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Electron spin resonance. Volume 11B : a review of recent literature to mid-1988
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Electron Spin Resonance Volume I 1 B

A Specialist Periodical Report

Electron Spin Resonance Volume 11B

A Review of Recent Literature to mid4988 Senior Reporter M. C. R. Symons, FRS, Department of Chemistry, University of L eicester Reporters

G.R. Eaton, University of Denver, USA S.S. Eaton. University of Colorado at Denver, USA J.F. Gibson, Imperial College, London G.R. Hanson, University of Queensland, Australia J.A. Howard, National Research Council, Ottawa, Canada A. Hudson, University of Sussex K.T. Knecht. NIHINIEHS, North Carolina, USA K.R. Maples, NIHINIEHS, North Carolina, USA R . P. Mason, NIHINIEHS, North Carolina, USA B. Mile, National Research Council, Ottawa, Canada J.-M. Spaeth, University of Paderborn, FRG G.L. Wilson, Monash University, Clayton, Australia

@

ROYAL SOCIETY OF CHEMISTRY

ISBN 0-85 186-871- 1 ISSN 030 1-0074

Copyright 0 1989 The Royal Society of Chemistry

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

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

Printed in Great Britain at the Alden Press, Oxford, London and Northampton

Foreword

Volume 1 1 B i s c o m p l i m e n t a r y t o V o l u m e 1 1 A p u b l i s h e d a y e a r a g o . A summary o f t h e c o n t e n t s o f V o l u m e 1 1 A i s i n c l u d e d i n t h e ' C o n t e n t s ' I call a t t e n t i o n t o Chapter 2 on section. I n particular, ' T h e o r e t i c a l A s p e c t s o f ESR' ( A . Hudson) which i s o b v i o u s l y o f r e l e v a n c e t o a l l ESR s p e c t r o s c o p i s t s . We h a v e c o n t i n u e d t h e e x p e r i m e n t o f r a t h e r a r b i t r a r i l y d i v i d i n g t h e s e r e p o r t s i n t o t w o , Volume A d e a l i n g w i t h O r g a n i c a n d Bioo r g a n i c t o p i c s a n d Volume B w i t h I n o r g a n i c a n d B i o - i n o r g a n i c t o p i c s . This p r o c e d u r e d i v i d e s t h e l o a d of work n i c e l y , and s l o t s i n w i t h t h e A n n u a l I n t e r n a t i o n a l ESR C o n f e r e n c e s o r g a n i s e d b y T h e R o y a l Society of Chemistry. I t i s v i t a l t h a t we m a i n t a i n s a l e s o f t h e s e v o l u m e s a t a maximum l e v e l i f t h i s s e r i e s i s t o c o n t i n u e . So I u r g e a l l t h o s e who r e a d t h i s f a r t o buy a copy! A s a l w a y s , I am m o s t g r a t e f u l t o a l l c o n t r i b u t o r s f o r t h e i r c o n t i n u e d h e l p w i t h t h i s t a s k . a n d e s p e c i a l l y f o r p r o d u c i n g camerar e a d y copy. I a m a l s o g r a t e f u l t o t h o s e ( t h e m a j o r i t y ) who m e t t h e e x a c t i n g d e a d l i n e , w h i c h was S e p t e m b e r 1 9 8 8 . Most o f t h e C h a p t e r s i n t h i s Volume a r e o n g o i n g f r o m Volume 10B. In addition, there are special chapters on 'Free Radical Mason, K.R. M a p l e s a n d K.T. M e t a b o l i t e s ' d e t e c t e d i n v i v o (R.P. Knecht), on ' I n o r g a n i c and O r g a n o m e t a l l i c R a d i c a l s and C l u s t e r s p r e p a r e d i n a R o t a t i n g C r y o s t a t by Metal V a p o u r T e c h n i q u e s ' by J . A . Howard a n d B. M i l e , a n d o n ' C o m p l e x e s o f P a r a m a g n e t i c M e t a l s w i t h P a r a m a g n e t i c L i g a n d s ' b y S.S. E a t o n a n d G . R . E a t o n . Martyn C.R. Symons

Contents

CHAPTER

1

In V i v o Detection of Free Radical Metabolites by Spin Trapping B y R.P.

1

3

1 1

S p i n T r a p p i n g I n Vivo

2

2.1 2.2

2 2

CHAPTW

2

Spin Trapping Difficulties

Folch Extraction

3

3.1

3 4 4

Halogenated Carbon-centred Radicals Lipid-derived Radicals Advantages and L i m i t a t i o n s

5

Biological Fluids 4.1 4.2 4.3 4.4 4.5

5

Knecht

The Problem Approaches

3.2 3.3 4

M a p l e s , a n d K.T.

Introduction

1.1 1.2 2

M a s o n , K.R.

New Approach Urine Perfusate Bile Blood

Conclusion

8

References

9

Theoretical Aspects of E.S.R. By A . H u d s o n

1

Introduction

11

2

A p p l i c a t i o n s o f Quantum C h e m i s t r y

12

3

S p i n R e l a x a t i o n and L i n e Broadening E f f e c t s

13

4

CIDEP a n d R e l a t e d P h e n o m e n a

17

5

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

17

References

20

vii

...

Contents

Vlll

CHAPTER

3

Transition Metal Ions B y J.F. G i b s o n

1

Introduction

24

2

Selected Topics

25

Techniques Theory Simulation of S p e c t r a Jahn-Teller E f f e c t s Mixed V a l e n c e Complexes Phase T r a n s i t i o n s Spin Crossovers. Superconductors Glasses Paramagnetic Ligands Semiquinones Nitroxyls Binuclear and Oligonuclear Complexes H o m o b i n u c l e a r , Cu-Cu Homobinuclear, O t h e r Metals Heterobinuclear Oligonuclear

25 26 28 28 31 33 37 39 40 42 42 44 45 45 47 50 51

s

52

3

= 1/2

d1 Configuration T e r v a l e n t T i t a n i u m , Zirconium and Hafnium Q u a d r i v a l e n t Vanadium Q u i n q u e v a l e n t Chromium, Molybdenum a n d T u n g s t e n S e x i v a l e n t Manganese, Technetium and Rhenium

d5 Configuration

52 52 53 55 57 58

U n i v a l e n t Chromium and Molybdenum 58 B i v a l e n t M a n g a n e s e , T e r v a l e n t I r o n a n d R u t h e n i u m 59

d7 C o n f i g u r a t i o n

60

Z e r o v a l e n t M a n g a n e s e a n d R h e n i u m , U n i v a l e n t I r o n 60 B i v a l e n t C o b a l t , Rhodium and I r i d i u m 62 T e r v a l e n t Nickel, Palladium and Platinum 63

d9 Configuration B i v a l e n t Copper Z e r o v a l e n t C o b a l t and Rhodium, U n i v a l e n t N i c k e l and Palladium

4

65 65 70

S = l

71

d2 Configuration

71

Q u a d r i v a l e n t Chromium

d8 Configuration Bivalent Nickel

71 71 71

ix

Contents 5

s = 2

72

d4 Configuration

72 72

B i v a l e n t Chromium

73

d6 C o n f i g u r a t i o n

73

Bivalent Iron 6

73

S = 312

73

d3 Configuration

T e r v a l e n t Chromium and Molybdenum, Vanadium

Bivalent

d7 C o n f i g u r a t i o n

Bivalent Cobalt 7

74 75

d5 Configuration

75

References

4

74

S = 512

S u p e r p o s i t i o n Model B i v a l e n t Manganese Tervalent Iron

CHAPTER

73

75 77 79 81

Recent Developments of ENDOR Spectroscopy in the Study of Defects in Solids By J.-M. S p a e t h

1

Introduction

89

2

S t r u c t u r e D e t e r m i n a t i o n o f D e f e c t s by M a g n e t i c Resonance

90

H y p e r f i n e a n d S u p e r h y p e r f i n e S t r u c t u r e o f ESR Spectra

92

E l e c t r o n N u c l e a r D o u b l e R e s o n a n c e (ENDOR)

97

3 4

4.1 4.2 4.3 4.4 5

97 100 113 117

A d v a n c e d ENDOR M e t h o d s

120

E N D O R - i n d u c e d ESR DOUBLE-ENDOR

120 125

5.1 5.2

6

S t a t i o n a r y ENDOR A n a l y s i s o f ENDOR S p e c t r a Experimental Aspects D y n a m i c a l E f f e c t s , Defect R e a c t i o n s a n d E n e r g y L e v e l s s t u d i e d b y ENDOR

O p t i c a l l y d e t e c t e d ENDOR

130

Contents

X

7

CHAPTER

5

Conclusions

133

References

134

Inorganic and Organometallic Radicals and Clusters p r e p a r e d i n a R o t a t i n g C r y o s t a t b y Metal Vapour Techniques By J . A . H o w a r d a n d B. M i l e

1

2

Introduction

136

1.1 1.2

136 137 137 139

3.2

3.3 3.4 4

141 141 142 143

Group 1 G r o u p 11 G r o u p 13 Pentamers and/or Septamers

143 144 145 145

Monoligand I n t e r m e d i a t e s

147

4.1 4.2 4.3 4.4

147 148 149 152 152 154 154 154 154 154 157 158 161 162 163 164 164 164

4.5 4.6

5

Group 1 G r o u p 11 O t h e r Atoms

Clusters 3.1

140 141

Atoms

2.1 2.2 2.3

3

P r e v i o u s Reviews and Books Experimental Procedures 1.2.1 T h e R o t a t i n g C r y o s t a t 1.2.2 Reaction Conditions 1.2.3 A n a l y s i s o f E.S.R. S p e c t r a o f M e t a l Atoms, C l u s t e r s a n d A d d u c t s

0

c8 Acetylenes Cyanides 4.4.1 HCN 4.4.2 A l k y l C y a n i d e s Alkyl Isocyanides Alkenes 4.6.1 G r o u p 13 4.6.1.1 E t h y l e n e 4.6.1.2 A1[CqHg] 4.6.1.3 H i g h e r A l k e n e s 4.6.1.4 B u t a - 1 , 3 - d i e n e 4.6.1.5 B e n z e n e 4.6.1.6 A l l e n e 4.6.2 G r o u p 11 4.6.2.1 A r e n e s 4.6.2.2 A l l e n e s

Diligand Intermediates

5.1 5.2

co Alkynes

5.2.1

Acetylenes

165 165 166 166

Contents

x1 6

7

T r i l i g a n d Complexes

167

6.1 6.2

167 168

CU(CO)~ Ag(CO)3

Tetraligand Intermediates

169

7.1 7.2

169 170

Co(CO)4 Rh(CO),

170

References

CHAPTER

6

Inorganic and Organometallic Radicals B y M a r t y n C.R.

1

2

3

4

Symons

Introduction

175

1.1 1.2

175 176

Books and R e v i e w s Techniques

Trapped and S o l v a t e d E l e c t r o n s

177

2.1 2.2 2.3

177 177 178

Models f o r E l e c t r o n s i n F l u i d s Electrons i n Fluids Electrons i n Solids

Atoms, M o n a t o m i c I o n s a n d R e l a t e d C e n t r e s

180

3.1 3.2

180 181

Muonium A t o m s Hydrogen Atoms

Diatomic R a d i c a l s and Radical-Ions

(AB)

183

4.1

Introduction

183

4.2

oZ1 C e n t r e s

183

4.3

(nxlnyl)

184

Centres

184 184

5

6

4.6

nX*l C e n t r e s

186

4.7

nx*2ny*1 C e n t r e s

186

T r i a t o m i c R a d i c a l s (AB2) a n d R e l a t e d S p e c i e s

188

5.1 5.2 5.3 5.4

188 188 188 189

Hydrogen Adducts L i n e a r A-A-B C e n t r e s Bent 17-Electron Centres 19-Electron Centres

Tetra-atomic 6.1

R a d i c a l s (AB3) a n d R e l a t e d S p e c i e s

Hydrogen C o n t a i n i n g C e n t r e s

189 189

Contents

xii 6.2 7

Penta-atomic Radicals ( A B 4 ) 7.1 7.2

8

9

11

12

and Related Species

Electron-Loss Centres Electron-Capture Centres

8.1 8.2 8.3 8.4 8.5

192 192 193 194 194

Introduction Carbon-Centred Radicals Nitrogen-Centred Radicals Oxygen-Centred Radicals Sulphur-Centred Radicals

Radicals in Inorganic Materials

195

9.1 9.2 9.3

195 195 196 196 196 197

Introduction Silica and Other Oxides Carbon, Silicon and Germanium 9.3.1 Diamond Silicon 9.3.2 III-V Compounds

The Use of Spin-Traps

198

10.1 10.2 10.3

198 198 198

Introduction Oxy-Radicals Other Inorganic Radicals

Metal Carbonyls and Related Species

199

11.1 11.2 11.3

199 199 199

Introduction Silver Carbonyls Chromium Carbonyls

Gas Phase Radicals and Ions

200

12.1 12.2

200

Introduction The Hydrogen Molecular Ion in its Dissociation Limit E.s.r. Studies LMR Studies 12.4.1 Atoms and Diatomic Radicals 12.4.2 Triatomic Radicals The Azide Radical 12.4.3

References

7

190 191 192

12.3 12.4

CHAPTER

190

Other Radicals

9.4

10

189

25-Electron Centres

200 201 202 202 202 202 204

Metalloproteins By G.R.

H a n s o n a n d G.L.

Wilson

1

Introduction

209

2

Copper Proteins

209

2.1

209

Type 1 Copper Proteins

...

Contents

Xlll

2.2

Type 2 Copper Proteins

210

2.3

Multi-centred Copper Proteins

211

Iron Proteins

213

3.1

Non-Heme Iron Proteins

213

3.2

Heme Iron Proteins

219

Iron Sulfur Proteins 4.1

[Fe - S 4 ] ( 3 t 9 2 t )

4.2

[ Fe2 - S 2 ] ( 2 + 9 1 t )

4.3

[Feg - S 3 / 4 1 ( 3 + J + ; 1 + 9 0 ) Proteins

4.4 5

6

223

Cluster Containing Proteins Cluster Containing Proteins

[Fe4 - S 4 ] ( 3 + , 2 + ; 2 + 9 1 + ) Proteins

224 2 24

Cluster Containing 224

Cluster Containing 225

Hydrogenase and Other Nickel Containing Enzymes

226

5.1

Nickel Iron-Sulfur Hydrogenases

226

5.2

Iron-Sulfur Hydrogenases

228

5.3

Nickel Enzymes

228

Molybdenum Enzymes

229

6.1

Molybdenum Iron Cluster Enzymes

229

6.2

Mononuclear Oxomolybdenum Enzymes

230

7

Vanadium Enzymes

232

8

Cobalt Enzymes

233

9

Manganese Enzymes

234

Paramagnetic Metal Substituted Enzymes

234

10.1

Substituted Zinc Enzymes

234

10.2

Other Metallosubstituted Enzymes

235

10.3

Extrinsic Metal Binding Site in Biological Molecules

237

10

11

12

Mitochondria1 Enzymes

238

11.1

Succinate Dehydrogenase

239

11.2

Cytochrome Oxidase

239

Photosynthetic Enzymes

241

12.1

243

Photosystem I

Contents

xiv 12.2

P h o t o s y s t e m I1

249

References

CHAPTER

8

244

Complexes of Paramagnetic Metals with Paramagnetic Ligands B y S a n d r a S. E a t o n a n d G a r e t h R. E a t o n

1

Introduction

258

2

Complexes w i t h S p i n - l a b e l e d Ligands

259

3

2.1

Metals w i t h S = 1 / 2 2.1.1 C o p p e r (11) 2.1.2 Silver(I1) 2.1.3 Vanadyl 2.1.4 Low s p i n C o b a l t ( I 1 ) 2.1.5 Low s p i n I r o n ( I I 1 )

2.2 2.3 2.4 2.5

Metal Metal Metal Metal

Complexes w i t h N i t r o x y l R a d i c a l s C o o r d i n a t e d v i t h e N i t r o x y l Oxygen

263 263 263 264 264

Vanadium Manganese Cobalt Nickel Copper Lanthanides

264 265 266 266 266 268

Semiquinone Complexes

269

3.1 3.2 3.3 3.4 3.5 3.6 4

S = 1, N i c k e l ( I 1 ) S = 3 / 2 , Chromium(III), h.s. C o b a l t ( I 1 ) S = 512, Manganese(II), h.s. I r o n ( I I 1 ) S = 7 / 2 , Gadolinium(II1)

259 260 262 262 262 263

4.1 4.2 4.3 4.4 4.5 4.6 4.7

Vanadium Chromium Manganese Iron Cobalt Nickel Copper

References

AUTHOR INDEX

270 270 271 27 1 271 272 272 274

279

Summarised C o n t e n t s o f Volume 11A CHAPTER 1

Organic Radicals i n Solution By B . J .

Tabner

1) I n t r o d u c t i o n ; 2 ) C a r b o n - c e n t r e d R a d i c a l s ; 3) N i t r o g e n c e n t r e d R a d i c a l s ; 4 ) O x y g e n - c e n t r e d R a d i c a l s ; 5) N i t r o x y Radicals; 6) S u l p h u r - c e n t r e d R a d i c a l s ; 7) R a d i c a l C a t i o n s ; 8) R a d i c a l A n i o n s ; 9 ) CIDEP; R e f e r e n c e s CHAPTER 2

T h e o r e t i c a l A s p e c t s o f E.S.R. By A . H u d s o n

1) I n t r o d u c t i o n ; 2) Numerical Methods and S p e c t r a l A n a l y s i s ; 3) S p i n R e l a x a t i o n a n d L i n e B r o a d e n i n g E f f e c t s ; 4 ) CIDEP a n d R e l a t e d P h e n o m e n a ; 5 ) P u l s e d E.S.R. S p e c t r o s c o p y ; 6) A p p l i c a t i o n s o f Quantum C h e m i s t r y ; R e f e r e n c e s CHAPTER 3

S p i n L a b e l s : B i o l o g i c a l Membranes By C h i n g - S a n L a i

1) I n t r o d u c t i o n ; 2 ) P r o t e i n s ; 3 ) N u c l e i c A c i d s ; 4 ) P r o p e r t i e s of Model and B i o l o g i c a l Membranes; 5) L i p i d P r o t e i n I n t e r a c t i o n ; 6 ) C e l l u l a r Membrane Dynamics; 7 ) M o d i f i c a t i o n o f M e mb r a n e F u n c t i o n s ; 8) M i s c e l l a n e o u s ; 9 ) Synthesis; References CHAPTER 4

Free Radical S t u d i e s i n Biology and Medicine By N.J.F.

Dodd

1) I n t r o d u c t i o n ; 2 ) T i s s u e s ; 3 ) R a d i a t i o n E f f e c t s i n B i o l o g i c a l M o l e c u l e s ; 4 ) R a d i c a l R e a c t i o n of Drugs and T o x i c Chemicals; 5) E n z y m e s ; 6 ) Oxygen R a d i c a l s ; 7 ) O t h e r Systems; References CHAPTER 5

E.S.R. o f t h e C o n f o r m a t i o n o f 5- a n d 6 - M e m b e r e d N i t r o x i d e (Aminoxyl) R a d i c a l s By A . R o c k e n b a u e r ,

M.

G y G r , H.O.

