p e n t a n e r a d i c a l - c a t i o n

shows c o u p l i n g t o o n l y two p r o t o n s b u t a l l t e n p r o t o n s become e q u i v a l e n t a t 100 K . 1 9 1 r 1 9 4 A t 141 K t h e s p e c t r u m of t h e c y c l o h e x a n e r a d i c a l cation has hyperfine coupling t o t h e s i x equatorial protons ( 4 . 3 m T ) , t h e s i x a x i a l p r o t o n s n o t i n t e r a c t i n g t o any s i g n i f i c a n t e x t e n t . 94 T h i s s p e c i e s l o s e s h y d r o g e n when e x p o s e d t o v i s i b l e l i g h t t o g i v e a s p e c t r u m w i t h a ( 2 H ) 5 . 5 0 , a ( 2 H ) 2 . 2 0 , and a ( 2 H ) 0.90 mT a s s i g n e d t o t h e c y c l o h e x e n e r a d i c a l - c a t i o n . The r a d i c a l - c a t i o n s of m e t h y l - s u b s t i t u t e d b e n z e n e s a r e r e a d i l y The s p e c t r a o f t h e t o l u e n e and p r e p a r e d by r a d i o l y s i s i n FCC13. p - x y l e n e r a d i c a l - c a t i o n s a r e b o t h c h a r a c t e r i z e d by a l a r g e c o u p l i n g t o t h e m e t h y l p r o t o n s [ ( a ( 3 H ) 1 . 8 5 a n d a ( 6 H ) 1 . 8 2 mT r e s p e c t -

ively]. These r e s u l t s e s t a b l i s h t h a t 4-71 d e l o c a l i z a t i o n i n v o l v e s e l e c t r o n d o n a t i o n from t h e C-H a - o r b i t a l s i n t o t h e b e n z e n e ring. Very s i m i l a r r e s u l t s a r e o b t a i n e d f o r t h e s e two r a d i c a l c a t i o n s a n d f o r t h o s e o f p- a n d m-xylene i n CF3CCl3.Ig6 The e . s . r . s p e c t r a o f t h e r a d i c a l - c a t i o n s o f h a l o b e n z e n e s a r e c h a r a c t e r i z e d by a l a r g e c o u p l i n g t o t h e h a l o g e n and c o n f i r m t h a t t h e s e s p e c i e s h a v e a f a v o u r e d m o l e c u l a r o r b i t a l which p l a c e s t h e maximum s p i n - d e n s i t y on t h e h a l o g e n . l g 7 The o b s e r v a t i o n o f t h e e . s . r . s p e c t r u m of t h e r a d i c a l - c a t i o n of t e t r a f l u o r o e t h e n e i s of i n t e r e s t due t o t h e u n c e r t a i n t y i n assigning a structure t o its radical-anion. The s p e c t r u m o f t h e radical-cation indicates a l l four fluorine nuclei are equivalent, b u t a l s o shows an a d d i t i o n a l 1 : l d o u b l e t which must o r i g i n a t e from c o u p l i n g t o a I 9 F n u c l e u s of a m a t r i x (FCC13) m o l e c u l e . 1 9 8 These results i n d i c a t e t h a t t h e radical-cation has a planar a - s t r u c t u r e . I n v i e w of t h e s e results it i s i n t e r e s t i n g t o n o t e t h a t INDO calcul a t i o n s s u p p o r t t h e c o n c e p t o f a c h a i r form f o r t h e c o r r e s p o n d i n g r a d i c a l - a n i on. 99

200

Electron Spin Resonance

The s p e c t r u m o b t a i n e d f o l l o w i n g y - i r r a d i a t i o n o f a c e t a l d e h y d e i n FCC13 c o n s i s t s o f a d o u b l e t o f The s p e c t r u m o f t h e CD3CH0 r a d i c a l - c a t i o n and some o t h e r r e l a t e d r a d i c a l - c a t i o n s similarly suggest t h a t the hyperfine quartet structure originates from i n t e r a c t i o n between t h e r a d i c a l - c a t i o n and h a l o g e n a t o m s o f t h e m a t r i x . 2 0 0 p2O2 These c o n c l u s i o n s a r e s u p p o r t e d by Symons z t al. who s u g g e s t t h a t t h e m a t r i x i n t e r a c t i o n i n v o l v e s f o r m a t i o n o f a weak a-bond l o c a l i z e d between t h e p a r e n t c a t i o n and a s i n g l e c h l o r i n e atom.203 The r a d i c a l - c a t i o n of m e t h y l f o r m a t e a p p e a r s t o show a much s t r o n g e r i n t e r a c t i o n w i t h a c h l o r i n e atom o f a m a t r i x molecule.204 A f u r t h e r p o i n t of i n t e r e s t a r i s e s i n t h i s l a t t e r c a s e , however, a s upon warming ( t o 1 4 0 K) t h e s p e c t r u m c h a n g e s

[a(H) 0 . 5 6 , ~ ( ~ H c H 2 . 3 3 , and a(HcH3) 0 . 4 0 mT1 t o t h a t O f t h e more 3 s t a b l e n - s t r u c t u r e i n w h i c h t h e Me g r o u p i s n o t r o t a t i n g f r e e l y . The e . s . r s p e c t r a o f t h e r a d i c a l - c a t i o n s o f f u r a n , t h i o p h e n e , a n d p y r r o l e h a v e been o b s e r v e d i n d i f f e r e n t m a t r i c e s . Symons e t al. h a v e employed FCC13205 and S h i o t a n i et a. h a v e , i n a d d i t i o n , employed m a t r i c e s s u c h a s c y c l o - C 6 F l 2 and c y c l o C6F1 1CF3.206 The s p l i t t i n g c o n s t a n t s r e p o r t e d by t h e s e two r e s e a r c h groups a g r e e w i t h i n e x p e r i m e n t a l e r r o r , and r e v e a l t h a t t h e s e r a d i c a l - c a t i o n s h a v e a common s t r u c t u r e i n which t h e n a t u r e o f t h e atom o r g r o u p a t t h e 1 - p o s i t i o n h a s l i t t l e i n f l u e n c e . I n t h e r a d i c a l - c a t i o n s of t h e m e t h y l d e r i v a t i v e s o f f u r a n a n d thiophene a l l t h e methyl protons a r e e q u i v a l e n t i n d i c a t i n g r a p i d r o t a t i o n . 206 The r a n g e o f r a d i c a l - c a t i o n s o f e t h e r s formed by r a d i o l y s i s h a s now been e x t e n d e d t o i n c l u d e b o t h v i n y 1 2 0 7 and c y c l i c ethers.207,208 The s p e c t r u m of E t O E H = k H 2 c o n s i s t s o f a b a s i c t r i p l e t Ca(2H) 1.94 mT1 a s s i g n e d t o t h e m e t h y l e n e p r o t o n s . 2 0 7 An a d d i t i o n a l t r i p l e t , a ( 2 H ) 0 . 3 5 mT, a p p a r e n t upon a n n e a l i n g , i s a s s i g n e d t o t h e CH2 p r o t o n s o f t h e E t g r o u p . I r r a d i a t i o n of ( M e 0 I 2 C H C H = C H 2 p r o d u c e s a s p e c t r u m [a(H) 1.9 and a(3H) 2.4 mT1 c o n s i s t e n t w i t h (MeOI2tkHMe which would i n v o l v e a 1-3 h y d r o g e n s h i f t . Both r e s e a r c h groups have s t u d i e d t h e r a d i c a l - c a t i o n s d e r i v e d f rom t h e c y c l i c e t h e r s o x i r a n e and o x i t a n e .207 p2O8 These two r a d i c a l - c a t i o n s h a v e s u r p r i s i n g l y d i f f e r e n t s p l i t t i n g constants. The s p e c t r u m o b t a i n e d by i r r a d i a t i o n o f o x i r a n e ( 6 5 ) h a s a ( 4 H ) 1 . 6 4 mT a n d t h a t from o x i t a n e ( 6 7 ) h a s a ( 4 H ) 6.56 a n d a ( 2 H ) 1 . 0 8 mT. The h y p e r f i n e p a r a m e t e r o f t h e f o r m e r h a s l e d Wang e t a l . t o s u g g e s t t h e ring-opened s t r u c t u r e ( 6 6 ) which is i s o e l e c t r o n i c and i s o s t r u c t u r a l w i t h t h e a l l y 1 r a d i c a l [which h a s

20 1

5 . Orgunic Radicals in Solutioii

1 . 4 8 and 1 . 3 9 It is interesting t o note t h a t t h e e l e c t r o n i c a b s o r p t i o n spectrum of t h i s s p e c i e s s u p p o r t s t h i s conc l ~ s i o n . The ~ ~ s~p e c t r u m of t h e f i v e - m e m b e r e d r i n g 1 , 3 - d i o x o l a n e

a(2H)

r a d i c a l - c a t i o n h a s a n - s t r u c t u r e w i t h t h e u n p a i r e d e l e c t r o n del o c a l i z e d o v e r t h e -0-CH2-0u n i t [a(2H) 15.3 and a(4H) 1.1 1 n T 1 . ~ ” The s i x membered r i n g a n a l o g u e , 1 , 3 - d i o x a c y c l o h e x a n e , a p p e a r s t o have a very s i m i l a r s t r u c t u r e [a(2H)

(65)

a.1 4 . 0 I ~ T I . ~ ”

(67)

(66)

Some n o v e l r a d i c a l - c a t i o n s h a v e b e e n p r e p a r e d by t h e r a d i o l y s i s o f c a r b o n y l compounds. The r a d i c a l - c a t i o n o f 2 , 4 - d i methylpentan-3-one

has an unusually l a r g e proton hyperfine coupling

[ a ( 4 H ) 1.52 mT1. T h i s i n d i c a t e s a p r e f e r r e d c o n f o r m a t i o n w i t h one s t r o n g l y coupled proton from each methyl group.212 S t r o n g longrange hyperfine coupling is a l s o observed i n t h e 2,2,6,6-tetrad e u t e r i o c y c l o h e x a n o n e r a d i c a l - c a t i o n [ ( a ( 2 H ) 2.75 mT1. This c o u p l i n g i s a s s i g n e d t o t h e e q u a t o r i a l p r o t o n s a t C-3 a n d C-5, The s p e c t r u m o f t h e p - b e n z o r a t h e r t h a n t h o s e a t C-2 a n d C-6. q u i n o n e r a d i c a l - c a t i o n [ a ( 2 H ) 1 . 8 6 mT1 i n d i c a t e s t h a t i t d o e s n o t remain symmetric b u t d i s t o r t s w i t h t h e unpaired e l e c t r o n l a r g e l y c o n f i n e d t o o n e o x y g e n a t o m . 2 1 3 The r a d i c a l - c a t i o n s of t h e k e t e n e s ( R 1 R 2 C = C = 0 ) a r e c h a r a c t e r i z e d by s i m i l a r s p l i t t i n g c o n s t a n t s t y p i c a l o f o r d i n a r y . r r - r a d i c a l s w i t h a f a i r l y l a r g e s p i n - d e n s i t y on t h e pz o r b i t a l o f t h e a - c a r b o n R a d i o l y s i s of a range of e s t e r s ( R 1 C 0 2 R 2 , R 1 = H , Me, o r E t a n d

R 2 = Me o r E t ) p r o d u c e s

1~

radical-cations

with the hyperfine

A f e a t u r e r e v e a l e d by t h e s e s p e c t r a c o u p l i n g t o R2 p r e d o m i n a n t . * I 5 i s t h a t t h e a c i d Me g r o u p ( R ’ ) r o t a t e s f r e e l y w h e r e a s t h e a l c o h o l i c Me g r o u p ( R 2 ) d o e s n o t , s u g g e s t i n g i n t r a m o l e c u l a r b o n d i n g b e t w e e n

t h i s group and t h e carbonyl oxygen. radical-cations

Of related interest are the

o f t h e v i n y l monomers, m e t h y l m e t h a c r y l a t e a n d

The s p e c t r u m o f t h e m e t h y l m e t h a c r y l a t e rnethylacrylate.216 r a d i c a l - c a t i o n h a s a ( 4 H ) 1 . 5 5 mT, b u t e x p e r i m e n t s on t h e d e u t e r i a t e d monomer show t h a t t h e r e i s no c o u p l i n g t o t h e m e t h y l p r o t o n s of t h e ester group.

These s p e c i e s a g a i n appear t o be

TI

radical-

c a t i o n s w i t h two o f t h e m e t h y l g r o u p p r o t o n s a c c i d e n t a l l y e q u i v a l e n t t o t h e two o l e f i n i c p r o t o n s . The p r i n c i p a l h y p e r f i n e s t r u c t u r e o b s e r v e d i n some S i M e 3 - s u b s t i t u t e d a l k e n e s a l s o r e s u l t s f r o m

202

Electron Spin Resonance

c o u p l i n g t o t h e o l e f i n i c pr ot ons.217 The c o m p a r a t i v e l y l a r g e proton coupling i s probably a consequence of a c o n s i d e r a b l e t w i s t i n g from p l a n a r i t y i n t h e s e s p e c i e s . The f i r s t r e p o r t s h a v e a p p e a r e d o f r a d i c a l - c a t i o n s produced by t h e r a d i o l y s i s o f some n i t r o a l k a n e s and o f some amides. The n i t r o a l k a n e r a d i c a l - c a t i o n s h a v e a s m a l l 14N h y p e r f i n e c o u p l i n g w i t h a s i n g l y o c c u p i e d m o l e c u l a r o r b i t a l c o n f i n e d t o oxygen.218 The spectrum o f a rearrangement product (OkOR)+ is a l s o observed i n t h e s e e x p e r i m e n t s . The s p e c t r a o f t h e two amide r a d i c a l - c a t i o n s , Me2fiC(0)H+ and Me2fiC(0)Me+, a r e v i r t u a l l y i d e n t i c a l w i t h well d e f i n e d s e p t e t s from two e q u i v a l e n t m e t h y l g r o u p s i n d i c a t i n g a h i g h s p i n - d e n s i t y on t h e Me2” u n i t . 2 1 9 However, t h e s p e c t r u m o f t h e t h i o a m i d e r a d i c a l - c a t i o n , Me2k( S)H + , i n d i c a t e s a s i g n i f i c a n t s h i f t i n s p i n - d e n s i t y from n i t r o g e n t o s u l p h u r . The r a d i c a l c a t i o n s o f t h e p e r o x i d e ( But 0) 2 and t h e p e r s u l p h i d e (ButSI2 h a v e b e e n r e p o r t e d . 2 2 0 These h a v e v e r y s i m i l a r e.s.r. s p e c t r a i n t e r p r e t e d i n terms o f p l a n a r s t r u c t u r e s , t h u s m aximising t h e n-bonding e n e r g y . P r o g r e s s h a s a l s o been made on some o r g a n o m e t a l l i c s p e c i e s . The r a d i c a l - c a t i o n s o f SnMe4221 ,222 and of PbMe4221 b o t h have s i m i l a r e.s.r. s p e c t r a w i t h h y p e r f i n e coupling t o one methyl group only. A l l t h e r a d i c a l - c a t i o n s so f a r d e s c r i b e d h a v e been p r e p a r e d by r a d i o l y s i s i n FCC13, o r s i m i l a r s o l v e n t s , a t low t e m p e r a t u r e . The r e m a i n i n g p a r t o f t h i s s e c t i o n o f t h e r e p o r t now c o n c e n t r a t e s on r a d i c a l - c a t i o n s produced by c h e m i c a l o r e l e c t r o c h e m i c a l methods. S e v e r a l s u c h s t u d i e s have c o n c e n t r a t e d on v a r i o u s c y c l o b u t a d i e n e radical-cations. The p h o t o l y s i s of t h e aluminium h a l i d e complexes o f t e t r a m e t h y l - and tetraethyl-cyclobutadiene p r o d u c e s t h e c o r r e s p o n d i ng r a d i c a l -ca t i o n s 223 The t e t r a m e t h y 1c y c l obu t a d i e n e r a d i c a l - c a t i o n h a s a(12,B-H) 0.875 mT and t h e t e t r a e t h y l - d e r i v a t . i v e h a s a(8,B-H) 0.769 and a ( 1 2 H ) 0.002 mT. S i m i l a r s p e c i e s ( 6 8 ) a r e formed by t h e p h o t o l y s i s o f t h e u-complex formed between t h e d i m e r i z e d d i a l k y l a l k y n e s and A l C 1 3 ( i n CH2C12) .224*225 A m i x e d a l k y n e R1C:CR2 c o u l d g i v e b o t h t h e & ( 6 8 ) and trams ( 6 9 ) i s o m e r s , w h e r e a s a m i x t u r e o f t h e two a l k y n e s R’CECR’ and R 2 C X R 2 s h o u l d g i v e o n l y t h e c i s i s o m e r . Hence t h e e . s . r. s p e c t r u m o b t a i n e d form MeC CBut is a m i x t u r e o f two s p e c i e s w i t h a ( 6 H ) 0.900 and a ( 1 8 H ) 0.020 mT and w i t h a ( 6 H ) 0.800 and a ( 1 8 H ) 0.024 mT r e s p e c t i v e l y . 2 2 4 On t h e o t h e r hand a m i x t u r e o f MeCZCMe and ButC~CBut g i v e s one s p e c i e s w i t h t h e l a t t e r s e t o f s p l i t t i n g c o n s t a n t s . The t e t r a - t b u t y l c y c l o b u t a d i e n e r a d i c a l - c a t i o n i s formed Ca(36H) 0.030 mT1 by

.

5: Organic Radicals in Solution

203

t h e o n e - e l e c t r o n o x i d a t i o n of tetra-t-butyltetrahedrane ( 7 0 ) . 226 The cycloheptenecyclobutadiene r a d i c a l - c a t i o n ( 7 1 ) undergoes r i n g i n v e r s i o n w i t h k = 0.7 2 0.4 x lo8 s-' a t 196 K a s does t h e bicycloheptenecyclobutadiene r a d i c a l - c a t i o n (72) w i t h t h e two c y c l o h e p t e n e r i n g s moving i n d e p e n d e n t l y . 227 The t e t r a - t - b u t y l a l l e n e r a d i c a l - c a t i o n , [ (Me3C)2C:C:C(CMe3)21;, i s produced by e l e c t r o c h e m i c a l o x i d a t i o n i n CH2C12 a t 193 K and h a s t h e u n p a i r e d e l e c t r o n d e l o c a l i z e d o v e r a l l t h e p - o r b i t a l s Ia(36H) 0.09 mT1 .228

R2

I

R1

A method h a s been proposed t o i n t e r p r e t t h e s p e c t r a of t h e b e n z o ( a ) p y r e n e r a d i c a l - c a t i o n which i n v o l v e s t h e s t u d y of a l l 12 monomethyl d e r i v a t i v e s . 229 T h e method is based upon t h e assumption t h a t m e t h y l - s u b s t i t u t i o n does n o t change t h e sum o f t h e o t h e r s p l i t t i n g c o n s t a n t s and t h a t t h e s p l i t t i n g c o n s t a n t of t h e Me group i s s i m i l a r t o t h e p r o t o n i t h a s r e p l a c e d . The e.s.r s p e c t r a o f t h e r a d i c a l - c a t i o n s of t h e t h r e e [2.2lpyrenophanes have been i n t e r p r e t e d w i t h t h e a i d of ENDOR a l t h o u g h a unique assignment of a l l t h e s p l i t t i n g c o n s t a n t s i s r a t h e r d i f f i c u l t . 2 3 0 The somewhat s m a l l e r hexamethyl- and o c t a m e t h y l - s u b s t i t u t e d C2.2lparacyclophane r a d i c a l - c a t i o n s have been i n v e s t i g a t e d . 231 The c o u p l i n g t o t h e methyl p r o t o n s i s much g r e a t e r t h a n t h a t t o t h e CH2 p r o t o n s i n these species. The s i n g l e r i n g h e t e r o c y c l i c r a d i c a l - c a t i o n s of 1 , 2 , 4 , 5 - t e t r a z i n e s ( 7 3 ) have been prepared by o n e - e l e c t r o n o x i d a t i o n . 2 3 2 T h e i r s p e c t r a , depending upon t h e n a t u r e of R 1 , R3, R4, and R6, have a(l-,4-N) 0.75 and a(2-,5-N) 0;45 mT w i t h h y p e r f i n e c o u p l i n g t o p r o t o n s of t h e s u b s t i t u e n t a l s o observed where a p p r o p r i a t e . The s p l i t t i n g c o n s t a n t s i n t h e r a d i c a l - c a t i o n s of A,B'-disubsti-

Electron Spin Resonance

204

t u t e d - 4 , 4 ‘ - b i p y r i d y l i u m d i c h l o r i d e have been a s s i g n e d from an ENDOR study of some of i t s d e u t e r i a t e d compounds.233 I t i s i n t e r e s t i n g t o n o t e t h a t t h e magnitudes o f a(2-HI and a(3-H) a r e r e v e r s e d i n t h e d i p h e n y l - and dimethyl- s p e c i e s probably a s a r e s u l t of s t e r i c c o n s i d e r a t i o n s . The spectrum of t h e l!l,l!11-bis-di-(4-fluorophenyl)s p e c i e s h a s a l s o been i n t e r p r e t e d w i t h t h e a i d of ENDOR.234 This r a d i c a l - c a t i o n d i m e r i z e s a t low t e m p e r a t u r e (AHo -43 kJ mol” 1. The r a d i c a l - c a t i o n of l!l,l!l,l!l’,lt-tetramethylbenzidine,formed by p h o t o i o n i z a t i o n i n a n i o n i c vesicles, h a s also been r e p o r t e d . 235 Another r e l a t e d i n v e s t i g a t i o n i s t h a t of t h e m u l t i - r i n g e d indolizino~6,5,4,3-aijlquinoline ( 7 4 ) r a d i c a l - c a t i o n and t h o s e of some of i t s d e r i v a t i v e s . 2 3 6 The magnitude of t h e observed s p l i t t i n g c o n s t a n t s ( a g a i n determined w i t h t h e a i d of ENDOR) i n d i c a t e t h a t t h e u n p a i r e d e l e c t r o n is e s s e n t i a l l y c o n f i n e d t o t h e carbon perimeter. R4

I (74)

-N

E l e c t r o c h e m i c a l o x i d a t i o n of 1-nitroso-2,2,6,6,-tetramethylp i p e r i d i n e i n a c e t o n i t r i l e a t 243 K r e a d i l y produces t h e r a d i c a l cation.237 ,238 T h i s s p e c i e s , a 0 - r a d i c a l i s o e l e c t r o n i c w i t h iminoxy r a d i c a l s , h a s a(N) 4.50 and 0.658, a ( 8 H ) 0.282, and a(6H) 0.094 mT. The r a d i c a l - c a t i o n s of t r i a l k y l s u l p h e n a m i d e s such a s (CH214NSMe can a l s o be prepared by e l e c t r o c h e m i c a l o x i d a t i o n . 2 3 9 Two d i f f e r e n t methylene p r o t o n s p l i t t i n g c o n s t a n t s a r e observed f o r [(CH2l4NSMe1;, a(2H) 2.17 and 1.82 mT, i n d i c a t i n g t h a t t h e r e is a l a r g e b a r r i e r t o r o t a t i o n a b o u t t h e N-S bond. The r a d i c a l - c a t i o n o f t e t r a m e t h y l t e t r a z e n e h a s been r e p o r t e d b u t it i s found t o decompose t o ( Me2NNMe21 240 Chromans a r e i m p o r t a n t a n t i - o x i d a n t s i n b i o l o g i c a l systems and t h e i r o x i d a t i o n t o form r a d i c a l - c a t i o n s i s t h e r e f o r e of i n t e r e s t . S u t c l i f f e st al. have s t u d i e d t h e r a d i c a l - c a t i o n s of t h r e e t r i c y c l i c chromans [such a s (75)1.241 The e.s.r. spectrum of ( 7 5 ) shows an a l t e r n a t i n g l i n e w i d t h e f f e c t d u e t o i n t e r c o n v e r s i o n between i d e n t i c a l conformers ( A H 50 kJ mo1-l). The spectrum h a s been i n t e r p r e t e d w i t h t h e a i d of ENDOR and T R I P L E r e s o n a n c e experi m e n t s and r e v e a l s t h a t t h e m a j o r i t y of t h e u n p a i r e d e l e c t r o n

;.

5: Organic Radicals in Solution

205

d e n s i t y i s c l o s e t o t h e a r o m a t i c r i n g . I n c o n t r a s t t h e spectrum of t h e r a d i c a l - c a t i o n [(76), X = CH2, R = Me3 shows no t e m p e r a t u r e dependence a s t h e i n t e r c o n v e r s i o n i s r a p i d on t h e e.s.r t i m e s c a l e . The r a d i c a l - c a t i o n [(76), X = 0, R = H I i s one of s e v e r a l r e l a t e d s p e c i e s whose e . s . r . s p e c t r a have been s t u d i e d . 2 4 2 The o b s e r v e d s p l i t t i n g c o n s t a n t s i n t h e s e l a t t e r s p e c i e s have been a s s i g n e d w i t h t h e a i d of MO and I N D O c a l c u l a t i o n s .

