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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 *
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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
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u,
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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
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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
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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 ,
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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
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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
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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
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-
[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
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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 .
-
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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
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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 .
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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
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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
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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
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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
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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
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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 ,
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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),
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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 -
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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,
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( 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 .
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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-
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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
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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
.
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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
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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
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192
Electron Spin Resonance
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(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
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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
+
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Me3CO’
+
ROOH
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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
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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
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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)
.’’’
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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
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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-
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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
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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.
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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
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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
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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 . ~ ~ ~
.
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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 ,
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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.
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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
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~-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
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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
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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.
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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@!