Cyclic

H a n k o v s z k y and K . H i d e g

1) I n t r o d u c t i o n ; 2 ) C o m p u t e r S i m u l a t i o n o f S p e c t r a ; 3 ) E.S.R. S p e c t r o s c o p i c Data f o r N i t r o x i d e R a d i c a l s ; 4 ) P y r a m i d a l o r Out-of-Plane D i s t o r t i o n of t h e C(C)NO Group; 5) Ring C o n f o r m a t i o n ; 6) C o n c l u s i o n ; R e f e r e n c e s AUTHOR INDEX

xv

1 ln Vivo Detection of Free Radical Metabolites by Spin Trapping BY R. P. MASON, K. R. MAPLES AND K. T. KNECHT 1 Introduction 1.1 The Problem.- In vivo detection of free radical metabolites is

a very challenging task that has only recently been undertaken. One reason for the late development of this area is that most biochemicals, as opposed to drugs and industrial chemicals, are not easily metabolized through free radical intermediates. In addition, detection of something as ephemeral as a free radical inside a whole animal is inherently not easy. Production rates of free radicals in animals are slow in comparison to chemical systems; therefore, the highest possible sensitivity is of paramount importance. Water, with its high dielectric constant, is the worst solvent for ESR spectroscopy in that only very small samples can be studied. This decreases the molar sensitivity of biological samples just when sensitivity is needed most. However, unless free radical metabolites can be demonstrated with a whole animal, there will always be some question as to their actual existence in biology. 1.2 Approaches.- Over the last thirty years, several approaches have been tried to circumvent the problem of working with aqueous samples. Freezing water lowers its dielectric constant so that larger samples can be studied. The freeze quench technique is useful for enzymes where solutions can be frozen in milliseconds.’ Frozen tissues, however, must be ground to fit into ESR sample tubes, and this leads to mechanically induced radicals or artifacts.2 In addition, the resulting powder spectra are poorly resolved, and their interpretation in complex biological systems is very difficult, if not impossible. Lyophilized, or freeze-dried tissue is plagued by the same problems of artifacts and poor resolution. r 4 Low-frequency ESR enables the study of larger samples, and perhaps even small animals could be studied directly, that is, with in vivo spectroscopy. Unfortunately, sensitivity is 1

2

Electron Spin Resonance

strongly dependent on frequency, and low-frequency instruments are unlikely to achieve the molar sensitivity of the standard X-band instruments. Spin trapping, in that it ideally integrates free radicals formed over time, appears to be the most attractive approach to the detection of free radicals in vivo. Since the concentration of naturally occurring radicals in body tissues is generally near the sensitivity limit of ESR spectroscopy, the spintrapping technique is not limited by background signals. 2 Spin Trappinq In Vivo 2.1 Spin Trappinq.- The technique of spin trapping involves the

addition of a reactive, primary free radical across the double bond of a diamagnetic compound, the spin trap, to form a more persistent, secondary free radical, the radical adduct. This technique allows the indirect detection of primary free radicals that cannot be directly observed by conventional ESR due to low steadystate concentrations or to very short radical relaxation times, which lead to very broad lines.5 To date, all in vivo spin trapping investigations have used the nitrone spin traps, phenyl-tert-butylnitrone (PBN), a-2,4,6-trimethoxy-PBN ((MeO)3 PBN) and 5,5-dimethyl-l-pyrroline Eoxide (DMPO). In most cases, radical adducts of nitrone spin traps produce six-line ESR spectra. The hyperfine splittings arise from the nitrogen and p-hydrogen of the spin trap rather than from atoms of the primary radical. Identification of the trapped radical species, therefore, depends on a careful comparison of hyperfine splitting values with those of reference nitroxides analyzed under exactly the same experimental conditions. If the radical is trapped at a nucleus with nonzero spin such as I4N, then additional hyperfine splittings occur which greatly facilitate the identification of the radical adduct. A very useful approach for the radical adducts of C- and O-centered free radicals is isotopic substitution at this center in the radical precursor with 13C or l70, respectively. Difficulties.- Production of a radical adduct stable enough to be detected in biological samples is a major difficulty, but other factors must also be considered. when using the spin-trapping technique in whole animals, spin traps may interfere with the 2.2

1: Detection of Free Radical Metabolites by Spin Trapping

3

experimental system by inhibiting or stimulating enzymes, or by producing toxicity. The latter possibility has not seemed to affect in vivo work to date, although this issue has not been directly addressed in detail. There are several good reviews of spin trapping,5-14 which address in vitro applications of this technique. For discussions of spin traps as enzyme inhibitors or enzyme substrates, or of more general problems of spin trapping such as artifacts, the reader is referred to these reviews. 3

Folch Extraction

3.1 Haloqenated Carbon-centered Radicals.- The first in vivo experiment was reported in 1979 when the spin trap PBN and carbon tetrachloride were given to a rat through a stomach tube.15 After 2 hours the liver was extracted with a mixture of methanol and chloroform (the Folch extraction for lipids), and the ESR spectrum of the chloroform layer was taken. The spectrum was assigned by comparing experimental hyperfine coupling constants with those of the PBN/'CC13 radical adduct generated in a microsoma1 system or by photolysis of carbon tetrachloride. More definitive identification of this species as the PBN/'CC13 radical adduct was later made using 13C carbon tetrachloride, which produces an additional splitting in the radical adduct spectrum.16'17 Carbon-centered radical adducts from other halogenated hydrocarbons have also been detected in the organic extracts of livers from treated animals. Of clinical interest have been studies with the volatile anesthetic halothane, which produces hepatitis in humans. Under hypoxic conditions, halothane produced both liver damage in phenobarbital-pretreated rats and free radicals, which were trapped by PBN and extracted from the liver.18-20 The trapped radical species has not yet been identified unambiguously, but reductive debromination is probably responsible for the reported carbon-centered free radical formation. Chloroform, iodoform, bromoform, and bromodichloromethane are other compounds that are converted to free radicals in vivo. 21 Administration of carbon tetrachloride and PBN to gerbils results in detectable free radical adducts in Folch extracts of liver, kidney, heart, lung, testis, brain, and blood, with signal intensities of spectra decreasing in the order given. In Folch extracts of liver, PBN/'CC13 was iden-

Electron Spin Resonance

4

tified, but no assignment of radical identity in other tissues could be made. 22 3.2 Lipid-derived Radicals.- Halocarbon free radical formation results in lipid peroxidation, and it is logical to expect that the spin adducts detected after administration of carbon tetrachloride, which do not exhibit 3C hyperfine coupling from 3CC14, are derived from carbon- or oxygen-centered lipid radicals. Such lipid-derived radical adducts have been detected in vitro by several investigators.l6 23-25 The extracted livers of rats treated with (CH30)3PBN and carbon tetrachloride yield a carbon tetrachloride-dependent, non-carbon tetrachloride-derived species assigned to a lipid-derived radical.25 However, O-demethylation of the spin trap precludes a simple explanation of these latter studies. Definitive identification of lipid radicals is inherently difficult, since I3C-labelled fatty acids are unavailable and chromatography or mass spectroscopy of these heterogeneous radical adducts is a formidable task. Lipid peroxidation is itself an important toxicological process, and lipid radical adducts may be the only evidence of initiating radical species that do not form stable adducts themselves. For instance, 3-methylindole is metabolized in vitro to form a PBN/nitrogen-centered free radical. In vivo, however, Folch extracts of lungs from goats treated with 3-methylindole and PBN yielded only a carbon-centered lipid radical adduct. Radical adduct detection varied inversely with in vivo levels of GSH, as was suggested with experiments with the GSH precursor cysteine and the GSH-depleting agent diethylmaleate.2 6 In a similar manner, carbon-centered lipid-derived radical adducts have been extracted from the brain, spleen, liver, and heart, but not the kidney, of rats dosed with PBN and exposed to a non-lethal burst of An ethanol and high fat diet has also been gamma-irradiati~n.~~ used to produce the (CH30)3PBN/lipid radical adduct in Folch extracts of hearts and livers from animals treated in vivo.28

'

'

3.3 Advantaqes and Limitations.- In all of the in vivo investigations of carbon-centered free radicals from halocarbons and lipids, the Folch (1:2 methano1:chloroform) extraction was used. The greatest advantage of the Folch extraction is that the radical adduct is removed from the high dielectric biological tissues,

1: Detection of Free Radical Metabolites by Spin Trapping

enabling the use of larger can be easily concentrated The greatest limitation of polar radical adducts such radicals can be detected. 4

5

sample volumes. In addition, the sample by evaporation of the chloroform layer. organic extraction is that only nonas those of 'CC13 and lipid-derived

Bioloqical Fluids

4.1 New Approach.- Our approach to in vivo spin trapping has been to examine biological fluids directly for spin adducts using the TMl10 ESR cavity and a 17 millimeter flat cell which gives the largest possible aqueous sample size in the active region of the cavity, about 100 pl. This approach gives us high molar sensitivity and high resolution. Oxygen solubility in water is only about 250 pM, so degassing of samples is not usually necessary, but sometimes will narrow the sharpest lines. No background signals other than the ascorbate semidione doublet and Mn2+ (only in bile) have been detected. The biological fluids are not extracted, and the difficult question of what happens during extraction other than a physical separation is avoided. The detection of free radical metabolites in urine, blood or bile is little different from the detection of the products of drug metabolism by HPLC as practiced in pharmacology departments or by the pharmaceutical industry.

4.2 Urine.- When this new approach was used with rats administered carbon tetrachloride and PBN, a novel radical adduct, PBN/'C02-, was detected in the urine of living rats treated with carbon tetrachloride and PBN.29 Use of 13C carbon tetrachloride proved that this radical adduct, like PBN/'CC13, was carbon tetrachloridederived. PBN/'C02 was also detected in the effluent perfusate of livers which were perfused with carbon tetrachloride and PBN, and in the urine of rats after administration of bromotrichloromethane and PBN. 30 Bromotrichloromethane is metabolized by the same pathway and to the same metabolites as carbon tetrachloride, but is more readily dehalogenated due to the relative weakness of the C-Br bond. 4.3 Perfusate.- Hepatocellular necrosis in perfused liver, as measured by LDH release from lysed cells, occurs after the infusion of CC14 or CBrC13 and follows the appearance of PBN/'C02-.30

6

Electron Spin Resonance

Perfusion of the liver with nitrogen-saturated instead of oxygensaturated buffer accelerates this LDH release. The concentration of PBN/'CO~- in perfusate at the beginning of lysis is statistically correlated with the amount of time required until LDH is detected.30 The higher the radical concentration the shorter the time to lysis. Correlation does not imply causation; therefore, the stable trapped product PBN/'C02- can only be considered a marker for some more reactive species actually responsible for membrana damage. Nevertheless, the detection of a radical adduct does imply production of reactive free radical. metabolites in vivo, and the correlation of radical adduct production with an index of toxicity is consistent with a free radical-mediated pathology. In these experiments, PBN (5 mM) did not appear to prevent membranedamaging free radical reactions from occurring, but this can be explained by the very low rates of radical trapping characteristic of PBN.31 The U . S . Food and Drug Administration and Environmental Protection Agency require companies to identify the urinary metabolites derived from drugs and pesticides. As the above studies suggest, such requirements may be extended to the spin-trapped products of free radical metabolism as well. Unlike the stable productF of detoxification, which can be detected by conventional analytical techniques, the detection of radical adducts proves the formation of highly reactive intermediates. Free radicals can clearly react as easily with cellular constituents as with spin traps and thus could cause damage in vivo. 4 . 4 Bile.- Both PBN/'CC13 and PBN/'C02- were detected in bile samples collected at multiple timepoints after treatment of living rats with PBN intraperitoneally and carbon tetrachloride intragastrically.3 2 In vivo manipulations such as low oxygen tension of inspired air or phenobarbital pretreatment, known to increase the toxicity of carbon tetrachloride, also produce qualitative and quantitative changes in the ESR signals detected. Either hypoxia or phenobarbital induction was required for the detection of PBN/'C02-. Both treatments also increased the biliary concentration of PBN/'CC13. In principle, ionic, polar, and nonpolar radical adducts can be detected in bile, because, in addition to the aqueous phase, the biliary micelles provide a hydrophobic environment.32

I : Detection of’ Free Radical Metabolites by Spin Trapping

7

4.5 Blood.- The reaction of oxyhemoglobin with phenylhydrazine and hydrazine-based drugs within red blood cells induces a series of processes which leads to destruction of the cell and results in hemolytic anemia. Considerable evidence obtained from in vitro ESR investigations implicates free radicals in the events contributing to red blood cell hemolysis. An immobilized radical adduct (aNZz = 31.8 G and aNzz = 9.5 G) is formed in the blood of rats which received an intraperitoneal injection of DMPO followed by an intragastric dose of phenylhydrazine. This immobilized radical adduct is detected when phenylhydrazine is administered at a dose of only 1 mg/kg. Hydrazine itself gives a weaker spectrum of the same species. The immobilized radical adduct co-chromatographs with oxyhemoglobin and can be detected in vitro using purified rat hemoglobin, phenylhydrazine, and DMPO. The sulfhydryl reagents, iodoacetamide, maleimide, and N-ethylmaleimide all inhibit phenylhydrazinedependent radical adduct formation when whole rat blood is treated -in vitro. This sulfhydryl-dependent radical adduct has been assigned to a DMPO/hemoglobin thiyl radical adduct. This is the first report of free radical formation from a biological macromolecule formed as a consequence of free radical metabolism. In addition, PBN could replace DMPO in vivo to yield the PBN/hemoglobin thiyl radical adduct, aNzz = 30.8 G.34 The DMPO/phenyl radical adduct was also detected in chloroform earacts of whole blood in accord with in vitro results.33 In subsequent work, the DMPO/hemoglobin thiyl radical adduct was detected in the blood of rats following the administration of some hydrazine-based drugs.34 The drugs examined were hydralazine, iproniazid, isoniazid, and phenelzine. Of the four drugs, only iproniazid and phenelzine were able to induce DMPO/hemoglobin thiyl radical adduct formation in vivo, whereas only hydralazine and phenelzine were able to form this adduct in vitro. The in vivo iproniazid-induced radical adduct formation was decreased by pretreating the rats with bis-para-nitrophenylphosphate, an arylamidase inhibitor. These results support the argument that iproniazid is hydrolyzed in the liver to a more reactive metabolite, most likely isopropyl hydrazine, which is subsequently released into the blood stream. In contrast, phenylhydrazine and phenelzine react directly with red blood cells to yield the DMPO/hemoglobin thiyl radical adduct. As

EIectron Spin Resonance

8

hydralazine did not yield this adduct in vivo, we proposed that hydralazine is metabolized in vivo into a less reactive compound, possibly via acetylation. In summary, a DMPO/hemoglobin thiyl radical adduct has been detected in vivo. This species is formed by the reaction of phenylhydrazine,3 3 ' 34 and some hydrazine-based drugs34 with oxyhemoglobin. 5 Conclusion

In conclusion, organic extraction is limited to the detection of non-polar radical adducts such as those of 'CC13 and lipid-derived radicals, whereas the ionic radical adducts of 'COz- and thiylhemoglobin can only be detected in biological fluids such as urine and blood, respectively. In principle, ionic, polar, and non-polar radical adducts can be detected in bile. Although to date most in vivo investigations of free radical metabolite formation have been limited to the halogenated hydrocarbons, enough additional examples are known to demonstrate that this technique may be widely applicable. If this is the case, then the detection and identification of free radical metabolites in vivo should be more useful to the understanding and even to the prediction of toxicities than are traditional analytical techniques, which are inherently limited to the detection of stable metabolites. In addition, both experience and chemical intuition would suggest that any free radicals detected in vivo would be of great toxicological significance. In contrast, the products of metabolism detectable by HPLC may be related to detoxification rather than to toxicity, but are not necessarily related to either.

9

1: Detection of Free Radical Metabolites by Spin Trapping 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

R.C.Bray, in 'Biological Magnetic Resonance', ed. L.J.Betliner and J. Reuben, Plenum, New York, 1980, Vol. 2, p. 45. J.E.Baker, C.C.Felix, G.N.Olinger, and B.Kalyanaraman, Proc. Natl. Acad. Sci. USA, 1988, 85, 2786. H.M.Swartz. in 'Biological Applications of Electron Spin Resonance', ed. __ H.M.Swartz, J.R.Bolton, and D.C.Borg, Wiley-Interscience, New York, 1972, p. 155. D.C.Borg, in 'Free Radicals in Biology', ed. W.A.Pryor, Academic Press, New York, 1976, Vol. I, p. 69. E.G.Janzen, in 'Free Radicals in Biology', ed. W.A.Pryor, Academic Press, New York, 1980, Vol. IV, p. 115. E.Finkelstein, G.M.Rosen, and E.J.Rauckman, Arch. Biochem. Biophys., 1980, 200, 1. =.Perkins, Advan. Phys. Org. Chem., 1980, 11, 1. G.R.Buettner, in 'Superoxide Dismutase', ed. L.W.Oberley, CRC Press, Florida, 1982, Vol. 11, p. 63. B.Kalyanaraman, in 'Rev. Biochem. Toxicol.', ed. E.Hodgson, J.R.Bend, and R.M.Philpot, Elsevier Biomedical, New York, 1982, Vol. 4, p. 73. R.P.Mason, in 'Free Radicals in Biology', ed. W.A.Pryor, Academic Press, New York, 1982, Vol. V, p . 161. R.P.Mason, in 'Spin Labeling in Pharmacology', ed. J.L.Holtzman, Academic Press, New York, 1984, p. 87. R.P.Mason and C.Mottley, in 'Electron Spin Resonance', ed. M.C.R.Symons, Royal Society of Chemistry, London, 1987, Vol. 10B, p. 185. M.J.Davies, Chem. Phys. Lipids, 1987, 54, 149. C.Mottley and R.P.Mason in 'Spin Labeling: Theory and Applications 111', ed. L.J.Berliner and J.Reuben, Plenum Publ., New York, 1989, in press. E.K.Lai, P.B.McCay, T.Noguchi, and K.-L.Fong, Biochem. Pharmacol., 1979, 28, 2231. J.L.Poyer, P.B.McCay, E.K.Lai, E.G.Janzen, and E.R.Davis, Biochem. Biophys. Res. Commun., 1980, 94, 1154. E.Albano, K.A.K.Lott, T.F.Slater, A.Stier, M.C.R.Symons, and A.Tomasi, Biochem. J., 1982, 204, 593. J.L.Poyer, P.B.McCay, C.C.Weddle, and P.E.Downs, Biochem. Pharmacol., 1981, 30, 1517. J.L.Plummer, A.L.J.Beckwith, F.N.Bastin, J.F.Adams, M.J.Cousins, and P.Hal1, Anesthesiology, 1982, 57, 160. K.Fujii, M.Morio, H.Kikuchi, S.Ishihara, M.Okida, and F.Ficor, Life Sci., 1984, 2, 463. A.Tomasi, E.Albano. F.Biasi, T.F.Slater, V.Vannini, and M.U.Dianzani, Chem.-Biol. Interactions, 1985, 55, 303. F.F.Ahmad, D.L.Cowen, A.Y.Sun, Life Sci., 1987, 5, 2469. B.Kalyanaraman, R.P.Mason, E.Perez-Reyes, C.F.Chignel1, C.R.Wolf, and R.M.Philpot, Biochem. Biophys. Res. C o m m m , 1979, E, 1065. A.Tomasi, S.Billing, A.Garner, T.F.Slater, E.Albano, Chem.-Biol. Interact., 1983, 46, 353. P.B.McCay, E.K.Lai, J.L.Poyer, C.M.DuBose, and E.G.Janzen, J. Biol. Chern., 1984, 9, 2135. S.Kubow, E.G.Janzen, and T.M.Bray, J.Biol. Chem., 1984, 9, 4447. E.K.Lai, C.Crossley, R.Sridhar, H.P.Misra, E.G.Janzen, and P.B.McCay, Arch. Biochem. Biophys., 1986, 244, 156. L.A.Reinke, E.K.Lai, C.M.Dubose, and P.B.McCay, Proc. Natl. Acad. Sci. USA, 1987, 86, 9223. H.D.Connor, R.G.Thurman, M.D.Galizi, and R.P.Mason, J. Biol. Chem., 1986, 261, 4542. L.B.LaCagnin, H.D.Connor, R.P.Mason, an(iR.G.Thurman, Mol. Pharmacol., 1988, 22, 351. R.G.Gasanov and R.Kh.Freidlina, Russian Chem. Rev., 1987, 264.

x,

10 32 33 34

Electron Spin Resonance K.T.Knecht and R.P.Mason, Drug Metab. and Dispos., 1988, in press. K.R.Maples, S.J.Jordan, and R.P.Mason, Mol. Pharmacol., 1988, 33, 344. K.R.Maples, S.J.Jordan, and R.P.Mason, Drug Metab. and Dispos., 1988, in press.

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

-1 Introduction ~

-

~

This topic is normally covered in the volumes of this series devoted to organic and bio-organic subjects, most recently in Volume 1 1 A * . The transfer t o the inorganic and bio-inorganic series should enable me to cover 'Triplets and Biradicals' in Volume 12A. A s a result the present chapter covers the period from early 1987, rather than the normal two years. The impact of computers on e.5.r.

has been as marked a s in

other areas of human endeavour. One result of the Information Technology Revolution is that those of us who thought that EPR stood for electron paramagnetic resonance

, are

now also familiar

with the East Pacific Rise, ethylene propylene rubber, epoxy-phenolic resins, and electrochemical potentiokinetic reactivation. ESR sometimes standi for erythrocyte setting rate, edge-stabilised ribbon, or experimental storage ring. It was, nevertheless, disappointing to find that a paper on 'The EPR Experiment

-

Thoughts about the "Loophole" ' involved Einstein,

Podol sky, and Rosen. There have been several attempts in recent years to develop a computer program which will objectively analyse a complex and noisy solution e.6.r.

spectrum and I shall survey recent progress in a

later saction. For those involved in teaching e.s.r.,

the

traditional approach t o spectrum interpretation is well described in an article in the 'Journal of Chemical Education'2. Another article3 in the same journal describes how it is possible to integrate the teaching of Huckel theory and e.5.r

in a spectroscopy

course. Computer programs are used to calculate the HMO coefficients and simulate e.s.r.

spectra.

Other recent review articles of relevance to this chapter cover flash photolysis e = s = r = * ,forbidden hyperf ine transitions in metal i onss, 1 ong-range intramolecular el ectron-electron exchange interactions*, spin-polari sed triplets in 1 i quid crystal s 7 , the 11

12

Electron Spin Resonance

use of electron spin-echo spectroscopy in studying photoexcited tripletsa, the time-resolved e-5.r.

of intersystem crossing and

energy transfer processes9, and electron relaxationlo. The concept of the spin Hamiltonian ha5 been the subject of an extensive and critical reviewla. The constraints imposed on spin-Hamiltonian parameters by crystallographic site symmetry have been presented a5 a set of tables and are particularly useful for high-spin ions and/or low-symmetry sitesl2. & method ha5 been given for extracting the zero-field splitting tensor for an S = 3/2 centre from rotation data1=.

2 gpplications of Ruantum chemistry A feature of the period under review has been the large

number of papers dealing with clusters and defect centres in crystals. Extensive studies have been made of the ground-state potential energy surfaces of trimeric species containing Cu, Ag and 6U".

Ab initio calculations predictlS a 'Al ground state +or

A13 in disagreement with a matrix isolation e.5.r.

favoured a spin-quartet state.

study which

a general approach, based on &a

initio SCF calculations on large atomic clusters, ha5 been described for modeling point defects in quartzl4. Ab initio results have been presented for defect-pair complexes in silicon17. Multiple scattering X f f results have been employed t o investigate the superfine interactions of a Si dangling-bond defect at the Si02/Si(111) interfacelo and t o model deep levels in

Si :Pd19. Unrestricted Hartree Fock (UHF) calculations have been made for nitrogen in diamond20 and restricted Hartree Fock (RHF) calculations for a radiation induced defect in paratellurite21. 6 pseudopotential method has applied to impurity states in Si-Ge CNDO has been used t o study the structure of adsorbed oxygen species on a titanium dioxide catalysta3 and of vanadium containing heteropol y aci dsze. Quite large molecular systems are now accessible t o ab initio SCF/CI calculations.

It has been shown for example that the

unpaired electron in a diaquatetrakis(carboxylato)dirhodium(l+) complex occupies a Rh-Rh Ic orbitalzo. Large scale calculations have been used t o investigate the energy hypersurface for bridging in 8-haloethyl radicalszb. The results are in general agreement with e.s.r.

data. UHF SCF methods have been applied to methoxyl

substituted semi q u i n o n e ~and ~ ~ methyl benzoate radical ani onszo.