(75)

(76)

S e v e r a l r e p o r t s have appeared o f r a d i c a l - c a t i o n s c o n t a i n i n g v a r i o u s h e t e r o a t o m s , t h e most w i d e l y s t u d i e d b e i n g t h o s e of some phenothiazines with s i d e chain s u b s t i t u e n t s a t t h e nitrogen atom.243*244 The s p e c t r a of t h e s e s p e c i e s a r e n a t u r a l l y r a t h e r complex and t h e s p l i t t i n g c o n s t a n t s have been a s s i g n e d by comparison w i t h t h o s e of t h e 10-methylphenothiazine r a d i c a l - c a t i o n . The s p e c t r a e x h i b i t l i n e w i d t h a l t e r n a t i o n due t o i n t e r c o n v e r s i o n between two conformers when t h e s i d e c h a i n is -CH2CHMeCH2NMe2 b u t n o t when i t i s -CH2CH2CH2NMe2. The r a d i c a l - c a t i o n s of t h e r e l a t e d 10-methylphenoxazine h a s a l s o been r e p o r t e d 2 4 5 a s have t h o s e of some b e n z o t h i a z o l i n e s . 246 The s i m p l e r t h i a n t h r e n e r a d i c a l - c a t i o n h a s been employed a s a model t o s t u d y t h e i n t e r a c t i o n between r a d i c a l - c a t i o n s and f l u o r i n e - s u b s t i t u t e d a c e t i c a c i d s . 2 4 7 The r a t e of decay of t h i s r a d i c a l - c a t i o n by s e l f - r e a c t i o n d e c r e a s e s a s t h e d e g r e e of f l u o r i n e - s u b s t i t u t i o n i n c r e a s e s . The r a d i c a l - c a t i o n o f lt8:4,5-bis(dise1eno)naphthalene h a s now been added t o t h o s e of a w i d e range of b r i d g e d n a p h t h a l e n e s whose e.s. r. s p e c t r a have been observed.248 O f t h e r a d i c a l - c a t i o n s of t h e analogous S, Se, and Te b r i d g e d t e t r a c e n e s h y p e r f i n e c o u p l i n g was r e s o l v e d o n l y i n t h a t of 5,6:11,12-bis(dithio)tetracene La(8H) 0.055 mTl.249 One might e x p e c t two s e t s of f o u r e q u i v a l e n t p r o t o n s i n t h i s s p e c i e s but they a r e not resolved within t h e s p e c t r a l l i n e width.

206

Electron Spin Resonance

B

Radical-anions

Compared w i t h t h e number of r e p o r t s of r a d i c a l - c a t i o n s prepared by r a d i o l y s i s , t h e r e p o r t s of new r a d i c a l - a n i o n s p r e p a r e d by t h i s same t e c h n i q u e have been very few indeed. However, they have produced some q u i t e i n t e r e s t i n g results. P r e v i o u s e f f o r t s t o produce methylene r a d i c a l - a n i o n s , ' C R z - , have been u n s u c c e s s f u l b u t Symons now r e p o r t s t h e i r p r e p a r a t i o n by t h e r a d i o l y s i s of l i t h i u m a l k y l ~ . ~ ~The ' e . s . r . spectrum of 'CH2- c o n s i s t s of a 1:2:1 t r i p l e t without coupling t o 6 L i o r 7 L i d e s p i t e i t s probable existence a s an i o n p a i r . The v a l u e o f a(2H) (2.04 mT) i n t h i s s p e c i e s i s c l o s e t o t h a t f o r t h e i s o e l e c t r o n i c 'NH2 r a d i c a l (2.4 mT). In a n o t h e r i n t e r e s t i n g paper Symons r e p o r t s t h a t ( NkBr)' i s n o t * i s o s t r u c t u r a l w i t h "02 and ' C O z - , b u t h a s a l i n e a r a - s t r u c t u r e w i t h t h e u n p a i r e d e l e c t r o n almost e q u a l l y s h a r e d by t h e carbon and bromine o r b i t a l s . 2 5 1 T h i s s p e c i e s , however, undergoes an irrev e r s i b l e change when warmed t o 140 K t o g i v e a spectrum w i t h a reduced h y p e r f i n e c o u p l i n g t o *'Br b u t a much enhanced c o u p l i n g t o 14N c o n s i s t e n t w i t h a b e n t s t r u c t u r e w i t h t h e unpaired e l e c t r o n i n a d e l o c a l i z e d T* o r b i t a l . P e r f l u o r i n a t e d o r g a n i c r a d i c a l s o f t e n have a s t r u c t u r e d i f f e r e n t t o t h a t of t h e i r hydrogen-containing c o u n t e r p a r t s . I n d i l u t e s o l u t i o n ( i n , f o r u,2 - m e t h y l t e t r a h y d r o f u r a n ) a t 77 K r a d i o l y s i s of m e t h y l i s o c y a n a t e g i v e s a s i m p l e e . s . r . spectrum [a(N) m. 0.7 mT1 a s s i g n e d t o (MeNeO)'.252 Upon p h o t o l y s i s t h e r a d i c a l - a n i o n g i v e s Me' and c y a n a t e i o n s . The octafluorocyclooctatetraene r a d i c a l - a n i o n h a s now been p r e p a r e d by r a d i o l y s i s i n 2 - m e t h y l t e t r a h y d r o f ~ r a n . ~ I~ t~ h a s e i g h t e q u i v a l e n t f l u o r i n e atoms Ca(819F) 1.092 mT1 c o n f i r m i n g t h a t i t h a s a p l a n a r s t r u c t u r e a s h a s t h e p r e v i o u s l y o b s e r v e d r a d i c a l - a n i o n of c y c l o octatetraene. Only two p a p e r s have appeared d e a l i n g w i t h r a d i c a l - a n i o n s of benzoquinones, b u t both p r o v i d e some i n t e r e s t i n g r e s u l t s . The f i r s t c o n c e r n s t h e r e a c t i o n of d i a l k a l i - m e t a l 3 , 5 - d i - t - b u t y l c a t e c h o l a t e s w i t h 'naked' permanganate i o n s (h, a s o l u t i o n of potassium permanganate i n dimethylformamide i n t h e p r e s e n c e of 18-crown-6) .254 The second c o n c e r n s t h e s i m i l a r d i a l k a l i - m e t a l c a t e c h o l a t e r a d i c a l - a n i o n Ca(2H) 0.08 mT1 formed a s a common f r a g m e n t a t i o n product d u r i n g t h e o x i d a t i v e d e g r a d a t i o n of s a c c h a r i d e s , c e l l u l o s e , and s t a r c h . 2 5 5 The s p l i t t i n g c o n s t a n t s of t h e r a d i c a l - a n i o n s o f 1,4-naphthoquinone s u b s t i t u t e d i n t h e 2 - p o s i t i o n

5: Organic Radicals in Solution w i t h a n a z a c r o w n e t h e r (tetraoxomonoaza-15-crown-5)

207

d e p e n d upon t h e r a d i u s a n d c h a r g e o f t h e c a t i o n i n t h e crown e t h e r The v a r i a t i o n s i n a(H) c a n b e r a t i o n a l i z e d i n terms o f c h a n g e s i n t h e s p i n p o p u l a t i o n r e s u l t i n g from t h e i n f l u e n c e o f t h e c a t i o n . The r a d i c a l - a n i o n s o f some 2 , 3 - d i s u b s t i t u t e d 1 , 4 - n a p h t h o q u i n o n e s , p r e p a r e d by e l e c t r o c h e m i c a l r e d u c t i o n , h a v e b e e n r e p o r t e d . 257 Several papers have appeared dealing with t h e radical-anions of v a r i o u s c h l o r o - ,258-261 m e t h y l - , 2 5 9 9 2 6 1 a n d m e t h ~ x y -s u~ b~s t~i t u t e d naphthazarins. The e.s.r. s p e c t r a o f these r a d i c a l - a n i o n s a p p e a r i n d e p e n d e n t o f t h e p r e p a r a t i o n method employed a n d show cons i s t e n t e v i d e n c e f o r a s t r o n g O - H ~ l ~ ~ l ~ Obond w i t h t h e h y d r o g e n atom i n t h e m i d d l e o f t h e 0~~~110 segment. Hyperfine c o u p l i n g t o t h e methoxy g r o u p i s n o t o b s e r v e d , b u t t h e m e t h y l g r o u p a p p e a r s t o r o t a t e freely d e s p i t e being p o t e n t i a l l y hindered. Another group of r a d i c a l - a n i o n s h a v i n g a q u i n o n e t y p e s t r u c t u r e a r e some t e t r a c y a n o a r e n o q u i n o d i m e t h a n e s r e p o r t e d by G e r s o n et a1.262 T h e r e a r e a number o f r o u t e s a v a i l a b l e f o r t h e p r e p a r a t i o n of t h e s e r a d i c a l a n i o n s d e p e n d i n g upon t h e a v a i l a b i l i t y o f t h e p a r e n t m o l e c u l e o r its dihydro-derivative. The e. s. r. s p e c t r a o f t h e s e species h a v e b e e n i n t e r p r e t e d w i t h t h e a i d o f ENDOR and t h e y c a n be f u r t h e r reduced t o form r a d i c a l - t r i a n i o n s which have r e l a t i v e l y simple e . s . r . s p e c t r a w i t h no a p p a r e n t c o u p l i n g t o 1 4 N . The l i n e s h a p e i n t h e e . s . r . s p e c t r a o f r a d i c a l - a n i o n s o f t e n i n d i c a t e s t h e p r e s e n c e o f r e s t r i c t e d r o t a t i o n and s e v e r a l f u r t h e r e x a m p l e s o f t h i s e f f e c t h a v e been r e p o r t e d . When t h e r a d i c a l - a n i o n i s prepared e l e c t r o c h e m i c a l l y t h e e q u i l i b r i u m b e t w e e n two r o t a m e r s c a n be s t u d i e d . However, when t h e r a d i c a l - a n i o n i s p r e p a r e d by a l k a l i metal r e d u c t i o n a complex s e t o f e q u i l i b r i a a r e p r e s e n t [ p o s s i b l y i n v o l v i n g ' f r e e ' ( l ) , ' s o l v e n t - ~ e p a r a t e d ~ ( 2 ) a, n d ' c o n t a c t ' ( 3 ) i o n p a i r s 1 a s i l l u s t r a t e d below. C l e a r l y w i t h 22 r a t e c o n s t a n t s a n d s i x sets of h y p e r f i n e s p l i t t i n g c o n s t a n t s r e q u i r e d t h e c o m p l e t e a n a l y s i s o f s u c h a s y s t e m would be v i r t u a l l y impossible. An a n a l y s i s o f t h e e x c h a n g e b r o a d e n e d s p e c t r u m o f 3 - n i t r o b e n z a l d e h y d e ( p r e p a r e d by e l e c t r o c h e m i c a l r e d u c t i o n ) y i e l d s a n a c t i v a t i o n e n t h a l p y o f 1 6 . 4 kJ mol" f o r i n t e r c o n v e r s i o n of t h e & isomer t o t h e trans isomer.263 L i k e w i s e t h e a c t i v a t i o n e n e r g y f o r i n t e r n a l r o t a t i o n i n t h e radical-anion of 3-nitroacetophenone has been d e t e r m i n e d i n d i m e t h y l f o r m a m i d e (20 kJ mol") .264 I n t h e p-nitrobenzophenone r a d i c a l - a n i o n it a p p e a r s t h a t b o t h r i n g s undergo h i n d e r e d r o t a t i o n and t h a t i n t h e 4-nitrobenzophenone r a d i c a l - a n i o n t h e u n p a i r e d e l e c t r o n d e n s i t y i s l o c a l i z e d on t h e

208

Electron Spin Resonance

p - n i t r o p h e n y l moiety.265 A b a r r i e r t o r o t a t i o n is a l s o found i n some m e t h y l - s u b s t i t u t e d m - t e r p h e n y l r a d i c a l - a n i o n s . 2 6 6 The i n t e r p r e t a t i o n o f t h e e.s.r. s p e c t r u m o f t h e u n s y m m e t r i c a l l y s u b s t i t u t e d 21,41-dimethyl-m-terphenyl r a d i c a l - a n i o n i n d i c a t e s t h a t it behaves l i k e a p h e n y l - s u b s t i t u t e d b i p h e n y l .

x

11

11

Linewidth effects can a l s o a p p e a r i n e.s.r. s p e c t r a a s a c o n s e q u e n c e o f c a t i o n movement b e t w e e n e q u i v a l e n t s i t e s i n t h e radical-anion. In t h e radical-anions of dinitrobenzenes t h e c a t i o n i s u s u a l l y s t r o n g l y a s s o c i a t e d w i t h o n e o f t h e n i t r o groups.267 However, upon t h e a d d i t i o n o f a common i o n ( e . g . NaBPh4) c a t i o n e x c h a n g e o c c u r s a t a f r e q u e n c y c o m e n s u r a t e w i t h t h e e . s . r . times c a l e (Ea m . 20 kJ mol”) The a d d i t i o n o f crown e t h e r s c a n o f t e n i n f l u e n c e t h e r a t e of c a t i o n movement a s r e p o r t e d f o r t h e 5 , 1 2 - d i h y d r o t e t r a c e n e r a d i c a l - a n i o n . 268 L i n e w i d t h a1t e r n a t i o n is o b s e r v e d when L i + i s t h e c a t i o n i n t h e p r e s e n c e o f dibenzo-18-crown-6 (Ea 1 3 . 5 kJ mo1-l) b u t n o t i n i t s a b s e n c e . T h i s r e s u l t i n d i c a t e s s i g n i f i c a n t c o m p l e x a t i o n o f L i + by 18-crown-6 t y p e p o l y e t h e r s , a s do similar r e s u l t s o b t a i n e d f o r t h e f l u o r e n e r a d i c a l - a n i o n i n t h e p r e s e n c e of 18-crown-6 a n d d i c y c l o h e x y l - 1 8 - c r o w n - 6 .269 I n t h i s l a t t e r s y s t e m t h e s p e c t r a o b s e r v e d a t l o w t e m p e r a t u r e (180-220 K) r e s u l t from t h e p r e s e n c e of a t l e a s t two d i s t i n c t s p e c i e s . The e.s.r. s p e c t r a o f a w i d e r a n g e o f e l e c t r o c h e m i c a l l y g e n e r ated 5 - s u b s t i t u t e d 2 - n i t r o f u r a n r a d i c a l - a n i o n s have been i n v e s t i gated.270 These g e n e r a l l y h a v e a l a r g e c o u p l i n g t o 14N w i t h t h e v a l u e of a(3-HI l e a s t s e n s i t i v e t o t h e n a t u r e of t h e s u b s t i t u e n t . The v a l u e o f a(4-HI decreases w i t h s u b s t i t u e n t i n t h e s e q u e n c e CH3Ca(4-H) 0.585 mT1 > H > COO’ > CF3 > CN > COOCH3 > Coca3 > CHO > N02[a(4-H) 0.109 mT1. The s u b s t i t u t i o n o f a v i n y l e n e g r o u p (-CR1:CR2R3) a t t h e 5 - p o s i t i o n r e s u l t s i n a decrease i n u n p a i r e d e l e c t r o n d e n s i t y i n t h e f u r a n r i n g a s a r e s u l t o f some d e l o c a l i z a t i o n onto t h e vinylene group. Similar s p l i t t i n g c o n s t a n t s have been observed f o r -C(N02) :CHAr s u b s t i t u e n t s a t t h e 5 - p o ~ i t i o n . ~ ~ ~

.

5: Organic Radicals in Solution

209

A l l these radical-anions a r e relatively unstable.

Numerous r e p o r t s h a v e been made o f r a d i c a l - a n i o n s o b t a i n e d by reduction of nitrobenzenes including t h a t of nitrobenzene i t s e l f .272,273 When p r e p a r e d by r e d u c t i o n i n l i q u i d ammonia t h e e.s.r. s p e c t r u m of t h 8 n i t r o b e n z e n e r a d i c a l - a n i o n r e v e a l s n o e v i d e n c e f o r i o n p a i r f o r m a t i o n . 2 7 3 However, t h e a d d i t i o n of NaI r e s u l t s i n a n i n c r e a s e i n and linewidth a l t e r n a t i o n becomes e v i d e n t i n t h e s p e c t r u m . The i o n a s s o c i a t i o n c o n s t a n t , K a , i s s e v e r a l o r d e r s o f m a g n i t u d e smaller i n l i q u i d ammonia (m. 3 x t h a n it is, f o r example, i n t e t r a h y d r o f u r a n (m. 1 5 6 ) . T h e s e r e s u l t s a r e u n d o u b t e d l y i n f l u e n c e d by a s t r o n g i n t e r a c t i o n between t h e Na+ i o n and t h e n i t r o g e n l o n e - p a i r i n t h e former s o l v e n t . The p - c y a n o n i t r o b e n z e n e r a d i c a l - a n i o n ( i n hexamethylphosphoramide) h a s p r o v e d a s u c c e s s f u l s y s t e m f o r t h e s t u d y of h y d r o g e n b o n d i n g ( t o E t O H ) . 2 7 4 The s p l i t t i n g c o n s t a n t s o f t h e r a d i c a l - a n i o n d i f f e r c o n s i d e r a b l y i n t h e presence and absence of h y d r o g e n b o n d i n g a n d i t h a s p r o v e d p o s s i b l e t o d e t e r m i n e t h e e n t h a l p y o f a c t i v a t i o n f o r hydrogen-bond f o r m a t i o n ( 1 7 kJ mol" 1. O t h e r s i m i l a r s t u d i e s i n c l u d e t h o s e o f some o t h e r s i m p l e s u b s t i a n d some c y c l i c a c e t a l s u b s t i t u t e d 2 7 6 n i t r o b e n z e n e radical-anions. I n m- a n d p - n i t r o p h e n y l a c e t y l e n e r a d i c a l - a n i o n s t h e m a j o r i t y of t h e u n p a i r e d e l e c t r o n d e n s i t y resides on t h e n i t r o group.277 R e d u c t i o n o f h a l o - g e n e r a t e d d e r i v a t i v e s o f these compounds, however, g i v e s t h e s p e c t r u m o f t h e p a r e n t r a d i c a l o n l y , due t o r a p i d halogen-hydrogen replacement. I n nitrophenylamine r a d i c a l - a n i o n s t h e u n p a i r e d e l e c t r o n resides i n t h e n i t r o s u b s t i t u t e d r i n g only with observed s p l i t t i n g c o n s t a n t s similar t o t h e c o r r e s p o n d i n g n i t r o a n i l i n e ~ . ~ ~Other ' n i t r o radical-anions s t u d i e d i n c l u d e t h o s e o f some n i t r o - s u b s t i t u t e d t h i o u r e i d e ~a n~d ~ ~ m o r p h o l i n e s ( w h e r e t h e u n p a i r e d e l e c t r o n i s a g a i n l o c a l i z e d on t h e a r o m a t i c r i n g ) .2*0 It h a s b e e n p r o p o s e d t h a t t h e e l e c t r o c h e m i c a l r e d u c t i o n o f n i t r o compounds, R N 0 2 , t o h y d r o x y l a m i n e s , RNHOH, i n v o l v e s a n i t r o s o i n t e r m e d i a t e , RNO, w h i c h s h o u l d be more r e a d i l y r e d u c e d t h a n t h e p a r e n t n i t r o compound. Some e v i d e n c e f o r t h e f o r m a t i o n o f n i t r o s o i n t e r m e d i a t e s h a s now b e e n o b t a i n e d d u r i n g t h e r e d u c t i o n o f n i t r o d u r e n e , t - n i t r o b u t a n e , a n d p - n i t r o t o l u e n e i n t h e p r e s e n c e of a l k y l h a l i d e s . 281 D u r i n g e l e c t r o l y s i s t h e s p e c t r u m o f t h e r a d i c a l - a n i o n o f t h e n i t r o compound i s o b s e r v e d , b u t upon d i s c o n n e c t i o n of t h e applied voltage t h e spectrum of t h e corresponding RN(b)R1 appears. The s p e c t r a o f t h e r a d i c a l - a n i o n s of s e v e r a l common s p i n - t r a p s ,

2 10

Electron Spin Resonance

s u c h a s n i t r o s o d u r e n e a n d 2,4,6,-tri-t-butylnitrosobenzene a r e reported.282 The s p e c t r u m of t h e f l u o r e n o n e r a d i c a l - a n i o n i n t h e p r e s e n c e o f b u l k y c o u n t e r i o n s , s u c h a s Bun3S+, Bun4N+, and (n-C12H25)3(Me)N+, shows a marked d e p e n d e n c e upon t h e n a t u r e o f t h e c a t i o n t h u s i n d i c a t i n g t h a t i n benzene it e x i s t s a s a n i n t i m a t e i o n pair.283 The r e a c t i o n o f b e n z o i c a c i d s , w i t h phenylmagnesium b r o m i d e s i n E t 2 0 g i v e s t h e benzophenone, o r a s u b s t i t u t e d b e n z o p h e n o n e , r a d i c a l - a n i o n . 284 The a d d i t i o n o f o t h e r s o l v e n t s does n o t i n f l u e n c e t h e r e a c t i o n , b u t can i n f l u e n c e t h e magnitude o f t h e s p l i t t i n g constants due t o t h e d i f f e r e n c e i n t h e s o l v a t i o n e n v i r o n m e n t s o f t h e Mg2+ c o u n t e r i o n . From a c a r e f u l s t u d y o f t h e k e l a t i v e i n f l u e n c e o f t h e K+ and Mg2+ c o u n t e r i o n s on t h e s p e c t r a l p a r a m e t e r s o f t h e benzophenone r a d i c a l - a n i o n i t h a s b e e n d e d u c e d t h a t t h e c a r b o n y l 13C s p l i t t i n g c o n s t a n t h a s a p o s i t i v e s i g n . 2 8 5 G e n e r a l l y t h e u n p a i r e d e l e c t r o n i n 1,3-dioxo-2-indanylpyridinium compounds p r e f e r s t h e p h t h a l o y l m o i e t y b u t t h e s u b s t i t u t i o n o f a n e l e c t r o n w i t h d r a w i n g g r o u p s u c h a s CN i n t h e p y r i d i n i u m r i n g c a n reverse t h i s situation.286 The r a d i c a l - a n i o n s o f some n o v e l a n d q u i t e i n t e r e s t i n g c y c l o o c t a t e t r a e n e s have been r e p o r t e d . These i n c l u d e naphthalene and a n t h r a c e n e s u b s t i t u t e d c y c l o o c t a t e t r a e n e r a d i c a l - a n i o n s t h e e.s.r. spectra o f which i n d i c a t e t h a t t h e unpaired e l e c t r o n d e n s i t y i s l a r g e l y c o n f i n e d t o t h e c y c l o o c t a t e t r a e n e ring.287 The a n t h r a c e n e d e r i v a t i v e is p a r t i c u l a r l y i n t e r e s t i n g a s it forms a r a d i c a l t r i a n i o n t h e s p e c t r u m of which h a s n o t y e t b e e n i n t e r p r e t e d . The 3,8-dimethyl-2-methoxyazocine r a d i c a l - a n i o n ( 7 7 ) h a s a n e.s.r. spectrum s i m i l a r t o t h a t of o t h e r monosubstituted cyclooctatetraenes.288 The s p l i t t i n g c o n s t a n t s i n t h i s s p e c i e s h a v e b e e n The e.s.r a n d ENDOR a s s i g n e d w i t h t h e a i d of I N D O c a l c u l a t i o n s . s p e c t r a of t h e n o v e l r a d i c a l - a n i o n s o f 5 , 6 - d i d e h y d r o - (78) a n d 5,6,9,10-tetradehydro-benzocyclooctene h a v e b e e n o b s e r v e d . 2 8 9 The s p l i t t i n g c o n s t a n t s h a v e b e e n a s s i g n e d w i t h t h e a i d o f MO c a l c u l a t i o n s w h i c h s u g g e s t t h a t t h e eight-membered r i n g i s e s s e n t i a l l y planar.