2: Theoretical Aspects of E.S.R.

13

Optimised geometries have been obtained for RSSH anion radicalszr and the ammonia-boryl radical which has a non-planar radical centre on boronsu. Numerical wavefunctions have been used to calculate isotropic and anisotropic hyperfine splitting constants for BeH, MgH, CN, and CP31. Amongst semi-empirical methods, MNDO ha5 been used t o show that the preferred structure of C2H2Sz+involves a 4-membered ringsz, to study the structures of dithiadiazolyl and trithiadiazolylium free radicals33 and mono- and dithiosemidione radical anions3',

and to calculate coupling constants for the

cubane radical Cationse'.In contrast

tc3

the MNDO method, UHF/AMl

calculations give a good account of twisting in alkyl-substituted olefin cation radicalss'. The INDO method ha5 been used to assign a structure to a radical in irradiated cellu10ses7, to study rotational isomers of the butane radical cations8, the radical anion of hexachloroethanes', and radicals derived from y-lactone~'~. It would appear that molecular geometries, obtained by optimising the fit of INDO calculated coupling constants t o experiment, are superior to those obtained by energy minirni~ation~~. It is a feature of many recent papers that MNDO and/or ab initio methods are used t o obtain a molecular geometry by energy minimisation. The spin densities

for that geometry are then calculated by the INDO

method. There ha5 been continued interest in Jahn-Teller distortions of the benzene radical anion.', benzenes's,

the cations of substituted

and the anions of substi tut:ed tetra- and

pentaf1uorobenzenes4'.

The transient spectra of the latter were

recorded using optically detected (OD) e.5.r. Relativistic effects are expected to be important for radicals containing heavy elements and appropriate corrections have been made in a multiple scattering calculation on Ag(C0)S. estimated '07Ag and

The

hyperfine tensors are in good

agreement with experiment. The unpaired electron has about 40% silver Sp, characteras. 3 Spin Relaxation and Line Broadeninq Effects

Following previous practice, this section includes a survey of the use of spin probes in the study of colloids, liquid crystals, gels, glasses, and micelles, but not of biological

Electron Spin Resonance

14

applications. T h e investigation of molecular mobility in supercooled 1 i qui d s and gl a s s e s h a s been revi ewed*b. Previous1 y described correction procedures for inhomogeneous line broadening have been extended t o cover doxyl-labeled

a1 kyl chains47. A

method for determining diffusion coefficients from t h e concentration dependence of e.5.r.

linewidths h a 5 been illustrated

by a study of a nitroxide dissolved in di-butyl phthalate crystals+B'. A new probe for studying liquid crystals h a s been prepared by

labeling 4-butoxy-4'-acetylazobenzene

and used t o study t h e

molecular dynamics in t h e smectic A phasekT. An androstane nitroxide spin probe w a s used in a study of t h e smectic G phase of

N-(4-butoxybenzylidene)-4'-butylani1ineso. number of small probes h a s been

The alignment of a

measured in nematic MBBCIs*. T h e

s a m e host was employed in a n investigation of silver(I1) dithiocarbamate complexessz. T h e ordering of nitroxides in a liquid crystal host h a 5 a l s o been studiedJs using 2-mm

e.5.r.

ENDOR spectra have been reported for s o m e biradicalsB+, for

semiquinone radical ionssB, and for a cholertane spin probe dissolved in liquid crystalssb. Both isolated radicals and radical pairs have been detected in inclusion compounds of nitroxides in cyclodextrineB7. The

-

motion of t h e p-benzosemiquinone

radical anion is restricted in t h e

cavity of 3-cyclodextrin but it rotates freely in t h e B-

and

Y-cyclodextrin cavitiesBo. Heisenberg spin exchange r a t e s have been reported for nitroxides complexed with Y-cyclodextrinaT, It has been pointed out that t h e formation of nitroxide radical pairs in polymers can lead t o features that resemble t h e superimposed spectra of probes in different motional regimes. S o m e previously published papers need reassessment in t h e light of t h i s cautionary note"". The

mobility of four different spin probes in

isotactic polypropylene h a s been interpreted in terms of free volume theory*'. Anionic probes interact electrostatically with t h e amino end groups in nylons and can b e used t o monitor t h e mobility of t h e end groupsb2. T h e effect of water in t h i s system h a s also been investigatedA3. Other work on nylon includes t h e effects of hydrogen bonds on probe mobilityb* and a comparison of translational and rotational diffusion6a. Probes possessing an amide group similarly exhibit interactions with nylon films*". T h e glass transition temperature o f poly(viny1acetate)

h a s been

studied a s a function o f both temperature and pressure using small

2: Theoretical Aspects of E.S.R.

15

probes67. Motional behavi our above and be1 o w t h e gl a 5 5 transi ti o n temperature h a s also been monitored in crosslinked epoxy resins, bisphenol A polycarbonate, and poly300 The

yield in this type of reaction can also be influenced by using high microwave power at certain magnetic f ieldslzo.

5 Numerical Methods and Spectral Analysis

Electron Spin Resonance

18

An on-line computer is an integral part the modern e.s.r. spectrometer and this section will be mainly concerned with the application of new numerical methods, rather than routine applications. There have been further contihutions on the effects of "9-strain" on metal loprotein

the computer

analysis of the spectra of spin labels in membranes130, and the analysis of electron spin echo envelope modulation in doped c r y ~ t a l s ~and ~ l polycrystall ine ~ a m p l e 8 ~The ~ ~ .complex nature of systems for which it is now possible to analyse polycrystalline spectra is well illustrated by a study of rhenium(IV1 in titanium dioxidelJ3. The paramagnetic centre has S = 3/2, rhenium has two isotopes each with

I = 512, and the

interactions exhibit a large anisotropy giving spectral widths of "4500 G at X-band and y1Q500 I3 at El-band.

In its simplest form, the use of computers in spectral analysis involves the comparison of a computed spectrum with an experimental trace. The parameters are varied until a good visual fit is obtained. At a more sophisticated level the experimental trace is digitised and a least squares analysis is performed t o obtain a best fit. The Gauss-Newton method is not ideally suited to such a process and alternative numerical methods have been investigated. The Simplex algorithm has been used to fit quite complicated solution spectra with up t o 17 adjustable parameters including benztalpyrenyl-6-oxy which has 1 1 hyperfine splittings. The starting values are obtained by visual matching until at least the wings of the spectrum are reasonably well reproduced. The values are then refined by the Simplex algorithm13*. The procedure takes 0.5-2 h on the microcomputer used by Beckwith and Brumby (compatible with an IBM XT). The method has been applied in a second paper to determine the relative concentrations of radicals in a mixture133. It is thus an alternative t o double integration. The actual spectra are calculated using a fast Fourier Transform Method with allowance for modulation broadening. Corrections can also be made for baseline drift. EInother approach to the problem of finding a global minimum on a multidimensional surface is to use the Monte-Carlo method. KirstelS* has tackled a variety

of

problems including isotropic

spectra with constant linewidtha, spectra with asymmetric (ml-dependent) linewidths, spectra with alternating linewidths due to dynamic effects, and powder spectra. A s an example the solution

2: Theoretical Aspects of E.S.R.

19

spectrum of the l,9-dichlorophenalenyl

radical has been analysed

using proton couplings from the ENDDR spectrum as starting parameters. The chlorine splittinqs and linewidth were obtained by fitting the e.5.r.

spectrum. &symmetric linewidth variations, which

are important in spin probe studies, can be incorporated into the procedure by allowing the linewidth to vary quadratically with the nuclear spin quantum number. The use of either the Simplex or Monte Carlo methods as outlined above requires an initial analysis of the spectrum to provide starting values for the iterative process. Ideally these would be provided by a computer analysis of the spectrum. A first-order isotropic e.s.r.

spectrum with no linewidth variations

exhibits high symmetry and Luders has shown how this may be exploited to transform the spectrum and estimate the hyperfine splittings137. The spectrum consists of subspectra produced by successive splitting of the original e.s.r.

line. The spectrum is

"rolled up" on a cylinder with radius R and overlapping areas are added t o give the transformed spectrum.

If R is correctly chosen,

the overlap of the ends of the transformed trace is the same as the overlap of the subspectra in the original, and the transformed spectrum is symmetrical. R is then related to a coupling constant. This is a neat idea but is probably of limited applicability. The methods I have reviewed up to now have all been illustrated by application to relatively noise free spectra. The analysis of noisy spectra is another matter. The problem of image enhancement has received a great deal of attention in recent years. In particular, the maximum entropy method has been applied in a wide variety of situations including cleaning up n.m.r.

spectra.

Some initial investigations by Jacksonlyn, indicate that the

method is also likely to be of value in improving the presentation of e.5.r.

spectra. Examples are given of noise reduction,

resolution enhancement, and the successi ve eval uat i on of coup1 i ng constants using autocorrelation. The maximum entropy method is powerful, but the computational demands are high, and

those

contemplating it5 use should beware that they are not employing a "sledgehammer to crack a nut". A wide variety of numerical algorithms have been applied to the analysis of e.s.r.

spectra over

recent years and there is a need for a comparative study o f their efficiency and computational demands. Are they of widespead applicability or do they only work with selected examples?

Electron Spin Resonance

20 References

14.

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21

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Electron Spin Resonance M.J.

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23

3 Transition Metal Ions BY J. F. GIBSON

1 Introduction

"By the early 1970s the generally accepted view of e.s.r. was that the spin Hamiltonian and the various crystal and ligand field theories then current were adequate to cover most circumstances. But several new trends may be discerned since then, away from the determination of spin Hamiltonian parameters of metal complexes towards a deeper underlying concern about the origin of lineshapes and dynamical behaviour." So writes John Pilbrow in the introduction to his refreshing review, before embarking upon identifying these trends which include: the connection between field- and frequency-swept e.s.r. and that asymmg strain and linewidth anisotetric lines should be expected: ropy: the true origin of the 1/g factor and also the fact that it is not necessary to use it; the improved resolution of the parallel copper hyperfine lines at lower frequencies; computer simulation of e.s.r. line profiles; and the inadequacy of purely static models of the ligand field. The present author has picked out the same theme concerning several of these trends in a review of the application of e.s.r. to metalloenzymes, an area which is covered by others in this series.2 This report is a reflection of the physical and chemical transition metal e.s.r. literature during the period mid-1986 to mid-1988 and as will be seen does include some of the themes mentioned above. Here we mention books and reviews. Pulse methods are slow in coming to the fore and although the chemists are lagging behind the biologists in this area, some advances in chemistry are being made. Zeolites for example are an important source of experimentation to which ESEEM spectrometry is being applied. Larry Kevan has reviewed the new geometrical information about catalytically active surface systems3 including Cu2+ in A , X and Y zeolites; Ni+ produced by photoreduction of Ni2+, and Rh2+ in zeolites; and Pd' and Pd3+ in X zeolites. See also Shpiro e t a l . 24

3: Transition Metal Ions

for the last mentioned metal4 and Che and Bonneriot for more conventional applications to surfaces and catalytic oxide supports5. Volume 22 of Metal Ions in Biological Systems is devoted to the There application of resonance methods to paramagnetic species. is more there on ESEEM' but ENDOR7 and e.s.r. are also discussed. The emphasis is on the application to biological systems e.g. I 7 O linebroadening of manganese nucleotide spectra' but the chapters on the application of microwave power saturation methods' and the use of e.s.r. for studying solution equilibria" are more applied to low molecular weight complexes. The solution equilibria studied up till now have been rather simple systems which may be described by a single equilibrium constant but in fact, Harald Gampp tells us" that e.s.r. is well suited for studying complicated systems too, including those where minor species occur and where the individual spectra are strongly overlapping; this is an area which will probably see rapid progress in the future. Buckmaster has given an extensive review of e.s.r. in transition metal ions which includes many which we miss out here, namely the lanthanides and actinides' while Misra has reviewed the effects of paramagnetic host lattices on their transition metal ionsq2. E.s.r. is becoming popular in the geosciences13 and archeologyl'l as a dating technique covering the time range beyond the upper limitation of 50,000 years for radiocarbon (I4C1 dating. The principle is to detect and quantify paramagnetic defects that have been created by natural radiation or chemical reaction. Those applications which involve transition metals include for example the oxidation of iron and copper in biological materials; manganese(I1) dating of microfossils in the deep sea core which relates to time by a temperature dependent diffusion process (past temperatures may be assessed in such an environment); the with Y3+ in irradiated carbonates; and other association of C0:metals such as Ti3+ in connection with other centres in minerals such as quartz which are age dependent. A new technique to judge the time elapsed after bleeding, has been described. It relies upon the oxidation of Fe2+ in coagulated blood which gives first an axial g,, = 6 signal and later a rhombic geff = 4.3 signal and appears to be useful up to a time of about 800 hourst5. 2 Selected Topics Techniques.- Electron spin echo envelope modulation continues to grow in stature. The general theory of ESEEM including nuclear

25

26

Electron Spin Resonance

'

quadrupole interaction is reviewed. Approximation methods for inclusion of the nuclear quadrupole interaction whether smaller or comparable with the nuclear Zeeman interaction are included; exact analytical expressions and numerical diagonalization procedures are also given. Experimental applications to single crystal studies of copper-doped nickel dithiocarbamates and maleonitriledithiolates are extensively reviewed and the exact diagonalization method has been used for 27Al ( I = 5/21 nuclear quadInformation rupole interaction for copper exchanged zeolites. from powder samples is less readily retrievable but in cases where the quadrupole coupling is larger or comparable with the Zeeman interaction, which is often the case for 14N, some hyperfine and quadrupole information may be available. Flanagan et a l . have developed an orientation-selective ESEEM technique in which the variation of the ESEEM pattern with irradiation posIt should find wideition within the e.s.r. line is studied.'* spread application in the assignment of nuclear modulation effects, the analysis of quadrupole coupling constants and the Any species for determination of metal-ligand complex geometry. which a single interaction dominates the dispersion of the e.s.r. powder pattern is suitable but their example concerns myoglobin and determines the orientation of the imidazole ring relative to the principal axes of the low spin d 5 Fe3+ g tensor. Simulations of I4N ESEEM patterns for the S = $, I = 1 spin system with isotropic hyperfine structure, and the dependence of these patterns on quadrupole coupling, hyperfine coupling and the external field have also been theoretically treated. Experiments in which uniaxial compression , either with2' or without2' a simultaneous electric field , have been described: removal of heat using a flow system has allowed modulation ampli-

'

'

tudes as high as 4 mT22; the application of thermally detected e.s.r. to transition metal impurities in 111-V crystals has been reviewed23; and the observation of a 2-dimensional image of two tubes containing solutions of a copper complex and of a vanadyl complex has removed a previously perceived barrier to e.s.r. imaging of species with multiline spectra. 24 Theory.- In a paper which sets out the spatial equivalence but the magnetic inequivalence of the two 170 atoms in a C Z v metal superoxide, the spin-Hamiltonian tensor components have been derived entirely by using symmetry arguments and with no assumptions concerning the molecular electronic ground state. They have

3: Transition Metal Ions

been compared with those derived in terms of m.0. coefficients and found to be of the same form. The inequivalence arises in the non-collinearity o f the principal axes o f the I 7 O hyperfine tensors. 25 In another comparison with the conventional spinHamiltonian, this time of s = 3/2 or 5 / 2 ions in strong axial crystal fields, exact expressions for the resonance fields for hyperf ine transitions within the HS = I * ' / 2 > doublet were obtained, and found to agree well with numerical diagonalization of the appropriate spin-Hamiltonian matrix; the application was primarily to manganese-doped SrTi03. 26 In contrast, Rudowicz has given a critical review of the spin Hamiltonian concept. After first overviewing (the two approaches to the spin Hamiltonian, namely, the "derivational" one resulting in the microscopic spin Hamiltonian and the "constructional" one resulting in the generalized spin Hamiltonian, he details misconceptions concerning the relationships between the zero field splitting and the crystal field Hamiltonian on the one hand and with the spin-spin Hamiltonian on the other. The reviewer goes on to systematize the operators, the forms of the spin Hamiltonian parameters and the axis systems commonly used. General relations for conversion of the zero field splitting parameters of several operator notations to the Stevens notation ( 8 ; ) are given and several pitfalls in these notations, as well as errors in the literature are pointed out.27 The author's expertise in this area is well demonstrated by his papers on a generalized net charge compensation model in which the zero field splitting Hamiltonian for a charge-compensated e.s.r. defect centre is partitioned into two separate zero-field-splitting Hamiltonians, one for a non-locally compensated centre and another which describes the net effect of charge compensation. The e.s.r. data analysed were for Cr3+ in several A2MFq and A2MC14 crystals28 and for Cr3+ and Fe3+ in AMF3 crystals2' ( A is an alkali metal, M is an alkaline earth). The Slater d orbital is not always capable of yielding good spin-orbit fine structure and although the Watson s.c.f. d orbital can be made much better, good agreement with experiment is often still lacking. For these reasons, Zhao and his co-workers have introduced three approximate conditions concerned with the spin-orbit coupling coefficient and with overlap which have resulted in a rather successful though quite empirical theory to describe optical, magnetic and high pressure properties of crystals containing transition metal ions such as Cr3+, Mn2+, Fe2+,

27

28

Electron Spin Resonance

Co2+, Ni2+ and Cu2+. In this u-K-a model, a correlation between dipole moment, covalency and polarizability is proposed for a given type of crystal. Semi empirical relationships are proposed between the spin-Hamiltonian parameters and the ligand-field parameters of the ion; once such relationships are established, no further fitting parameters are needed. For recent publications see references 30, 31, 32, 33. Several other papers have started from the same theoretical standpoint. Thus the zerofield splitting of the ferrous ion in F ~ s ~ F ~ .34~ and H ~ Oin orthopyroxene (MgFe)Si03;35 g and D for Cr3+ in a-Li103;36 and g and A for Cu2+ in K2Cd( SO4) 2. 6H20 37 have all been predicted theoretically and compared favourably with experimental data. Magnetic and spectroscopic data for the complexes C U ( L H ) ~ C ~ ~ have been used to estimate the angular overlap parameters and hence metal-ligand distances (the LH are pyrazole derivatives characterized by various alkyl groups in the 3, 4 and 5 positions). The radial parts of the overlap integrals between the 36 orbitals on copper and the p orbitals of a number of coordinating atoms have been calculated and the set of transferable parameters thus obtained are recommended for use in interpreting experimental data for other complexes of copper(I1) with pyrazole derivatives which are r-donor ligands. 38 of spectra. Recent understanding of the nature of f ield-swept e. s .r . has led to the following simulation method. 39 The spin Hamiltonian is solved at various frequencies and the resonance frequency difference vo(B) as a function of 8 , determined at each point across a line whose centre, vo(B) is vc (the constant applied frequency). The detected absorption signal is then given by a summation over all angles and M I values of a function of g12, the powder averaged intensity factor due to anisotropy, of the lineshape fCvc - vo(B)l defined in frequency units, and of the linewidth in frequency units. This differs from the usual representation where the shape function is written in terms of magnetic field variables and must be divided by the g factor appropriate to each line centre.

Simulation

Jahn-Teller Effects.- The molecular and electronic structure of a Jahn-Teller ion surrounded by six equivalent ligands and having cubic symmetry may be described in terms of coupling between the doubly degenerate electronic Eg and vibrational c9 functions. Taking linear coupling terms only, leads to the well-known Mex-

3: Transition Metal Ions

ican hat potential surface whose perimeter becomes warped corresponding to three equivalent minima when higher-order coupling terms are included. At low temperature a six-coordinate complex may be frozen into one of these minima if random strain lowers any one minimum with respect to the other two. This gives rise to the static Jahn-Teller effect and to an e.s.r. spectrum that will be a superposition of the spectra having symmetry axes Parallel to I, x and y. Riley et have modified the potential surface to allow for the uniaxial strain in their lattice of interest, KZZn(Cu)F4 and further, have introduced a parameter r l which measures displacement in the presence of a harmonic angular variation from the equilibrium position in the "well" of the "Mexican hat" potential surface. n-dependent contributions to the energy are then calculated in terms of p and b, the warping and strain energies and of f the force constant of the angular vibration. With the proviso that the octahedral complex can be considered alone, i . e . uncoupled from the lattice which it resides in, a numerical approach to the calculation of the vibronic Hamiltonian is then given for this dynamic Jahn-Teller effect. The results for K2Zn(Cu)F4 are interpreted in terms of a predominantly d12 ground state for the [CuF6] 4- guest species with a small admixture of dX2-,2 caused by vibronic coupling. In lattices of lower symmetry, the x, y and z molecular axes are no longer symmetry related and the guest ion in each site has rhombic g and A tensors. In such a case, on warming, the parameters related to one molecular axis would be expected to remain essentially temperature independent while those for the other two axes would approach one another. Such behaviour was reported by Silver and Getz (SG) in their early work on the [Cu(H20)6I2+ moiety in the potassium Tutton salt4' assuming a temperature dependence given by a weighted average of the Boltzmann population distribution between the three levels. Riley et a l . (RHM)42 have re-examined this salt using, in marked contrast to exchange between different static distortions, a Boltzmann distribution over the vibronic energy levels of the potential surface, wherein vibronic coupling causes variable admixtures ( d Z 2 and dX2-,2) of the electronic ground states. In order to do this they extended the vibronic coupling model4' to allow the treatment of this six-coordinate copper(I1) complex to be considered as perturbed by an orthorhombic lattice strain. Their results do not invalidate the frequently used SG model, but do point the way in which the new approach might be better. The RHM calculations

29

30

Electron Spin Resonance

for example suggest that the SG model works very well for the potassium Tutton salt but increasingly less well for the Rb+, NH4+ and Cs+ salts. The basic reason for this is that the SG model depends on the vibronic wave functions being strongly localized in separate minima, and this cannot always be the case. At higher temperatures, thermal population of the higher, less localized levels allows rapid exchange away from the three wells, so that an isotropic signal is observed. Such an explanation has to account for the single been put forward by Bersuker et isotropic line observed in garnets containing octahedral Co2+ or Cu2+ or tetrahedral Ti3+ or Cr5+ impurities. De has shown, for another [ CUF6] 4- chromophore, this time in ZnTiF6.6H20 that powder studies can show whether a given distortion is static or dynamic. The distinction is made using linewidth studies of the high temperature isotropic spectrum which is seen in this system, as a function of temperature. An Orbach-dependent spin-lattice relaxation time was observed and related to an excited vibronic level above either (2A2 or 2A1) if the ground state is static or above 2E if dynamic.44 An interesting example of a transition from static to a partially dynamic Jahn-Teller distortion, which extends over a wide temperature range is to be found in the study of bis(methoxyacetato1 diaquocopper ( I1 1 .45 The geometry of the Ni06 chromophore of the host lattice is not a perfect octahedron because there are different oxygen donor ligands (methoxy, water and carboxylate in order of increasing ligand field strength). Doping with Cu2+ introduces a degenerate ground state however and a further (JahnTeller) distortion is then additionally expected for the Cu06 chromophore. The low temperature Cu-0 bond distances are derived after correction with respect to the Ni-0 bond lengths using a model which assumes orthorhombic strain proportional to bond strength. The geometric changes of the Cu2+ polyhedra are then related to the changing e.s.r. spectrum as a function of temperature on the assumption that the dynamic averaging process which takes place at the higher temperature is restricted to the plane containing the copper-methoxy and copper-water directions. Other transitions from the static to the dynamic state may be noted: the influence of the second coordination sphere in Na2Zn(Cu1DP2.7H20 (DP = p~ridine-2~6-dicarboxylate) ;46 the first coordination sphere in Zn(Cu)(hi~tidine)~ where displacement of water at high temperatures permits rotation of histidine molec-

3: Transition Metal Ions

ules;47 and a linear dependence of gll related to a slowing down of rotation of the tetramethylammonium ions (TM) in Cu(TMI2Br4. 48 Mixed Valence Complexes.- The first successful simulation of solution e.s.r. spectra of mixed valence transition metal complexes has been claimed by The general approach is to calculate the positions of the transition metal hyperfine lines which are expected when the unpaired electron is trapped on one of the metal ions, then calculate the spectrum for each combination of two nuclear spin states using modified Bloch equations originally derived to describe the n.m.r. lineshape for a jumping spin, and finally to add up the spectra for all possible combinations of the nuclear spin states. It applies to mixed-valence complexes in which the electron hops from one metal to another rather than to those in which the electron is static on one metal atom or delocalized over them all. In an application to binuclear systems with equivalent copper ions, typical seven-line published spectra which are asymmetrically broadened at low field were successfully simulated. From the simulations were derived the transition probabilities and activation energies appropriate to the hopping process from copper(1) to copper(I1). The same hopping electron theory was applied to e.s.r. simulations of mixed-valence heteropolyanions containing vanadium(1V) and vanadium(V) In this case both the 15-line and 22-line spectra, obtained when a heteropolyanion containing either two ( C P V ~ W ~ ~ O or ~ ~three I ~ - ()[P2v3w15062]9-) equivalent vanadium(I1) ions is reduced, were simulated. Successful simulations were also derived for the 36-line and 43-line spectra from the protonated V3 species ( [HP2V3W15062]8- and [HSiV3W9040]6-) in which the V06 octahedra are either edge- or corner-shared. Seven adjustable parameters were used to simulate the spectrum being: g; a ; and the three coefficients of a quadratic in m which describes the linewidth; plus p the electron hopping rate; and r a factor which varies between 1 corresponding to the trapped electron and 0.5 for complete delocalization. The isopolytungstate CWlo03214on the other hand after photoreduction yields a complex in which the unpaired electron is delocalized and to which such simulations would be inapplicable. A crystal containing [W1o032l5was studied at 77 K and yielded superhyperfine interaction from eight magnetically equivalent 'H atoms : these are the hydrogenbonding water protons bonded to terminal oxygen atoms at the eight W06 octahedral sites. The unpaired electron is clearly

31

32

Electron Spin Resonance

delocalized over these sites quite probably through four nearly linear W-0-W bridges in an orbital embracing four sets of O=W-0W=O multiple bonds.51 The first true mixed-valent rhodium(I1)-rhodium(II1) complex is claimed as one of two geometric isomers of [Rh2(ap)41+ (ap = 2-anilinopyridinate)52. Dirhodium (Rhz') complexes have now been identified in which the odd-electron spin density is equally I 1 split into a 1 :2 : 1 triplet1 , unequally distribd i ~ t r i b u t e d(~g ~ (gI1 split into a 1 : 1 : 1 : 1 doublet of doublets), and now, localized on only one (g,, split into a doublet). The axial polarization of the unpaired electron in this molecule appears to be enhanced by unequal equatorial and axial bonding on the two rhodium ions brought about by the nature of the bridging A spectrum previously assigned as [W(CN) (H20)12- has ligand. been reassigned recently as the mixed valence dimer [(CN)7W(V)CNW(1V) (CN)7]6-.55 [ LFeS2FeL]2- complexes (L = a phenolate thiolate chelate1 having oxygen and sulphur donors as their terminal ligands have On been prepared and studied by a range of physical methods.56 chemical reduction e.s . r. spectra corresponding to [LFeS2FeL] 3 are seen and may be compared with those of the reduced Rieske proteins. Phenolate is shown to be very effective in lowering g 2 and g3 by approximately the required amount and thereby provides good evidence to suggest that the low g values of the Rieske protein can result from oxygenous ligation even though nitrogenous coordination has already been indicated by ENDOR and ESEEM experiments in some cases. Mixed valence complexes of manganese are currently attracting considerable attention because such species have been observed by e.s.r. in the working enzymes of chloroplasts. The photosynthetic oxygen-evolving complex (OEC) requires four manganese ions to convert water to oxygen and exists in five states so -+ . . . s5 only one of which (S2) is known to be e.s.r. active. Br~dvig~~ has forwarded a single centre model to account for the observed multiline spectrum and the geff = 4.1 feature which is also seen. In this model the multiline pattern arises from a low-lying excited state of a cluster that has an S = 312 ground state; the geff = 4.1 signal arises from a perturbed conformation of the cluster. Recently, a proposal that a mononuclear manganese centre which is in an electron-transfer equilibrium with a mixedvalence dimer has appeared.58 However the idea of a monomer in equilibrium with a trimer is also valid and Li et a i . claim the

3: Transition Metal Ions

33

homogeneous mixed-valence manganese trimer with S = 3/2 gives the geff = 4 signal." The molecule is linear Mn(III)-Mn(II)-Mn(III) and centrosymmetric. No special relationship to the spectroscopically observed S2 state of the OEC is claimed however and quite rightly so since only when chloride is absent is the geff = 4.1 signal seen in the biological system; in the natural system, where chloride is present, the multiline spectrum of S2 is seen. A more realistic comparison may be made with the oxide-bridged tetranuclear manganese cluster reported by Bashkin et In this molecule a Mn(III)Mn(IV) pair ( S = 1/2) is thought to be coupled to a Mn(III)Mn(III) pair ( S = 0 ) to give an overall s = 1/2 state and a multiline spectrum somewhat reminiscent to that of S2. Two other, binuclear, systems may also serve as reference compounds. 62 63 first which

'

Phase Transitions.- Many types of transition, for example structural, cooperative Jahn-Teller, ferroelectric, ferroelastic, magnetic, liquid crystal, superconducting, occur as a function of some perturbing influence such as temperature or pressure. The current literature reflects widespread use of e.s.r. to monitor such transitions, the majority of which are structural in nature. They include changes of crystal morphology ,64 65 66 67 commensurate to incommensurate, 68 tetrahedral to octahedral ions , 69 ferrodistortive to anti-f errodistortive , 70 and several changes in hydrogen bonding, 6 4 i 7 1i 7 2 group rotations47i48i73i74and transla t i o n ~ . ~ ~ The , ~subject ~ has been reviewed.75 76 Various aspects of the e.s.r. method have been utilized to monitor the transition which takes place. These include: changes in linew i d t h ; 4 7 i 4 8 i 7 7 i 7 8 i 7 9 changes in anisotropy or in the g tensor,47,4',69,70,'0,81 appearance of new lines;44,77,7','2 variations in intensity; 6 6 1 8 3 changes in fine structure73174 or hyperfine structure; 6 4 1 6 5 171 and where such changes are too small to be reliable, hyperfine shifts seen by ENDOR.67 The great majority of applications rely on doping a diamagnetic lattice with a paramagnetic ion though in a few cases a pure paramagnetic lattice may be studied4" 6 9 17 0 1 8 5 and occasionally irradiation techniques are used to create the centre which then acts as a probe to the diamagnetic host lattice. 7 1 7 2 r 8 6 With doping, the question arises as to whether the dopant truly responds to the driving force of a structural transition in a host crystal, since local distortions around the probe ion may prevent a perfect coupling to the host. The Fe3+ ions which substitute la4

34

Electron Spin Resonance

for Ti4+ in BaTi03 for example participate by at least one order of magnitude less in the collective Ti4+ motions, driven by the structural phase transitions. 6 5 Nevertheless dopants are frequently used and are almost invariably of half-integral spin or a multiple thereof (i.e. d', d 3 , d 5 or d 9 ) presumably because variations with temperature for such ions cause fewer complications due to changing relaxation behaviour than do whole-integer spins. We now give some detailed examples of the use of S = 1/2, 3/2 or 5 / 2 centres, in that order. The monohydrate of vanadium-doped zinc(I1)aquathiosemicarbazidediacetate displays e.s.r. spectra of two centres in which V02+ has displaced Zn2+, but which differ in the orientations of the V-0 bond. E.s.r. showed that one centre converted to the other as a function of temperature and that the hyperfine structure of both is split below 198 K indicating a reduction in symmetry from orthorhombic to monoclinic. Only one conformation was revealed by X-ray structural data however, and no phase transition. This apparent conflict was explained when it was learned that the process of irradiation by X-rays leads to hydrogen bond breaking which results also in only one impurity centre and no splitting of the hyperfine structure below Tc in the vanadiumdoped sample. 64 In a series of compounds in which the anion, [VO2F4I3-, is constant but which have cations of Na' and NH4+ in varying proportions, it is found that hydrogen bonding due to ammonium protons has a strong influence on the fluxional behaviour of the anions and on the structural phase transitions observed with varying temperature. The [V02F4l4- ion, produced in small quantities by y-irradiation, has made possible these observations since characteristic isotropic or anisotropic v4+ are observed sometimes at one and the same time. Thus phase transitions in CNH4I3 V02F4, NaCNH412V02F4 and CNH4I2VO2F3 are assigned to changes in this type of hydrogen bonding. In the compounds Ka2[NH4]V02F4 and Na3V02F4, on the other hand, the [V02F4]3- units are found to be freely reorienting and these compounds do not exhibit any phase transitions. 72 y-irradiation of [Moo4] 2--doped crystals of KH2POq produces the paramagnetic centre [Moo4] 3-. A quintet from four equivalent protons at high temperatures collapses to a triplet from only the two closest protons as the temperature is lowered but it occurs at a temperature above the ferroelectric phase transition Tc while the crystal is still in the ric phase. This appears to be because the proton

paraelectmotion is

3: Transition Metal Ions

35

frozen on the e.s.r. timescale even before the transition temperLiCsS04, similarly irradiated, has been ature Tc is reached. the host to [CrO4I3- whose spectrum, together with that of 'NH3 which was present at the same time, were followed in temperature and used to display a ferroelastic transition from the orthorhombic PCmn phase to the monoclinic 1 9 2 ~ / r phase 1 near 202 K.86 The structural changes of [NH4]2Pb[Cu(N02)6] during the phase transitions at 316, 287 and 9 5 K have been studied using a single crystal e.s.r. technique. The lowest temperature phase (IV) has been shown to be anti-ferrodistortive with two crystallographically non-equivalent [Cu (NO2)6]4- octahedra elongated along the [loo] and [OIO] crystal axes. The anisotropy changes from g , , >

'

to g,, < gL on passing into the intermediate phases I11 and I1 in which the crystal is known to be tetragonally compressed along the c axis. In the highest temperature phase (I), an isotropic g value was obtained corresponding to a three-dimensional dynamic Four Jahn-Teller distortion of the [ Cu (NO21 614- octahedra.70 phases were also shown to exist in [ C U ( T M ) ~ B ~ ~ ] (TM = tetramethylammonium) between room temperature and 230 K and were followed by abrupt changes in g factors and linewidth. An increase in linewidth as the temperature was lowered from 120 K downwards was attributed to a slowing down of rotation of the tetramethylammonium ions.48 In a comparison of several copper(I1) amino acid complexes either as frozen water solutions or when doped in the corresponding zinc(I1) complexes the e.s.r. spectra were closely similar to each other with the exception of the copper histidine complex where a strong g shift and copper hyperfine change was observed, and related to a phase transition at -2OOC. The anomalies were explained by a static stabilization of a square of nitrogens around copper(I1) in a host lattice for which coordination around zinc(I1) is tetrahedral. Water binds directly to the copper at low temperatures but its displacement at higher temperatures permits rotation of the histidine mole c u l e ~ .Earlier ~~ work which reported no phase change in copperdoped calcium cadmium acetate hexahydrate has been updated first by Misra and Kumara4 who found a reversible phase change at 130 K and subsequently by Sikdar and who reported instead two successive phase transitions at 133 and 128 K. The trigonal to monoclinic phase transition in ZnTiF6.6H20 has been studied in depth by De.66 An interesting observation which still requires an explanation is that the transition temperature is found to be gL

36

Electron Spin Resonance

lowered in the powdered material relative to the value found for the single crystal.4 4 Although the alums have been well studied, there are still some questions open, notably those related to the nature of the successive phase transitions and the structure of the low temperature phases. In the case of Cr3+ : KAl ( S O 4 ) 2. 12H20, the high agree with earlier temperature results of Bielecki et published work by others but their low temperature patterns between 4 . 2 K and the known phase transition at 5 8 K differ from those of others at 1.3 K and their conclusion is that there must Room tempbe another phase transition between 1.3 K and 4 . 2 K. erature e.s.r. spectra of Cr3+ in Rb2CdC14 (K2NiF4 type structure) indicate axial symmetry. Cooling caused an abrupt change to orthorhombic symmetry at 1 4 3 K. In contrast with Cr3+ in the corresponding caesium salt, this spectrum shows that in the rubidium salt, the e.s.r. spectrum represents a centre with nonlocal charge compensation when Cr3+ substitutes for Cd2+ in the tetragonally compressed octahedron of chloride ions. Similarly, strong delocalization occurs when chromium(II1) is doped into RbCdF3 (perovskite type structure). In this case the same type of structural transition was shown up by the rich I9F hyperfine structure which was seen as far away from the chromium(II1) as the third shell of I9F neighbours. ENDOR (both of the central nucleus and of the ligands) was found best to show up this phase transition clearly.67 An unusual d 3 ion , manganese (IV) was used, presumably for its identical charge, in substituting for titanium(IV1 to study the ferroelectric phase transition in the perovskite BaTi03.76 Both Mn2+ and Fe3+ substitute for Cd2+ in RbCdF3 which undergoes a structural phase transition from cubic to tetragonal symmetry at T , = 124 K. From their ENDOR study on the I9F nuclei in this lattice, Studzinski and Spaeth, in keeping with the warning earlier, conclude that the misfit between the probe ion and the substituted host ion should always be kept as small as possible in order to prevent the influence of a lattice relaxation about the probe ion. They found this influence could be seen out to the third shell of neighbouring fluorines but nevertheless found, with Mn2+ as the dopant, that all of the important features of the phase transition could be detected (first order character, temperature dependence, relative strengths of intraand interplane forces1 . 65 The phase transition in tris-sarcosine calcium chloride crystals at 130 K has been a subject of many

3: Transition Metal Ions

37

investigations using techniques such as dielectric measurements, Raman scattering, infrared and e.s.r. spectra; several suggestions for the nature of the transition have been made. The most recent e.s.r. evidence has used a Mn2+ doping which shows that the dynamics of the ligand field are reflected sensitively by the fine structure tensor. This particular dopant exhibits a rather simple spectrum being constant in g and A but having an anisotropic D tensor which shows a significant change with temperature. A broadening above T c in the paraelectric phase becomes a splitting below T , from two opposite domains in the ferroelectric phase. The results were attributed to a changeover from displacive to order-disorder character in the transition region. 77 A similar splitting was seen in manganese-doped ammonium sulThe earlier work being fragmented and inconclusive, phate . 82 Misra and Korczak have reinvestigated manganese-doped langbeinite Cd2 ( N H 4 ) ( SO4 1 (abbreviated as CAS ) ?3 They have reported detailed Hamiltonian parameters for the room temperature spectrum and shown from its conversion to a more complex spectrum that the phase transition occurs at 9 4 . 5 K. The low temperature spectrum was too complex for analysis but a comparison with V02+-doped CAS suggests that the phase transition is the result of a slowing down and ultimate freezing of the groups. When the ammonium ions in this lattice are replaced by thallium we have Cd2T12(S04)3 (CTS). A phase transition at 9 4 . 5 K is again found but in contrast to CAS this lattice has two other transitions at higher temperatures. The room temperature Hamiltonian parameters '

only, were reported in detail. 7 4 S p i n Crossovers. In a study designed to see how chemical, structural and bonding effects can influence the spin crossover phen-

omenon a range of six-coordinate d 5 iron(II1) Schiff base complexes of type [Fe(X-~alen)(L)~]Yhas been synthesized and studied." X-salen is a ring-substituted I02N2) chelate; L is an Y = clo4-, BF4-, P F ~ - ,BPh4-. In imidazole or pyrazole base; general, the transition from a high-spin to a low-spin state is accompanied by a change from a broad spectrum with several features and shoulders (from 6A1) changing to a strong rhombic signal near g = 2 (from 2T2). Subtle variations in the components of the complex i.e. 102N2) from the chelate; IN2) from the base; and changes in the counter ion, can lead to dramatic changes in the spin state encompassing pure high spin, pure low spin, and high spin low spin. Clearly this combination of

+

Electron Spin Resonance

38

{02N41 donor atoms produces an overall ligand field close to

the

+ 1/2 crossover

point. The inter-conversion rates between s = 5 / 2 and s = 1/2 have also been studied in related molecules and shown to be very sensitive to small molecular and structural changes. 89 With X-salen = salophen 1X = N,N' -phenylenebis (salicylideniminato)} and NO in place of neutral base and anion, we have a new donor set {02N3) and an interesting change to a different crossover. Magnetic susceptibility indicates a change from S = 3/2 to s = 1/2 at 180 K as the temperature is lowered. The corresponding e.s.r. change is from a broad featureless line to a sharp axially symmetric spectrum. The molecular orbital treatment regards the low spin state as Fe(I1)NO with six electrons in the d shell of the metal and one electron in the antibonding my* orbital on NO. Thus the spin crossover consists of a transfer from a filled d X y orbital to a d,2 i.e. d y2 z d x z d x y y * 5/2

--+ d~zd~zd,y"y

"

?d z 2 .

There are only a few examples in the literature of changes produced in the stepwise formation of complexes and most of these involve iron(I1). What appears to be the first example with cobalt(I1) has recently been documented by Garcia-Espafia et a l . using violuric acid H3vi as the ligand. They demonstrated that when cobalt(I1) is titrated with this ligand no e.s.r. spectra were observed (because the high spin form relaxes too fast) until the green complex [Co(H,vi),]was formed when a low-spin cobalt(I1) spectrum was detected at 130 K. In these examples and in others, spin crossovers in iron(I1) or other first row transition metal ions with d4 to d7 electron configurations have been detected directly using some property of the metal ion undergoing the transition. However there is a growing number of applications of a method wherein a paramagnetic ion is doped into the host lattice hopefully to report on such a magnetic transition indirectly, as it does often quite readily, for structural phase transitions. The novelty of this method is claimed by Vreugdenhil et a 1 . 9 2 who doped copper into [Fe(NCS)2(btrl21.H2O (btr = 4,4'-bi-1,2,4-triazole). The transition in this compound occurs at 144.5 K and since the low-spin state is diamagnetic, the dopant gives a well-resolved e.s.r. spectrum below this temperature. Preliminary results indicate that manganese(I1)-doped [Fe(NCS)2(btr)21.H20 also shows an abrupt change at T , . Their work is not unique however because Ozarowski et a l . have used manganese(I1) to probe the high-spin low-spin 'transition of FeL2 (NCS) (L = 2,2 ' -bi-2-thiazoline). 93 In their exp-

6

3: Transition Metal Ions

eriment five groups of six lines became poorly resolved as the temperature was lowered until at 170 K the spectrum abruptly changed to one with sharp features. The manganese ion here is high-spin at all times and indicates the spin crossover behaviour of its host probably via the effect of the changing ferrous alternatively however, the paramagnetism on the manganese T I ; broadening could possibly arise from strain effects causing a distribution of zero-field interactions in the manganese(I1) complex throughout the crystal. An additional point of interest comes out of this paper in that it supplies the first case of two polymorphs of an iron(I1) compound in which spin-crossover behaviour is seen in only one of them. The behaviour described above was for polymorph A while in polymorph B the spectrum of only one molecular entity was observed at all temperatures and all orientations. Superconductors. Epoch-making discoveries of high T, superconducting oxides, the La-Ba-Cu-0 and Y-Ba-Cu-0 systems are giving rise to opportunities to reconsider the mechanism of high T, superconductivity. Now that an extremely high T, over 90 K has been obtained in the Y-Ba-Cu-0 system, electron pairing via nonphonon mechanisms seems likely and in several models, electron correlations are considered crucial for the mechanism. Experimentally it is found that samples which are sufficiently annealed show a weak paramagnetism but no e.s.r. signal while for samples on which the heat treatment was insufficient, several components of e.s.r. signals have been observed. Thus Kanoda e t a1.94 found a typical axial copper(I1) powder pattern in YZBaCu05 at temperaBelow this temptures from room temperature downwards to 7 0 K. erature the signal broadens, diminishes and becomes less anisotropic before disappearing abruptly at about 20 K. These observations were interpreted as an antiferromagnetic spin correlation with a transition temperature which is higher than that determined by the Faraday susceptibility technique (14 K) and less good mainly for reasons of errors in integration of a Lorentzian-type Other relineshape within too small a field range (100 mT). s e a r c h e r ~ ~ ~too , ~have ~ , ~shown ~ that there is a resonance associated with the green coloured insulator Y2BaCu05. The e.s.r. signals are typical of localized electrons on tetragonal (axially symmetric) cupric ions but they relate to a phase which is not responsible for the superconductivity and they are insensitive to T,. Typically the signal anisotropy collapses to a single line

39

40

Electron Spin Resonance

as the temperature is lowered and this then becomes weaker and The general features of vanishes at temperatures down to 10 K. this behaviour are agreed while the temperatures for the observed changes differ, presumably through slight variations in composition. Similar spectra are seen in the black-coloured superconducting formg5I YBa2Cu307-x at temperatures above T , but they are a factor of 3000 down in intensity because they are only due to itinerant d electrons of copper in this lattice. In fact a plot of e.s.r. intensity against temperature shows three transitions at 91 K, 78 K and 30 K with the paramagnetism from the Cu2+ ions only completely vanishing below 10 Kg8. The system appears to consist of a linear combination of superconducting and normal states below the high onset temperature of about 91 K and it is possible that some of the Cu2+ ions are converted into Cu' which has a metallic character. Another orthorhombic, type of localized Cu2+ signal appears as the number of oxygen vacancies increases on increasing x in the above formula though as expected, this situation is reversed on annealing in an oxygen atmosBelow T , this signal decays and more or less disappears suddenly at about 84 K owing to the Meissner effect in the superconducting state. Dopings with other metals abound e.9. Cr, Mn, Fe, Ni, Co or Zn in the superconductor E U B ~ ~ C U ~ 9O9 ~and - ~Cr , in YBa2Cu307-x95. E.s.r. is reported in these systems but at this stage it is thought likely that the signals arise from small amounts of an additional phase and may not be related to the superconducting mechanism.

here.'^

Glasses.- Glasses are a natural subject for e.s.r. study since transition metal compounds are commonly found as impurities in the parent oxides, fluorides etc. which make up the glass. Alternatively they may be deliberately doped in. Being present in small amounts they are magnetically diluted naturally and thus offer first, the means of recognizing which metal is present, second, how much and third, through an understanding of their local environment, the possibility to probe the structure of the glass. All the first row transition metals are commonly found in glasses with the emphasis being on chromium, manganese and iron. The subject, with particular reference to phosphate, fluorophosphate and fluoride glasses has been recently reviewed100. An important concept to grasp is that of variable environment which of necessity requires the use of a range of spin Hamiltonian

.