5 : Organic Radicals in Solution

21 1

R e s e a r c h on t h e r a d i c a l - a n i o n s o f c y c l o p h a n e s c o n t i n u e s and t h e i n t e r e s t e d r e a d e r may f i n d a r e c e n t review on t h e s e s p e c i e s v a l u a b l e . 290 The r e d u c t i o n o f a l l t h e m u l t i p l y - b r i d g e d [ 2 n l c y c l o p h a n e s h a s now been c o v e r e d a n d summarised i n a r e c e n t p u b l i c a t i o n . 2 9 1 They a p p e a r t o f a l l i n t o t h r e e c a t e g o r i e s . Some c y c l o p h a n e s r e d u c e t o t h e c o r r e s p o n d i n g r a d i c a l - a n i o n , some r e a c t t o g i v e secondary s p e c i e s (such a s t h e tetrahydropyrene r a d i c a l a n i o n ) , a n d o t h e r s do n o t a p p e a r t o r e a c t t o a n y m e a s u r a b l e e x t e n t . Superphane ( 7 9 ) r e a c t s w i t h a s o l u t i o n of potassium i n 1,2-dim e t h o x y e t h a n e when i r r a d i a t e d w i t h v i s i b l e l i g h t t o g i v e t h e r a d i c a l - a n i o n [a(12H) 0.185 rnT3 . 2 9 2 C o u p l i n g t o o n l y 1 2 p r o t o n s i n d i c a t e s t h a t t h e u n p a i r e d e l e c t r o n i s l o c a l i z e d i n o n l y o n e of t h e benzene r i n g s . The i r r a d i a t i o n t e c h n i q u e d e s c r i b e d a b o v e c a n a l s o be u s e d t o o b t a i n s p e c t r a o f m e t h y l e n e r a d i c a l - a n i o n s . The 1,n-di-9-anthrylalkanes (80, n = 2 t o 4 ) a l l reduce t o t h e corres p o n d i n g r a d i c a l - a n i o n s and r a d i c a l - t r i a n i o n ~ . ~The ~ ~ radicala n i o n s have t h e unpaired e l e c t r o n d e l o c a l i z e d over both a n t h r a c e n e m o i e t i e s i n 1,2-dimethoxyethane/hexamethylphosphoramide [a(4H) 0 . 1 8 4 , 0 . 1 2 1 , 0 . 0 9 8 , 0 . 0 5 9 , and 0 . 0 9 6 a n d a ( 2 H ) 0.264 mT1 b u t i t i s d e l o c a l i z e d o v e r o n l y one m o i e t y i n t h e r a d i c a l - t r i a n i o n [a(4H) 0 . 2 8 3 and 0 . 1 4 6 , a ( 2 H ) 0.228 and 0 . 0 2 9 , and a(H) 0 . 4 7 2 m T 1 . 2 9 3 * 2 9 4

(79) (80)

The r a d i c a l - a n i o n s o f two a n t i a r o m a t i c a n n u l e n e s , t h o s e of [ 8 1 - and [ 1 6 1 - a n n u l e n e h a v e been r e p o r t e d p r e v i o u s l y . However, t h e r a d i c a l - a n i o n o f t h e l m i s s i n g l [ 1 2 l a n n u l e n e h a s now been reported.295

The a n n u l e n e i s p r e p a r e d

U

p h o t o i r r a d i a t i o n of

212

Electron Spin Resonance

~-tricycloC6.4.O.Oldodeca-2,4,6,1O-tetraene a t 163 K f o l l o w e d by e l e c t r o c h e m i c a l o r a l k a l i - m e t a l r e d u c t i o n below 2 3 3 K. I t s e. s . r. s p e c t r u m h a s been i n t e r p r e t e d i n t e r m s of a ( 3 H ) 0 . 1 5 3 , a ( 6 H ) 0 . 0 2 2 5 , a n d a ( 3 H ) 0.130 mT. The r a d i c a l - a n i o n s and r a d i c a l t r i a n i o n s of l a r g e p o l y c y c l i c c o n j u g a t e d h y d r o c a r b o n s s u c h a s o c t a l e n e ( 8 1 1 , d i b e n z C c , j l o c t a l e n e ( 8 2 1 , a n d dicyclohepta[cd,ghlpental e n e ( 8 3 ) h a v e been r e p o r t e d . 2 9 4 3 2 9 6 The s p i n d i s t r i b u t i o n i n t h e r a d i c a l - a n i o n s and r a d i c a l - t r i a n i o n s a r e , of c o u r s e , q u i t e d i f f e r e n t , and t h e s p e c t r u m of (8313' i s i n t e r p r e t e d i n terms o f s i x s p l i t t i n g constants indicating t h a t the alkali-metal counterion p o l a r i z e s t h e spin-densi t y . The r a d i c a l - a n i o n s of some cyano-subs t i t u t e d a c e n a p h t h a l e n e s and a c e n a p h t h y l e n e s have been p r e p a r e d by e l ec t r o c hem i c a l r e d u c t i on i n dime t h y 1f o rmam i d e 97 O f p a r t i c u l a r i n t e r e s t i s t h e 1,2-dibromo-5,6-dicyanoacenaphthylene r a d i c a l - a n i o n which a p p e a r s t o be r e a s o n a b l y p e r s i s t e n t .

.

The r a n g e o f s u l p h u r h e t e r o c y c l i c r a d i c a l - a n i o n s h a s now been e x t e n d e d t o i n c l u d e t h o s e o f some Me3Si- and C N - s u b s t i t u t e d t h i o p h e n e s p r e p a r e d from t h e p a r e n t m o l e c u l e s by p o t a s s i u m - m e t a l r e d u c t i o n i n t e t r a h y d r ~ f u r a n . ~The ~ ~ r a d i c a l - a n i o n of dibenzot h i o p h e n e - 5 , 5 - d i o x i d e h a s been o b s e r v e d d u r i n g t h e r e a c t i o n o f t h e I t~ i~s n o t c l e a r i f t h e parent molecule with a l k o x i d e ~ . ~ r a d i c a l - a n i o n l i e s on t h e r e a c t i o n pathway t o t h e r i n g - o p e n e d products, The g l y o x a l d i i m i n e , B d N : CHCH: NBut, i s r e d u c e d by p o t a s s i u m i n 1 , 2 - d i m e t h o x y e t h a n e t o t h e r a d i ~ a l - a n i o n . ~ " The e . s . r . s p e c t r u m i n d i c a t e s t h a t b o t h t h e 1 - i s o m e r Ca(2N) 0 . 5 6 a n d a ( 2 H ) 0 . 4 4 mT1 a n d t h e & i s o m e r [ a ( 2 N ) 0 . 5 6 , a ( 2 H ) 0 . 4 4 , and a(39K+) 0 . 1 5 mT1 a r e p r e s e n t . P y r i d i n i u m m e t h y l i d e s can a l s o be r e d u c e d by potassium ( i n 1,2-dimethoxyethane) t o g i v e t h e r e l a t i v e l y s h o r t l i v e d radical-anions [ f o r e.g,, t h e radical-anion of pyridinium bis(methoxycarbony1)methylide h a s a(N) 0 . 5 5 2 and a(H) 0 . 3 7 4 , 0 . 3 2 6 , 0 . 2 0 5 , 0 . 1 6 1 , and 0 . 0 9 1 1 n T 1 . ~ The ~ ~ l a c k of e q u i v a l e n t r i n g p r o t o n

5 : Organic Radicals in SoEution

213

s p l i t t i n g c o n s t a n t s is n o t unusual i n radical-anions of s i m i l a r s y s t e m s s u c h a s (PhCHOIf. McLachlan MO c a l c u l a t i o n s h a v e b e e n employed t o a s s i g n t h e s e s p l i t t i n g c o n s t a n t s . An u n u s u a l r a n g e o f r a d i c a l ' c o m p l e x e s ' of 4 , 4 ' - b i p y r i d i n e h a v e b e e n d e s c r i b e d . 302 T h e s e c o m p l e x e s [(RnM+)L(M+Rn)l' a r e formed i n t h e r e a c t i o n of 4 , 4 ' - b i p y r i d i n e a n d o r g a n o m e t a l l i c compounds s u c h a s BePh2, ZnPh2, B E t 3 , GaMe w i t h p o t a s s i u m i n t e t r a h y d r o f u r a n . C o u p l i n g t o " 8 , 67Zn, and 39/71Ga i s o b s e r v e d a s a p p r o p r i a t e , w i t h t h e m a g n i t u d e o f a(N) l a r g e l y d e t e r m i n e d by t h e n a t u r e o f t h e c o o r d i n a t e d m e t a l . The s i m i l a r r e a c t i o n of Mo(C0I6 and W ( C O I 6 w i t h 4 , 4 ' - b i p y r i d i n e and p o t a s s i u m i n t h e p r e s e n c e of e x c e s s p h o s p h a n e , p h o s p h i t e , o r a r s a n e l i g a n d s g i v e s [ (XR3)(C0>4M-L-M(C0)4(XR3) Ir.303 E x t e n s i v e c o u p l i n g t o a l l t h e magnetic n u c l e i i s observed i n t h e s e s p e c i e s w i t h t h e i n t e r p r e t a t i o n of t h e i r s p e c t r a s u g g e s t i n g a conformation a t t h e metal centre. I n t h e l a s t volume o f t h i s s e r i e s I r e p o r t e d t h a t t h e o b s e r v a t i o n o f t h e s p e c t r a o f r a d i c a l s d u r i n g t h e c o u r s e o f some r e a c t i o n s provided evidence f o r a s i n g l e e l e c t r o n t r a n s f e r mechanism ( E q u a t i o n 1 0 ) . The i n t e r p r e t a t i o n o f t h e o b s e r v e d s p e c t r a h a s n o t a l w a y s been a c h i e v e d , however, and h e n c e t h e i d e n t i t y of t h e r a d i c a l s h a s n o t been f i r m l y e s t a b l i s h e d . Evidence for several further reactions possibly involving single electron t r a n s f e r mechanisms h a s now been p r e s e n t e d . T h e s e i n c l u d e t h e C l a i s e n c o n d e n s a t i o n , 304 t h e C a n n i z z a r o r e a c t i o n , 305 t h e a l d o l c o n d e n s a t i o n , 306 and t h e b e n z i l i c e s t e r r e a r r a n g e m e n t ,307 a n d c h a r a c t e r i z a t i o n o f t h e e . s . r . a c t i v e s p e c i e s i n some o f t h e s e r e a c t i o n s h a s now been p r o m i s e d . 3 0 6 I n t h e meantime Kaim e t a l . h a v e c h a r a c t e r i z e d t h e s p e c i e s formed i n t h e s i n g l e e l e c t r o n t r a n s f e r r e a c t i o n s o f A l H 3 and L i B H E t 3 w i t h n i t r o g e n - c o n t a i n i n g heterocycle^.^^^^^^^ I n t h e c a s e of r e a c t i o n s o f A l H 3 t h e s e s p e c i e s a r e t h e r a d i c a l - a n i o n complexes of e i t h e r A l H 3 o r AlH2+,308 a n d i n t h e c a s e of L i B H E t 3 t h e y a r e t h e r a d i c a l - a n i o n c o m p l e x e s of BEt3.309 I n t h e l a t t e r c a s e t h e s p e c i e s h a v e been i d e n t i f i e d w i t h t h e a i d o f ENDOR and T R I P L E r e s o n a n c e s p e c t r a . R

+

MX +[R',M;]

+[Rf,M+l

+

X'

(10)

To c o n c l u d e t h i s s e c t i o n I h a v e g a t h e r e d t o g e t h e r some o f t h e p a p e r s which i n v o l v e r a d i c a l - a n i o n s o f s p e c i e s c o n t a i n i n g P , S i , a n d B. The p h o s p h o r o u s c o n t a i n i n g r a d i c a l - a n i o n s o f Ph3P, Ph3P0,

Electron Spin Resonance

214

and Ph3PS a r e a l l r e a d i l y p r e p a r e d by e l e c t r o c h e m i c a l r e d u c t i o n i n dimethylformamide. C o u p l i n g t o 3 1 P i s e v i d e n t i n a l l of t h e i r s p e c t r a w i t h a(9-H) > a(p-H) >> a ( a ~ - H ) . ~ l ' The same t e c h n i q u e h a s b e e n employed t o p r e p a r e t h e r a d i c a l - a n i o n s o f di-PR2 s u b s t i t u t e d b e n z e n e s and b i p h e n y l s . 3 1 1 The v a l u e o f a(31P) i s m a r k e d l y t e m p e r a t u r e dependent and l i n e w i d t h a l t e r n a t i o n i n t h e s p e c t r a i n d i c a t e s r e s t r i c t e d r o t a t i o n a b o u t t h e Caryl-P bond. The o r g a n o s i l a n e (84) i s reduced t o i t s radical-anion with potassium i n 1 , 2 - d i m e t h o ~ y e t h a n e . ~ ' ~A t low t e m p e r a t u r e s (a. 193 K ) t h e m e t h y l e n e p r o t o n s become i n e q u i v a l e n t [a(H) 0.0459 and 0 . 0 4 9 7 mT1 d u e t o t h e ' f l i p p i n g ' of t h e m e t h y l e n e b r i d g e . The r a d i c a l - a n i o n s o f some o t h e r f o u r - a n d f ive-membered s i l i c o n - c o n t a i n i n g m o l e c u l e s h a v e a l s o b e e n r e p o r t e d . 313 F i n a l l y t h e t e t r a - t - b u t y l d i b o r a n e This r a d i c a l - a n i o n h a s a ( 3 6 H ) 0 . 0 5 4 a n d a ( 2 1 1 B > 0.144 mT.314 r a d i c a l - a n i o n was g e n e r a t e d by r e a c t i o n o f t h e d i - t - b u t y l c h l o r o - o r d i - t - b u t y l b r o m o - b o r a n e (But2BX, X = C1 o r B r ) w i t h Na/K a l l o y i n tetrahydrofuran. The s i m i l a r r e a c t i o n o f d i - t - b u t y l d i c h l o r o d i b o r a n e w i t h Na/K a l l o y g i v e s (But4B4)' I a ( 3 6 H ) 0.031 and a ( 4 l ' B ) 0 . 1 2 0 mT1 .315 Me3Si,

4fe Z S

~

SiMeg

LiMe

2

-

215

5: Organic Radicals in Solution

1

2 3 4 5 6 7 8 9

B. J. Tabner, i n ' E l e c t r o n Spin Resonance', ed. P.B.Ayscough ( S p e c i d l i s t P e r i o d i c a l Reports), The Royal Society of Chemistry, London, 1983, Vol. 8, p. 243. W.M.Gulick, Ugadbcm RevL, 1983, 8, 33. M.C.R.Symons, ReD. Prop. Sect. c. , 1981 (Publ. 19821, 151. R.A.Jackson, &nu. Fkp. Pron. C l a w , . S e c t . BL, 1981 (Publ. 19821, 81. R.A.Jackson, &uu. W.P r Q & S w i I . . Sect. B, , 1982 (Publ. 19831, 29, 69. S.Brumby, W g , 1983, 8, 1. R. A. Jackson, 3 9 1983, 523. K.Ohno, ,1982, 5p, 145. F.Momo, G. A. Ranieri, and A.Sotgiu, b u t . Enhanced SDectroso. 1983, 1,

w.

u.

a, a,

.-

79. 10 11

12

R. N. Bagchi, T. L. Henderson, and F. L. Wal ter,

-,

1982, l2, 175.

J. D. Lipscomb and R. W. Salo, lbmput. Wanted S.Dectrosc.p 1983, 1, 11. R. Schul t z , G. Hurst, T. E. T h i e r e t , and R. W. K r e i l i c k , ,1983 ,

!i3, 303-

13 14 15 16 17 18 19 20

G. J. Hormann and B.M.Peake, J. MagQ, ResqlL, 1983, s, 121. S.I.Weissman, 1982, 33, 301. C. P. Poole and H. A.Farach, -osc. Rev,, 1983, 1p, 167. S.Basu and K. A.McLauchlan, ,1983, 51, 335. S. Basu, K. A.McLauohlan, and A. J. D. R i t c h i e , 1983, 29, 95. R. Z. Sagdeev, W.Moeh1, and K.Mobius, J. Phvs. C-, 1983, fi, 31 83. L.Zhongli and J . K . %Wan, SOcL, 1983, 1 111, 2480. B.B.Adeleke, D.Weir, M.C.Depew, and J.K.S.Wan, J. C w , 1984, 62,

,-

-a&,

-.

117. 21 22 23 24 25 26 27 28 29 30

A.Kanemoto, S.Niizuma, S.Konishi, and H.Kokubun, Soc. JDn., 1983, 16, 46. Y.N.Molin and 0. A. Anisimov, Radiat. Phvs. C h , 1983, 2 L 77. S.Brumby, ,1983, 81, 1917. G.Placucci and L.Grossi, ,1982, 112, 375. K.U.Ingold, D.C.Nonhebe1, and T.A.Wildman, J. PWS. C h , 1984, 88, 1675. U. Aeberhard, R. Keese, E. Stamm, U. C.Voegeli, W. Lau, and J. K. Kochi, Helv.

-,

1983, 66, 2740.

K. S c h l o s s e r , J. &&, S t r u c t . , 1982, 95, 221. L-M.Wu and H. F i s c h e r , m v . CHlm.,1983, 46, 138. K. K. K a r u k s t i s and P.Smith, ,1983, 52, 202. A. Hasegawa, T. Wakabayashi, M. Hayashi, and M. C. R. Symons,

,-

1983,

I p s

941 *

38

A.Ferse, R.Vetter, and A.Bart1, 2 . Ch-, 1983, 21, 254. R.Stoesser and U.Proesch, Z., 1983, 23, 382. G . C i r e l l i , T. K.Ha, R. Meyer, and H. H.Gunthard, them., 1982, I2, 15. G. C i r e l l i , A. Russu, and H. H. Gunthard, J. P h o t o c b , 1983, 22, 123. A. J.Bloodworth, A.G. Davies, R. A. Savva, and J. N. Winter, m e t . C b L , 1983, 253, 1. S.Brumby, J. C b b Sac.. - C , 1982, 677. N.M. Shishlov, V. N. Korobeinikova, V. A. Mazunov, A. A. Panasenko, and R.A.Sadykov, JWm, Vvs. Enern., 1983, lT., 179. , M. J. Davies, B. C.Gilbert, and R. 0. C. Norman, 1

39

B. C. G i l b e r t , R. O.C. Norman, P. S. Williams, and J. N.Winter,

40 41

D.Behar, R.W.Fessenden, and J. P. Hornak, C.Chatgilialoglu, K.U. Ingold, I.Tse-Sheepy,

42

1983, fi, 1077. R.G. Gasanov, R.G. Petrova, and R. K. F r e i d l i n a , J z v . &ad. Khim, 1982, 2596.

31 32 33 34 35 36 37

1983, 731

-

1982, 1439.

u. , -P and J.Warkentin,

1982, a,267. J.

m,

Ser,

216

43

Electron Spin Resonance D.C.Nonhebe1, C. J.Suckling,

and J.C.Walton,

Tetrahedron L e t t . ,

1982, 23,

4477. 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

H.G.Korth, R. Sustmann, R.Merenyi, and H.G.Viehe, J. Chem. SOC.. P e r k i n w s . 3 t 1983, 67M.Lehni and H.Fischer, &&. J. Chem. Kinet. , 1983, E,733. C. C h a t g i l i a l o g l u , K. U. I n g o l d , and J. C. Scaiano, J. Am. Chem. SOC,9 1983,

lQ!i, 3292. V. L. Vyazovkin, B. V. Bol 'shakov, and V. A. Tolkachev, Chem. Phvs,, 1983, E , 11. S. Basu, K. A.McLauchlan, and G. R. S e a l y , J. Phvs. E t 1983, 16,767. D.Beckert and K.Mehler, FXD. Tech. Phvs, , 1983, XL, 397. D.Beckert and K.Mehler, Ber. Bynsennes. PhVS. C h , 1983, &L, 587H.G. Korth, P.Lomes, W. S i c k i n g , and R.Sustmann, b t . J. Chem. Kinet,, 1983, l!i,267. D.A.Trifunac, NATO Adv. Studv I n s t . S e r . . S e r . C , 1982, 84, 347. S-W.Ko, A.C.Ling, R.J.Waltman, and J.Bargon, , 1983, 2, 67. K.H.Lee and S.Brumby, J. Chem. Soc.. P e r k i n T r a n s . 3 , 1983, 1071. N. A. Batyrbaev, V. V.Zorin, A. P.Moravskii, V. F. Shuvalov, S. S. Z l o t s k i i , and D.L. Rakhmankulov, zb. Obshch. K U , 1983, 53, 416. B. C.Gilbert, D. M. King, and C. B. Thomas, J . Chem. SOC.. P e r k i n Trans. 2, 1983, 675. T.Kudo and H.Heusinger, Carbohvdr. Res. , 1983, 23, 41. L.Lunazzi, G. P l a c u c c i , and L.Grossi, T e t r a h edron, 1983, 39, 159. J.D. Rich and R.West, J. Am. C h a . SoCL9 1983, Illfit 5211. A. A l b e r t i , A.Hudson, and G. F. P e d u l l i , .Tetrahedron, 1982, 3, 3749. P.Wuensche, H.K.Roth, and J . L i n k e , Pnlvm. P h o t o c b 9 1984, 4, 95. D. L. Kleyer, R. C.Haltiwanger, and T.H. Koch, J. Ora. Chem., 1983, 4&, 147. R. J. Himmelsbach, A. D.Barone, D. L. Kleyer, and T. H. Koch, J. Ora. C b , 1983, M,2989. A.T.Tegowski and D.W.Pratt, C Soc,, 1984, 1Q6,64. C.Roberts and J.C.Wdlton,.nikreP na.J 7 , 1983, 879.

.-

A.G. Davies, J. A. A. Hawari, M. Grignon-Dubois, and M. P e r e y r e , .-Omanornet. Chem., 1983, 25, 29. B.Maillard and J.C.Walton, C 1983, 900. T. Kawamura, Y. Sugiyama, and T. Yonezawa, Bull. Chem. SOC. Jm. 9 1983,

*

16,

2919. 69

J. Kamphuis, R. G.Visser, H. J. T. Bos, A.Mackor, and A. H. Huizer, Tetrahedron

Lett., 1983, 24, 2299. 70

J.C.Bevington,

P.F.Fridd,

and B. J. Tabner, J. Chem. Soc.. P e r k i n T r a n a - 2 ,

1982, 1389. 71 72

T.L.Simandi, A.Rockenbauer, and F.Tudos, Eur. Polvm. J. 9 1983, 19,427. T. I.Vakul'skaya, V. A. Lopyrev, and M. G.Voronkov, Dokl. Aka d. Nauk SSSR ,

73

J.A.Baban,

1983, 212, 387. M.D.Cook,

and B.P. Roberts, J. Chem. SOC.. P e r k i n Trans. 3 , 1982,

1247. 74

A.G.Davies,

J.Y.Nedelec,

and R . S u t c l i f f e ,

~,

P k'

T

1983, 209.

75 76 77 78

T.Ozawa and T.Kwan, c c , A.G.Davies and R . S u t c l i f f e , J. Chem. SOC. Per k i n H. G. Korth, P.Lommes, and R. Sustmann, J. Am. c m . D.Griller, D.C.Nonhebe1, and J.C.Walton, J. Chem.

1983, 1373.

79

D.Griller, D.C.Nonhebe1,

1983, 80. Trans. 3, 1982, 1483. SOCL, 1984, 1114, 663. Soc.. P e r k i n Tm

ns.,

and J. C.Walton, J. Chem. Sac.,, - C

1982, 1059. 80 81

J. C.Walton, J. M a n . ResoRr, 1983, 52, 241. E.Bascetta, F.D.Gunstone, and J.C.Walton, J. Chem. SOC.. Per k i n Trans. 3 ,

1983, 603-

82 83 84

P.Smith and C.I.Weathers, L , .J 1984, %, 77. H. J. Dern, F.Lange, and R. Sustmann, Chem. Ber. 8 1983, 1l6,3316. A.G.Davies, E.Lusztyk, J.Lusztyk, V.P. J.Marti, R.J.H.Clark, and M.J.Stead, J. Chem. SOC.. Per k i n Trans. 3 , 1983, 669.