3: Transition Metal Ions

parameters to describe the presence of a given impurity. Thus the glass formed from CaF2-BaF2-A1F3 may be envisaged as formed by chains of octahedra with randomly distributed lengths and orientations, the average number of shared fluorine atoms being two, with the connection between the octahedra being either c i s or trans"'. The divalent ions are situated between these chains in order to create a three dimensional network of the glass. The e.s.r. spectra corroborate these assumptions. Manganese(I1) for example, being spherically symmetric, is a very good probe ion and must substitute for calcium or barium in this system since a distribution of fine structure parameters is observed, as expected from a large range of different coordinations. Copper(I1) is less good since its Jahn-Teller character governs the geometry of the occupied site. Nevertheless there is some variation of symmetry with this ion too indicating the occupation of calcium and barium sites provided the ion can adopt an environment with two long Cu-F bonds. Without doubt, chromium(II1) and iron(II1) occupy the aluminium(II1) sites i . e . within the chains. Small linewidths in these cases imply a fairly symmetrical environment and an unchanging coordination number i .e. 6. lo' Similar results have been found for the BaF2-InF3 system containing ZnF2' PbF2, YF3 or ThF4 additives. Manganese substitutes for Ba2+ in between the chain-forming [ InF6] 3- octahedra, while chromium substitutes for In3+ as indicated by the similarity between the glass spectrum and those of crystals containing octahedral [ InF6] 3- units. Io2 Fluorozirconate glasses have potential applications as materials for low-loss infrared fibres whose efficiency is a function of their impurity contents. E.s.r. has provided not only a measure of the Fe3+ and Mn2+ content of such glasses but also information about the glass structure. Io3 Thus for example, from the measured g values of 2.0, 4.28 and a broad signal stretching from 1 to 10, it was concluded that the bulk of the signal was due to Fe3+ sites with rhombic distortion as is required to incorporate Fe3+ at Zr4+ sites as [FeF6I3-. Vanadyl doping provided a partial confirmation of such octahedra by the observation of fluorine hyperfine structure consistent with a square of four fluoride ions in the equatorial plane. lo4 Tetragonally elongated Cu2+' tetragonally compressed Ti3+ and undistorted octahedra from Mn2+, Co2+ and Ni2+ in ZrF4 have also been seen. lo' Curiously, reducing conditions were necessary for the nickel-doped glass but Ni2+ was apparently, not reduced to Ni+, which with s = 1/2 would have been observable by e.s.r., even

41

Electron Spin Resonance

42

though

copper

for example gives no signal when reduced because In general, e.s.r. results were in good agreement with optical data for all of these ions. In a subject area where broad lines prevail, the new experimental technique of laser irradiation holds out some promise. For chromium(II1) for example the observed spreads in the 0 (and € 1 values were respectively 0.15 to 0.4 cm-‘ (0.3 to 0.8 cm-’) with. without irradiation and 0.2 to 0.4 cm-I (0.4 to 0.8 cm”) This selective line sharpening reveals a dependence of optical characteristics on site structure since those ions with the lower D and E values are removed from the e.s.r. spectrum by laser excitation. 100 Cu’ prevails.

Paramagnetic Ligand8.- Apart from molecules in. which the paramagnetic ligand NO is directly bound to a metal (these are dealt with in later sections), I select out two main groups of molecules whose paramagnetism originates at least in part, in the ligand. These are complexes which contain semiquinones or related molecules as ligands, and complexes which contain nitroxyls (spin labels) attached to a ligand. Before dealing with these, I mention the review which concerns itself with stable radicalcontaining complexing agents which could be useful for the e.s.r. determination of metalslo6 and a group of papers describing lowIn some of these, valent metals with strong II acceptor ligands. metal hyperfine structure is so small that the paramagnetic entity should be considerd a radical since its unpaired electron in others the unpaired electron is metalis ligand-based; lo7-11 centred. 12, l 3 In one of the latter, though the origin of the paramagnetism is unknown, the species responsible was shown to be a novel three-membered PtC2 metallacycle formed by sideways addition of a substituted acetylene to a platinum(I1) dihydride. l 3

’ ’



Semiquinones. In the period of this review several papers have described experiments with 1,2-benzoquinone (here denoted as Q ) ; its partially reduced form the semiquinone (SQ); or its fully reduced form, 1,2-benzenediol (denoted CAT for catechol). The molecules were substituted by tertiary butyl groups in either the 3,5 or the 3,6 positions. Two kinds of reaction were found to take place between planar cobalt (11) complexes and 3, 5-di-tBu-oquinone .I1 4

43

3: Transition Metal Ions

Co(1I)L + Q

Co(1II)LSQ

(1)

Co(I1)L + Q

[Co(III)L]+ + SQ'

(2)

Here Co(1I)L is a cobalt(I1) dithiolato complex and the reaction in equation (1) above is the first known example of an oxidative addition reaction of this type of complex. The simple electron transfer in the second equation was also observed but found to be thermodynamically less favourable. Such is not always the case however. When the reduced form of this quinone, namely 3 , 5-di-tBu-catechol, is condensed with ammonia, a new tridentate biquinone (denoted CAT-N-BQ) is formed. This ligand, when complexed with various divalent metal ions, binds in the ( - 1 ) form, (M(II)(CAT-N-BQ)2 (M = Ni, Cu, Fe, Zn, Cd, Mg). But intramolecular electron transfer does take place, and quite readily, with certain metals. Thus manganese(I1) yields Mn(1V)(CAT-N-SQI2 which is regarded as a spin-spin coupled system in which the d 3 ion of manganese(1V) is strongly coupled to two S = 1/2 semiquinone ligands to give finally a complex with one unpaired electron. The related complex with cobalt(I1) requires the transfer of only one electron to form Co(III)(CAT-N-BQ)(CATN-SQ). Here the only paramagnetic entity is the semiquinone CATN-SQ with S = 1/2 and the unpaired electron is, not surprizingly, strongly ligand-localized as indicated by the weak cobalt hyperfine structure. E.s.r. verifies that the unpaired electron in Mn(CAT-N-SQI2 is metal-localized. Similar behaviour was found in a related, but bidentate donor, containing the same quinone; two electrons were transferred from molybdenum, one from chromium. Reactions between the isomeric 3 , 6-di-tBu-o-quinone (Q) with V(COI6 'I7 or M o ( C O ) ~'I8 also have been followed by e.s.r. For example, the tris chelate of vanadium was considered as either [V(III) (SQ)3]o or [V(V) (CAT)2SQ]o. The latter was preferred, in keeping with the following reduction sequence

which places unpaired electrons on the metal since here was found to contain V4+ (not V02+) with one d,2-,2. The alternative sequence would have been e [V(SQ)3]O

_i)

e

[VCAT(SQ)2]-'

+

(3) the dianion electron in

e

[V(CAT)2SQ]-2

+

[V(CAT)3]-3 (4)

44

Electron Spin Resonance

but the electron delocalization on the ligands here is keeping with the observed spectra.

not

in

Paramagnetic metals have been coordinated to about 130 spin-labelled ligands: the majority of these have been to copper (11) I9-l2' but others, also with relatively slow electron spin relaxation rates have included silver ( 11 1 , I 2 O low-spin cobalt(I1) 120, vanadyl(1V) 120f12' and chromium(II1) 12' , 122. In some instances, coordination to the metal takes place via the oxygen of the nitroxyl group in which case ferro- or antiferromagnetic interaction is likely to be large. As an example, which incidentally indicates that e.s.r. can be a useful tool for examining the nitroxyl complexes of metals with an even number of unpaired electrons, take the tetraphenylporphyrin complex of manganese(II1) to which various nitroxyl radicals have been coordinated

Nitroxyls.

via the nitroxyl oxygen. 23 Antif erromagnetic coupling between the S = 2, Mn(II1) and the S = 1/2, nitroxyl states resulted in an s = 3/2 ground state with characteristic geff values of 2 and 4. Electron-electron spin-spin coupling between slowly relaxing inequivalent unpaired electrons results in AB splitting of the e.s.r. signa1,s. The interaction comprises two parts, dipolar coupling and exchange. The latter is a function of the bonding between the two paramagnetic centres while the dipolar contribution depends upon their internuclear distance. Fluid solution spectra for a series of spin-labelled porphyrins in which the metal to label distance was varied, provide examples of iron nitroxyl interaction ranging from weak to relatively strong. In the complexes with weak iron-nitroxyl interaction, the iron relaxation rate was sufficiently fast to collapse the spin-spin splitting of the nitroxyl signal. As the strength of the interaction increased, the nitroxyl line broadened and shifted downfield. For two of the complexes the iron-nitroxyl interaction was sufficiently strong to cause averaging of the iron 1-1/2> to I +I/2 > and the nitroxyl transitions. 120 In a further paper designed to investigate the effect of the metal relaxation rate, ( Ti1 1 , frozen solutions of one of these iron porphyrins was studied.'24 At 8 K the relaxation rates are slow and the spinspin splitting is resolved but it collapses as the temperature is raised due to increasing rates of relaxation of the high-spin iron(II1). The complex may be described as Fe(TPP)X where X is F, C1 or Br. The temperatures at which the splitting of the nitr-

3: Transition Metal Ions

oxyl signal was collapsed increased in the order X = Br < C1 < F. The iron relaxation rate therefore increases in the reverse order which is anticipated since T 1 - ’ is proportional to D and the zero field splitting is expected to increase in the order F < C1 < Br. It is possible, where spin-spin interaction is entirely dipolar, to calculate the distance between the two spins and it is always tempting to do this also when the coupling is weak, thus ignoring any effects of exchange. There are checks however which should be made before making distance measurements; these include measurement of the intensity of the half-field transitions and analysis of resolved splittings. A new method has recently been proposed for distinguishing between these two contributions to the spin-spin interaction.1 2 ’ The idea is that, because the pathway is important, an extra CH2 group between the metal ion and the nitroxyl radical is likely to cause a substantial decrease in the exchange interaction. The same group should cause only a small increase in the internuclear distance for interspin distances in the range of 0.7 to 1.0 nm and therefore cause only a small change in the dipolar interaction. Two derivatives of EDTA were therefore synthesized each containing an NO group. They differed by one CH2 group in the bonding pathway between the metal and the nitroxyl radical. Eight different paramagnetic metals were then examined bound to the EDTA ligands. Small differences between the two ligands when bound to copper(I1) indicated that the spin-spin interaction was primarily dipolar, while large differences for Ni(II1, Fe(III1, Mn(II), Gd(II1) and vanadyl were indicators that a substantial exchange interaction was being attenuated by the extra CH2 group. The key workers in this area, the interaction of spin labels with transition metals , have recently reviewed it. 25 Binuclear and Oligonuclear Complexes.- H o r n o b i n u c l e a r , C u - C u . Gatteschi has briefly reviewed small metal ion clusters and their e . s .r . spectra, 126 and the Florence group have continued their search to understand the mechanism for anisotropic exchange, usually using single crystals. 27 Thus the polycrystalline powder ][PP~ pz~ ] = pyrazolate spectra of [ C U , ( H ~ B ( ~ Z ) ~ ) ~ ( ~ Z ) ~ C ~where are characteristic of a triplet spin state but no reasonable fit of the spectra could be obtained assuming collinearity of g and 0. Single crystals were therefore studied and this non-collinearity was shown to be an indicator that exchange contributions to the anisotropic spin-spin interaction are operative. The zero

45

Electron Spin Resonance

46

field tensor D is generally written as the sum of two contributions Ddip and gex. The latter, the anisotropic exchange contribution, may be estimated from the experimentally observed D if D ~ can ~ be P calculated where the structure is known. This procedure was followed for the pyrazolato-bridged complex followed by further analysis which allowed the extraction of ferromagnetic contributions to J even though the overall exchange interaction is known to be quite strongly antiferromagnetic. The anisotropic exchange interactions were shown to be mainly determined by the interaction between the ground x y state on one ion with excited xz and/or y t states on the other in direct consequence of the presence of a bridging chloride ion. Bencini et a l . have made a similar study on a single crystal of the bis(pyridine N-oxide) bridged complex which appears to have the longest Cu-Cu distance so far studied for this type of The coupling is antiferromagnetic and while g Z z is molecule’28. is not. This is deduced to be the along the Cu-Cu direction, ,,D result of a finite Dex having a sizeable Dyz component and from it an estimate of the corresponding anisotropic contribution to J was made. D,, has usually been found parallel to ,g in similar complexes and its deviation here might be due to the relatively increasing importance of D y z which does not diminish so quickly with increasing internuclear distance. In other cases as D,, antiferromagnetic exchange was found to be transmitted via bridging c a r b o ~ y l a t e s ’ ~and ~ the o-hydroxy oxygens of a Schiff base ligand derived from salicylaldehyde and o-hydroxypropiophenone. 3’ Bridging methoxy groups on the other hand are responsible for ferromagnetic interaction in the dimeric anion [(dnpI2Cu(OMel2Cu(dnp1 212- where dnp = 2 , 4- or 2 , 6-dinitrophenol. Two magnetic centres were found according to the crystal structures adopted either side of the phase transition. J and D values as high as 1 3 0 cm-’ and 1.26 cm-’ respectively were observed and though D did not change markedly over the three similar compounds which were studied, J did and was found to be very sensitive to slight changes in structure. This was predicted by theoretical calculations which assume the magnitude of the isotropic exchange interaction in the ground state J,2-,2,,2-,2 to be again the sum of competing contributions of different signs.8 5 The Monash group continue to be active in this area bringing their expertise of computer simulation well to the fore in interpretation. Frozen solution triplet state spectra of Cu(LI2 where HL = 2-(2‘-



3: Transition Metal Ions

pyridylmethylenehydrazonomethy1)phenol show that ferromagnetic dimers are present at low temperature though monomers only are present in mobile solution. Linewidths are evaluated in this computation for the computed AH, = t l and t2 transitions and this makes possible a comparison of peak intensities within the AH, = 1 1 and 12 regions. A J value of about 30 cm-’ was assumed. The copper ions are found to be largely magnetically dipole-dipole coupled and internuclear distances calculated to be in close agreement to dimeric phthalocyanine species where the copper

atoms also are in a square of four nitrogens.13’ In many cases exchange terms are small due to large coppercopper separations. The effect of varying the distance has been nicely demonstrated in some interesting molecules in which two tetraaza macrocycles, each containing a metal ion are separated by bridges of different length. The strength of the magnetic dipolar interaction between the two centres was shown to decrease as the bridge was lengthened from -(CH2I2- to -CH2PhCH2- 132. Likewise when two copper(I1) ions reside in the two separated tetraazacyclotetradecane macrocyclic rings which comprise bicyclam they are sufficiently well separated to appear as monomers. 133 Weak interactions were seen in a few out of over 80 8diketone adducts , 3 4 in several carboxylato-bridged structures with apically bound antipyrinel 35 and in some elegant molecules formed by the Schiff base template condensation on various metal ions of 2,6-diacetylpyridine with the diamine 3,3’-diaminopropylamine. 1 3 6 This condensation led to macrocyclic complexes of the 28-membered ligand H2L1 which were then used in transmetallation reactions to form binuclear copper macrocycles. Cu2 (H2L11 (NCS)q for example was prepared in this way from the mononuclear The binuclear nature of the Sr (H2L1) ( BPh4) and copper acetate. products was shown by the presence of a seven-line hyperfine structure in the g,, region and also at half field though only an extremely small zero field splitting was seen. This was as expected from a purely dipolar interaction at the likely internuclear distance of 0.7-0.8 nm for a pair of copper(I1) ions. For two copper ions in a smaller macrocycle see next section. other metals. The same diacetylpyridine as above in L 1 has also been condensed on Pb(NCS) with l15-diaminopentane and 1,6-diaminohexane. 37 The resultant 24- or 26-membered macrocycle (L2 or L3 respectively) contained two lead atoms bridged by a single thiocyanate ion. Such macrocycles have been transmetalHornobinuclear,

47

48

Electron Spin Resonance

lated with Cu(I1) or Co(I1) to form binuclear complexes in which each metal ion is six-coordinate and connected by two thiocyanate A weak antiferromagnetic interaction was bridges to the other. observed in the smaller macrocycle containing two copper ions but virtually no interaction in the other dicopper macrocycle nor in either of the two dicobalt(I1) macrocycles which were low-spin and high-spin respectively. This is a curious result because even the larger macrocycle is likely to hold the metal ions no further apart than about 0.5 nm. Methylene-bridged complexes of manganese, cobalt and rhodium have been shown to undergo reversible one-electron processes to anions and cations at readily accessible potentials. E.s.r. of t[CpMn(C0)2]2(p-CH2))+ for example indicates, by its Il-line hyperfine structure, the interaction of two equivalent manganese ions required by delocalization in the Mn-C-Mn bridge. The anion 4[CpRh(CO)]2(u-CH2)]was similarly assigned but with less certainty from a frozen solution spectrum. 1 3 8 The claim is made for the electrochemical one-electron oxidation of a bis rhodium(1) dimer represented as [Rh2(dimen)2(dppm)212+, to the corresponding [Rh(I)Rh(II)]3+ species described as a d7-d8 radical. The e.s.r. of a frozen solution could be described as near-axially symmetric with a five-line hyperfine structure in the gL region and a triplet on g,,. Since dppm Cbis(diphenylphosphino)methane] contains two phosphorus atoms ( I = 1 /2), since Io3Rh also has I = 1/2, and since the complex is quite likely to display near-axial symmetry, several suggestions may be put forward to explain this spectrum other than that put forward by Boyd et a l . 13' who assign the quintet as due to interaction with four phosphorus atoms a n d to an unsplit line but ignore the likelihood of Io3Rh structure in this region. Further consideration of this spectrum seems called for. Bridges other than methylene have been used to connect these same two [ C P ( C O ) ~ M ~fragments. I These bridges include the ions CMe3COl-, Cl-adamantyl oxidel-, Cimidazolatel-, [Me3CC(CN)2]- and [HC(CNI2]-. Oxidation by O 2 or Pb02 leads again to eleven-line hyperfine structure from two equivalent manganese ions. Spin delocalized mixed-valence dimers of this sort should exhibit half the metal isotope coupling constant of the corresponding mononuclear systems, thereby leaving the total spectral width unchanged. This rule is obeyed for the last three bridges mentioned above even up to intermetal distances of about 0.8 nm, but not for the first two, the alkoxides. The special reduction of the hyperfine

3: Transition Metal Ions

structure for these is thought to arise, through the steric demands of the bridging oxygen, from weak metal-metal bonding i .e. the formation of a triangulo species.62 The interaction of MgMe2 with MOC14 (M = W or Mo) leads to the tetrahydroparamagnetic species [(Me4M0)2Mg(thf)4] (thf = furan)I4O. The e.s.r. spectrum of [(Me4Wo)2Mg(thf)4] is typically S = 1 / 2 and similar to that of the corresponding monomeric C(Me4WO)Li(thf)2J except that it appears twice over, being split by a long range magnetic dipole-dipole interaction. Hyperfine splitting due to a single 183W ( I = 1/21 nucleus is apparent on each feature. The X-ray crystallographic structure of the tungbridged sten compound shows square pyramidal [WOMe4]- units ( d l by magnesium which is in an octahedron of oxygen atoms: the tungstens are spaced at 0.748 nm. Spectra of the molybdenum species are more complicated due to narrower g anisotropy coupled with the more extensive hyperfine structure which is expected However the appearance of from 95M0 and 97M0 (both I = 5 / 2 1 . four dominant peaks in the spectra of both the magnesium and the lithium solvates correlates with the assumption that both molybdenum complexes are dimers in solution. The compounds Ni2(formI4 and Pd2(formI4 where form = formamidinato {[(P-CH~C~H~)N]~CH)have been prepared and characterized by several physical methods including X-ray crystallography. Their oxidation products [M2 (form)4]+ have been studied by e.s .r . and while the spectrum of the nickel complex is indicative, as expected, of axial symmetry, that of the palladium dimer was only a symmetric line. SCF-X,-SW calculations showed that in each case the half-occupied orbital is 6b1. This predicts 42% metal d,,, character for the nickel compound in agreement with the observed anisotropy and 30% for the palladium compound which does not agree with the observed single line. The suggestion is made therefore that the unpaired electron in [Pa2(form)4]+ is in fact not in 6b, as calculated but ligand-based in an orbital of very similar energy. 4 1 The synthesis of the simple dinuclear complex of chromium(II1) [ (H3N)3Cr(OH)3Cr(NH3)3]3+ has been achieved after efforts spanning many years. 142 The X-ray crystallographic structure shows the cation to be bridged by three hydroxyl groups as is the case for the well-known corresponding cobalt complex. The e.s.r. spectra of the chromium compound doped into the cobalt compound were studied as a powder down to 9 7 K and revealed that antiferromagnetic coupling leaves the s = 0 state lowest and that the

49

50

Electron Spin Resonance

e.s.r. signal comes from the quintet state, S = 2 corresponding to J = 122 cm-I in good agreement with the magnetic susceptibility data. Exchange-coupled Cu(I1)-Mn(I1) pairs are seen in manganese-doped [ c ~ ( p y O ) C l ~ ( M e ~ S 0and ) ] ~ are characteristic of a quintet ( S =2) state. At 7 7 K they appear as though doped in a diamagnetic host but as the temperature is raised the superimposed features of the pure copper-copper dimer become dominant. The principal components of 9 and D were estimated using the experimental spin Hamiltonian parameters, those of the pure compound and those assumed for manganese. 28 Large positivelycharged macrocycles would be expected to form insoluble 1:l precipitates with large negatively-charged macrocycles. Skorobogaty et a l . showed that this was indeed the case with tmtppa = N,N',N",N"'-tetramethyltetra-2,3-pyridine porphyrazine and tspc = 3,10,17,24-tetrasulphonatophthalocyanine and used e.s.r. on the transition metal complexes of these ions to seek for preferential packing in the salts and magnetic interactions between the metal Heterobinuclear.

ions. 43 Heteronuclear interactions were observed as broad e.s.r. spectral features in most combinations of Mtmtppa/M'tspc where M = Cu(II), Co(II), Ni(II), VO(1V) and M' = VO(II), Cu(II1, Ni(II), Co(I1) and probably arises from some form of preferential stacking of like macrocyclic units which allows magnetic interactions between the metal ion centres. It is interesting but not easy to explain that in three cases, no interaction between the dissimilar ions was seen. These were M,M' = VO(IV), Ni(I1); VO(IV1, Co(I1); and Co(II),Ni(II). Similarly, no Cu(II)-Ni(II) interaction was seen in the bicyclam which gave the Cu(I1)-Cu(I1) interaction described earlier, 1 3 3 nor would it in all likelihood in the smaller of the two macrocycles L2 and L3 mentioned in the previous section.137 The reason for this last statement is that when [Pb2L2l4+ is transmetallated with excess Ni(I1) , Fe(I1) or Mn(I1) no homobinuclear species are obtained; the heterobinuclear [PbML2I4+ complexes are formed instead. It seems likely that steric constraints forbid the entry of two transition metal ions in this macrocycle though one is allowed if accompanied by the sterically less-demanding Pb(I1) ion. Thus Pb(I1) is unlikely to be displaced by Cu (111 , theref ore the [CuNiL2] 4+ species is unlikely even to form. However the interesting possibility that Pb(I1) could be displaced by a sterically accommodating trans-

3: Transition Metal Ions

ition metal ion remains, and this ligand could be a source future heterodinuclear studies.

51

for

Oligonuclear. Trimeric clusters are the next simplest after dimers and Kokoszka et al. have found a nice system for their This is [ (WgAs0331 zCu3 (H20)2 ] 2 - which contains the study. 4 4 three copper(I1) ions in an isosceles triangle capped on either side by a large heteropolyanion which has the effect of creating magnetic dilution and minimizing inter-cluster interactions. The spin states expected are S = 3/2 and two s = 1/2 doublets. The presence of fine structure showed the quartet state to be occupied and D was found to be small indicating relatively weak intracluster interactions too. An unusual and unexpected lowfield line at about g = 10 which, below 130 K, first increases in intensity and then diminishes until it disappears at 10 K is not understood but is tentatively suggested as being interdoublet or doublet-to-quartet in origin. Some other triangular Cu3 clusters

and some Ni3 clusters have also been reported. 14’ Some linear trinuclear species based upon connected macrocycles and abbreviated as MnCuMn and NiCuNi have been synthesized and studied by magnetic susceptibility and e. s .r. techniques. 4 6 They have a high-spin multiplicity in the ground state and ferromagnetic-like behaviour in the low-temperature range because the two terminal ions are held together ferromagnetically as a result of antiferromagnetic coupling between each and the central ion. Thus the ground states for these (5/2)(1/2)(5/2) and (1)(1/2)(1) systems are S = 9/2 and S = 3/2 respectively. E.s.r. of the former is a single strong line assigned to the AMs = a 1 transitions with various shoulders assigned to the forbidden transitions AMs = a2, t3 and a4. The explanation for the spectrum of NiCuNi is more convincing since its asymmetry resembles the pattern seen for S = 3/2 when 1 0 1 > hv and the signal arises from the la1/2> Kramers doublet. Among the organometallics, a clever synthesis design has yielded a triangular MnFeFe cluster capped by a triply bridging NO group. On reduction, the mixed metal cluster is paramagnetic and e.s.r. provides good evidence that the unpaired electron occupies a non-degenerate HOMO which is primarily antibonding between the two iron atoms since only a single line is seen with evidence of hyperfine coupling from neither 55Mn nor I4N. 147 The cubane-like [Fe4S5(Cp)4Jn cluster has also been studied and shown to exist over the six oxidation states which range from n = -2 to

Electron Spin Resonance

52

+3. This molecule is related to the well known ferredoxin models except that one of the bridging sulphur atoms is replaced by S2.

X-band e.s.r. spectra have been seen for n = - 1 , +I and +3. They are all different, but all S = 1 / 2 presumably through antiferromagnetic coupling over the Fe4S5 core. The n = - 1 compound, like the ferredoxins needs temperatures below 77 K but the other two can be seen at higher temperatures because they relax more slowly. 48 In contrast , the tetrameric oxygen-bridged Cuq04 core of (Cu4L4) where L is a ligand derived from pyridoxal and aminoalcohols, the exchange coupling is ferromagnetic with S = 2 . 14' The magnetic interactions between local moments on transition metal ions in some structures arise from indirect exchange involving conduction electrons and cover quite large distances. Rhodium(I)bis-4,4'-diisocyanobiphenyl chloride polymers for example contain "graphite-like" two dimensional laminae which are stacked in such a way that the d,2 orbitals on the rhodium ions overlap with the expectation that the resultant delocalization of electrons gives rise to a one-dimensional metal. Surprizingly the substance is weakly paramagnetic and gives two e.s.r. signals. 50 Likewise copper phthalocyanine iodide is a molecular metal whose conducting stacks incorporate a I-dimensional array of copper(I1) ions strongly coupled to conduction electrons. Below 2 0 K the e.s.r. g value of the coupled system increases anomalously and linewidth measurements indicate the existence of a transition at 8 K when the signal broadens and becomes unobservable. This work has been reviewed. 52 3 s = 1/2

1

Configuration.- T e r v a l e n t T i t a n i u m , Z i r c o n i u m a n d H a f n i u m . Stabilization of tervalent group IV metals with cyclopentadienyl or substituted cyclopentadienyl is common practice as in (CpI2TiL. The chemistry seems to be with variations in the ligand L. = ]L~ -can act as a The hydrocarbyl dianion [ R C H - C ~ H ~ - C ~ H ~ - C H R bidentate ligand through the benzylic C-centres, and, by virtue of the flexibility associated with torsion along the biphenyl axis, in so doing it can accommodate a wide variation in metal size without major departure from tetrahedral geometry. After reduction by NaCCIOH8], the e.s.r. spectra of (CpI2ML, M = Ti, Zr, Hf show hyperfine structure from protons as well as the metals, but an absence of 3 1 P hfs in the presence of PMe3 showed that the tetrahedral geometry of the parent molecule had remained d

3: Transition Metal Ions

intact. The spectra showed that these metallacycles were all stable at -8OOC and that some of them were stable for days at room temperature. They were also invaluable for suggesting several reaction schemes using these ligands . 53 The nature of the Ti(II1) species formed on reaction of (CpI2TiCl2 with SPPh2H has also been deduced by its rich hyperfine coupling from 47Ti, 49Ti, 3 1 P and 'H, to be monomeric with a single phosphorus atom and ten protons. On this basis the formula Cp2Ti(III)SPPh2 was suggested; its formation presumably occurs via reductive elimination of (SPPh2)2 from the initially formed Ti") species. The paramagnetic species is seemingly reactive enough to extract a sulphur from any available source to form, as shown again by e.s.r., the dithiophosphinate (CpI2Ti(III)S2PPh2. Several similar reactions in which Ph was replaced by Cy; S by Se; and Ti by Zr or Hf were recorded but with these chalcogenide ligands, no corresponding redox chemistry was observed for Zr and Hf complexes. 154 The use of chelating amido ligands has enabled the preparation of a new range of ternary derivatives of electropositive transition metals including Ti(II1). Thus the e.s.r. of C12TiN(CH2CH2NEt2)2 confirms the presence of monomeric titanium(II1) which is presumably fivecoordinate and trigonal-bipyramidal; this was the structure adopted for C12VN(CH2CH2NEt2)2. 155 Vanadium. Although many areas of 0x0 vanadium(IV1 chemistry are well explored, there are relatively few structurally characterized compounds with sulphur donor ligands. It is thus appropriate that Collison e t a l . have reported the X-ray structural characterization of CPPh4]2[VO(mnt)2] (mnt = 1,2Quadrivalent

dicyanoethylene-1,2-dithiolate = maleonitriledithiolate) and its single crystal spectrum both as the pure salt and diluted in the isomorphous molybdenyl compound CPPh4]2[MoO(mnt)2]. The anion as

expected is essentially square pyramidal in shape. The detailed e.s.r. analyses for both crystals are extremely similar and reveal no surprizes. The magnetically dilute system yielded additionally a quadrupole term which was reduced (as was, by implication its electric field gradient also reduced) in the CVOS4I2- structure relative to that of a similar CV0O4l2- environment, in keeping with the expected increase in covalence from sulphur. 156 From the same school comes a similar , comprehensive and detailed study (would that such were the norm!) this time using 2-methylquinolin-8-olate ( = mquin) as the ligand. 157 The

53

54

Electron Spin Resonance

single-crystal e.s.r. spectra of [ V O ( m q ~ i n ) ~ both ] undiluted and diluted in [ G a C l ( m q ~ i n ) ~ are ] consistent with retention of C2 symmetry in the undiluted system. The ground state is a mixture of d X 2 - , 2 , d x y and d,2 in this point group and the metal Coefficients of the antibonding molecular orbitals were determined using an angular overlap approach from the combined e.s.r. and electronic spectra. Based only upon polycrystalline e.s.r. spectra together with electronic spectra, the standard semi-empirical evaluation of molecular orbital parameters, spin-orbit, dipolar and Fermicontact terms has been carried out for V02+ in several other lattices. These were potassium and ammonium oxalates, potassium hydrogen oxalate, strontium tartrate, cadmium ammonium sulphate and cadmium potassium sulphate. g value variation was insignificant among these compounds as expected since they only possess oxygen donors and therefore have similar electronegativities, crystal field splittings, n bonding effects etc. However variation in " A was significant possibly due to 3 d x y , 4s mixing in the lattices with lower symmetry but possibly also due to hydrogen bonding and its effect in lowering the energies of the electronic absorptions. 58 Among other molecular compounds for which the vanadium e.s.r. signal has been a useful tool we mention the heteropolyanion [ PV2M010040]5- which yields a spectrum either from V02+ or from V4+ after photoreduction;15' various phosphates and phosphonic esters;'60 and the vanadyl ion in porphyrinic structures in tar sandsq6' and asphaltenes (an ENDOR study 1 . 62 Turning now away from molecular compounds towards the other major area of interest, meaning doping into existing lattices, one important area is that of the zeolites. Simultaneous introduction of Cu(I1) and Cr(V) by solid phase reaction into the zeolite ZSM-5 leaves Cu2+ and Cr5+ initially occupying the cationic positions in a statistical distribution. Subsequent interaction with Cr03 and then with CuO causes displacement of most of the Cr5+ by Cu2+. If the subsequent interaction is with V205 rather than Cr03 then the same sequence leads to the displacement by Cu2+ of V4+ rather than Cr5+. However the Cu2+ may be expelled by reduction with hydrogen after which increasing the temperature to 800'C in a hydrogen atmosphere causes the cation positions to be occupied once more by V4+ ions.163 Similar experiments have been used to follow Mo(V). 164 E.s.r. is shown by such work as extremely powerful in following the migration of

3: Transition Metal Ions

55

metal ions from the outer surface acid sites where they are first trapped, into the internal channels of the zeolite. Furthermore the strong influence of oxygen adsorption on hyperfine structure is also a valuable indicator of site occupancy and removal therefrom, especially for Cr(V), Mo(V) and V(1V). Single crystals of the mineral wavellite A13(OH)3(P04)2.5H20 were shown, principally by determining the orientation of ,g and A,, of the most prominent centre which is present, to contain substituting for [A1OHI2+. In fact there are two crystallographically inequivalent aluminium ions each with two OH groups in their first coordination spheres but Vassilikou-Dova e t a l .

V02+

have shown that the substitution of V02+ has taken place with the shortest AlOH. Comparison has also been made with V02+ in zoisite where there were two high intensity centres, one of which again derives from V=O substituting for A1-OH, the other for Al0.165 The same group has applied its skills to the rare

mineral apophyllite, KCa4FSi8020.8H20 which has a somewhat unusual sheet structure in which two V4+ centres have been assigned previously as V02+ replacing Ca2+ and K+ respectively. On charge compensation and symmetry grounds, Vassilikou-Dova and Lehmann felt the latter replacement unlikely and therefore repeated the work, 166 but were unable to fault it significantly. The fact that in high-silica zeolites has been ref erred to in the previous section. 163 64 The e.s.r. of CrOQ- has been reported extensively in the literature; CrOz- readily substitutes for SO$- and ionizing radiation may be used to convert this into CrOi- together with SOG. If e.s.r. can be detected at room temperature the ground state is the singly occupied d X 2 - , 2 orbital, the centre possesses near-axial symmetry, and there is usually hyperfine interaction Two recent examples are the LiKS04 host in with nearby protons. which the unpaired electron couples with two 6Li atoms to give a 1 : 2:3 :2: 1 quintet; 1 6 7 and the LiCsS04 host in which interaction with a single proton is assumed.86 A more interesting paper on this subject has recently appeared describing [CrO4I3- in the Na2S04 host.168 Here we see that when the host crystal is grown at 4OoC, the centre which is formed on y-irradiation is the one described above i.e. d x 2 - y 2 and is unique since interacts with two H atoms is distinguishable at most orientations. This signal is denoted centre A, it is observable at room temperature and its Quinquevalent

cr(V)

and

Chromium,

Molybdenum and T u n g s t e n .

Mo(V) may substitute for the

cation

56

Electron Spin Resonance

molecular symmetry is lower than that of SO:in the host. If the crystal is grown at 8OoC however, the [CrO4I3- has a d Z 2 ground state with no proton hyperfine structure. E.s.r. in this species, centre B, is only seen at 77 K. The presence of the two hydrogens in centre A could presumably arise easily if the chromate ion was doped as HCrOq but the authors of this article prefer the idea that they could arise from water inclusion. The inversion of the ground state orbitals from z2(B) to x2-y2 ( A ) and the concomitant lowering of site symmetry in A are in all probability due to the electrostatic interaction between Cr5+ and these protons which force the Cr5+ to move away from its central tetrahedron. 6 8 position in the SO:Of biological interest are the Cr(V) centres in the oxochromium(V)tetraphenylporphyrin-p-nitrobenzoate complex and those which form when Cr(V1) reacts with certain sugars. Sharp e.s.r. lines showing 53Cr hyperfine structure and with g values In the former case the compound is of than 2.0 are seen. erest as a possible active intermediate for a cytochrome

less intP450

model because it converts benzene to cyclohexanol and cyclohexanone. 16' In the latter case the reaction between Cr(V1) and lactose yields an e.s.r. signal from Cr(V) which is similar to that seen when Cr(V1) reacts with milk and which then eventually decays to chromium(II1). The relevance of this observation is connected with the candidacy of Cr(V)/lactose intermediates as possible ultimate forms of carcinogenic chromium compounds. 70 The low A values (95M0, 97M0) and relatively high g values of the transient Mo(V) states that are produced during turnover of the enzyme xanthine oxidase, are usually ascribed to the presence of thiolate ligands coordinated on an 0x0-Mo(V) centre, but a terminal sulphido ligand has also been proposed. The first mononuclear Mo(V) complex with a terminal sulphido ligand, [HB(Me2C3N2H)31MoSC12, has now been synthesized and comparison of its e.s.r. with that of [HB(Me2C3N2H)3]M~OC12 shows that in this case g values have actually decreased and A values remained more or less constant on exchanging Mo=O for Mo=S. Three mechanisms whereby g might increase with such an exchange are discussed, and though these might be operative, the inescapable conclusion is that they cannot be the dominant factors, at least when comparing these two molecules. Some paramagnetic oxomethyl complexes of W(V) and Mo(V) have been studied. [(Me4WO)Li(thfl2l (thf = tetrahydrofuran) in frozen solution shows g anisotropy and hyperfine structure both from

3: Trunsition Metal Ions

a single Ia3W ( I = 1/21 nucleus and (probably) also from a single 'Li ( I = 3/21 nucleus. The complexes formed when lithium is replaced by magnesium and when tungsten is replaced by molybdenum are dimeric. 140 When CW(CN)a13- in aqueous solution is irradiated using the appropriate filters a gradual decay in the e.s.r. absorption leads mostly to the formation of diamagnetic [W(CNIaI4- but about 10% of the products are paramagnetic and based upon CW(CN),I2- in the form of its reaction products with added anion (C1-, Br- , Ng-, SCN- or [:Fe ( CN 1 3- 1 , dioxygen or water. The mixed-valence dimer C (CN)7W(V)CNW(IV) (CNI7l6- was also found to be present but [W(CN)7]2- itself was detected directly only on photolysis at 77 Sexivalent Manganese, T e c h n e t i u m and Rhenium. Vacancy complexes have been created by the doping of [MnO4I2- into KBr, RbBr and KI and the spin Hamiltonian parameters for these centres reported. 72 Some data have recently been published concerning the anions [TCNXqI- (X = halogen) and their relationship to similar Group VI and Group VII ions. For example, resolved superhyperfine structure due to a fifth halogen ligand has been observed on the features associated with g , , for the [CrOF5I2- and [ReOF5]- complexes. However it does not appear to have been observed for other systems particularly where chlorine and bromine are the halogens and [TcNX4]- we now learn is no exception.173 No superhyperfine structure was seen from chlorine, nor from nitrogen, nor from bromine trans to nitrogen but it was observed from the four

equatorial Br- ions and the best simulations were achieved assuming two pairs of two equivalent Br- ions along X and Y rather than models assuming four equivalent Br- ions and an isotropic interaction in the X Y plane. Thus it is proposed that [TcNX5l2dissociates completely in solution and that the position trans to N is left vacant or is coordinated by a solvent molecule as expected from the strong t r a n s effect of nitrogen. E.s.r. has been an invaluable tool in the characterization of paramagnetic rhenium organometallics. The tetragonal pyramidal ReOR4 compounds (R = mesityl, o-tolyl or o-methoxyphenyl) have been analysed by their mobile and frozen solution spectra at both X- and Q-bands17'l as have the tetrahedral Re02(mesityl)2 174 and Re02(xylyl)2 175 complexes which occupy a unique position among the known oxoorganorhenium species. In all cases, well-resolved spectra were greatly complicated particularly in the perpendic-

57

58

Electron Spin Resonance

ular region by off-axis transitions and by the presence of many small lines attributed to AMI = 1 or 2 as encountered in earlier rhenium(V1) , d 1 complexes. Such observations are very typical for rhenium spectra because of its very large rhenium hyperfine coupling constant. Computer fitting of the spectra showed marginally improved fits if a small rhombic component was introduced and some nuclear quadrupole interaction assumed, to account for the slightly uneven spacing of the perpendicular hyperfine lines. d5 Configuration.- A theoretical paper based upon earlier exit assumes perimental data for d5 complexes has been reported; the equivalence of a hole in d 6 , with an earlier d 1 energy level derivation. The new calculation obtains two parameters simultaneously namely k the orbital reduction factor and A the rhombicity (0 to 1/3).176