217

5: Organic Radicals in Solution

T.Chen, F.Graf, and H.H.Gunthard, Chem., 1983, E,165. R. N. Sangst er, K. P.Madden, and R.H. S c h u l e r , U h k , 1983, 2395. R.H. Schul er and H. Taniguchi, -AChem., 1984, 1507. 87 M.Kira, H.Sugiyama, and H.Sakurai, Soc,, 1983, lQ5, 6436. 88 H. Sakurai, M. Kira, and H. Suglyama, Chem., 1983, 599. 89 K.Akiyama, S.Tero-Kubota, and Y. Ikegami, 1983, 1pli, 90 3601. O.W.Kolling, W . C h , 1983, 55, 143. 91 J.C. Walton, J. Chem. Soc.. Per, 1983, 1043. 92 L. N.Markovskii, P. P. Kornuta, L. S. Kachkovskaya, and O.M. Polumbrik, 93 Lett., 1983, 1, 143. H.Zeldes and R. Li v i n g st o n , 1982, fi, 1254. 94 M. Zamkanei, J. H. Kaiser, H.Birkhofer, H. D.Beckhaus, and C. Ruechardt, Chem. 95 1983, J S - 9 3216. V.I.Savin, V.I.Morozov, and Y.P.Kitaev, ,1983, 19,977. 96 R.Livingston and H.Zeldes, J. Phvs. -, 1983, &, 1086. 97 J. M. Dust and D. R. Arnold, ,1983, 1pfi, 1221. 98 K.H.Lee and S.Brumby, SOC.. p 1982, 1537. 99 100 A.Alberti, G.Seconi, G.F.Pedulli, and A.Degl'lnnocenti, JArgamnEL chem., 1983, 253, 291. 1983, 118, 3828. 101 B.W.Smith, C.Saint, and G.D.Branum, J. O m . , 102 R.Leardini, A. Tundo, G.Zanardi, and G. F. P e d u l l i , d d W m , b k b 1983, 285. 103 R. L e a r d i n i , A. Tundo, G. Zanardi, and G. F. P e d u l l i , Tetrahedron, 1983, 3p, 2715. SOC,, 1983, 1Qfi, 104 C.Hass, B.Kirste, H.Kurreck, and G.Schloemp, 7375. 1983, 105 R.C.Haddon, A.M.Hirani, N.J.Kroloff, and J.H.Marshal1, J. O m . C,118, 2115. 106 A. G. Davles, J. A. A. Hanari, and M. W h i t e f i e l d , T e t r a h e d r o n L e t t . , 1983, 4465. 107 K.Tajima, H. Shimizu, T.Morita, H. Tmoda, K.Mukai, and K. I s h i z u , R89on.q 1983, 2.L 376. 108 E.C. Ashby, R. N. D eP r i e st , A. Tuncay, and S. S r i v a s t a v a , Tetrahedron L e t t . , 1982, 23, 5251. 1983 , 251, 53. 109 A.G. Davies and J. A. A . H a w a r i , J. Orsannmet. ,C110 Z. K. Kasymbekova, A. I. Prokof'ev, N.N.Bubnov, S. P. Solodovnikov, V. I.Bregadze, V. T. Kampel, M.V. P e t r i a s h v i l i , 1.N. Godovikov, and H. I.Kabachnik, 2 v . a ,1983, 316. 111 K. Sarbasov, S. P. Solodovnikov, B. L. Rtmanskii, N. N. Bubnov, and A. I. Prokof ' e v , &v. Ser. 1982, 1509. 112 A. K. Chekalov, A. I. Prokof ev, N. N. Bubnov, S. P. Solodovnlkov, A. A. Zhdanov, and Ser. ghipr., 1983, 1184. M.I.Kabachnik, . maaoaeb.t, 1983, 2!&, 199. 113 A.Alberti and A.Hudson, JO G&&, 1983, 261. 114 A . A l b e r t i and G.F.Pedulli, J.et. , 1983, 39 3943. 115 M. T.Caproiu, N. Negoita, and A. T.Balaban, 1984, &, 1142. 116 M.Novak and B. A.Brodeur, J. OrR. C&, 117 B. S. Taraselchuk, A. I.Belozerov, E. P. Sanaeva, and K. P.Butln, Zh. O m . , 1983, 1p, 561. Soc.. P e r k i n . 2, 118 J.C.Brand, B. P. Roberts, and J.N.Winter, J. C& 1983, 261. 119 S.A.Fairhurst, R.S.Pilkington, and L . H. S u t c l i f f e , Trans., 1983, 19, 439. 120 S. A . Fai rhurst , R.S. P i l k i n g t o n , and L. H. S u t c l i f f e , J. C b . Soc.. -F Trans., 1983, ILP, 925. 121 K. S c h l o s s e r and S. Steenken, J-&&l&& SOC., 1983, 1pli, 1504. 122 Y.Mlura, H.Asada, K K i n o s h i t a , and K.Ohta, J. Phvs. Che,ih 1983, &, 3450. 123 Y.Miura, T.Kunishi, and M. Ki n o sh i t a , GW&,L-I&L, 1983, 885. 1983, 313. 124 A. A l b e r t i and A.Hudson, J., - rO 125 F.A.Neugebauer and R.Siege1, m e w . C h , 1983, E,329.

85

86

m,

m,

-

w,

w,

mu.,

a,

w,

a,

a,

Electron Spin Resonance

218

a,

126 127 128

a,

129 130 131 132 133 134 135 9

136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 1 57 158 159 160 16 1 162 163 164 165 166 167 168

L.S. Podenko, A.K. Chirkov, and V. P. Shchipanov, Zh. S t r uk t . K h k , 1982, 84. A.Alberti and G.F.Pedulli, J. OrR,anmet. C a , 1983, M, 2544. M. T.Caproiu, M.Elian, N.Grecu, N.Negoita, and A.Balaban, J. Chem. SOC.. ans. 7 , 1983, 591. M.D. Cook, L. L.Ng, and B. P. Roberts, Tetrahedron Lett,, 1983, 3761. J.A.Howard, R . S u t c l i f f e , and B.Mile, J. Phvs. CheqL, 1984, 88, 171. J.Lub, M.L.Beekes, and T. J.de Boer, 1 k 9 1983, 721. H. Teeninga and J.B.F.N. Engberts, J. Ora. C h e ~ ,1983, U, 537. R. S u t c l i f f e , M.Anpo, A.Stolow, and K. U. I n g o l d , Chem. SOC.9 1982, ULy, 6064. J.C.Brand and B. P. Roberts, J. Chem. Soc.. Per k i n T r u , 1982, 1549. J. E l o r a n t a , E.Hamalainen, E.Salo, R.Makela, and U.Kekalainen, Acta. €k& 1983, 831, 383. J. Tsuchiya, E . N i k i , and Y.Kamiya, , 1983, 56, 229. M.Matsuo, S.Matsumoto, and T.Ozawa, &a. Uagn, Reson., 1983, 261. T.Doba, G.W.Burton, and K.U. Ingold,, J 1983, lQ5, 6505. J.Tsuchiya, Sono >hS I 1982, U , 213. K.Mukai, C.Morimoto, and K.Ishizu, T e t r a h e d r o n L e t t . , 1983, &, 5099. K.Mukai, M. Yamashita, K. Ueda, K. Tajima, and K. I s h i z u , J. Phvs. C h e L , 1983, BL 1338. D. E.Wellman, K. R. L a s s i l a , and R.West, J. O m . ChePL, 1984, Us 965. J.Herdan, A. T.Balaban, N. Negoita, and N.Grecu, Rev. Roum. C h k , 1983, 28, 129. V.Fischer, W.Buehler, and K.Scheffler, 2. Naliuforsch.. T e i l 9, 1983, 570 V.Fischer and K.Scheffler, Z. Naturforsch.. T e i l A, 1983, 3 U , 68. E. P. Ivakhnenko, Zh. O m . Kh inL, 1983, 1p, 886. F.R.Hewgil1 and F.Legge, k T , 1982, 2863. F. R.Hewgil1 and F.Legge, J. C b . SOC.. P e r k i n T r a n n , 1983, 653. P. P e l i k a n , A.Tkac, L. Omelka, and A. S t a s k o , O m . Magn, R e s m , 1982, 212, 205. L. Omelka, A. Tkac, L. J i r a c k o v a , and J. P o s p i s i l , pER. M a n . Reson., 1982, 1p, 153. 0. N. J e n s e n and J. A. Pedersen, Tetrahedron, 1983, 39, 1609. F. J.Rakoczi, T.K.Ha, and H.H.Gunthard, &em. P h v L 9 19839 111, 273. W.A.Pryor, D.G.Prier, and D.F.Church, J. Am. C h e m . S O G , 1983, XE, 2883. E.Niki, S.Yokoi, J.Tsuchiya, and Y.Kamiya, J. Am. Chem. SOC.r 1983, U ! L t 1498. J. E.Bennett, G.Brunton, A. R. F o r r e s t e r , and J. D . F u l l e r t o n , J. Chem. SOC.. -Trans.., 1983, 1477. C. C h a t g i l i a l o g l u and K. U. I n g o l d , J. Phvs. C h e L , 1982, 86, 4372. L.H.Sutcliffe and A.Zilnyk, J. Cheer. SOC.. Far-s. 1 , 1982, 28, 3499. E.G. Janzen, D.Rehorek, and H. J. S t r o n k s , J. M a a a ResQa 9 1984, 56, 174. E.G. Janzen, G.A.Coulter, U.M.Oehler, and J. P.Bergsma, Can. J. C h , 1982, 4a, 2725. Y.Kotake and K.Kuwata, W l . Chem. SOC. JDn. 9 1982, !i!L 3686. Y.Ito, J. C k m - P h y ~ , 1983, 2650. R.Hendriquez, M. J . P e r k i n s , and D. Griller, Can. J. C b , 1984, f& 139. M.Kira, H.Osawa, and H.Sakurai, Je..t C h e b , 1983, 51. L.Grossi, L.Lunazzi, and G.Placucci, J. Chem. SOC.. Per k i n Trans. 7 9 1983, 1831. K.Abe, H.Suezawa, M.Hirota, and T . I s h i i , J. C b SOC.. P e r k i n Trans. 2, 1984, 29. H. Chandra, I . M . T.Davidson, and M. C. R. Symons, J. Chem. SOC.. P e rk i n T r m 2, 1982, 1353. J. E.Bennett, G.Brunton, A. R . F o r r e s t e r , and J . D . F u l l e r t o n , J. C a . Soc. P e r k i n Trans. 2 , 1983, 1481. E.G.Janzen and G.A.Coulter, &J& Chem. SOC,, 1984, 1Qh, 1962.

a,

m,


N-O-. However, the spectrum was very asymmetric in the case of system 11. The radicals are postulated to be formed by transfer of an oxygen atom from a nitro group of I or 11. Brown et al. 86 have studied the kinetics of the reactions between amines and epoxy resins and related the results to the microstructure of the m i n e cured epoxy. 3.2. Heterogeneous Chain Growth.- Heterogeneous polymerization is becoming increasingly important because of its obvious commercial relevance and applications. In this review we will limit our comments to two major and significant areas: a) graft copolymerization and b) POlymeriZatiOn catalysis. Graft copolymerization involves polymerization of monomer to polymer through initiating sites on a preformed polymer. This process can be carried out in homogeneous solution phase, but use of a gaseous phase monomer is becoming more common. An area which is receiving much attention is the use of preformed polymer films where reactions can occur much more rapidly at the surface than in the bulk polymer. This field of surface coatings is an immensely important one, and an area in which much interest is being shown.

Electron Spin Resonance

238

Most grafting is initiated by ionising radiation, gamma rays or electron beams, or UV irradiation. However, the use of plasma initiation seemsto be increasingly studied and this area has been covered in an earlier section. 3.2.1 Gra,ft Copolymerization. Most studies make use of polymer substrates which are readily available synthetically or occur as natural polymers,such as cellulose, dextran, etc. Morgan et a1.87 have studied the grafting of acrylonitrile onto low density polyethylene powder,initiatedby 0.8 MeV electron irradiation. The percent graft was investigated along with the concentration of radical forms by varying several reaction parameters, e.g. air, vacuum, dry oxygen and dose rate. ConverselY, Ogiwara et a1.88 have used a two step process to graft acrylamide to low density polyethylene coated with Ph2C0. Initially the P E film was grafted by photoinitiation to one of acrylic acid, methacrylic acid, acrylonitrile or methyl methacrylate and this substrate was then further grafted with acrylamide. Most efficient secondary grafting was obtained in the P E /methacrylic acid system. The reaction mechanism was postulated from the free radicals observed. Sat0 and Otsu are continuing their studies of substituted poly (acrylamides). The systems produced a stable propagating radical, the reaction of which can be studied with other monomers. The propagating radicals of 2-methylacrylamide and x-methylmetha~rylamide~’have been reacted with a series of binary mixtures of vinyl monomers and the resulting radical species identified. Studies on the propagating radical of E-phenylmethacrylamide, produced by photoinitiation of the benzene solution of the monomer and tert-butyl peroxide have been reported90 These propagating radicals reacted with methyiacrylate and methacrylate to form block copolymers. Tatsumi and Yamamoto91’92 have studied a series of popcorn polymerizations by e.s.r. using a variety of seed copolymers and monomers. The spectra obtained were analysed and discussed.

.

3.2.2 Polymerization Catalysis. As in previous reviews, the number of papers involving polymers and metal atoms continues to be

6: Applications of ESR in Polymer Chemistry

239

significant, covering a range of studies from the structures of the coordination complexes involved in polymerization catalysis to the interaction of metals with polymers. Wichterlova et al.93 have studied the effect of the chromium ions in chromium zeolite catalysts and the extent of polymerization of ethylene. A study of chromium catalysts for ethylene has also been reported by Chien and Haller94 The kinetics of the polymerization of styrene using the Cu2C12 + A1Et3 catalyst system has been studied by Pandya et al.95, who, on the basis of e.s.r. results, propose a different mechanism to that described for the CuC12/A1Et3X (X = Br, C1) system. Studies of the polymerization of butadiene with a nickeldinaphthanate/A1Et3/BF30Et2 catalyst have been reported96

.

.

4 Polymer Structure, Interactions and Properties Many properties of polymer systems have been correlated with the e.8.r. spectra of the polymer. Properties reported have included molecular ratio, conformation and bonding with metal atoms. A particularly important class of polymers that continues to receive much attention is semiconducting polymer systems in which the observed paramagnetism can yield information on polymer microstructure and mobility. 4.1 Conductive Polymers.4.1.1 Polyacetylenes. A variety of properties of polyacetylenes have been examined using e.s.r. in conjunction with other techniques. Several papers have examined the roles played by or dopants such as compounds of arsenic97f98f99, ironloo or iodine101’102. The results of these studies indicate the presence of both localized and ‘itinerant’ sites for the paramagnetic centres. The nature of the paramagnetic sites in undoped polyacetylenes has also been examined103,104,105 as have 106 these sites in stretched polyacetylene films Schen et a1.1°7 have reported studies of the kinetics and mechanism of polymerization of acetylene. Radioactively labelled inhibitors were used and the polymerization was quenched with labelled carbon monoxide, thus allowing the active sites to

.

Electron Spin Resonance

240

be identified. When combined with earlier e.s.r. studies, these results allow conclusions to be drawn about the catalytically active species in the polymerization. The cis-trans isomerization in polyacetylene also continues to be investigated. It has been reportedlo8 that the presence of oxygen accelerates the isomerization, probably through interaction with the double bonds. The adsorption of oxygen at soliton sites has also been investigated by Genoud et a1 109using e. s. r .In another study, Bernier et al. 'lo suggested that degradation processes are certainly involved, and they obtained an activation energy close to 30 k cal mole' from e.s.r. studies for the thermal isomerization.

.

.

4.1.2 Poly (p-phenylene) E. s.r. studies'" on poly (p-phenylene) have shown that the nature of the paramagnetic centres in this polymer are strongly dependent upon its mode of preparation and on the polymer's thermal history. Doping of the annealed polymer with SbF4 leads to a reduction in the line width of the single-line e.s.r. spectrum'12, and the appearance of this narrower signal appears to be closely related to an increase in the conductance of the polymer. The effect of deuteration on polymer properties has has been also been reported' 13. Poly (p-tetradeuterophenylene) shown to have a different colour, solubility and concentration of paramagnetic centres from that found for the undeuterated polymer. Polypyrroles. Neutral polypyrrole exhibits two distinct lines at room temperature. One line is narrow with an intensity of one spin per 1000 residues, while the other is broader with an intensity of 10 spins per 1000 residues. Devreux et al.114 showed that there is probably a maximum in the spin susceptibility versus temperature curve, with a sharp decrease observed for T < 30 K. Oxygen doping increases the intensity of the narrow signa1115' 'I6. The conductance of polypyrrole was found to increase in the early stages of oxidation, whereas the changes in the e.s.r. and optical properties occur in the latter stages of oxidation, when no further changes in the conduction take place. The early stages of oxidation are believed to lead to an ionic polymer, and, during the latter stages, some chemistry at nitrogen atoms of the pyrrole rings is involved. 4.1.3

e.8.r.

6: Applications of ESR in Polymer Chemistry

24 1

polypyrrole

4.1.4 Other Systems. Snow and Griffith”’ have studied the polymerization of poly-butadiyne on a variety of polymeric substrates. The results indicated that the post polymerization of terminal acetylenic groups led to a more highly aromatized and complex polymer and was dependent on the polymer substrate. Tourillon et a1.118 have shown that the polymers formed from 3-methylthiophene and 3,4-dimethylthiophene doped with B U ~ N + C F ~ S O ~ exhibit quasi-metallic behaviour with the electrons moving along the carbon backbone of the polymer. The incorporation of carbon fibres into polymer systems and their resulting reaction and structure have been reported. Fialkov et al.ll’ have reported the effects of oxygen on the e.s.r. spectra of formaldehyde-furfural-phenol copolymers with 20% carbon. Differences in the e.s.r. spectra were related to the amount of adsorption of oxygen onto the carbon. Breedon Jones et a1.l2’ have reported the dependence of the structure of carbon fibres on their method of preparation. The structure and relaxation mechanism of polymers in solution have been studied by Mita et a1.121 who reported relaxation experiments in polystyrene and their dependence as a function of viscosity of Iwai et al. 122 have studied exciplex the solution. formation and energy transfer in p-(dimethylamino)styrene-9vinylphenanthrene copolymer in various polar solvents. 4.2 Polymer/Metal Interaction.- The structures and interaction between polymers and metals continues to be of interest. Krivos et al.123 have reported the e.s.r. spectra of Mn2+ ions absorbed on granulated nylon and polypropylene fibres. The distribution of Cu2+ ions on a series of cation exchange

242

Electron Spin Resonance

membranes has been reported by Vasquez et al. 124 The microstructure of a series of sulfonated polystyrene ionomers has been studied by Weiss et al.125 after different thermal treatments. The microstructure was found to be dependent Mn, Na or Zn) and is on the nature of the cation present (i& explained in terms of enhanced diffusion of ionic species at elevated temperatures. References 1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

1983, 21, 957. R.Basheer and M.Dole, J.Polym.Sci.,Polym.Phys.Ed., 1983, 21, 111 R.Basheer and M.Dole, J.Polym.Sci.,Polym.Phys.Ed., 1984, 22, 1313. R.Basheer and M.Dole, J.Polym.Sci.,Polym.Phys.Ed., Aires), M.Dole and R.A.Welch, An.Acad.Nac.Cienc.Exactas,Fis.Nat.(Buenos 1982, 34, 139. A.Plonka, J.Polym.Sci.,Polym.Phys.Ed., 1983, 3, 1011. J.Sohma, Pure Appl.Chern., 1983, 55, 1595. P.Wuensche, J.Limburg and H.K.Roth, J.Macromol.Sci.,Phys., 1983, 169. Y.M.Pleskachevskii, T.M.Kachalova and V.P.Sel'kin, Dokl.Akad.Nauk BSSR, 1983, 27, 235. T.Hashimoto, K.Ogita, S.Umemoto and T.Sakai, J.Polym.Sci.,Polym.Phys.Ed., 1983, 21, 1347. J.L.Williams and T.S.Dunn, Radiat.Phys.Chem., 1983, 22, 209. M.R.Murthy and S.Radhakrishna, Pramana, 1983, 20, 8 5 7 M.R.Murthy and S.Radhakrishna, Indian J.Pure Appl.Phys., 1983, 21, 5 8 2 . M.Tabata, G.Nilsson, A.Lund and J.Sohma, J.Polym.Sci.,Polym.Chem.Ed., 1983, 21, 3257. M.I. Bitenbaev and A.I.Polyakov, 1zv.Akad.Nauk Kaz.SSR,Ser.Fiz.-Mat., 1983, (6)s 16. H.Xue and J.Qian, Huaxue Xuebao, 1983, 41, 692. H.Xue and J.Qian, Huaxue Xuebao. 1983, 41, 1038. M.R.Kawamra, P.R.Crippa and S.Isotani, Int.J.Biol.Macromol., 1983, 5, 312. M.I.Chipara, I.Bunget, R.Georgescu, E.Georgescu and I.Vilcov, Nucl.1nstrum. Methods Phys.Res., 1983, 209-210, 395. K.Sultanov, D.S.Khamidov, U.A.Azizov and Kh.U.Usmanov, Vysokomol.Soedin., Ser.B, 1983, 25, 6. A.N.Rukhlya, E3. Petryaev and O.I.Shadyro, Zh.Prikl.Khim. (Leningrad), 1983, 56, 954. J.J.Raffi and J.P.L.Agne1, J.Phys.Chem., 1983, 87, 2369. S.N.Bhattacharyya and D.Maldas, J.Polym.Sci.,Polym.Chem.Ed., 1983, 21, 3291. V.N.Khabarov, L.L.Koz1ov and G.M.Panchenkov, Zh.Prikl.Spektrosk., 1983, 2, 444. K.Setoyama, Nippon Sanshigaku Zasshi, 1982, 51, 305. T.N.Bowmer, J.R.Brown, E.Grespos and J.H.O'Donnel1, Proc.IUPAC, I.U.P.A.C., Macromol.Symp.,28th, 1982, 445. A.Ferse, R.Vetter and A.Bart1, Z.Chem., 1983, 23, 254. G.M.Kent, Report, 1982, N-8316395. (CA 100:69141). K.R.Schaffer, R.E.Fornes, R.D.Gilbert and J.D.Memory, Polymer, 1984, 25, 54. A.N.Malakhova, T.S.Men'shchikova, S.L.Panasyuk, A.A.Persinen and T.P. Tolmacheva, Deposited Doc., 1981, SPSTL 850 Khp-D81. (CA 98:54986). A.Torikai and K.Fueki, Polym.Photochem., 1982, 2, 297. A.Merlin and J.P.Fouassier, Angew.Makromol.Chem., 1982, 108,185.

e,

-

243

6: Applications of ESR in Polymer Chemistry 32 33 34 35 36 37 38 39

40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

K.Setoyama, Nippon Sanshigaku Zasshi, 1982, 51, 271. E.Y.Davydov, G.B.Pariiskii and D.Y.Toptygin, VysokoPaol.Soedin.,Ser.A, 1983, 25, 144. H.Egawa, M.Ishikawa, M.Tsunooka, T.Ueda and M.Tanaka, J.Polym.Sci.,Polym. Chem,Ed., 1983, 21, 479. G.S.Zhdanov, A.S.Smolyanskii, L.G.Khamidova and V.K.Milinchuk, Khim.Vys. Energ., 1983, 17,318. T.A.Skowronski, J.F.Rabek and B.Raanby, Polym.Photochem., 1983, 2, 341. J.F.Rabek, "Experimental Methods in Photochemistry and Photophysics", Wiley, Chichester, 1982. A.Torikai, K.Suzuki and K.Pueki, Polym.Photochem., 1983, 3, 379. O.Hinojosa, J.A.Harris and R.R.Benerito, Book Pap.,Natl.Tech.Conf.-AATCC, 1983, 246. 182 H.Kacemarek, F.Rozploch and A.Kaminska, Angew.Makromol.Chem., 1983, 2, A.L.Karasev, A.S.Kolotilldn and E.I.Mal.'tsev, Proc.Tihany Symp .Radiat.Chem. 1982, 1983, 2, 859. J.Bargon, Proc.IUPACs1.U.P.A.C.,Macromol.Symp.,28th, 1982, 297. 1983, 17 47. G.S.Zhdanov, L.G.Khamidova and V.K.Milinchuk, Khim.Vys.Ener S.Poulin-Dandurand, M.R.Wertheimer and A.Yelon, Polymer, 1933, 2,1581. K.Ohno, N.Ishii and J.Sohma, Jpn.J.Appl.Phys.,Part 1, 1983, 22, 996. N.Hozumi, T.Takao, Y.Kasama and Y.Ohki, Jpn.J.Appl.Phys.,Part 1, 1983, 22, 636. J.Janca, A.Talsky and M.Necasova, Folia Fac.Sci.Nat.Univ.Purkynianae Brun., 1982, 23, 69. O.Hinojosa, T.L.Ward and R.R.Benerito, ACS Symp.Ser., 1983, g,225. R.Song, S.Yue, J.Liu, J.Chen, Z.Chen and X.Liu, Gaodeng Xuexiao Huaxue Xuebao, 1983, 4, 139. G.Legeay, J.C.Brosse and F.Epaillard, Vide, Couches Minces, 1982, 212, 213. B.A.Gorelik, D.Aneli and E.Semenenko, Vysokomol.Soedin.,Ser.A, 1982, 24, 2532. D.N.S.Hon, J.App1. D.N.S.Hon and K.S. T.Saita and O.Matumura, Jpn.J.Apm B.E.Krisyuk, E.V.Polianc1 Smirnov, Vysokomol.Soedin. ,Ser.A, 1983, 25, 2036. Y.Ogo, Y.Okuri and Y.Miura, Zairyo, 1983, 32, 808. 1.M.Brown and T.C.Sandreczki, Report, 1982, UCRL-15512. (CA 99:141431). D.Roylance, Int.J.Fract., 1983, 21, 107. C.Simionescu, C.Vasiliu-Oprea and C.Negulianu, Rev.Roum.Chim., 1982, 27, 1055. K.L.DeVries and M.Igarashi, Proc.IUPAC, I.U.P.A.C.,Macromol.Symp.,28th, 1982, 795. M.Igarashi and K.L.DeVries, Polymer, 1983, 24, 1035. M. Igarashi, J .Polym. Sci. ,Polym. Chem. Ed. , 1983, 21, 2405. B.Banerjee, Kautsch.Gd,Kunstst., 1984, 37, 21. E.V.Dovbii, I.F.Khudoshev, A.T.Kalashnik, S.P.Papkov, A.V.Volokhina and G.Kudryavtsev, Khim.Voloha, 1983, (A), 21. J.H.O'Donnel1, B.McGarvey and H.Morawetz, J.Am.Chem.Soc., 1964, 86, 2322. V.R.P.Verneker and R.Vasanthakumari, J.Polym.Sci.,Polym.Chem.Ed., 1983, 21, 1657. J.H.O'Donnel1, P.J.Pomery and R.D.Sothman. Makromol.Chem., 1980, 181,409. H.Kumakura, T.Fujimura and I.Kaetsu, Eur.Polym.J., 1983, 2,621. B.W.Dodson and C.Arnold, J.Phys.Chem., 1983, 87, 3023. S.Kilic and B.M.Baysa1, Makromol.Chem., 1983, 184, 1919. M.Kamachi, H.Umetani, Y.Kuwae and S.Nozakura, Polym.J.(Tokyo), 1983.

-

-

.,

Electron Spin Resonance

244

72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108

H.Sixl, &Gross and W.Neumann, Stud.Inorg.Chem., 1983, 2, 493. P.Wuensche,H.K.Roth and J.Linke, Polym.Photochem., 1984, A, 95. B.Yamada, A.Matsumoto and T.Otsu, J.Polym.Sci.,Polym.Chem.Ed., 1983, 21, 2241. D.Behar, R.W.Fessenden and J.P.Hornak, Radiat.Phys.Chem., 1982, 2, 267. M.N.Masterova, E.M.Fedorova, V.B.Golubev and V.P.Zubov, Vestn.Mosk.Univ., Ser.2: Khim., 1983, 24, 176. T.Ota, Kenk,u Hokaku-Asahi Garasu Kogyo Gijutsu Shoreikai, 1983, 42, 65. A.Bukowski and T.Milczarska, J.Appl.Polym.Sci., 1983, 28, 1001. T.Ouchi, Y.Hosaka, M.Imoto and R.Konaka, MakromolGhem.,Rapid.Commun., 1983, 4, 263. T.Nonaka, H.Nishida, R.Tagawa and H.Egawa, Chem.Lett., 1982, (9), 1385. V.B.Golubev, I.V.Semenikhina, I.L.Stoyachenko, B.B.Patra, E.Y.Kositsyna and V.P.Zubov, Vysokomol.Soedin.,Ser.A, 1983, 25, 1230. S.Talapatra, S.K.Saha, S.K.Chakravarti and S.C.Guhaniyogi, Polym.Bul1. (Berlin), 1983, 10,21. M.J.Ballard, R.G.Gilbert, D.H.Napper, J.H.O'Donnel1 and P.J.Pomery, Macromolecules, 1984, 5, 504. Y.V.Vishev, E.G.Furman, A.P.Meleshevich, V.M.Kuznetsova and O.A.Valatina, Vysokomol.Soedin.,Ser.A, 1983, 2,970. K.Kawamura, Bull.Chem.Soc.Jpn., 1983, 56, 676. I.M.Brown, A.C.Lind and T.C.Sandreczki, Report, 1981, MDC-QO759. (CA 97: 217279). P.W.Morgan and J.C.Corelli, J.Appl.Polym.Sci., 1983, 28, 1879. Y.Ogiwara, M.Takumi and H.Kubota, J.Appl.Polym.Sci., 1982, 27,37430 T.Sato, T.Iwaki and T.Otsu, J.Polym.Sci.,Polym.Chem.Ed., 1983, 21, 943. T.Sato, Y.Yutani and T.Otsu, Polymer, 1983, 2,1018. M.Tatsumi and S.Yamamoto, Nippon Kagaku Kaishi, 1983,(8),1236. M.Tatsumi and S.Yamamoto, Polym.Bul1. (Berlin), 1983, 10,452. B.Wichterlova, S.Beran, Z.Tvaruzkova and J.Novakova, Geterog.Katal., 1983, 5, 291. S.H.Chien and G.L.Haller, Bull.Inst.Chem.,Acad.Sin., 1982, 2,15. M.V.Pandya, D.D.Deshpande and N.M.Desai, J.Appl.Polym.Sci., 1982, 27, 4861. S.Hua and X.Tang, Jilin Daxue Ziran Kexue Xuebao, 1982, f4), 91. D.Davidov, S.Roth, W.Neumann and H.Sixl, 2.Phys.B: Condens.Matter, 1983, 51, 145. S.Roth, K.Ehinger, K.Menke. M.Peo and R.J.Schweiser, J.Phya,Colloq., 1983, 69. D.Davidov, S.Roth and W.Nemnn, J.Phys.,Colloq., 1983, 295. Y.W.Park, J.C.Woo, K.H.Yoo, W.K.Han, C.H.Choi, T.Kobayashi and H.Shirakawa, Solid State Commun., 1983, 46, 731. J.M.Pochan and H.W.Gibson, Polym.Comun., 1983, 24, 322. J.M.Warakomski, J.C.W.Chien and F.E.Karasz, Polym.Prepr.(Am.Chem.Soc.,Div. Polym.Chem.), 1981, 22, 402. M.Mehring, H.Seide1, W.Mueller and G.Wegner, Solid State Conrmun., 1983, 45, 1075. A.Bart1, G.Freudenberg, J.Froehner, B.Pietrass and L.Wucke1, Makromol.Chem., 1983, 184,2187. K.Kume , K.Mizuno , K.Mizoguchi , K.Nomura , J Tanaka M.Tanaka, H.Fujimoto and H.Shirakawa, J.Magn.Magn.Mater., 1983, 31-34, 1151. S.Kuroda, M.Tokumoto, N.Kinoshita, T.Ishiguro and H.Shirakawa, J.Magn.Magn. Mater., 1983, 31-34, 1149. M.A.Schen, F.E.Karasz and J.C.W.Chien, J.Polym.Sci.,Polym.Chem.Ed., 1983, 21, 2787. A.Zanobi and L.D'Ilario, J.Phys.,Colloq., 1983, 309.

-

.

-

6: Applications of ESR in Polymer Chemistry

245

109 F.Genoud, M.Nechtschein, K.Holczer, F.Devreux and M.Guglielmi, Mol.Cryst. Liq.Cryst., 1983, 96, 161. 110 P.Bernier, S.Lefrant, M.Rolland, M.Aldiosi, M.Galtier, A.Montaner, C.Linaya and F.Schue, J.Electron.Mater., 1983, 2, 289. 111 F.Barbarin, G.Berthet, J.P.Blanc, C.Fabre, J.P.Germain, M.Hamdi and H.Robert, S th.Met., 1983, 6, 53. 112 M.Sato, K.h?riyama and K.Sokno, Makromol.Chem., 1983, 184,2241. 113 C.F.Hsing, I.Khoury, M.D.Bezoari and P.Kovacic, J.Polym.Sci.,Polym.Chem.Ed., 1982, 3, 3313. 114 F.Devrew, F.Genoud, M.Nechtschein, J.P.Travers and G.Bidan, J.Phys., Colloq., 1983, 621. 115 P.Pfluger, M.Krounbi, G.B.Street and G.Weiser, Report, 1983, TR-14. (CA 99: 213242). 116 P.Pfluger, M.Krounbi, G.B.Street and G.Weiser, J.Chem.Phys., 1983, 78, 3212 117 A.W.Snow and J.R.Griffith, Proc. UIPAC, I.U.P.A.C.,Macromol.Symp.,28th,1982, 432. 118 G.Tourillon, D.Gourier, F.Garnier and D.Vivien, J.Phys.Chem., 1984, 88, 1049. 119 A.S.Fialkov, L.S.Tyan and R.N.Sibgatullina, Dokl.Akad.Nauk SSSR, 1982, 267, 422. 120 J.Breedon Jones and L.S.Singer, Carbon, 1982, 20, 379. 121 N.Kasparyan-Tardiveau, B.Valeur, L.Monnerie and I.Mita, Polymer, 1983, 24, 205. 1983, 122 K.Iwai, Y.Itoh, M.Furue and S.Nozakura, J.Polym.Sci.,Polym.Chem.Ed., 21, 2439. 469. 123 B.J.Krivos, V.Hronsky and M.Rakos, Czech.J.Phys., 1983, B, 124 R.Vasquez, J.Avalos, F.Volino, M.Pineri and D.Galland, J.Appl.Polym.Sci., 1983, 28, 1093. 125. R.A.WeGs, J.Lefelar and H.Toriumi, J.Polym. Sci. ,Polym.Lett .Ed. , 1983, 21, 661.

-

7 Spin Labels: Biological Systems BY CHING-SAN LA1 1

Introduction

This chapter reviews the recent applications o f spin label i t c o m b i n e s C h a p t e r s 1 0 and 11 o f t h e p r e c e d i n g v o l u m e o f t h i s s e r i e s . The m o s t e x c i t i n g development d u r i n g t h i s r e v i e w P e r i o d i s t h e improvement i n t h e d e t e c t i o n o f d i s p e r s i o n e.s.r. signal. R e c e n t a p p l i c a t i o n s o f c o n v e n t i o n a l and s a t u r a t i o n t r a n s f e r e l e c t r o n s p i n resonance (ST-e.s.r.) methods f o r biomembrane r e s e a r c h h a v e b e e n r e v i e w e d b y Fung and J o h n s o n , l and Hemmingaz; t h e f o r m e r e m p h a s i z e s t e c h n i q u e s , and t h e l a t t e r t h e i n t e r p r e t a t i o n and a n a l y s i s o f t h e s p e c t r a l l i n e s h a p e s . C u r t a i n and G o r d o n 3 h a v e w r i t t e n a c h a p t e r on t h e a p p l i c a t i o n s o f s p i n l a b e l s p e c t r o s c o p y f o r biomeabrane s t u d i e s e s p e c i a l l y f o r b i o l o g i s t s who h a v e l i t t l e b a c k g r o u n d i n p h y s i c a l c h e m i s t r y and q u a n t u m m e c h a n i c s . A d i s c u s s i o n on s p i n l a b e l i n v e s t i g a t i o n o f l i p i d p r o t e i n i n t e r a c t i o n s was p r e s e n t e d b y N a r ~ h . ~ The u s e o f s p i n l a b e l e.s.r. s p e c t r o s c o p y f o r p r o t e i n and enzyme r e s e a r c h a l s o has been reviewed . 5 $ 6

spectroscopy f o r b i o l o g i c a l studies;

2 2.1

Instrumentation

-

Recent Developments I n 1972, Hyde and D a l t o n 7 showed t h a t t h e

d i s p e r s i o n d i s p l a y ( U i ) i s s e n s i t i v e t o motions i n t h e range 1 0 - 7 t o 1 0 - 4 s e c and t h e r e f o r e o f f e r s a p o s s i b l e a l t e r n a t i v e t o ST-e.s.r.

f o r t h e d e t e r m i n a t i o n o f v e r y slow motions.

The U i

d i s p l a y has s e v e r a l advantages o v e r t h e a b s o r p t i o n d i s p l a y i n c l u d i n g s i g n a l s t h a t a r e g e n e r a l l y 3 t o 1 0 t i m e s s t r o n g e r and d o n o t s a t u r a t e e a s i l y b y m i c r o w a v e p o w e r , and a l i n e s h a p e t h a t i s s i m p l e w i t h e a s i e r a n a l y s i s and s i m u l a t i o n .

However, b e c a u s e

FM n o i s e from k l y s t r o n s degrades t h e s i g n a l - t o - n o i s e

d e t e c t i o n o f t h e U i d i s p l a y i n e.s.r. l o n g e x i s t i n g problem. Over t h e p r e s e n t r e v i e w p e r i o d ,

ratio,

s p e c t r o s c o p y h a s been a s e v e r a l groups have i n d e -

p e n d e n t l y r e p o r t e d t h e t e c h n i c a l advances f o r t h e d e t e c t i o n o f the U i signal.

Hyde and h i s c o l l e a g i r e s 8 h a v e d e s c r i b e d t h e u s e 246

[For references see p . 284

247

7: Spin Labels: Biological Systems o f a l o o p - g a p r e s o n a t o r f o r t h e d e t e c t i o n o f U i s i g n a l and r e p o r t e d a 7 0 0 - f o l d improvement i n t h e s i g n a l - t o - n o i s e r a t i o c o m p a r e d w i t h t h a t o f a t y p i c a l TE102 c a v i t y r e s o n a t o r . The g r e a t improvement i s a t t r i b u t e d t o h i g h e r energy d e n s i t y , l o w e r Thomas e t a l . 9 h a v e u s e d r e s o n a t o r Q, and h i g h e r f i l l i n g f a c t o r . t h i s new U i d i s p l a y t o s t u d y t h e s p i n - l a b e l l e d m u s c l e f i b e r and f o u n d t h a t t h e s e n s i t i v i t y o f t h e U i d i s p l a y i s m o r e t h a n 100 t i m e s b e t t e r t h a n t h a t o f V; resonant cavity.

display obtained i n a conventional

Unexpectedly,

h a s an a d d i t i o n a l a d v a n t a g e : component of t h e e . s . r .

they noted t h a t the U i display

t h a t i s , t h e weakly immobilized

spectrum o f s p i n - l a b e l l e d muscle f i b e r

c a n be s e l e c t i v e l y suppressed i n U i d i s p l a y ,

eliminating the

No d o u b t more w i l l b e h e a r d o f need f o r chemical t r e a t m e n t s . t h i s p o t e n t i a l l y important technique. S e h r e t a1.,10 o n t h e o t h e r hand, h a v e d e v e l o p e d a b a l a n c e d c a v i t y i n w h i c h k l y s t r o n FM n o i s e i s s u p p r e s s e d a t t h e m a g i c t e e u s i n g t h e p h a s e s h i f t e r and a t t e n u a t o r .

Using t h i s c a v i t y ,

they

r e p o r t e d a p r o f o u n d improvement i n t h e s i g n a l - t o - n o i s e r a t i o f o r t h e Ili s i g n a l o f f a t t y a c i d s p i n l a b e l - - b o v i n e c o m p l e x e s i n 80% g l y c e r o l .

The U i S T - e . s . r .

serum a l b u m i n

s i g n a l was

c a l i b r a t e d f o r i s o t r o p i c m o t i o n i n t h e r a n g e 1 0 - 7 t o 10-4 s e c . The p o t e n t i a l u s e f u l n e s s o f U i d i s p l a y f o r t h e s t u d y o f membrane d y n a m i c s was d i s c u s s e d .

F a j e r and M a r s h 1 1 h a v e a l s o

d e s c r i b e d a somewhat new m e t h o d f o r a n a l y z i n g m o l e c u l a r m o t i o n s I n t h i s method, a d i f f e r e n c e spectra. u s i n g U i ST-e.s.r. s p e c t r u m was c a l c u l a t e d b e t w e e n t h e i n t e g r a t e d f i r s t h a r m o n i c , p h a s e , a b s o r p t i o n s p e c t r u m and t h e f i r s t h a r m o n i c , 90' o u t o f

i n

phase, d i s p e r s i o n spectrum. I s a e v - I v a n o v e t a1 .I2 h a v e d e s c r i b e d an u l t r a h i g h - f r e q u e n c y e.s.r. spectrometer which provides e f f e c t i v e suppression o f l o w - f r e q u e n c y k l y s t r o n F M n o i s e and p e r m i t s t h e d e t e c t i o n o f b o t h a b s o r p t i o n and d i s p e r s i o n s i g n a l s . U s i n g t h i s new s p e c t r o m e t e r , t h e y r e p o r t e d t h e d e t e c t i o n of an aqueous s o l u t i o n o f t h e n i t r o x i d e r a d i c a l , 10-6 M, w i t h i n 5 0 ms r e c o r d i n g t i m e i n a c u v e t t e w i t h v o l u m e 0 . 1 cm3.

2.2

Techniques

-

I r e l a n d e t a l . 1 3 have d e s c r i b e d t h e con-

s t r u c t i o n o f an i n e x p e n s i v e d a t a a c q u i s i t i o n s y s t e m f o r a V a r i a n E - l i n e C e n t u r y s e r i e s e . s . r . s p e c t r o m e t e r i n t e r f a c i n g w i t h an Apple I 1 p l u s computer. I t i s p o s s i b l e t o perform computerc o n t r o l l e d t i m e a v e r a g i n g u s i n g t h i s s y s t e m w i t h o u t a n y mod-

248

Electron Spin Resonance

i f i c a t i o n of hardwares. T h e y have used t h i s s y s t e m t o analyze t h e e.s.r. s p e c t r a o f d i l u t e s p i n - l a b e l l e d n u c l e i c acid solution. A f t e r t h e ST-e.s.r. method w a s introduced by Thomas, Dalton and H y d e in 1976, at least t w o c a l i b r a t i o n p r o b l e m s w e r e s o o n realized: namely, t h e p r e c i s e d e t e r m i n a t i o n o f t h e m i c r o w a v e m a g n e t i c f i e l d Hi at t h e s a m p l e p o s i t i o n and t h e c o r r e c t p h a s e s e t t i n g o n t h e p h a s e - s e n s i t i v e d e t e c t o r . V i s t n e s and D a l t o n 1 4 h a v e reported t w o different m e t h o d s t o i m p r o v e H1 d e t e r m i n a t i o n . Both m e t h o d s r e q u i r e t h e use o f small c r y s t a l s o f NMP-TCNQ ( N - m e t h y l p h e n a z i n i u m tetracyanoquinodimethan). T h e f i r s t method is based o n t h e f a c t t h a t t h e e.s.r. linewidth o f N M P - T C N Q i n c r e a s e s w i t h i n c r e a s i n g Hi. T h e second method is based o n a magnetization hysteresis (MH) parameter obtained from a MH s p e c t r u m which is c a l c u l a t e d from t w o e.s.r. s p e c t r a d e t e c t e d with a 90" difference in phase setting. They claimed that t h e l i n e w i d t h method is useful f o r Hi d e t e r m i n a t i o n in t h e 0.1 t o 0.5 G and t h e MH m e t h o d is useful f o r t h e r e g i o n s 0.02 t o 0.1 6. In ST-e.s.r., a variation o f 2 t o 3" in t h e p h a s e s e t t i n g c o u l d g i v e an e r r o r o f up t o 30% in t h e estimated c o r r e l a t i o n t i m e s . To i m p r o v e t h e phase setting, Vistnesl5 h a s introduced m a g n e t i z a t i o n h y s t e r e s i s ( M H ) p a r a m e t e r s based o n MH spectra. T h e a u t h o r noted that t h e s e p a r a m e t e r s are p h a s e invariant w h i l e s e n s i t i v e t o rotational c o r r e l a t i o n t i m e s i n t h e r a n g e 1 0 - 7 t o 1 0 - 3 sec. R o b i n s o n 1 6 has also published a n o t e o n t h e improvement o f p h a s e s e t t i n g in ST-e.s.r. m e t h o d . His method is based o n t h e d e f i n i t i o n o f o n e additional field p o s i t i o n at which Thomas, Dalton and Hyde's qotional p a r a m e t e r s b e c o m e p h a s e i n d e p e n d e n t . O t h e r potential d i f f i c u l t i e s in t h e a n a l y s e s o f ST-e.s.r. s p e c t r a have also been discussed by Delmelle.17 He showed t h a t Thomas, Dalton and Hyde's q o t i o n a l p a r a m e t e r s c a n be affected b y t h e effective H i , cavity configuratioo, dielectric properties of s o l v e n t s as well as Zeeman m o d u l a t i o n amplitude. T h e author r e c o m m e n d e d e x p e r i m e n t a l i s t s t o c o n s t r u c t a set o f r e f e r e n c e s p e c t r a and c a l i b r a t i o n c u r v e s adapted t o their own e x p e r i m e n t a l conditions. C o m p u t e r s i m u l a t i o n o f Q-band n i t r o x i d e e.s.r. s p e c t r a i n f r o z e n s o l u t i o n h a s been s h o w n t o b e a valuable w a y o f d e t e r m i n i n g m a g n e t i c parameters, s u c h as A and g tensors.18 Bales and W i l l e t t l g h a v e presented some e x p e r i m e n t a l d a t a t o v e r i f y t h e i r p r e v i o u s published t h e o r y o n t h e m e a s u r e m e n t o f s p i n e x c h a n g e f r e q u e n c i e s f r o m e.s.r. s p e c t r a o f n i t r o x i d e radical s p i n l a b e l s

7: Spin Labels: Biological Systems

249

in d i l u t e a q u e o u s solution. T h e s p i n e x c h a n g e c o n s t a n t o f n i t r o x i d e r a d i c a l s with oxygen in a q u e o u s s o l u t i o n also h a s b e e n d e t e r m i n e d by t h e a n a l y s i s of l i r ~ e s h a p e s . ~ ~ 3

Protein

3.1 M e m b r a n e s - H e r z et a1.21922 h a v e reported t h e s e l e c t i v e m o d i f i c a t i o n o f carboxyl r e s i d u e s o f b a c t e r i o r h o d o p s i n in p u r p l e m e m b r a n e s with T e m p a m i n e spin label using [ E E O Q (N-(ethoxycarbonyl)-2-ethoxy-l,2-dihydroquinoline] as t h e c o u p l i n g agent. Based o n t h e a c c e s s i b i l i t y o f n i t r o x i d e m o i e t i e s t o t h e param a g n e t i c b r o a d e n i n g agents, t h e y s u g g e s t e d t h a t s o m e carboxyl g r o u p s o f t h e p r o t e i n m a y be buried w i t h i n h y d r o p h o b i c domains, at least 16 A f r o m t h e m e m b r a n e surface. R i f k i n d et al.23 r e p o r t e d p r e v i o u s l y that m a l e i m i d e s p i n l a b e l s c o v a l e n t l y bound t o t h e e r y t h r o c y t e m e m b r a n e g i v e r i s e t o a c o m p o s i t e e.s.r. s p e c t r u m c o m p r i s i n g t h r e e c o m p o n e n t s , n a m e l y , W and S, c o m m o n l y r e f e r r i n g t o a s w e a k l y and s t r o n g l y immobilized c o m p o n e n t s , and an additional c o m p o n e n t which is u n d e t e c t a b l e by e.s.r. m e t h o d s d u e t o labelled f r e e sulfhydryl g r o u p s t h a t are c l o s e enough t o c a u s e dipolar i n t e r a c t i o n . They c l a i m e d t o d e t e c t t h i s t h i r d e.s.r. silent c o m p o n e n t at e l e v a t e d t e m p e r a t u r e s and t h u s cautioned t h e i n t e r p r e t a t i o n o f m a l e i m i d e syin-1 abel led e r y t h r o c y t e m e m b r a n e data. Fung and J o h n s o n 2 4 h a v e p u b l i s h e d a short n o t e t o d i s p u t e R i f k i n d et al.'s c o n c l u s i o n s . T h e i r r e s u l t s s h o w e d no c h a n g e in t h e c o n c e n t r a t i o n o f s p i n l a b e l s bound t o t h e e r y t h r o c y t e m e m b r a n e at elevated t e m p e r atures. T h e y c o n c l u d e d that t h e i n c r e a s e in e.s.r. signal a m p l i t u d e at elevated t e m p e r a t u r e reported by Rifkind et al. w a s mainly due t o a temperature-dependent dielectric-induced change in c a v i t y s e n s i t i v i t y . F u n g 2 5 h a s a l s o w r i t t e n a b r i e f review o n a n a l y s i s of s p i n - l a b e l l e d e r y t h r o c y t e membranes. T h e e.s.r. s p e c t r u m o f i o d o a c e t a m i d e spin-labelled s a r c o plasmic r e t i c u l u m A T P a s e exhibited a s u b s t r a t e induced b r o a d e n i n g w h i c h w a s g r e a t l y e n h a n c e d when C a 2 + w a s present.26 A d d i t i o n o f substrates triggered the conformational change o f ATPase enzyme t o f o r m a t r a n s i t i o n c o m p l e x which f a v o r e d phosphoryl transfer. Other ST-e.s.r. s t u d i e s o f m a l e i m i d e spin-labelled s a r c o p l a s m i c r e t i c u l u m v e s i c l e s also h a v e been r e p ~ r t e d . * ~ - ~ g

I

Electron Spin Resonance

250

A s p i n - l a b e l l e d d e r i v a t i v e o f ATP (1) was p r e p a r e d f o r t h e

s t u d y o f t h e i n t e r a c t i o n o f A T P w i t h t h e a c t i v e s i t e o f t h e Ca2+, Mgz+-ATPase o f r a b b i t s a r c o p l a s m i c r e t i c u l u m . 3 0 The e . s . r .

s i g n a l a m p l i t u d e o f 1 i n aqueous s o l u t i o n was r e d u c e d m a r k e d l y b y t h e a d d i t i o n o f Mn2+, and Mn2+.

s u g q e s t i n q t h e complex f o r m a t i o n between 1

A d d i t i o n o f ATPase i n t o t h e r e a c t i o n m i x t u r e r e s t o r e d

t h e o r i g i n a l e.s.r.

s i g n a l o f 1.