Univalent Chromium and Molybdenum. Hyperfine coupling in paramagnetic bis(arene) transition metal complexes has been studied extensively. Thus the 3d,2 + a l g direct spin delocalization mechanism accounts for the a-delocalization between the singly occupied Cr(3dz2) orbital and the six equivalent CH fragments of the q6-arene ring. Elschenbroich et a l . have cleverly tested this mechanism by perturbing the [Cr(ben~ene)~]+cation with a strap connecting the two rings together so that they tilt open as though hinged.177 They observed a 'H hyperfine structure corresponding to two sets of four equivalent ring protons which shows that the hyperfine coupling is sensitive to tilting. However g1 did not split into g, and gy so it would appear that the degeneracy of the unoccupied orbitals e2,, and elg is not significantly lifted by this experiment. It may be that this a-delocalization is reasonably well understood, but a question which has long remained unresolved is the mechanism of spin transfer to the methyl substituents in these complexes. Hyperconjugation is possibly important and since this effect is governed by the angle between the d,2 orbital and the CH bond Elschenbroich et al. have made further studies on molecules in which this angle is well defined e.g. by attachment of a four-membered ring to the edge of the benzene ring.178 They were able to apportion the methylene coupling constants of this cyclobutane into a conformation-dependent part and a conformation-independent part. The parameters when combined gave good agreement with the hyperfine coupling constant of a related molecule possessing a freely rotating

3: Transition Metal Ions

methyl group thus lending support to the original proposal of hyperconjugative spin transfer to explain the sign and magnitude of hydrogen coupling constants in such molecules. The same group has co-condensed arsabenzene with chromium to give a sandwich structure, which after oxidation by dioxygen, gave an e.s.r. signal typical of a bis ( q6-arene) chromium cation. This not too surprizingly indicates that the singly occupied HOMO in this particular radical cation bis ( r16-arsabenzene)chromium (I1 has high ligand character. 17' In addition to arenes, cyanide also is very good at stabilizing low oxidation states of Group VI metals. Magnetic moments and e.s.r. data of the mixed-ligand cyanonitrosyl complexes formulated as [Cr(NO)(CN)2L2H20] for example, (L = a substituted pyridine base) are consistent with a low-spin d 5 configuration''* as are some mixed carbonyl-isocyanide-phosphine complexes of molybdenum. The latter, formulated as [Mo (CO1 ( CNR 1 (PR ' 1 1' are 17-electron cations whose solution spectra reveal a rich hyperfine structure from the two active molybdenum isotopes and

*

from two equivalent 3 ' ~ nuclei. Bivalent Manganese, Tervalent Iron a n d Ruthenium. N-substituted p-phenylenediamines are well known for their ease of oxidation to free radicals of the Wurster's blue type, but they can also act as ligands. Gross and Kaim have studied the interaction of electron-rich molecules of this type with the dicarbonylcyclopentadienylmanganese(I1) fragment which is a good CI and n acceptor. 182 The e. s . r . spectra are characterised by reduced metal hyperfine structure and detectable ligand superhyperfine structure. Effectively, ligand to metal charge transfer is taking place in the ground state as the metal oxidizes the ligand, giving it Wurster's blue character. The authors describe the ligands as non-innocent and the molecules as intermediates in reactions leading to diamagnetic binuclear p-quinone diimine or nitrene complexes. The tkis(salicylaldehydehydrazonato)iron(III) in dmf has a complex e.s.r. which simplifies to a simple orthorhombic S = 1/2 spectrum typical of low-spin d 5 , by adding pyridine. A new method for simulating such spectra has been described in an earlier ~ection.~' A similar spectrum is found to grow in the spin crossover system, [Fe(3-MeOSPH)2]PF6 (SPH2 = salicylidene-2pyridylhydrazine) as the geff = 4 . 2 (high-spin) signal diminishes on cooling. A Griffith-type analysis was used to derive the en-

59

60

Electron Spin Resonance

ergies of the three Kramers doublets and conclude that the electron is essentially in a d x y orbital." Ruthenium(II1) complexes of the general form [RuL2Cl(solvent)] have been made, (L is the NS chelating donor formed by deprotonation of 2-mercapto-3-phenyl-4-quinazolinone). Magnetic susceptibility measurements indicate a low-spin configuration as do the g values which are typically highly anisotropic S = 1/2. The experimental g data were used as input in a program of the Hudson and Kennedy type which details all 48 possible correspondences of the g tensor with x , y and z and of the signs of its principal components. Selection of the one, most realistic solution led to a knowledge of the wave function coefficients corresponding to the lowest Kramers doublet, the orbital reduction factor k and the tetragonal ( A / h ) and rhombic ( V / h ) distortions. 1 8 3 d7 Configuration. -

Zerovalent

Manganese and

Rhenium,

Univalent

The bis(salicylaldehydehydrazonato)manganese(II) molecule is, like its tris iron(II1) counterpart mentioned above, normally high-spin. In this case Tirant e t a l . have reduced with excess hydrazine to a state which gives well resolved gL and g,, regions showing manganese hyperf ine structure from manganese ( 0 ), d7. Use of hydroxylamine, borohydride or dithionite takes the reduction only to the manganese(1) stage.39 Photolysis of the M-M bond ( M = Mn, Re) in molecules of the type (CO)5MM(CO)3L (L = N,N'-tB~1,4-diaza-If3-butadiene) might be expected to yield fragments In fact the photolysis products have containing Mn(0) or Re(0). Iron.

similar i.e.

wise,

g values and on this basis are described as M+(C0I3L' M+ with an unpaired electron on the ligand since otherthe higher spin orbit coupling on the heavier metal would

d6

have caused a bigger g shift. 184 However the e. s .r. spectra show extensive hyperfine couplings not only with I4N and 'H but also with 55Mn or I8'Re and '*'Re. This is evidence for some contribution of the M ( 0 ) d7 state and more evidence is provided by their own explanation which assumes spin density on the metal and equal g shifts because the M=Re compound has larger contributions but which balance each other because they are opposite in sign. They argue that g for both complexes would be raised from the free spin value by mixing in with a low-lying doubly occupied state, gRe the more so because it has the higher spin orbit coupling constant. The unpaired electron in the SOMO of the rhenium complex however interacts also with a higher-lying empty orbital which gives a contribution to the g shift of opposite sign.

3: Transition Metal Ions

Glidewell and Johnson have recently identified several paramagnetic mononuclear complexes of general formula [Fe(NO)2X2]-. Thus the Roussin methyl ester Fe2(SMeI2(N0l4 reacted with Br- or I- to give [Fe(NO)2X2]- (X = Br-, I-) and reactions with NO2-, N3- and NCO- gave similar products. An interesting variation was observed with SCN- in that in the reaction with Fe2(SR)2(N0)4 (R = Me or tBu), first the S-bonded [Fe(N0)2(SR)(SCN)]was formed but this was then displaced by the N-bonded [Fe(N0)2(SR)(NCS)]-. and Sulphate and chromate yielded [Fe (NO1 ( SMe )OSOg]2A series of related [Fe(NO)2(SMe)OCr03]2respectively.185 Fe(1) d7 compounds have also been found by oxidation of Fe2(N0I4cations in which L may be C12. 1 8 6 These are the [Fe(N0)2LL'L"]+ a solvent or a monophosphine; L'L" may be a diphosphine. Three types of radical were found here and in all cases the two nitrosyl groups were equivalent and showed as a quintet. Thus a triplet of quintets was seen when the N of CH3CN as solvent interacted; a doublet of quintets showed interaction with one phosphine ligand; and a doublet of sextets which analysed as a doublet of doublet of quintets showed the two inequivalent phosphorus nuclei of a chelating diphosphine. These are trigonal bipyramidal 1 9 electron species as opposed to the earlier four coordinate and presumably approximately tetrahedral 17 electron [Fe(NO)2X2]' species but in each case the iron is formally d7 Fe+. E.s.r. has played an important part in all these studies, the various paramagnetic entities being recognized principally by their g values and their nitrogen hyperfine interactions. Belousov and Kolosova made similar heavy use of e.s.r. in their systematic study of the reactions between iron carbonyls or carbonylferrate anions such as [Fe(C0)4]2- on the one hand with nitro and nitroso compounds such as t B ~ N Oand CR2CN021- on the other; three different types of radical, two of them hitherto unknown, were recognized by their e . s . r . spectra. 8 7 Further experiments were then designed to produce the radicals by alternate routes and to identify them by spin trapping. The radicals finally featured in the coordination spheres in which the nitro compounds undergo successive deoxygenations to nitrenes followed by their subsequent carbonylation to isocyanates, and a scheme for this reductive carbonylation was proposed. Various thiols were shown to reduce [pentacyanonitrosylferrate12- which then lost CN- to give the square pyramidal iron(1)

61

62

Electron 'spin Resonance

complex [Fe(CN)qN0]2- which also is known by its characteristic e.s.r. spectrum. Its further reaction with RS-, O2 or loss of NO were then followed by e. s .r . 88 Bivalent Cobalt, Rhodium and I r i d i u m . The development Of model complexes for haem proteins that can provide insight into the binding and release of dioxygen presents a continuing challenge. Cobalt(I1) figures prominently here and e.s.r. is a beautiful indicator because distinct spectra are seen for both the deoxygenated and the oxygenated forms , of cobalt (I11 carriers. 89-194 In the presence of a nitrogenous base, the initial Co(1I)L complexes will show, by their I4N hyperfine structure, that either one, or in the case of the better donors, two of the base molecules will bind to the metal; the e.s.r. at this stage is typically that due to cobalt(I1) d 7 in the low-spin state. When exposed to a stream of oxygen gas, conversion to the dioxygen adduct takes place and the change may be monitored by the replacement of this signal by that of the adduct which is often described as a cobalt(II1) superoxide. See equation (5) base 02

[Co(II)L]

+ [Co(II)L

base]

[Co(II)LO2 base]

or [CO(III)LO~ base] (5) Most of these reactions take place in non-aqueous solvents because of insolubility or aggregation problems in water, but Evans and Wood have used a water soluble porphyrin in their reactions which are accordingly closer to the normal biological medium. The essential need for the base has been shown in a pretty experiment by Kimura et a l . who have succeeded in synthesizing the familiar cyclam macrocycle but bearing an appended imidazole that acts as an ideal donor from the axial direction. with the pendant imidazole, Co(I1)cyclam yields the paramagnetic 1:l Co(II)-o2 adduct; without it, it doesn' t . At low temperatures, oxygen uptake in these complexes is normally reversible but at room temperature there is a strong likelihood that dimerization might take place leading to the bridging peroxo complex as below, thus rendering the oxygen largely inaccessible.

*'

[Co(II)LO2base)

+ [base

LCo(III)02Co(III)L base]

(6)

Sensitivity to such dimerization appears to be very much diminished in the lacunar cyclidene complexes, (related to cyclam above but with a strap connected from one side of the macrocycle

3: Transition Metal Ions

63

to the other) which have been specially designed to provide a new 92 These ligands are bicyclic family of oxygen carriers. wherein one cycle accommodates the metal and the other the oxygen. The fact that the oxygen is bound to the metal on one side and is protected from its environment on the other apparently prevents the formation of the t~ peroxo complex shown in equation (6). Similar protection is afforded to the bis(dimethylg1yoximatol-cobalt(I1) complex [C0(1I)(dmgH)~] when synthesized in the a cages (supercages) of NaX zeolite. Hydration in this case, followed by oxygenation converts this complex to the superoxo species [Co(III)(dmgH)2.H20.02]- but such species are not free to diffuse and react with themselves, or with other species or with solvent, hence dimer formation as discussed above cannot take place. The oxy and the deoxy complexes, together with the intermediates described as distorted [Co(II) (dmgH)2] and [Co(II)(dmgH)2.2H20] were recognized by their e.s.r. parameters after comparison with those of model complexes, and a reaction sequence was thereby proposed and discussed. 93 Analogous chemistry has been shown for rhodium(I1) at least for the 2:1, base:metal complex. Dilithiophthalocyanine for example, PcLi2 and di-~-chloro-bis-(1,5-cyclooctadiene)rhodium [Rh(C0D)C1I2 react with nitrogenous bases L = butylamine (bu), pyridine (py), 4,4-bipyridine (bipy) and pyrazine (pyz) to form the corresponding hexacoordinated complexes PcRhL2. The g values of the pyridine solution spectrum for L = py, 2.46, 1.94 and 1.88 typify the spectra of these compounds. Likewise, rhodium(II1 species have been generated in Na-X zeolites. Their sites in the a and p cages have been identified by e.s.r. and ESEEM as have also the various adducts which result from subsequent adsorption of oxygen and also of water, ammonia, carbon monoxide, etc.Ig6 When NaCl or KC1 are doped with K3M(CNI6 (M = Co, Rh, Ir) and submitted to X-irradiation the low-spin [M(CN)4C12]4- is formed having 04,, symmetry and an electron in d , 2 . Hyperfine structure from two Na atoms was seen in NaCl but no K hyperfine structure in Kc1.1971198 I

Tervalent Nickel, Palladium and P l a t i n u m . There is a strong interest in the stabilization of high oxidation states of the first row elements and naturally ligands which are capable of releasing electron density back to the metal play a key role. Thus tervalent ions of the nickel group are commonly stabilized with dithiolenes, and two new ones have been synthesized. They

64

Electron Spin Resonance

Comparison of their g (R = Ph and OMe). are [Ni(S2C2(COR)2)2]values with those of other dithiolenes shows that the percentage metal character of the HOMO depends markedly on the substituent group present on the thiolene moiety. For these compounds it is shown that this percentage is proportional to < g > which surely means that there is no strong contribution to g from the sulphur and this is in keeping with the low spin-orbit coupling constant for sulphur. The similarity of the e.s.r. parameters found in this work relative to those found for nickel(II1) ion in several hydrogenases is noted. Similar compounds in which sulphur has been replaced by selenium have also been studied.2o08201 Frozen solutions of [M(td~)~]- (M = Ni, Pt; tds = Cbis(trifluoromethy1)ethylene]diselenolato) give well-resolved spectra with rhombic symmetry like that found in many of the bis(dithio1ene) complexes. Relative to these, and with the exception of g, for [Pt(tds),]which goes down slightly, all g components move up, gY markedly so, upon substituting selenium for sulphur in these complexes. This might reflect the influence of the larger spinorbit coupling of the heavier chalcogen atoms as is the case for [Ni(bds)](bds = o-benzenediselenolato) when compared with its thiolato analogue2'' but Heuer et a 1 . feel it is more likely to result from differences in the pattern of molecular orbital energies.201 Rich hyperfine structure is seen from Ig5Pt in [Pt(tds1,l- but the smaller coupling constants here when compared with the analogous sulphur complex CPt(tfdl21- actually indicate a lower spin density on the metal when bound to selenium. In addition the resolved 77Se hyperfine structure from both [Ni(tdsI21- and CPt(td~)~]- indicate extensive delocalization, 70% in fact, of the unpaired electron on to the four selenium atoms. Nickel(II1) in a NiN402 environment has been reported (having Nq from azomethine and pyridine nitrogens; O2 from water). The e.s.r. spectra indicate S = 1/2, gl > gI1 and a d,2 ground state consistent with tetragonal symmetry and relatively weak axial bonds. This conclusion is supported by the addition of pyridine which rather strikingly converts the gI1 feature into a quintet as the two axially bound water molecules are displaced by pyridA s we have seen, nitrogen, sulphur and other (heavier) donors seem to be quite good for stabilizing nickel(II1). Oxygen is much less so and it is with interest that we read of the first discrete Ni (111lo6 species [Ni ( bipy02 31 3+ ( bipy02 = 2,2 ' -bipyridine-Ill'-dioxide) though it must be admitted it has only been generated electrochemically. The e.s.r. g values fall in the

3: Transition Metal Ions

correct range for Ni(II1) and the rhombic character is consistent with the a f g e b :e configuration for D 3 d after the expected JahnTeller distortion. As before, g , , g y > g , as expected for the d Z 2 ground state left after axial elongation. 203 Paramagnetic palladium species may also be generated, and have proved interesting and significant in the study of X zeolites. Both Pd(1) and Pd(II1) have been identified and located in the zeolite by introducing various adsorbates to determine the accessibility of the palladium species to adsorbates of different sizes. ESEEM experiments have been particularly useful in determining the number of molecules coordinated to such ions and their internuclear distances. In fact, when considering zeolites, adsorption of the Cu(I1) ion has been studied in most detail and it has been established that the size of the zeolite's major cation plays a controlling role in the coordination of water to the cupric ion and thus in controlling crowding within the cage. 204 Similarly for the palladium species, the major cation in an X-zeolite has a strong controlling effect.205 For example, by using either a Ca-X or Na-X zeolite it is possible to control the location of either Pd(1) or Pd(I1I) to be in either a hexagonal prism or in a @-cage near a six-ring. The subject has been reviewed. 206 d9 Configuration.- The chemistry of copper and silver in their higher oxidation states has been reviewed in a lengthy article which includes rather brief mention of the e.s.r. in copper dimers and in silver (I1) compounds. 207

Bivalent Copper. Copper is by far and away,the most popular of the transition elements for e.s.r. study; it gives good spectra so easily. The result is that many researchers who just happen to be working with copper feel that they have to run the e.s.r. spectrum and report it even though they have a very limited understanding of the technique and less realization of its limitations. The consequence is a vast literature much of which never should have been published. Referees are urged to be more strict. Most good work is on frozen solutions or magnetically dilute powders or crystals, but the study of equilibria in mobile solution is gaining ground" and I give a recent example. Malonic acid and acetylacetone were grafted on to silica and the equilibria between copper(I1) and these two acids studied both in this immobilized state and when free in solution.208 The equil-

65

66

Electron Spin Resonance

ibrium constants for the reactions of silica-grafted malonic acid are identical to the values obtained for the corresponding homogeneous system. This is not the case for grafted acetylacetone and nor are their copper(I1) e.s.r. spectra the same. The latter gives a powder-type spectrum with rhombic distortion. Clearly steric effects are controlling the movements of the immobilized acetylacetone complex and its equilibria but not apparently those of the malate. Zhou has derived expressions for the Hamiltonian parameters of Cu2+ which for their evaluation require the parameters A and e l t 1 / 2 > and lt5/2> la3/2> transitions. A satisfactory description of the interaction between the electric field and the Fe3+ at this site was obtained by inclusion in the Hamiltonian of terms up to fourth order in the local electric field268 Perturbation of a different sort was observed for Fe3+ in some 111-V compounds. As stated earlier, uniaxial compression doesn't affect Mn2+ in GaP but it does affect Fe3+ in GaAs and InP as observed by a shift in fine structure lines and in GaP too to an even greater degree, quite possibly because the centre may not be Fe3+ but a ferromagnetic interaction between Fe2+(3d6) and a hole2'. A strong line and satellite lines from a minor species are assigned respectively to a dimer and a monomeric complex in crystalline [ F e ( a ~ a c ) ~ (acac ] = acetylacetonato). It has been proposed earlier that two forms of Fe(acacI3 exist on the basis of the coexistence of keto and enol forms of the acac ligand. Hedewy and Hoffmann however assign the weaker of the two lines, which the Q band spectrum in particular shows to be nearly axial, as FeO(keto f ~ r m ) ~ ( e n o form). l This form grows at the expense of the dimer above a transition temperature at 353 K 269. Mention has earlier been made to the assignment of Fe3+ to the zr4+ sites as C F ~ F ~ in I ~fluorozirconate g1asses'O3. Quadrivalent ions are substituted by Fe3+ in natural amethyst too; the most prominent of three groups of lines in Korean natural amethyst is that of the S 1 centre due to substitutional Fe3+ at Si4+ sites.270 In these two examples iron has entered the framework of the host material and so it does in A and X-type zeolites too271, but here, two main signals were seen after Fe3+ exchange. A broad signal is attributed to the M s = 1-1/2> + 11/2> transition arising from ions within the supercage of the zeolite where Brownian motion of loose water molecules perturbs the coordination symmetry of the Fe(II1) ion and leads to a distribution in crystal field parameters; but there is also a geff = 4.3 signal from Fe3+ ions which specifically replace A13+ from the zeolitic framework. Iror1 is of course, a very important atom in biological matThe erials so naturally it features widely in model compounds.

+

iron complex of parabactin, a spermidine-containing catechol-type compound revealed a rather isotropic geff = 4.5 signal from the middle Kramers doublet of high-spin iron(II1). The slight diff-

3: Transition Metal Ions

81

erence between this and those (geff = 4.3) of the 3/1 catechol/ Fe(II1) complex and other spermidine catecholamide-Fe(II1) complexes is thought to indicate a different structure, possibly close to that of ferrimycobactin P with the conclusion that the ohydroxyphenyl-A2-oxazoline moiety in this molecule may control the stability of the iron(II1) complex and the ease of release of the iron atom.272 A similar e.s.r. spectrum (geff = 4.2) was seen from the peroxide complex [Fe(OEP)O2I2- (OEP = octaethylporphyrin) which is particularly interesting because simple ferric porphyrin complexes normally give a well-defined axial signal with g,, = 2, gL = 6. Analysis of this signal both from its width and from its temperature dependence yielded a lower The data thus measured limit for the rhombicity ratio € I D . together with those from magnetic susceptibility measurements and Mbssbauer spectra are consistent with a geometry in which the iron atom is significantly displaced out of the porphyrin plane towards the peroxoligand.273

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244 245 246 247 248 249 250 251

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4 Recent Developments of ENDOR Spectroscopy in the Study of Defects in Solids BY J.-M. SPAETH

1.