The a u t h o r s s u g g e s t e d t h a t t h e

b i n d i n g o f Mn2+-1 c o m p l e x t o ATPase i n c r e a s e s t h e d i s t a n c e between t h e s e two paramagnetic c e n t e r s , t h e r e b y r e d u c i n g t h e magnetic d i p o l e - d i p o l e i n t e r a c t i o n . G r o v e r and P i e t t e 3 1 h a v e used a s p i n - l a b e l l e d d e r i v a t i v e o f p - c h l o r o m e r c u r i b e n z o a t e t o m o d i f y t h e t h i o l g r o u p s o f N A D P H c y t o c h r o m e P-450 r e d u c t a s e . A t o t a l o f seven t h i o l qroups were l a b e l l e d b y t h e r e a g e n t . One o r two o f t h e s e s p i n - l a b e l l i n g s i t e s were l o c a t e d near t h e NADPH b i n d i n g s i t e and were s e n s i t i v e t o c h a n g e s i n t h e p h y s i c a l e n v i r o n m e n t o f t h e p r o t e i n . The s p i n l a b e l m o d i f i c a t i o n , h o w e v e r , r e s u l t e d i n a p a r t i a l d e p l e t i o n o f t h e FAD and FMN i n t h e enzyme molecules.

Popova e t a1.32 h a v e i n v e s t i g a t e d t h e a c t i v e s i t e o f

m i c r o s o m a l cytochrorne P-450 u s i n g a s e r i e s o f t h e i o d i n e c o n t a i n i n g s t a b l e i m i n o x y l r a d i c a l s w i t h various spacer d i s t a n c e s b e t w e e n t h e N-0 g r o u p and t h e i o d i n e atom.

The b i n d i n q o f

s p i n - l a b e l r a d i c a l s induced changes i n t h e o p t i c a l s p e c t r a o f F e 3 + l o c a t e d i n t h e a c t i v e s i t e o f c y t o c h r o m e P-450 and i n h i b i t e d t h e o x i d a t i o n o f b o t h t y p e 1 and t y p e 2 s u b s t r a t e s ,

suggesting

t h a t t h e r a d i c a l s a r e c o v a l e n t l y b o u n d t o t h e enzyme n e a r t h e active site.

7: Spin Labels: Biological Systems

25 1

B u t t e r f i e l d and h i s

worker^^^,^^ h a v e i n v e s t i g a t e d t h e

p h y s i c a l s t a t e o f t h e e r y t h r o c y t e membrane b y e i t h e r l a b e l l i n g s i a l i c a c i d r e s i d u e s o f membrane g l y c o p r o t e i n s and g l y c o l i p i d s w i t h Tempamine s p i n l a b e l t h r o u g h r e d u c t i v e a m i n a t i o n o r l a b e l l i n g membrane p r o t e i n s w i t h a m a l e i m i d e s p i n l a b e l .

Other

s p i n l a b e l 1 i n g o f membrane p r o t e i n s a l s o h a v e b e e n r e p o r t e d . 3 5 - 3 9 B e r l i n e r 4 0 has r e v i e w e d t h e t e c h n i q u e s f o r m o d i f y i n g t h e f r e e t h i o l s o f membrane p r o t e i n s w i t h s p i n - l a b e l r e a g e n t s and i n t r o duced a r e v e r s i b l e t h i o l - s p e c i f i c

s p i n l a b e l f o r biomembrane

research.

3.2

Blood

-

-l_l_

protein,

Plasma f i b r o n e c t i n ,

a high molecular weight glyco-

present i n a l l vertebrate blood,

i n c e l l adhesion,

plays important r o l e s

b l o o d c l o t t i n g and e m b r y o n i c d e v e l o p m e n t .

Lai

and T o o n e y 4 1 h a v e m o d i f i e d s e l e c t i v e l y t h e f r e e s u l f h y d r y l g r o u p s

o f t h e p r o t e i n w i t h a maleimide s p i n l a b e l . P a r a l l e l experiments w i t h c e l l a d h e s i o n a s s a y and CD s t u d i e s showed t h a t t h e s t r u c t u r a l and f u n c t i o n a l i n t e g r i t y o f t h e p r o t e i n i s p r e s e r v e d a f t e r spin-label

incorporation.

They d e m o n s t r a t e d t h a t p l a s m a f i b r o -

n e c t i n i s n o t a r i g i d , g l o b u l a r p r o t e i n b u t has a h i g h deqree o f intramolecular f l e x i b i l i t y .

O t h e r s t u d i e s on b l o o d - r e l a t e d

p r o t e i n s a l s o have been r e p 0 r t e d . ~ * - 4 6

3.3

Enzymes

-

E.s.r.

m e a s u r e m e n t s h a v e been e m p l o y e d t o s t u d y

t h e i n t e r a c t i o n b e t w e e n s p i n - l a b e l l e d f a t t y a c i d s and o x i d o reductases,

namely,

b a c t e r i a l l u c i f e r a s e and s o y b e a n l i p o x y -

genase.47 F r e e f a t t y a c i d s a r e i n h i b i t o r s o f t h e s e enzymes. B o t h 5 - d o x y 1 and 1 6 - d o x y l s t e a r i c a c i d s b o u n d s t r o n g l y t o t h e enzymes. The s p e c t r u m o f 1 6 - d o x y l s t e a r a t e b o u n d t o s o y b e a n l i p o x y g e n a s e showed a c o m p o s i t e o f t w o c o m p o n e n t s ; c o m p o n e n t and a b r o a d e . s . r .

a strongly immobilized

l i n e component.

This l a t t e r

c o m p o n e n t was n o t s e e n i n t h e s p e c t r u m f o r t h e same p r o b e b o u n d t o bacterial luciferase.

T h i s 1i n e broadening,

i n t e r e s t i n g phenomenon, however,

a potentially

was n o t d i s c u s s e d i n t h e p a p e r .

C h i c k e n p e p s i n o g e n was m o d i f i e d b y a s p i n - l a b e l l i n g r e a g e n t

( 2 ) r e a c t e d w i t h t h e amino g r o u p s . 4 8

A bond c l e a v a g e i n d u c e d b y

252

Electron Spin Resonance 0 II

C-

a c i d i f i c a t i o n produced a n o n l a b e l l e d ,

a c t i v e enzyme.

The

i n t e r p r e t a t i o n was t h a t t h e s i t e s o f s p i n - l a b e l l i n g a r e n e a r t h e e n z y m e ' s a m i n o - t e r m i n u s a t w h i c h t h e p e p t i d e l e a v e s t h e enzyme a f t e r a c i d i f i c a t i o n . The s p i n - l a b e l l e d p e p s i n o g e n a p p e a r s t o b e u s e f u l f o r t h e s t u d y o f t h e mechanism o f pepsinogen a c t i v a t i o n . The c o n f o r m a t i o n a l s t a t e s o f p a n c r e a t i c l i p a s e w e r e i n v e s t i g a t e d b y s p i n l a b e l methods.49

The e . s . r .

spectrum o f s p i n - l a b e l l e d

enzyme showed a s t r o n g l y i m m o b i l i z e d component,

indicating that

t h e l a b e l s are r i g i d l y attached t o t h e p r o t e i n .

The e f f e c t i v e

r o t a t i o n a l c o r r e l a t i o n t i m e o f s p i n - l a b e l l e d enzyme was i n g o o d a g r e e m e n t w i t h t h e c a l c u l a t e d c o r r e l a t i o n t i m e b a s e d on Stokes-Einstein equation.

the

The a u t h o r s c l a i m e d t h a t t h e p r o t e i n

behaves as a r i g i d sphere i n s o l u t i o n .

R i b u l o s e b i p h o s p h a t e c a r b o x y l ase p l a y s an i m p o r t a n t r o l e i n The e . s . r . signal o f m a l e i m i d e s p i n - l a b e l l e d enzyme r e d u c e d b y d i t h i o t h r e i t o l was t h e photosynthetic carbon r e d u c t i o n cycle.

p a r t i a l l y r e o x i d i z e d b y t h e enzyme a f t e r r e m o v a l o f d i t h i o t h r e i t o l .50,5l e.s.r.

The a u t h o r s a r g u e d t h a t a f t e r r e o x i d a t i o n a new

s i g n a l a p p e a r s w i t h an i s o t r o p i c h y p e r f i n e c o n s t a n t o f

16.0 G ( i n i t i a l A,

1 7 . 1 G ) and a go v a l u e a b o u t 0.002 s m a l l e r T h i s s i m u l t a n e o u s d e c r e a s e i n A, and

t h a n t h e i n i t i a l go v a l u e .

g o v a l u e s i s v e r y uncommon and c a n n o t be e x p l a i n e d b y c h a n g e s i n polarity,

as t h e a u t h o r s r i g h t l y p o i n t e d o u t .

c l a i m e d t h a t t h i s new e . s . r .

The a u t h o r s

s i g n a l w i t h s m a l l A,

and go v a l u e s

i s due t o a l t e r a t i o n o f t h e e l e c t r o n s t r u c t u r e o f t h e n i t r o x i d e N-0 g r o u p . However, t h e r e p o r t e r f e e l s t h a t a 3 5 GHz e . s . r . e x p e r i m e n t s h o u l d be u s e f u l f o r s t r e n g t h e n i n g t h e a u t h o r s ' c l a i m o f t h e a p p e a r a n c e o f a new e . s . r . Aconitase,

an Fe-S p r o t e i n ,

signal. contains a single r e a c t i v e

s u l f h y d r y l g r o u p a t t h e c a t a l y t i c s i t e o f t h e enzyme.

al.52

Dreyer

have m o d i f i e d t h i s r e a c t i v e s u l f h y d r y l group w i t h a

et

253

7: Spin Labels: Biological Systems spin-label

sulfhydryl reagent.

They e s t i q a t e d t h a t t h e i n t e r -

a c t i o n d i s t a n c e b e t w e e n t h e c e n t e r o f t h e Fe-S c l u s t e r and t h e s p i n l a b e l s i t e i s about 12

A.

The b i n d i n g o f t h r e e s p i n -

l a b e l l e d o l i g o p e p t i d e s t o t h e a c t i v e s i t e o f l e u c i n e aminop e p t i d a s e f r o m b o v i n e e y e l e n s was i n v e s t i g a t e d b y e . s . r . spectroscopy.53

An a p e r t u r e o f l e s s t h a n 7 . 2

A

was p r o p o s e d

f o r t h e e n t r a n c e t o t h e a c t i v e s i t e o f t h e enzyme.

The m o l e c u l a r

s t r u c t u r e o f carboxypeptidase A d u r i n g t h e e s t e r o l y t i c r e a c t i o n was c h a r a c t e r i z e d u s i n g a C o 2 + - s u b s t i t u t e d enzyme and a s p i n l a b e l e s t e r s u b s t r a t e ( 3 ) a t l o w t e m p e r a t ~ r e s . 5 ~ The a c t i v e s i t e

0

.H

H

/" II I -~--c~c-c-o-c-c-o1

O

II

I

H- C-H

1. 0

L

J

Co2+ and t h e n i t r o x i d e g r o u p w e r e e s t i m a t e d t o b e s e p a r a t e d b y

7.7

8.

O t h e r s p i n l a b e l s t u d i e s o f enzymes a l s o h a v e b e e n

r e p o r t e d .55-61

-

3.4 Muscle G r a c e f f a and L e h r e r 6 2 h a v e i n v e s t i g a t e d t h e e f f e c t o f t e m p e r a t u r e on c o n f o r m a t i o n a l s t a t e s o f t r o p o m y o s i n b y l a b e l l i n g t h e p r o t e i n w i t h a maleimide s p i n l a b e l . They

demonstrated t h a t a t p h y s i o l o g i c a l temperature t h e p r o t e i n undergoes t h e r m a l e q u i l i b r i u m between two c o n f o r m a t i o n a l s t a t e s . A s p i n t r a p p i n g method f o r s p i n l a b e l l i n g p r o t e i n s u l f h y d r y l

g r o u p s was d e s c r i b e d b y G r a c e f f a . 6 3

The m e t h o d i s b a s e d o n t h e

s p i n t r a p p i n g o f t h e t h i y l r a d i c a l g e n e r a t e d due t o t h e o x i d a t i o n

o f t h e s u l f h y d r y l g r o u p b y Ce4+.

R S . + phenyl-CH =

254

Electron Spin Resonance

T h e drawback o f t h i s method is that t h e l i f e t i m e s o f t h e s p i n a d d u c t n i t r o x i d e s a r e t o o short ( % o n e hour). Other s p i n l a b e l l e d m u s c l e p r o t e i n s also have been pub1 ished.64-65 T h e use o f conventional and ST-e.s.r. m e t h o d s in t h e study o f r o t a t i o n a l d y n a m i c s o f spin-labelled m u s c l e p r o t e i n s have been r e v i e w e d by T h o m a s .66

3.5 Others -

The cys 27 of calmodulin was chemically modified w i t h a m a l e i m i d e spin labe1.67 A d d i t i o n o f Mn2+ induced a d e c r e a s e in t h e e.s.r. signal a m p l i t u d e o f spin-labelled protein. T h e interaction d i s t a n c e betwfen t h e label and t h e bound Mn2+ w a s estimated t o be w i t h i n 8 A. S p i n label t e c h n i q u e s also h a v e been applied t o study t h e s t r u c t u r e and d y n a m i c s o f o t h e r p r o t e i n s y s t e m s including n e u r o t o x i n 6 8 - 7 0 and c a r d i a c f a t t y a c i d - b i n d i n g protein.71 T i m o f e e v 7 2 h a s presented a model t o d e s c r i b e t h e d y n a m i c behavior o f s i d e c h a i n s on g l o b u l a r proteins.

4 Nucleic Acid 4.1 DNA - Bobst and his c o w o r k e r s have c o n t i n u e d t h e i r s t u d i e s in t h e f i e l d o f spin-labelled n u c l e i c acids. T h e y h a v e s y n thesized a series o f nitroxide spin-labelled deoxyuridine trip h o s p h a t e s with v a r i o u s s p a c e r l e n g t h s between t h e n i t r o x i d e g r o u p and t h e s u l f u r l i n k a g e t o t h e deoxyuridine.73 T h e s e spin-labelled base analogs were enzymatically incorporated with t e r m i n a l t r a n s f e r a s e t o form a s p i n - l a b e l l e d p o l y ( d T ) which in t u r n was annealed with poly ( d A ) t o g e n e r a t e a DNA duplex. Based o n t h i s technique, t h e y reported t h a t DNA m o l e c u l e s in its B f o r m p o s s e s s a d e p t h o f t h e m a j o r g r o o v e o f about 8 A . T h i s v a l u e is i n good agreement with previous x - r a y studies. T h e y also h a v e r e p o r t e d t h e s y n t h e s i s o f a s p i n - l a b e l l e d d e r i v a t i v e o f 2'd e o x y u r i d i ne-5' -tr i p h o s p h a t e .74 T h i s sp i n-1 abell ed b a s e analog i n h i b i t e d t h e a c t i v i t i e s o f s o m e DNA and RNA polymerases. T h i s s p i n - l a b e l l e d analog, however, c o u l d also b e e n z y m a t i c a l l y incorporated into p o l y d e o x y t h y m i d y l i c acid by r e v e r s e t r a n s c r i p t a s e . T h e mechanism u n d e r l y i n g t h e h y p e r t h e r m i c p o t e n t i a t i o n o f b l e o m y c i n t o x i c i t y is still not known. C h a p m a n et a1.75 have l a b e l l e d DNA with s p i n label (4) and f o u n d t h a t t h e h y p e r t h e r m i a e n h a n c e s DNA-bleomycin interaction as evidenced f r o m an i n c r e a s e in t h e rotational c o r r e l a t i o n t i m e o f t h e label. T h e e.s.r.

7: Spin Labels: Biological Systems

s p e c t r u m o f s p i n - l a b e l l e d DNA, h o w e v e r , showed a c o m p o s i t e o f t w o c o m p o n e n t s ; a r e l a t i v e l y fast t u m b l i n g c o m p o n e n t s u p e r i m p o s e d w i t h a broad e.s.r. line. One h a s t o be c a u t i o u s in a t t e m p t i n g t o c a l c u l a t e e f f e c t i v e rotational c o r r e l a t i o n t i m e s based o n t h i s c o m p l i c a t e d 1 ineshape. A h y d r o x y a p a t i t e c h r o m a t o g r a p h y m e t h o d h a s been d e s c r i b e d f o r s e p a r a t i n g s p i n - l a b e l l e d D N A m o l e c u l e s f r o m t h e unbound s p i n labe176; t h e latter c o m p o n e n t is q e n e r a l l y d i f f i c u l t t o r e m o v e c o m p l e t e l y and m a y b e m i s i n t e r p r e t e d as a " w e a k l y immobilized" component. O t h e r s t u d i e s on e.s.r. s p e c t r o s c o p y o f DNA also h a v e been presented.77

-

4.2 Chromatin L a w r e n c e and his c o l l e a g u e 7 8 h a v e c o n t i n u e d t h e i r s t u d i e s on t h e d y n a m i c i n t e r a c t i o n between h i s t o n e H i and D N A using s p i n lab61 methods. L y s i n e and t y r o s i n e r e s i d u e s o f h i s t o n e H i w e r e c o v a l e n t l y modified with c o r r e s p o n d i n g s p i n label reagents. Judging from e.s.r. m e a s u r e m e n t s , t h e y r e p o r t e d t h a t t h e l y s i n e r e s i d u e s located near t h e C - and N-termini o f h i s t o n e H i p r o t e i n bind t o DNA o r Hi-depleted chromatin, w h e r e a s t h e t y r o s i n e r e s i d u e s located i n t h e c e n t r a l g l o b u l a r part o f t h e p r o t e i n interact o n l y with chromatin. S i m i l a r r e s u l t s w e r e r e p o r t e d by T u r a e v et a1.79 T h e y e x a m i n e d t h e i n t e r a c t i o n b e t w e e n h i s t o n e Hi and DNA by l a b e l l i n g t y r o s i n e r e s i d u e s o f t h e p r o t e i n with a t y r o s i n e - s p e c i f i c s p i n label reagent. A s s o c i a t i o n o f s p i n - l a b e l l e d h i s t o n e H i w i t h DNA produced no c h a n g e in t h e e.s.r. spectra, arguing that t y r o s i n e r e s i d u e s a r e not d i r e c t l y i n v o l v e d in D N A - h i s t o n e H i interaction. T h e i n t e r a c t i o n o f maleimide or cyanuric chloride spin-labelled CAMP-dependent p r o t e i n k i n a s e w i t h its substrate, h i s t o n e H i , a l s o h a s been r e p o r t e d .a0

255

Electron Spin Resonance

256

4.3 RNA - K a o et a1.81 have prepared a s e r i e s o f spin-labelled b a s e analogs c o n s i s t i n g o f a six-membered n i t r o x i d e radical with s p a c e r s o f various lengths attached to either p o s i t i o n 4 o r 5 o f t h e p y r i m i d i n e base. T h e s e s p i n - l a b e l l e d base a n a l o g s w e r e e n z y m a t i c a l l y incorporated into R N A molecules. Based o n a m o t i o n a l model, t h e y w e r e able t o s e p a r a t e t h e m o t i o n o f t h e b a s e f r o m t h e motion o f t h e spin label i n s i n g l e and d o u b l e strand R N A s . T h e y c o n c l u d e d that t h e b a s e s i n an RNA d u p l e x exhibit rotational m o t i o n s with a c o r r e l a t i o n t i m e about 4 n s . T h e s a m e g r o u p also has reported t h e s y n t h e s e s and biological a c t i v i t i e s o f o t h e r spin-labelled n u c l e i c A spectral s e p a r a t i o n m e t h o d was described t o d i f f e r e n t i a t e t w o c o n f o r m e r s o f s p i n 1 abelled t R N A P h e f r o m Escherichi a col i .84 T h e binding o f a d e n o s i n e appeared t o induce c h a n g e s in t h e c o n f o r m a t i o n a l s t a t e s o f p o l y ( U ) f r o m a f l e x i b l e coil t o a rigid helix as s t u d i e d by s p i n label methods.85

5 P ro p er ties of -

Phospholipid Bilayers

5.1 Lateral D i f f u s i o n - Davoust et al.86987 h a v e published t w o p a p e r s on t h e t h e o r y and a p p l i c a t i o n o f a n e w m e t h o d f o r d e t e r m i n i n g c o l l i s i o n f r e q u e n c i e s b e t w e e n spin-labelled m o l e c u l e s in m e m b r a n e s . T h e t h e o r y is based on t h e l i n e b r o a d e n i n g d u e t o H e i s e n b e r g spin e x c h a n g e that o c c u r s between 14N and 15N spinlabelled m o l e c u l e s in membranes. T h i s method i s a s i g n i f i c a n t i m p r o v e m e n t f r o m c o n v e n t i o n a l e.s.r. line b r o a d e n i n q m e t h o d s f o r m e a s u r i n g lateral d i f f u s i o n d e v e l o p e d by Devaux and McConnell,88 a n d Sackmann and T r a u b l e 8 8 about a d e c a d e ago. In an accomp a n y i n g paper,87 t h e a u t h o r s have d e m o n s t r a t e d t h e f e a s i b i l i t y o f t h i s n e w method in t h e s t u d y o f l i p i d - p r o t e i n i n t e r a c t i o n in a reconstituted membrane containing rhodopsin molecules. Their r e s u l t s indicated t h a t t h e c o l l i s i o n f r e q u e n c i e s o f lipid m o l e c u l e s at t h e h y d r o p h o b i c s u r f a c e o f m e m b r a n e p r o t e i n s are o f t h e s a m e o r d e r o f m a g n i t u d e as in t h e bulk lipid phase. T h i s r e s u l t d o e s not s u p p o r t t h e " b o u n d a r y lipid" c o n c e p t proposed by G r i f f i t h and Jost,88 but is in agreement with n.m.r. results. T h e r e p o r t e r anticipates that t h i s 1 4 N , 1 5 N d o u b l e - l a b e l l i n g method will b e becoming a w i d e l y used t o o l f o r i n v e s t i g a t i n g t h e i n t e r a c t i o n between s p i n - l a b e l l e d m o l e c u l e s in m e m b r a n e s i n t h e n e a r future.

7: Spin Labels: Biological Systems

F e i x et a1.89 h a v e i n d e p e n d e n t l y reported t h e use o f 14N, l 5 N d o u b l e - l a b e l l i n g method t o d e t e r m i n e lateral d i f f u s i o n r a t e s o f f a t t y acid s p i n l a b e l s in p h o s p h o l i p i d bilayers using ELDOR and s a t u r a t i o n r e c o v e r y e.s.r. s p e c t r o s c o p y . T h i s dual-label method, i m p r o v e d f r o m t h e i r p r e v i o u s ELOOR method,23 a p p e a r s t o e l i m i n a t e e l e c t r o n - n u c l e a r d i p o l e induced n u c l e a r relaxation as an E L D O R a c t i v e process. T h u s Heisenberg s p i n e x c h a n g e should be t h e d o m i n a n t E L D O R - a c t i v e process of interaction between l 4 N , 1 5 N spin-label pairs a l t h o u g h as t h e a u t h o r s pointed out, t h e contribution of pseudosecular electron-electron dipolar rel a x a t i o n which is again an ELDOR a c t i v e process is not yet clear. B y using t h i s dual-label ELDOR m e t h o d , t h e y have r e p o r t e d lateral d i f f u s i o n c o n s t a n t s o f f a t t y acid spin labels in D M P C membranes. Based o n t h e observed, r e l a t i v e l y high c o l l i s i o n f r e q u e n c i e s b e t w e e n 5-doxy1 and 16-doxylstearates, t h e y c o n c l u d e d that t h e t e r m i n a l methyl g r o u p s o f s t e a r i c acid acyl c h a i n s exhibit vertical f l u c t u a t i o n s t o w a r d t h e s u r f a c e o f DMPC b i l a y e r s . 5.2 P h a s e T r a n s i t i o n and P h a s e S e p a r a t i o n - H y d r o l y s i s o f phosphatidyl inosital t o diacylglycerol is c a t a l y z e d by p h o s p h a t i d y l i n o s i t o l - s p e c i f i c p h o s p h o l i p a s e C. T h i s s t e p a p p e a r s t o t r i g g e r v a r i o u s c e l l u l a r r e s p o n s e s t o e x o g e n o u s stimuli. Ohki e t a1.90 have e x a m i n e d t h e physical p r o p e r t i e s o f model m e m b r a n e s c o n t a i n i n g p h o s p h a t i d y l i n o s i t o l o r d i a c y l g l y c e r o l b y s p i n label m e t h o d s . M e m b r a n e p r o p e r t i e s i n c l u d i n g phase t r a n s i t i o n a n d p h a s e s e p a r a t i o n appeared t o d i f f e r in m e m b r a n e s c o n t a i n i n g p h o s p h a t i d y l i n o s i t o l versus m e m b r a n e s c o n t a i n i n q d i a c y l g l y c e r o l . T h e i r r e s u l t s s u p p o r t t h e c o n c e p t t h a t t h e c o n v e r s i o n o f phosphat i d y l i n o s i t o l into diacylglycerol i n d u c e s t h e c h a n g e s in t h e physical p r o p e r t i e s o f t h e membrane, t h e r e b y promoting its r e s p o n s e t o e x o g e n o u s stimuli. T h e o r i e n t a t i o n and motion o f t h e c h o l e s t a n e s p i n label in o r i e n t e d m u l t i l a y e r s o f DPPC and D M P C h a v e been examined.91 Below t h e p r e t r a n s i t i o n t e m p e r a t u r e s , t h e c h o l e s t a n e s p i n label w a s t i l t e d with an a n g l e o f about 30' w i t h respect t o t h e lipid b i l a y e r normal. T h i s tilted a n g l e d i s a p p e a r e d when p r e t r a n s i t i o n occurred. Meanwhile, t h e t w i s t i n g m o t i o n o f t h e label increased by o n e o r d e r o f m a g n i t u d e when t h e t e m p e r a t u r e r a i s e d f r o m below t h e p r e t r a n s i t i o n t o above t h e main t r a n s i t i o n t e m p e r a t u r e s .

25 7

Electron Spin Resonance

25 8

Suzuki e t a l .92 have p r e p a r e d a s p i n - 1 abel l e d g l y c o l i p i d ( 5 ) and e x a m i n e d t h e d i s t r i b u t i o n and m o t i o n o f 5 i n p h o s p h a t i d y l c h o l i n e l i p o s o m e s i n t h e p r e s e n c e and a b s e n c e o f c o n c a n a v a l i n A

bobob HOCH H2COH I

I I HCOH

0- CH I HOCH

0

I

H C

‘N

(con A).

The l i n e w i d t h o f e . s . r .

Me

I H

signal o f 5 incorporated i n t o

l i p o s o m e s was l i n e a r l y p r o p o r t i o n a l t o t h e % m o l a r r a t i o o f 5 t o phospholipids. The d a t a s h o u l d b e s u f f i c i e n t f o r c a l c u l a t i o n o f l a t e r a l d i f f u s i o n o f 5 i n membranes b y u s i n g Sackmann and T r a u b l e method. I t i s n o t o b v i o u s why t h i s c a l c u l a t i o n was n o t r e p o r t e d . A d d i t i o n o f Con A i n d u c e d a g g r e g a t i o n o f l i p o s o m e s c o n t a i n i n g 5. A d i r e c t i n t e r a c t i o n b e t w e e n Con A and 5 was p r o p o s e d t o a c c o u n t f o r t h e o b s e r v e d l i n e w i d t h i n c r e a s e due p o s s i b l y t o t h e i n c r e a s e i n s p i n - s p i n i n t e r a c t i o n between t h e probes. The e f f e c t o f Ca2+ o n t h e l a t e r a l d i s t r i b u t i o n o f l i p i d s i n

dipalmitoylphosphatidylethanolamine (DPPE)ldipalmitoylphosphat i d y l s e r i n e ( D P P S ) v e s i c l e membranes h a s been i n v e s t i g a t e d b y 1 i p i d c r o s s - 1 i n k i n g and s p i n l a b e l m e t h o d s . 9 3 The r e s u l t s i n d i c a t e d t h a t t h e f l u i d p h a s e o f t h e membrane l i p i d s i s e s s e n t i a l f o r Ca2+ t o i n d u c e c h a n g e s i n l a t e r a l l i p i d d i s t ribution.

O t h e r s p i n l a b e l s t u d i e s o f p h o s p h o l i p i d membranes

h a v e been r e p o r t e d . q 4 - 9 5 5.3

Oxygen D i f f u s i o n

-

S u b c z y n s k i and Hyde96 h a v e d e t e r m i n e d t h e

oxygen d i f f u s i o n i n v a r i o u s s o l v e n t s based on t h e b i m o l e c u l a r c o l l i s i o n t h e o r y u s i n g t h e Smoluchowski e q u a t i o n .

Even t h o u g h

259

7: Spin Labels: Biological Systems

t h e m a c r o s c o p i c v i s c o s i t y o f h y d r o c a r b o n s i s g e n e r a l l y 10 t o 100 t i m e s g r e a t e r t h a n t h a t o f water, t h e y r e p o r t e d t h a t oxygen d i f f u s i o n i n l o n g c h a i n h y d r o c a r b o n s i s a b o u t t h e same as t h a t i n water. T h i s work e s t a b l i s h e s s u c c e s s f u l l y t h e use o f t h e Smoluchowski e q u a t i o n f o r a n a l y z i n g t h e b i m o l e c u l a r c o l l i s i o n r a t e o f d i s s o l v e d oxygen m o l e c u l e s w i t h s p i n l a b e l s . Hyde

a1.97

et

h a v e w r i t t e n a b r i e f r e v i e w on s p i n l a b e l o x i m e t r y f o r t h e

measurement o f oxygen c o n c e n t r a t i o n i n b i o l o g i c a l samples.

5.4

Membrane p o t e n t i a l and ApH

-

A s p i n p r o b e m e t h o d was u s e d t o

m e a s u r e ApH a c r o s s t h e membrane o f e n v e l o p e v e s i c l e s p r e p a r e d f r o m w i l d - t y p e and m u t a n t c e l l s o f H a l o b a c t e r i u m h a l o b i u m . 9 8 The a c c u m u l a t i o n and r e l e a s e o f t h e p r o b e , 4 - a m i n o - 2 , 2 , 6 , 6 - t e t r a -

methyl-piperidine-N-oxyl, a c r o s s t h e membrane a p p e a r e d t o f o l l o w t h e f i r s t - o r d e r k i n e t i c s and r e f l e c t e d t h a t . o f t h e c h a n g e i n ApH. L i n e t al.99 have determined t h e s u r f a c e p o t e n t i a l o f t h e e r y t h r o c y t e membrane u s i n g a d u a l p r o b e method. T h i s method i s b a s e d o n t h e p a r t i t i o n i n g o f t h e c a t i o n i c (CAT,) (AN,)

and a n i o n i c

s p i n p r o b e s ( 6 and 7 ) ( w h e r e n i n d i c a t e s t h e number o f

c a r b o n atoms i n t h e a c y l c h a i n ) i n t h e membrane.

The

Me Me( CH,)

-0-P--0 n-1

Me (7)

p a r t i t i o n i n g o f t h e s e p r o b e s i n t h e membrane i s a f f e c t e d b y t h e surface potential. U s i n g t h i s d u a l p r o b e method, t h e y h a v e shown t h a t t h e r e i s no s i g n i f i c a n t e l e c t r i c a l p o t e n t i a l a t t h e e x t e r n a l bilayer-aqueous

i n t e r f a c e o f t h e e r y t h r o c y t e membrane,

arguing

t h a t b o t h n e g a t i v e l y c h a r g e d s i a l i c a c i d s and c h a r g e d g r o u p s o f membrane p r o t e i n s e x p o s e d o n t h e e x t e r n a l s u r f a c e make n o a p p r e c i a b l e c o n t r i b u t i o n t o t h e measured s u r f a c e p o t e n t i a l . However, an a s s u m p t i o n t h a t h a s t o b e made i n t h i s m e t h o d i s t h a t t h e f l i p - f l o p o f t h e s e p r o b e s a c r o s s t h e membrane m u s t b e

260

Electron Spin Resonance

n e g l i g i b l e d u r i n g t h e t i m e c o u r s e o f t h e experiments. T h i s a s s u m p t i o n a p p e a r s t o be correct f o r t h e spin p r o b e s used in t h i s s t u d y as d e m o n s t r a t e d p r e v i o u s l y by Mehl horn and Packer.100 H o w e v e r , at least o n e o f t h e s e spin probes, CAT12, has r e c e n t l y b e e n shown by H a s h i m o t o et a1.101 t o be p e r m e a b l e in mitoc h o n d r i a l m e m b r a n e s . T h e y c o n c l u d e d t h a t C A T 1 2 i s not a s u i t a b l e p r o b e f o r m e a s u r e m e n t of s u r f a c e potential in e n e r g i z e d m i t o c h o n d r i a . In addition, Birrell e t a1 .lo2 d e m o n s t r a t e d p r e v i o u s l y t h a t s o m e a m p h i p a t h i c long c h a i n h y d r o c a r b o n spin probes d o f l i p - f l o p across t h e m e m b r a n e at t l I 2 v a l u e s w i t h i n minutes. M e h l h o r n and P a c k e r 1 0 3 h a v e r e v i e w e d t h e recent a p p l i c a t i o n s o f s p i n labels t o t h e measurement o f e n e r g e t i c p a r a m e t e r s i n c l u d i n g m e m b r a n e potential, ApH and cell volume.

5.5 M e m b r a n e P e r m e a b i l i t y - D o l i c h o l s are p o l y i s o p r e n o i d l i p i d s p r e s e n t m a i n l y i n lysosomes, Golgi, and e n d o p l a s m i c reticulum o f t h e cell and appear t o p l a y important r o l e s in t h e g l y c o p r o t e i n b i o s y n t h e s i s . Lai and S c h u t z b a c h l 0 4 r e p o r t e d that d o l i c h o l i n d u c e s t h e l e a k a g e o f m e m b r a n e s in liposotnes c o m p o s e d o f p h o s p h a t i d y l e t h a n o l a m i n e and p h o s p h a t i d y l c h o l i n e by m e a s u r i n g t h e e n t r a p m e n t o f T e m p o c h o l i n e , a c a t i o n i c s p i n probe, in liposomes. T h e y hypothesized that t h e p r e s e n c e o f dolichol w i t h a 1 e n g t h . a f 100 A i n P E - c o n t a i n i n g m e m b r a n e s m a y e n h a n c e t h e f o r v a t i o n o f transmernbrane ion c h a n n e l s , t h e r e b y inducing m e m b r a n e leakage. T h e p e r m e a b i l i t y o f p h o s p h o l i p i d b i l a y e r s t o small inorg-anic i o n s s u c h as Na+ and K + is g e n e r a l l y very low, 10-12 t o 10-14 cm/sec. T h e proton p e r m e a b i l i t y , however, is varied from to 10-9 cm/sec and t h e r e a s o n f o r t h e w i d e v a r i a t i o n i s still not c e r t a i n . Using a m e m b r a n e - p e r m e a b l e p h o s p h o n i u m s p i n label, C a f i s o and H u b b e l l l o 5 r e p o r t e d t h a t t h e proton p e r m e a b i l i t y in e g g p h o s p h a t i d y l c h o l i n e and d i p h y t a n o y l p h o s p h a t i d y l c h o l i n e is a b o u t 5 f 2 x 1 0 - 7 cm/sec. T h e v a r i a t i o n in proton p e r m e a b i l i t y as reported i n t h e l i t e r a t u r e was d i s c u s s e d in d e t a i l . 6

1.i pid-Protei n Interact ion

-

6.1 Integral P r o t e i n s Silviiis et a1.106 have e x a m i n e d t h e i n t e r a c t i o n o f s p i n - l a b e l l e d p h o s p h a t i d y l c h o l i n e and s p i n 1 abel led cholesterol with t h e h y d r o p h o b i c s u r f a c e o f Caz+-ATPase

26 1

7: Spin Labels: Biological Systems i n sarcoplasrnic r e t i c u l u m .

C h o l e s t e r o l appeared t o have a d i r e c t

contact w i t h t h e hydrophobic surface o f t h e protein,

although i t s

i n t e r a c t i o n was w e a k e r t h a n t h a t o f p h o s p h a t i d y l c h o l i n e .

The

a c t i v i t y o f Caz+-ATPase o f s a r c o p l a s m i c r e t i c u l u m i n c o r p o r a t e d i n t o DMPC o r OPPC membranes was shown t o b e n o t a f f e c t e d b y e i t h e r t h e f l u i d i t y o f t h e boundary l i p i d o r t h e r o t a t i o n a l m o t i o n o f t h e p r o t e i n as d e t e r m i n e d b y b o t h c o n v e n t i o n a l a n d ST-e.s.r.

methods.107

I n contrast,

R i t o v e t a1.108 h a v e

i n v e s t i g a t e d t h e i n t e r a c t i o n b e t w e e n 3-acyl-2-(16-doxylpalmitoyl) phosphatidylchol ine,

a 1i p i d s p i n l a b e l ,

and Caz+-ATPase f r o m

s a r c o p l a s m i c r e t i c u l u m membranes o f r a b b i t and c a r p w h i t e s k e l e t a l muscles,

and r e p o r t e d t h a t t h e p r e s e n c e o f t h e p r o t e i n

r e d u c e s t h e p r o b e m o b i l i t y i n t h e r e c o n s t i t u t e d membranes.

The

Arrhenius p l o t s e x h i b i t e d breaks f o r t h e probe m o b i l i t y a t t h e same t e m p e r a t u r e s as t h o s e f o r ATPase a c t i v i t y . these discrepancies i s not clear.

The r e a s o n f o r

O t h e r s t u d i e s on l i p i d - p r o t e i n

i n t e r a c t i o n u s i n g s a r c o p l a s m i c r e t i c u l u m membranes a l s o h a v e b e e n r e p o r t e d . 109-111 Yang e t a 1 . 1 1 2 - 1 1 3

h a v e p u b l i s h e d t w o p a p e r s on t h e e f f e c t o f

Mg2+ o n 1 i p i d - p r o t e i n i n t e r a c t i o n i n r e c o n s t i t u t e d p o r c i n e h e a r t m i t o c h o n d r i a 1 H+-ATPase.

#g2+ seemed t o i n d u c e a d e c r e a s e i n t h e

f l u i d i t y o f s o y b e a n p h o s p h o l i p i d r e c o n s t i t u t e d enzyme a s determined b y both e.s.r.

and f l u o r e s c e n c e t e c h n i q u e s .

Parallel

e x p e r i m e n t s w i t h c i r c u l a r d i c h r o i s m showed an i n c r e a s e i n t h e a - h e l i c a l c o n t e n t o f t h e p r o t e i n i n t h e p r e s e n c e o f Mg2+. The a u t h o r s s p e c u l a t e d t h a t t h e a l t e r a t i o n o f membrane f l u i d i t y b y Mg2+ i s a c c o m p a n i e d b y a c o n f o r m a t i o n a l c h a n g e o f t h e H+-ATPase p r o t e i n t o a higher a c t i v i t y form. A v a r i e t y o f s p i n - l a b e l l e d p h o s p h o l i p i d s were i n c o r p o r a t e d

i n t o Na+, K+-ATPase membranes f r o m T o r p e d o m a r m o r a t o e l e c t r i c o r g a n a t r e l a t i v e l y h i g h c o n c e n t r a t i o n s (up t o 5 mol o f s p i n l a b e l s p e r 100 mol o f p h o s p h o l i p i d s ) . l 1 4

The e . s . r .

line

b r o a d e n i n g d u e t o s p i n - s p i n i n t e r a c t i o n was u s e d t o c a l c u l a t e t h e f r a c t i o n o f p h o s p h o l i p i d s t h a t were a s s o c i a t e d w i t h t h e b u l k l i p i d bilayer.

I t was f o u n d t h a t n e g a t i v e l y c h a r g e d s p i n -

labelled phospholipids,

s u c h as s p i n - l a b e l l e d p h o s p h a t i d y l s e r i n e

and s p i n - l a b e l l e d p h o s p h a t i d i c a c i d ,

are d i s t r i b u t e d i n t o both

t h e b i l a y e r and a n o t h e r c o m p a r t m e n t ,

t h e 1i p i d s h e l l s u r r o u n d i n g

t h e p r o t e i n , whereas n e u t r a l s p i n - l a b e l l e d p h o s p h o l i p i d s such as s p i n - l a b e l l e d p h o s p h a t i d y l c h o l i n e and s p i n - l a b e l l e d p h o s p h a t i d y l -

Electron Spin Resonance

262

ethanolamine tend t o d i s t r i b u t e m a i n l y i n t o t h e b i l a y e r .

A

s p e c i f i c i n t e r a c t i o n between n e g a t i v e l y charged p h o s p h o l i p i d s and Na+,

K+-ATPase

i n t h e membrane was s u g g e s t e d .

Fatty acid spin label incorporated i n t o phospholipid vesicles r e c o n s t i t u t e d c y t o c h r o m e P-450 was m o t i o n a l r e s t r i c t e d b y t h e protein.115

I t was p r o p o s e d t h a t t h e n e g a t i v e l y c h a r g e d c a r b o x y l

group o f f a t t y acid s p i n l a b e l i s i n v o l v e d i n t h e l i p i d - p r o t e i n interaction.

I t i s o f i n t e r e s t t h a t m i c r o s o m a l membranes

prepared from d i f f e r e n t r a b b i t s e x h i b i t a v a r i a b l e e x t e n t o f motional r e s t s r i c t i o n o f f a t t y acid spin labels.

The a u t h o r s

a r g u e d t h a t t h e v a r i a t i o n i s due t o v a r i o u s amounts o f l i p i d b r e a k d o w n p r o d u c t s i n m i c r o s o m a l membranes vJhich c o m p e t e w i t h f a t t y a c i d s p i n l a b e l t o b i n d t o c y t o c h r o m e P-450. Ubiquinol oxidase o f the mitochondrial electron t r a n s f e r c h a i n can be r e c o n s t i t u t e d f r o m u b i q u i n o l - c y t o c h r o m e c r e d u c t a s e ( c o m p l e x 1 1 1 ) and c y t o c h r o m e c o x i d a s e ( c o m p l e x I V ) membranes.

i n DMPC

The e n z y m a t i c a c t i v i t y o f c o m p l e x I 1 1 o r I V a l o n e was

independent o f l i p i d phase t r a n s i t i o n . 1 1 6

However,

the a c t i v i t y

o f u b i q u i n o l o x i d a s e was s t r o n g l y d e p e n d e n t upon t h e l i p i d p h a s e t r a n s i t i o n ; t h e a c t i v i t y decreased s h a r p l y below t h e phase transition. I n a separate experiment, t h e authors noted t h a t maleimide s p i n - l a b e l l e d cytochrome c d i f f u s e s s l o w l y i n a gel-phase l i p i d .

Because u b i q u i n o l o x i d a s e depends on c y t o c h r o m e

c f o r i t s a c t i v i t y , t h e authors proposed t h a t f r e e d i f f u s i o n o f c y t o c h r o m e c on t h e membrane s u r f a c e i s e s s e n t i a l f o r o v e r a l l electron transfer i n mitochondrial respiratory chain. R i t t m a n e t a1.117

have l a b e l l e d band 3 p r o t e i n o f e r y t h r o c y t e

membranes w i t h a 1 6 - d o x y l s t e a r a t e l i n k e d b y an e s t e r b o n d t o a maleimide o r a n i t r e n e residue. f r o m t h i s work:

Two m a j o r c o n c l u s i o n s w e r e made

t h e s p i n - l a b e l l e d f a t t y a c i d s a r e t r a p p e d between

p r o t e i n m o l e c u l e s and b a n d 3 t e n d s t o f o r m o l i g o m e r s a t p h y s i o l o g i c a l temperatures. I t was shown t h a t t h e a c t i v i t y o f membrane-bound

enzymes c a n

be a f f e c t e d b y t h e f a t t y a c i d c o m p o s i t i o n o f t h e p h o s p h o l i p i d bilayers.

M a t h u r e t a1.118 h a v e f o u n d t h a t a c y l - C o A :

cholesterol

a c y l t r a n s f e r a s e a c t i v i t y i s a f f e c t e d by changing t h e p h o s p h o l i p i d f a t t y a c y l c o m p o s i t i o n o f i s o l a t e d r a t l i v e r tnicrosomes t h r o u g h i n v i t r o p h o s p h a t i d y l c h o l i n e exchange. however,

showed

S p i n l a b e l measurements,

no d e t e c t a b l e c h a n g e s i n membrane f l u i d i t y .

263

7: Spin Labels: Biological Systems T h e i r r e s u l t s d o n o t s u p p o r t a c o r r e l a t i o n b e t w e e n membrane f l u i d i t y and membrane enzyme a c t i v i t y . B r o p h y e t a1.119 h a v e e s t i m a t e d t h e s t o i c h i o m e t r y o f t h e f i r s t s h e l l l i p i d on t h e s u r f a c e o f m y e l i n p r o t e o l i p i d p r o t e i n u s i n g s p i n - l a b e l l e d l i p i d s as p r o b e s . They showed t h a t o n l y 1 0 l i p i d s a r e bound t o o n e p r o t e i n m o l e c u l e ( - 25,000 d a l t o n ) . This l o w s t o i c h i o m e t r y o f b o u n d a r y l i p i d s seemed t o s u p p o r t t h e c o n t e n t i o n t h a t t h e p r o t e i n i n membranes i s i n a hexamer f o r m as s u g g e s t e d p r e v i o u s l y b y sedimentation-equilibrium Among v a r i o u s s p i n - l a b e l l e d l i p i d s t e s t e d ,

experiments.

spin-labelled acidic

p h o s p h o l i p i d s w e r e m o s t i m m o b i l i z e d on t h e s u r f a c e o f t h e protein,

i n d i c a t i n g a s p e c i f i c i t y of

lipid-protein interaction

f o r m y e l i n p r o t e o l i p i d p r o t e i n i n t h e membrane. The i n t e r a c t i o n o f c h o l e s t e r o l and l i p o p h i l i n i s o l a t e d f r o m b o v i n e m y e l i n i n o r i e n t e d p h o s p h o l i p i d b i l a y e r s was s t u d i e d u s i n g c h o l e s t a n e s p i n l a b e l as a p r o b e . 1 2 0

The u s e o f o r i e n t e d

b i l a y e r s p e r m i t s t h e separation o f t h e e.s.r. spectrum i n t o two c o m p o n e n t s : one o r i e n t e d s p i n l a b e l component a t t r i b u t e d t o t h e p r o b e s i n t h e b u l k l i p i d s and o n e u n o r i e n t e d s p i n l a b e l component due t o t h e p r e s e n c e o f t h e p r o t e i n .

A specific interaction

b e t w e e n c h o l e s t e r o l m o l e c u l e s and l i p o p h i l i n p r o t e i n was p r o p o s e d . 6.2

Peripheral Proteins

-

The e f f e c t s o f d i v a l e n t c a t i o n s and

m y e l i n b a s i c p r o t e i n on phase b e h a v i o r o f p h o s p h a t i d y l g l y c e r o l w e r e e v a l u a t e d b y s p i n l a b e l and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y methods.121 Myel i n b a s i c p r o t e i n i n t e r a c t e d e l e c t r o s t a t i c a l l y w i t h n e g a t i v e l y c h a r g e d p o l a r head g r o u p and h y d r o p h o b i c a l l y w i t h t h e l i p i d core through i t s hydrophobic residues buried i n the l i p i d bilayer. Band 4 . 1 p r o t e i n o f t h e e r y t h r o c y t e membrane h a s b e e n demonstrated t o b i n d s e l e c t i v e l y w i t h liposomes c o n t a i n i n g p h o s p h a t i d y l s e r i n e ( P S ) . I 2 2 The b i n d i n g , w h i c h t r i g g e r e d t h e r e l e a s e o f T e m p o c h o l i n e e n t r a p p e d i n P S - c o n t a i n i n g l i p o s o m e s , was The r e s u l t s s u g g e s t e d t h a t b a n d 4 . 1 p r o t e i n i n h i b i t e d b y Ca2+. r e g u l a t e s membrane p e r m e a b i l i t y , i m p l y i n g i t s f u n c t i o n a l r o l e i n t h e i n n e r l a y e r o f t h e e r y t h r o c y t e membrane c o n t a i n i n g PS domains.

264

Electron Spin Resonance

7

Membrane F l u i d i t y o f C e l l s

-

7.1 P r o l i f e r a t i n g Cells L a i e t a l . 1 2 3 have demonstrated t h a t p u r i f i e d plasma f i b r o n e c t i n absorbs t o t h e s u r f a c e o f m i c r o c a r r i e r s i n a s e r u m - f r e e medium and s u b s e q u e n t l y p r o m o t e s C H O c e l l spreading.

U s i n g f a t t y a c i d s p i n l a b e l s as p r o b e s , t h e y

r e p o r t e d t h a t CHO c e l l s s p r e a d i n g o n p l a s m a f i b r o n e c t i n - c o a t e d m i c r o c a r r i e r s h a v e a more r i g i d membrane compared t o CHO c e l l s i n suspension,

s u g g e s t i n g a c o r r e l a t i o n b e t w e e n c e l l s p r e a d i n g and

membrane f l u i d i t y . A c a t i o n i c s p i n p r o b e , CATlZ

( s e e 6 ) and 5 - d o x y l s t e a r a t e

w e r e u s e d t o s t u d y gram n e g a t i v e b a c t e r i a o u t e r membrane composed o f a n i o n i c 1 i p o p o l y s a c c a r i d e and p r o t e i n . 1 2 4

Both probes

r e v e a l e d a s t r u c t u r a l t r a n s i t i o n o c c u r r i n q a t 9'C membrane. o f CATl2

i n the outer

The a u t h o r s s t a t e d t h a t above 30°C t h e e . s . r .

signal

i n t h e o u t e r qembrane i s t o o b r o a d t o m e a s u r e 2Tll,

w h e r e a s a t t h e same t e m p e r a t u r e t h e 2Tll l i p o p o l y s a c c h a r i d e i s measurable. observation;

i n purified

o f CATl2

T h i s i s an i n t e r e s t i n g

i t i s known t h a t t h e e . s . r .

s i g n a l becomes s h a r p e r

a t h i g h t e m p e r a t u r e s because o f m o t i o n - i n d u c e d l i n e n a r r o w i n g . The o b s e r v e d l i n e b r o a d e n i n g a t h i g h t e m p e r a t u r e s i s o p p o s i t e t o what one would e x p e c t .

The r e p o r t e r s p e c u l a t e s t h a t t h e o b s e r v e d

l i n e b r o a d e n i n g may be due t o a s t r o n g s p i n - s p i n i n t e r a c t i o n o f CATl2

r a d i c a l s r e s u l t i n g from a temperature-dependent

l i p i d phase

s e p a r a t i o n i n t h e o u t e r membrane. Simon e t a1.125 and l i p i d d r o p l e t ; E.s.r.

have conpared t h e f l u i d i t y o f i n t a c t c e l l s i s o l a t e d froin f a t t y a c i d m o d i f i e d L1210 c e l l s .

s p e c t r a p r e s e n t e d i n t h i s work,

components:

however,

showed m u l t i -

a t l e a s t one i m m o b i l i z e d component and o n e f r e e s p i n

component superimposed w i t h a b r o a d e.s.r.

line.

Caution should

b e made i n a t t e m p t i n g t o m e a s u r e t h e i n n e r h y p e r f i n e s p l i t t i n g f r o m t h i s t y p e o f complicated spectrum. C h a r t 3,

The e . s . r .

y i e l d e d from t h e computer-subtracted

5-doxylstearate i n l i p i d droplets,

spectrum i n

spectrum f o r

l o o k s mare l i k e a s e c o n d

d e r i v a t i v e display rather than a f i r s t derivative display. I n response t o t e m p e r a t u r e changes i n environment,

bacteria

a r e known t o a d a p t t h e i r membrane l i p i d c o m p o s i t i o n b y a l t e r i n g t h e degree o f f a t t y acyl u n s a t u r a t i o n mediated b y desaturase enzymes.

F o o t e t a l . 1 2 6 have n o t e d a d i r e c t c o r r e l a t i o n between

7: Spin Labels: Biological Systems

t h e A9-desaturase activity of the psychrophilic bacterium ( M i c r o c o c c u s c r y o p h i l u s ) grown at v a r i o u s t e m p e r a t u r e s and t h e a d a p t a t i o n o f m e m b r a n e f l u i d i t y t o c h a n g e s in environmental t e m p e r a t u r e s . T h e i r e.s.r. d a t a i n d i c a t e d that t h e d e s a t u r a s e e n z y m e is associated with t h e b u l k l i p i d s rather t h a n with any s p e c i f i c t y p e s o f lipids in t h e membranes. The o u t e r and i n n e r m e m b r a n e s isolated f r o m t h e gram n e g a t i v e e x t r e m e l y t h e r m o p h i l i c b a c t e r i a ( T h e r m u s T h e r m o p h i l u s H B - 8 ) w e r e shown t o d i f f e r in m e m b r a n e f l u i d i t y as determined by s p i n label m e t h 0 d s . 1 2 ~ A c o r e l a t i o n was r e p o r t e d between t h e physical s t a t e o f t h e m e m b r a n e and t h e s w i m m i n g behavior o f T e t r a h y m e n a p y r i f o r m i s as s t u d i e d by using f a t t y acid s p i n label methods.128 O t h e r s p i n label s t u d y o n t h e o r d e r i n g o f l i p i d s in bacterial m e m b r a n e s w a s r e p o r t e d .I29 7.2 N o n p r o l i f e r a t i n g Cells - W h e t t o n et a1.130-131 h a v e p u b l i s h e d t w o p a p e r s o n t h e effect o f cholesterol c o n c e n t r a t i o n s o n a d e n y l a t e c y c l a s e activity in rat liver plasma m e m b r a n e s . T h e c o n c e n t r a t i o n s o f cholesterol in t h e p l a s m a m e m b r a n e s w e r e c o n t r o l l e d by i n c u b a t i n g t h e m e m b r a n e with either c h o l e s t e r o l r i c h o r c h o l e s t e r o l - p o o r l i p o s o m e s at 4 ' C . The activity of a d e n y l a t e c y c l a s e was inhibited e i t h e r by increasing o r d e p l e t i n g c h o l e s t e r o l c o n c e n t r a t i o n s in t h e membrane. However, it is r a t h e r interesting t h a t e i t h e r cholesterol d e p l e t i o n o r e n r i c h m e n t p r o d u c e s a d e c r e a s e in m e m b r a n e f l u i d i t y as probed by f a t t y acid s p i n labels. T h e reason is not yet known. A d i r e c t r e l a t i o n b e t w e e n c h o l e s t e r o l levels and a d e n y l a t e c y c l a s e a c t i v i t y in 1 iver plasma m e m b r a n e s was c l e a r l y demonstrated. T h e m i c r o v i l l u s s u r f a c e o f t h e small i n t e s t i n e p l a y s important r o l e s i n p r e v e n t i n g t h e penetration o f foreign s u b s t a n c e s such as proteins, t o x i n s and bacteria, etc. Pang et al.132 r e p o r t e d t h a t t h e m i c r o v i l l u s m e m b r a n e from n e w b o r n r a b b i t s is s i g n i f i c a n t l y m o r e disordered t h a n t h a t obtained f r o m adult r a b b i t s as m e a s u r e d b y f a t t y acid spin label methods. T h e y hypothesized t h a t t h e d i s o r d e r i n g o f l i p i d s m a y account f o r t h e profound m a c r o m o l e c u l e t r a n s p o r t d u r i n g t h e perinatal period. A T r i t o n X-100 d e r i v e d , o x y g e n - e v o l v i n g p h o t o s y s t e m I 1 h a s been illustrated t o c o n t a i n l e s s fluid m e m b r a n e t h a n t h e t h y l a k o i d b y using T E M P O p a r t i t i o n i n g and lipid spin label techniques.133 T h e binding o f f i b r i n o g e n t o

265

Electron Spin Resonance

266

human b l o o d p l a t e l e t membranes i n d u c e d a d e c r e a s e i n membrane f l u i d i t y as e v i d e n c e d f r o m b o t h e . s . r . and f l u o r e s c e n c e s t u d i e s . 1 3 4 O t h e r s t u d i e s on membrane f l u i d i t y o f c e l l s h a v e a l s o appeared ,135-137 8 8.1

M o d i f i c a k i o n o f Membrane F u n c t i o n s b y D r u g s

Anesthetics

-

Uncouplers o f o x i d a t i v e phosphorylation i n

m i t o c h o n d r i a a c t as p r o t o n o p h o r e s t h a t c a t a l y z e t h e t r a n s p o r t o f p r o t o n s a c r o s s t h e membrane, l e a d i n g t o t h e c o l l a p s e o f i n t u r n i s t h e d i r e c t d r i v i n g f o r c e f o r ATP s y n t h e s i s .

ATH w h i c h

R o t t e n b e r g 1 3 8 r e p o r t e d t h a t g e n e r a l a n e s t h e t i c s s u c h as c h l o r o f o r m and h a l o t h a n e b e h a v e l i k e u n c o u p l e r s i n m i t o c h o n d r i a , b u t w i t h o u t an e f f e c t o n

ATH.

I n a s e p a r a t e e x p e r i m e n t , he

showed t h a t t h e s e a n e s t h e t i c s i n c r e a s e t h e f l u i d i t y o f m i t o c h o n d r i a l membranes. However,

the uncoupling e f f e c t o f these

a n e s t h e t i c s d o e s n ' t appear t o be due t o t h e d i r e c t r e s u l t o f t h i s f l u i d i z i n g e f f e c t , b u t r a t h e r t o an e f f e c t o n p r o t e i n - p r o t e i n i n t e r a c t i o n i n m i t o c h o n d r i a 1 membranes. The f l u i d i z i n g e f f e c t o f f o u r l o c a l a n e s t h e t i c s i n c l u d i n g 1 i d o c a i n e , t e t r a c a i n e , d i b u c a i n e and h e p t a c a i n e o n r a t b r a i n membranes h a s b e e n e v a l u a t e d u s i n g f a t t y a c i d s p i n l a b e l m e t h o d s . l 3 9 The d i s o r d e r i n g e f f e c t was m o r e p r o f o u n d i n t h e h y d r o p h o b i c c o r e r e g i o n t h a n a t t h e p o l a r head g r o u p r e g i o n o f t h e membrane. The d i s o r d e r i n g e f f i c i e n c y o f t h e d r u g s seemed t o correspond t o t h e i r anesthetic potency. B r a i n synaptosomal p l a s m a membranes i s o l a t e d f r o m e t h a n o l - t r e a t e d m i c e w e r e r e p o r t e d t o b e more r i g i d c o m p a r e d t o t h o s e f r o m c o n t r o l m i c e u s i n g 1 2 - d o x y 1 s t e a r a t e as a p r o b e . 140 Two s p i n - l a b e l l e d l o c a l a n e s t h e t i c s w e r e u s e d t o i n v e s t i g a t e t h e e l e c t r o s t a t i c i n t e r a c t i o n s b e t w e e n l o c a l a n e s t h e t i c s and p h o s p h o l i p i d s and p r o t e i n s i n e r y t h r o c y t e membranes.141 T h e s e c a t i o n i c s p i n l a b e l analogs were demonstrated t o b i n d e l e c t r o s t a t i c a l l y w i t h a n i o n i c p h o s p h o l i p i d s as w e l l a s p r o t e i n s i n t h e membrane.

O t h e r s t u d i e s o n membrane e f f e c t s o f a n e s t h e t i c s a l s o

have appeared. 142-145 P o l y e t h y l e n e g l y c o l i s o n e o f w i d e l y used membrane f u s i o n agents.

By u s i n g TEMPO p a r t i t i o n i n g and f a t t y a c i d s p i n l a b e l

methods,

H e r r m a n n e t a1.146 showed t h a t t h e p r e s e n c e o f p o l y -

e t h y l e n e g l y c o l r e d u c e s t h e p o l a r i t y o f t h e aqueous s o l v e n t s o l u t i o n as w e l l as d e c r e a s e s t h e f l u i d i t y o f membrane v e s i c l e s .

7: Spin Labeb: Biological Systems The a u t h o r s p o s t u l a t e d t h a t a d i r e c t i n t e r a c t i o n o f t h e agent w i t h head groups o f p h o s p h o l i p i d s i s r e s p o n s i b l e f o r t h e o b s e r v e d effect. S i m i l a r o b s e r v a t i o n s a l s o have been r e p o r t e d b y S u r e w i c z l 4 7 and B o s s . 1 4 8 S u r e w i c z r e p o r t e d t h a t t h e a g e n t r i g i d i f i e s membrane l i p i d s and i n d u c e s changes i n t h e o r g a n i z a t i o n o f membrane p r o t e i n s b y u s i n g s p i n l a b e l m e t h o d s . On t h e o t h e r hand, Boss r e p o r t e d t h a t p o l y e t h y l e n e g l y c o l i n d u c e s a d e c r e a s e i n t h e membrane f l u i d i t y o f p l a n t p r o t o p l a s t s and c a u s e s t h e a p p e a r a n c e o f an i s o t r o p i c s i g n a l b y u s i n g 5 - d o x y l s t e a r a t e a s a probe.

The s i g n i f i c a n c e o f t h i s s m a l l i s o t r o p i c s i g n a l i s n o t

certain.

8.2

Others

-

T r i t o n and h i s c o l l e a g u e s h a v e c o n t i n u e d t h e i r

s t u d i e s on t h e membrane t a r g e t h y p o t h e s i s f o r a d r i a m y c i n , anthracycline antibiotic,

an

Membrane f l u i d i t y o f Sarcoma 180 c e l l s

e x p o s e d t o n o r m a l a e r a t i o n o r h y p o x i a was d e t e r m i n e d b y e . s . r . spectrosocpy w i t h t h e i n c o r p o r a t i o n o f 5-doxylstearate i n t o t h e c e l l . I 4 9 A c o r r e l a t i o n was n o t e d b e t w e e n membrane f l u i d i t y , h y p o x i a and s e n s i t i v i t y o f t h e c e l l t o a d r i a m y c i n . However, t h e p o s s i b l e c e l l c y c l e e f f e c t d u e t o h y p o x i c c o n d i t i o n s was n o t considered i n t h i s study. Both e.s.r. and f l u o r e s c e n c e d e p o l a r i z a t i o n m e a s u r e m e n t s showed t h a t t h e l i p i d p h a s e o f p l a s m a membranes f r o m P388 m u r i n e l e u k e m i a c e l l s i s more f l u i d t h a n t h a t o f a d o x o r u b i n - r e s i s t a n t sub1 i n e , P388/ADH ,150 The a u t h o r s s u g g e s t e d t h a t t h e d i f f e r e n c e i n membrane f l u i d i t y b e t w e e n t h e s e c e l l s i s r e l a t e d t o t h e d i f f e r e n c e i n i n t r a c e l l u l a r accumulation o f a n t h r a c y c l i n e drugs. 25-Hydroxycholesterol, a major auto-oxidation product o f c h o l e s t e r o l , i s known t o i n d u c e m o r p h o l o g i c a l c h a n g e s o f e r y t h r o c y t e s and t o b e t o x i c t o c u l t u r e d , a o r t i c s m o o t h m u s c l e c e l l s a t low c o n c e n t r a t i o n s . Benga and h i s c o l l e a g u e s 1 5 1 h a v e c o m p a r e d t h e e f f e c t s o f c h o l e s t e r o l and 2 5 - h y d r o x y c h o l e s t e r o 1 o n l i p o s o m e s p r e p a r e d f r o m e g g y o l k l e c i t h i n u s i n g TEMPO p a r t i t i o n i n g and f a t t y a c i d s p i n l a b e l m e t h o d s . b i o l o g i c a l e f f e c t s o f 25-hydroxycholestero1 were n o t d e t e c t e d b y s p i n l a b e l methods.

The p r o f o u n d

a t low c o n c e n t r a t i o n s

The mechanism o f h y p e r t h e r m i c c e l l k i l l i n g i s n o t y e t c l e a r . Lepock e t a l . 1 5 2 used e.s.r. and f l u o r e s c e n c e e n e r g y t r a n s f e r measurements t o i n v e s t i g a t e t h e r e l a t i o n between t h e p h y s i c a l s t a t e o f C h i n e s e h a m s t e r l u n g c e l l membranes and h y p e r t h e r m i c

Electron Spin Resonance

268

cell killing. I t was p r o p o s e d t h a t h y p e r t h e r m i a a f f e c t s t h e s t r u c t u r e and f u n c t i o n o f membrane p r o t e i n s r a t h e r t h a n membrane 1i p i d s . The i n d u c t i o n o f c y t o l y s i s b y i n c o r p o r a t i n g f r e e f a t t y a c i d s i n t o r a t t h y m o c y t e s and s p l e e n l y m p h o c y t e s and i s o l a t e d n u c l e i was e x a m i n e d b y B u r t o n and P i e t t e . 1 5 3

Only f r e e f a t t y acids o f

C-8 t o C-18 i n d u c e d c e l l l y s i s b u t n o t t h e i r e s t e r o r amide d e r i v a t i v e s as shown b y s p i n l a b e l m e t h o d s . Benga e t a1 . I s 4 r e p o r t e d t h a t c h l o r p r o m a z i n e r e d u c e s m a r k e d l y t h e e.s.r.

s i g n a l amplitude of maleimide s p i n - l a b e l l e d e r y t h r o -

c y t e membranes.

T h i s s i g n a l r e d u c t i o n was d e m o n s t r a t e d t o b e d u e

t o t h e d e s t r u c t i o n o f n i t r o x i d e f r e e r a d i c a l s by chlorpromazine. The w e a k l y i m m o b i l i z e d component, s t r o n g l y i m m o b i l i z e d component,

W, was r e d u c e d g r e a t e r t h a n t h e

S,

t h e r e b y v a r y i n g t h e W/S r a t i o .

The a u t h o r s c a u t i o n e d u s i n g s p i n l a b e l s t u d i e s t o i n t e r p r e t

c h l o r p r o m a z i n e - e r y t h r o c y t e membrane i n t e r a c t i o n . Fung e t a1 . I 5 5 d e m o n s t r a t e d t h a t s i c k l e h e m o g l o b i n b i n d s a f a c t o r o f 2 t i g h t e r t o m a l e i m i d e s p i n - l a b e l l e d human e r y t h r o c y t e membranes t h a n n o r m a l h e m o g l o b i n a t p h y s i o l o g i c a l p H .

The

a u t h o r s proposed t h a t t h i s s t r o n g i n t e r a c t i o n between s i c k l e h e m o g l o b i n w i t h menbranes may r e l a t e t o t h e f o r m a t i o n o f irreversibly sickled cells. E r y t h r o c y t e s d i f f e r i n g i n c e l l age were i s o l a t e d b y c e n t r i f u q a t i o n . 1 5 6

S p i n l a b e l and f l u o r e s c e n c e

m e a s u r e m e n t s showed t h a t t h e membrane m i c r o v i s c o s i t y i n c r e a s e s w i t h i n c r e a s i n g age o f t h e e r y t h r o c y t e s .

A n a l y s i s o f phospho-

l i p i d c o m p o s i t i o n o f e r y t h r o c y t e membranes r e v e a l e d an i n c r e a s e i n s p h i n g o m y e l i n c o n t e n t as w e l l as t h e r a t i o s o f c h o l e s t e r o l t o phosphatidylcholine.

These c h a n g e s may r e l a t e t o t h e o b s e r v e d

i n c r e a s e i n m i c r o v i s c o s i t y i n t h e membrane o f aged c e l l s . d e c r e a s e i n p e r m e a b i l i t y o f an a n i o n i c s p i n p r o b e ,

2,2,5,5-tetramethylpyrrolidine-l-oxyl,

A

3-carboxy-

was s u g g e s t e d t o b e

r e l a t e d t o t h e decrease i n t h e a c t i v i t y o f band 3 p r o t e i n d u r i n g r e d c e l l aging i n vivo.157

O t h e r s p i n l a b e l s t u d i e s on e r y t h r o -

c y t e membranes a1 s o h a v e b e e n r e p o r t e d . 1 5 8 - l f j 7 The e f f e c t s o f l i p i d p e r o x i d a t i o n o n t h e m o l e c u l a r o r g a n i z a t i o n o f Ca2+-ATPase f r o m r a b b i t s k e l e t a l m u s c l e s a r c o p l a s m i c r e t i c u l u m membranes w e r e i n v e s t i g a t e d b y e . s . r .

spectroscopy o f

b o t h s p i n p r o b e and s p i n l a b e l m e t h o d s . 1 6 8 A c c u m u l a t i o n o f l i p i d p e r o x i d a t i o n p r o d u c t s i n d u c e d an i n c r e a s e o f t h e h y d r o p h o b i c i t y c o n c o m i t t a n t l y w i t h a d e c r e a s e o f t h e m o b i 1 it y o f t h e Ca2+-ATPase

269

7: Spin Labels: Biological Systems

p o l y p e p t i d e c h a i n f r a g m e n t i m m e d i a t e l y a d j a c e n t t o t h e enzyme B r u c h and T h a y e r , l 6 9

active site.

t h a t following peroxidation,

o n t h e o t h e r hand,

h a v e shown

t h e membrane p r e p a r e d f r o m s o n i c a t e d

s o y b e a n p h o s p h o l i p i d v e s i c l e s hecomes m o r e r i g i d ,

particularly

i n i n t r a m e m b r a n e l o c a t i o n as m o n i t o r e d b y 1 2 - d o x y l s t e a r a t e .

They

p r o p o s e d t h a t l i p i d p e r o x i d a t i o n i n d u c e s a change o f t h e f l u i d i t y gradient o f l i p i d bilayers. P h l o r e t i n i s a w i d e l y used a g e n t f o r m o d i f y i n g p e r m e a b i l i t y o f c e l l membranes.

The e f f e c t o f p h l o r e t i n o n e r y t h r o c y t e

membranes i n c o r p o r a t e d w i t h 1 6 - d o x y l s t e a r a t e was shown t o b e

c o n c e n t r a t i o n - d e p e n d e n t , namely, p r o m o t i n q l i p i d o r d e r i n g a t l o w c o n c e n t r a t i o n s and i n d u c i n g l i p i d d i s o r d e r i n g a t h i g h c o n c e n t r a t i 0 n s . 1 ~ 0 The c o n c e n t r a t i o n - d e p e n d e n t

e f f e c t s may r e l a t e t o

t h e d i f f e r e n t i a l e f f e c t s o f p h l o r e t i n on membrane t r a n s p o r t and d i f f u s i o n processes. 4 spin-labelled

a n a l o g o f PAF-acether

i n f l a m m a t i o n and a n a p h y l a x i s ,

( 8 ) , a mediator o f

was s y n t h e s i z e d b y B e t t e and

The p h y s i c o c h e m i c a l p r o p e r t i e s o f 8 a p p e a r e d t o b e

Bienvenue.171

s i m i l a r t o those o f phosphatidylcholines. serum albumin,

Partitioning of 8 i n

m o d e l membranes and m i c e l l e s was i n v e s t i g a t e d .

The p r e l i m i n a r y r e s u l t showed t h a t t h i s s p i n - l a b e l

analog

t r ig g e r s p l a t e 1 e t a g g r e g a t ion.

The i n t e r a c t i o n o f p u r i f i e d M o j a v e t o x i n w i t h s y n a p t o s o m a l membranes f r o m r a t b r a i n was e x a m i n e d b y f a t t y a c i d s p i n l a b e l methods.172

The p e r t u r b a t i o n o f t h e t o x i n m o l e c u l e t o t h e

membrane s t r u c t u r e o c c u r r e d m a x i m a l l y a t C8-9 w h i c h i s a b o u t

12-14

A

i n t o t h e membrane.

It i s r a t h e r i n t e r e s t i n g t h a t t h i s

i n t e r a c t i o n distance i s correlated well with the depth o f t h e h y d r o p h o b i c p o c k e t shown f o r p a n c r e a t i c p h o s p h o l i p a s e A 2 w h i c h h a s a p o r t i o n o f amino a c i d s e q u e n c e h o m o l o g o u s t o t h e b a s i c subunit o f the toxin.

The e f f e c t s o f o t h e r a g e n t s on t h e

p h y s i c a l s t a t e s o f membranes a l s o h a v e been r e p o r t e d b y u s i n g s p i n l a b e l methods .173-183

Electron Spin Resonance

270

9-- I m m u n o l o g y A n g l i s t e r e t a l . I 8 4 have i n v e s t i g a t e d t h e s t r u c t u r e o f a n t i b o d y c o m b i n i n g s i t e s u s i n g d i f f e r e n c e n.m.r. spectra c o n s t r u c t e d f r o m t h e n.m.r. s p e c t r a o f m o n o c l o n a l Fab a n t i b o d y f r a g m e n t s w i t h and w i t h o u t a s p e c i f i c s p i n - l a b e l l e d d i n i t r o p h e n y l hapten (9).

E l e v e n a r o m a t i c amino a c i d s were f o u n d t o b e i n

NO, NO2@!