Introduction

Point d e f e c t s such as impurity atoms o r i o n s o r i n t r i n s i c d e f e c t s such as v a c a n c i e s ,

interstitials or anti-site

defects i n binary

crystals o r a g g r e g a t e s of t h e s e d e f e c t s v e r y o f t e n d e t e r m i n e t h e b u l k p r o p e r t i e s o f a s o l i d . T h i s i s t h e c a s e i n , e.g.,

i o n i c crys-

tals o r o x i d e s and is p a r t i c u l a r l y i m p o r t a n t t e c h n o l o g i c a l l y i n semiconductors. Therefore it is of prime i n t e r e s t f o r materials science

methods t h a t are a b l e

t o have

t o determine

the defect

structures. R a d i a t i o n damage t o s o l i d s and d e t e c t i o n of

ionising

r a d i a t i o n are o t h e r f i e l d s where knowledge of t h e s t r u c t u r e s of t h e d e f e c t s c r e a t e d and t h e i r i n t e r a c t i o n s is a b s o l u t e l y necessary. Magnetic resonance is t h e most powerful technique €or determining defect s t r u c t u r e s of point defects.

a widespread method,

E l e c t r o n s p i n r e s o n a n c e (ESR) i s

but unfortunately suitable only i n particularly

f a v o u r a b l e cases. It t u r n s o u t t h a t a c o m b i n a t i o n o f e l e c t r o n s p i n resonance and n u c l e a r magnetic resonance, e l e c t r o n n u c l e a r double r e s o n a n c e (ENDOR),

i s t h e most powerful t o o l ,

the s p e c i f i c d e f e c t under investigation.

provided it works f o r

I n cases of i n t e r e s t t o

m a t e r i a l s s c i e n c e , ENDOR s p e c t r a a r e m o s t l y v e r y c o m p l i c a t e d . T h i s

m a y b e t h e r e a s o n wh y ENDOR s p e c t r o s c o p y i n s o l i d s h a s n o t b e c o m e more popular. have,

Computer a s s i s t e d e x p e r i m e n t s and a n a l y s i s of

however,

t o g e t h e r w i t h a d v a n c e d ENDOR m e t h o d s ,

spectroscopy i n r e c e n t y e a r s q u i t e f e a s i b l e and of beyond

simple

model

cases,

for

which

t h i s

spectra

m a d e ENDOR

practical use

spectroscopy

was

o r i g i n a l l y a p p l i e d f o l l o w i n g i t s d i s c o v e r y i n 1959.l T h e aim o f

t h i s Chapter is t o d e s c r i b e d b r i e f l y t h e p r e s e n t

p o s s i b i l i t i e s o f ENDOR s p e c t r o s c o p y f o r i n v e s t i g a t i n g p o i n t d e f e c t s in solids.

It i s a t t e m p t e d t o d i s c u s s and i l l u s t r a t e what can be

done w i t h o u t g o i n g i n t o g r e a t d e t a i l , e s p e c i a l l y w i t h r e s p e c t t o a precise a n a l y s i s of t h e spectra,

which would be beyond t h e s c o p e of

t h i s Chapter. The e x a m p l e s used t o i l l u s t r a t e t h e methods are, f o r convenience, Therefore,

l a r g e l y t a k e n from r e c e n t work o f t h e P a d e r b o r n group.

t h i s is not a review article i n t h e sense of summarising

89

Electron Spin Resonance

90

I t is f u r t h e r m o r e assumed t h a t t h e

a l l work done i n t h e f i e l d .

reader is f a m i l i a r w i t h t h e b a s i c concepts and t h e r e a l i s a t i o n of e l e c t r o n s p i n r e s o n a n c e which w i l l n o t be d i s c u s s e d . The f u n d a m e n t a l ideas of

ENDOR w i l l

be

described as w e l l

briefly

as t h e

basic

m e t h o d s o f a n a l y s i n g ENDOR s p e c t r a . T h i s a r t i c l e d o e s n o t d e a l w i t h t h e p o s s i b i l i t y of ENDOR u s i n g e l e c t r o n s p i n e c h o m e t h o d s . advantages,

measuring

These methods have c e r t a i n

e s p e c i a l l y a t low frequencies, but have a l s o s e r i o u s

d i s a d v a n t a g e s , i n p a r t i c u l a r i n t h e r e g i m e o f b r o a d ESR l i n e s a n d h i g h ENDOR f r e q u e n c i e s .

T h e r e a d e r i s r e f e r r e d t o r e c e n t a r t i c l e s by

Schweiger f o r t h e s e e x p e r i m e n t a l methods.2

2. The

S t r u c t u r e D e t e r m i n a t i o n of D e f e c t s by Magnetic Resonance

possibility

of

determining

defect

structures

by

magnetic

resonance s p e c t r o s c o p y i s based on t h e measurement of t h e magnetic i n t e r a c t i o n b e t w e e n t h e m a g n e t i c mo me n t o f t h e u n p a i r e d e l e c t r o n ( s ) or hole(s)

of

t h e d e f e c t and t h e magnetic moments o f

belonging t o t h e ' i m p u r i t y '

atom of

the defect (if

nuclei

any) and

the

a t o m s of i t s s u r r o u n d i n g l a t t i c e . T h i s i n t e r a c t i o n i s u s u a l l y c a l l e d hyperfine

(hf) interaction i f

it occurs between the

unpaired

e l e c t r o n ( h o l e ) and t h e c e n t r a l n u c l e u s of an i m p u r i t y atom, i f s u c h

a d e f e c t is under study, interaction

or

ligand

and it is c a l l e d superhyperfine (shf)

hyperfine

(lhf)

interaction i f

occurs

it

b e t w e e n t h e m a g n e t i c mo me n t o f t h e u n p a i r e d e l e c t r o n ( h o l e ) a n d t h e m a g n e t i c moments of t h e n u c l e i of t h e s u r r o u n d i n g l a t t i c e . I n F i g u r e

1 a s c h e m a t i c r e p r e s e n t a t i o n of a d e f e c t is shown, electron

spin

and

the

nuclear

spins

are

i n which t h e

indicated.

The

hf

i n t e r a c t i o n i s o f t e n r e s o l v e d i n ESR a n d i f t h e i m p u r i t y n u c l e u s occurs i n s e v e r a l magnetic isotopes then a chemical i d e n t i f i c a t i o n

o f t h e i m p u r i t y i s e a s i l y p o s s i b l e by t a k i n g a d v a n t a g e o f t h e k n o w n magnetic moments,

t h e r a t i o of which d e t e r m i n e s t h e r a t i o of t h e

o b s e r v e d h f s p l i t t i n g s ( s e e b e l o w ) a n d b y m e a s u r i n g t h e ESR s i g n a l intensities, which r e f l e c t t h e n a t u r a l abundance of t h e d i f f e r e n t isotopes.

U n f o r t u n a t e l y t h e r e a r e i m p o r t a n t a n d common i m p u r i t i e s ,

l i k e oxygen,

w h e r e o v e r 99% o f t h e i s o t o p e s a r e n o n - m a g n e t i c .

s u c h a c a s e , a s p e c i f i c d o p i n g o f a m a g n e t i c i s o t o p e ( e .g ., be n e c e s s a r y . The g - f a c t o r s

(g-tensors)

of

In

170) may

t h e ESR s p e c t r a c o n t a i n a l s o

i n f o r m a t i o n on t h e d e f e c t s t r u c t u r e , m a i n l y o n i t s s y m m e t r y . F o r a p r e c i s e s t r u c t u r e determination, a p a r t from symmetry,

the g-factor

a n a l y s i s u s u a l l y does n o t y i e l d enough r e l i a b l e i n f o r m a t i o n ,

since

e x c i t e d s t a t e e n e r g i e s are i n v o l v e d i n t h e g - f a c t o r

which

analysis,

91

Figure 1 Schematic r e p r e s e n t a t i o n of a d e f e c t i n a s o l i d . The u n p a i r e d e l e c t r o n s p i n , t h e e l e c t r o n s p i n d e n s i t y d i s t r i b u t i o n and t h e n u c l e a r s p i n s of t h e l a t t i c e n u c l e i a r e i n d i c a t e d

are mostly not k n ~ w n . ~ - ~ I f i t were p o s s i b l e t o m e a s u r e t h e n u c l e a r m a g n e t i c r e s o n a n c e

(NMR) o f t h e n u c l e i o f t h e s u r r o u n d i n g l a t t i c e , t h e l o c a l m a g n e t i c f i e l d s s e e n by t h e n u c l e i w o u l d j u s t g i v e t h e r e q u i r e d i n f o r m a t i o n on t h e d e f e c t s t r u c t u r e ,

s i n c e it is a superposition of t h e applied

static magnetic f i e l d and t h e f i e l d e q u i v a l e n t of t h e s h f o r l h f interactions. together

If

with

s t r u c t u r e of

the

their

local

f i e l d s of

symmetry

all the nuclei

relation

to

were known

the defect site,

t h e d e f e c t including t h e presence of

l a t t i c e d i s t o r t i o n s c o u l d be d e r i v e d . U n f o r t u n a t e l y , h o w e v e r , protons

t o

measure

NMR,

while

one

deals

NMR is

lo1'

nuclei

with

defect

n o t s u f f i c i e n t l y s e n s i t i v e . One n e e d s a t l e a s t a b o u t like

the

vacancies and

c o n c e n t r a t i o n s o f 1 0 l 6 cm-3 a n d l e s s a n d s a m p l e v o l u m e s u s u a l l y w e l l b e l o w 1 cm3. I n ESR t h e s h f

is m o s t l y n o t w e l l r e s o l v e d .

If

a t a l l i t is

resolved usually only with t h e nearest neighbour nuclei.

Therefore,

92

Electron Spin Resonance

ESR s p e c t r o s c o p y i s o f t e n s u i t a b l e f o r i n p u r i t y i d e n t i f i c a t i o n b y a r e s o l v e d hf i n t e r a c t i o n , b u t n o t f o r a p r e c i s e m i c r o s c o p i c s t r u c t u r e r e c e n t l y i n GaAs s e v e r a l ESR s p e c t r a w e r e

determination. I n f a c t ,

o b s e r v e d w i t h t h e same c e n t r a l 7 5 A s

h f s p l i t t i n g a n d a l m o s t t h e same

l i n e width, which a l l belong t o d i f f e r e n t d e f e c t s t r u c t u r e s . These d i f f e r e n t d e f e c t s h a v e o n l y o n e common f e a t u r e ; a n a r s e n i c a n t i s i t e , i.e.,

a n a r s e n i c a t o m o n a Ga-site

is involved. However, t h e

r e s t o f t h e s t r u c t u r e i s d i f f e r e n t . 6 By m e a s u r i n g o n l y ESR o n e c a n be badly misled.

A

higher

resolution

for

the

shf

i n t e r a c t i o n s and

a

higher

s e n s i t i v i t y t h a n o b t a i n e d i n N M R i s a c h i e v e d b y m e a s u r i n g END0R.l The NMR t r a n s i t i o n o f

neighbour nuclei coupled t o the unpaired

electron causes a change of t h e e l e c t r o n s p i n p o l a r i s a t i o n under suitable

experimental conditions

( p a r t l y s a t u r a t e d ESR),

b e d e t e c t e d a s ENDOR s i g n a l s ( s e e b e l o w ) .

Thus,

which can

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

are d e t e c t e d u s i n g a q u a n t u m t r a n s f o r m a t i o n t o h i g h e r q u a n t a , t h e

ESR m i c r o w a v e q u a n t a , w h i c h r e s u l t s i n a h i g h e r s e n s i t i v i t y . It m u s t b e n o t e d t h a t by r e s o l v i n g t h e s h f i n t e r a c t i o n s a n d t h e l i g a n d q u a d r u p o l e i n t e r a c t i o n s one c a n d e t e r m i n e t h e number and t h e symmetry r e l a t i o n o f n e i g h b o u r n u c l e i w i t h r e s p e c t t o t h e d e f e c t centre and a l s o t h e coupling c o n s t a n t s such as shf and quadrupole constants.

It

is

not,

however,

possible

to

determine

from

the

coupling c o n s t a n t s t h e d i s t a n c e of t h e i d e n t i f i e d n u c l e i from t h e defect c e n t r e w i t h o u t h a v i n g a t h e o r e t i c a l wave f u n c t i o n f o r t h e defect electron (hole) o r without assumptions about t h e r a d i a l p a r t of t h i s wave f u n c t i o n ,

e.g.,

whether it f a l l s o f f monotonously w i t h

d i s t a n c e f r o m t h e d e f e c t c o r e . T h i s i s v e r y o f t e n t h e case b u t n o t always.

E s p e c i a l l y f o r s h a l l o w d e f e c t s i n s e m i c o n d u c t o r s t h e r e may

be o s c i l l a t i o n s o f t h e u n p a i r e d s p i n d e n s i t y w i t h d i s t a n c e . reasonable a s s u m p t i o n a b o u t t h i s r a d i a l p a r t c a n be made,

I f a

then a

clear d e f e c t model c a n be d e r i v e d u n l e s s t h e r e are s p e c i a l symmetry conditions i n which one cannot d e c i d e between p o s s i b l e sites. T h i s i s , e.g.,

t h e case f o r t h e s u b s t i t u t i o n a l a n d t h e t e t r a h e d r a l i n t e r -

s t i t i a l s i t e i n t h e d i a m o n d s t r u c t u r e . T h e r e t h e same s y m m e t r y t y p e of

neighbour nuclei occurs,

only w i t h a d i f f e r e n t sequence.

If

n o t h i n g is known a b o u t t h e wave f u n c t i o n , o n e c a n n o t d e c i d e o n t h e s i t e f r o m ENDOR a l o n e . S u c h a case o c c u r e d f o r i n s t a n c e r e c e n t l y f o r chalcogen d e f e c t s i n ~ i l i c o n . ~

3.

H y p e r f i n e and S u p e r h y p e r f i n e S t r u c t u r e of ESR S p e c t r a

ESR l i n e s c a n b e s p l i t b y h y p e r f i n e ( h f ) a n d s u p e r f i n e ( s h f ) interactions. It is t h i s s p l i t t i n g which g i v e s most i n f o r m a t i o n on

93

4: ENDOR Spectroscopy in the Study of Defects in Solids the

microscopic

nature

of

the

defects

in

ESR.

The

hf

and

shf

i n t e r a c t i o n s a r e d e s c r i b e d by t h e r e s p e c t i v e h f ( s h f ) t e n s o r s A a n d t h e s p i n Hamiltonian

T h e sum r u n s o v e r a l l n u c l e i w i t h w h i c h t h e s h f ( h f ) i n t e r a c t i o n i s

B e t h e B o h r m a g n e t o n , 8,

ge i s t h e e l e c t r o n i c g - f a c t o r ,

measured.

t h e n u c l e a r magneton and gI t h e n u c l e a r g - f a c t o r . For t h e s i m p l e case o f a n i s o t r o p i c g - f a c t o r , o n l y one n u c l e u s i n t e r a c t i n g i n p e r t u r b a t i o n

theory

S = 112 and f o r of

f i r s t order

o n e o b t a i n s f o r t h e ESR t r a n s i t i o n s f r e q u e n c i e s :

where A(8) d e n o t e s t h a t t h e h f i n t e r a c t i o n i s i n g e n e r a l a n g u l a r dependent,

i.e.,

it

depends on

the

relative

orientation of

the

e x t e r n a l magnetic f i e l d and t h e p r i n c i p a l a x e s of t h e hf t e n s o r . For t h e s i m p l e case o f

I = 112 t h e r e are two values of

the

n u c l e a r s p i n q u a n t u m n u m b e r m I = kl12. S i n c e i n t h e e x p e r i m e n t VESR

is k e p t c o n s t a n t , t h e r e are two r e s o n a n t f i e l d s , which are s e p a r a t e d by A ( e ) / g e B e F i g u r e 2 s h o w s how

an hf

i n t e r a c t i o n c a n be used

i d e n t i f i c a t i o n f o r t h e e x a m p l e o f Te'

for defect

d e f e c t s i n Si.8 92% o f t h e Te

i s o t o p e s a r e d i a m a g n e t i c a n d g i v e r i s e t o t h e c e n t r a l ESR l i n e n e a r

3500 G ( m e a s u r e d i n X - b a n d ) c o r r e s p o n d i n g t o g e = 2. T h e t w o m a g n e t i c i s o t o p e s 1 2 5 T e ( 7 % a b u n d a n t ) a n d 123Te ( 0 . 9 % a b u n d a n t ) b o t h have I = 1 / 2 a n d a c c o r d i n g t o e q u a t i o n (3.2) T h e ESR

line

intensities

relative

s p l i t t i n g s are i n t h e

m om ent s ( s e e b e l o w ) .

follow

the

ratio

a doublet splitting.

i s o t o p e abundance and of

the

the respective nuclear

Therefore t h e d e f e c t is unambiguously iden-

t i f i e d as b e i n g due t o Te i m p u r i t i e s .

However, no f u r t h e r s t r u c t u r e

due t o t h e s h f i n t e r a c t i o n s i s r e s o l v e d .

Therefore,

t h e s i t e o f Te'

i n t h e l a t t i c e c a n n o t b e d e t e r m i n e d f r o m ESR. F i g u r e 3 s h o w s t h e ESR s p e c t r u m o f

a t o m i c hydrogen on i n t e r -

s t i t i a l sites, and on c a t i o n and anion vacancy sites i n KCl.9

The

c e n t r a l h f s p l i t t i n g w i t h t h e p r o t o n (I = 112) i s i n a l l t h r e e cases p r a c t i c a l l y t h e same a n d n e a r l y t h a t o f t h e f r e e h y d r o g e n a t o m . O n l y for the i n t e r s t i t i a l site is a shf bours resolved.

i n t e r a c t i o n with nearest neigh-

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

ESR s p e c t r u m . T h e y c o u l d b e e s t a b l i s h e d o n l y b y r e s o l v i n g t h e s h f i n t e r a c t i o n s w i t h ENDOR e x p e r i m e n t s . " The i n t e n s i t y r a t i o o f t h e s h f l i n e s i s a c h a r a c t e r i s t i c f e a t u r e

Electron Spin Resonance

94

Si :Te+ 20 K

9.74 G Hz

-+ +/

123Te

(0.9Yo)

gain r e d u c e d

(‘ 1 2 4

125 Te

(7%) Te (92.1 Yo) 1

1

1

1

I

I

1

I

1

1

1

I

I

I

4000 G

3500

3000

Figure 2 ESR s p e c t r u m of Te’ c e n t r e s i n S i . The g - f a c t o r i s i s o t r o p i c . The h f s p l i t t i n g s a r e due t o t h e t w o m a g n e t i c i s o t o p e s l Z 5 T e and lz3Te w i t h I = 1/2. (After r e f . 8) and c a n b e u s e d t o d e t e r m i n e t h e number o f i n t e r a c t i n g n u c l e i a n d t h e i r s p i n . E s p e c i a l l y i f t h e r e a r e s e v e r a l n u c l e i w i t h t h e same s h f interaction,

f o r a specific orientation of

which can occur

m a g n e t i c f i e l d , o n e c a n d e f i n e a t o t a l n u c l e a r s p i n N.1, t o a ( 2 N I + 1 ) f o l d s p l i t t i n g o f t h e ESR s p e c t r u m .

the

which l e a d s

Figure 4 shows t h e

l e v e l s c h e m e a n d t h e t r a n s i t i o n s f o r N = 1, 2 a n d 3 i n t h e c a s e o f I = 1/2.

T h e r e l a t i v e ESR s i g n a l h e i g h t i s g i v e n b y t h e s t a t i s t i c a l

w e i g h t s o r d e g e n e r a c y o f t h e l e v e l s . F o r N = 3 t h e y a r e 1:3:3:1

for

the f o u r e q u i d i s t a n t shf l i n e s . For f o u r e q u i v a l e n t n u c l e i of I = 3/2

t h e w e i g h t s are 1:4:10:20:31:40:44:40:31:20:10:4:1 f o r t h e t h i r -

teen shf l i n e s . T h i s is what is observed i n F i g u r e 3 f o r t h e inters t i t i a l a t o m i c hydrogen i n KC1, f o u r n e a r e s t 35Cl

nuclei,

which i n t e r a c t s predominantly w i t h

which have I = 3/2.

The h f a n d s h f t e n s o r s a r e d e t e r m i n e d i n t h e i r p r i n c i p a l a x i s system

from

spectra.3-5.

an

analysis

of

the

angular

dependence

of

the

ESR

It is c o n v e n i e n t f o r t h e d i s c u s s i o n below t o decompose

4: E N D O R Spectroscopy in the Study of Defects in Solids

95

Figure 3 ESR s p e c t r a o f t h r e e a t o m i c hydrogen c e n t r e s i n K C I . B O / / < l O O > , T = 77 K. The hydrogen a t o m s occupy i n t e r s t i t i a l s i t e s ( H g - c e n t r e s ) , c a t io n v a c ancy s i t e s ( H g - c e n t r e s ) and anion vacancy s i t e s c e n t re s) . The s p l i t t i n g due 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 w i t h t h e proton i s almost t h a t o f t h e f r e e hydrogen atom. ( A f t e r ref. 9 )

(Hi-

t h e hf

(shf) t e n s o r s i n t o a n i s o t r o p i c and a n i s o t r o p i c p a r t accord-

ing to: &

+Ti)

A

=

b

= 1/2Bz,

(a.Y

b’ = 1/2(Bxx - Byy)

(3.3)

(3.4) (3.5)

where a is t h e i s o t r o p i c h f ( s h f ) c o n s t a n t ( F e r m i c o n t a c t term), b

is t h e a n i s o t r o p i c h f ( s h f ) c o n s t a n t a n d b’ d e s c r i b e s t h e d e v i a t i o n x, y , z i s t h e p r i n c i p a l a x i s

of t h e t e n s o r from a x i a l symmetry.

Electron Spin Resonance

96

M13

3 1

i2-

\

1

2

C Figure 4 E l e c t r o n Zeeman l e v e l s c h e m e f o r a n e l e c t r o n w i t h shf c o u p l i n g to t h r e e e q u i v a l e n t n u c l e i w i t h I = 1 / 2 a n d t h e r e s u l t i n g ESR transitions

system,

whereby t h e l a r g e s t i n t e r a c t i o n is a l o n g t h e z - d i r e c t i o n .

A d i s c u s s i o n of t h e i n t e r p r e t a t i o n of t h e hf beyond t h e s c o p e o f

t h i s Chapter.

However,

(shf) i n t e r a c t i o n is

t o f a c i l i t a t e under-

s t a n d i n g of t h e f o l l o w i n g d i s c u s s i o n s some i n t r o d u c t o r y r e m a r k s are necessary .3,4 The h f

i n t e r a c t i o n c o n s t a n t s a r e d e t e r m i n e d by t h e e l e c t r o n i c

wave f u n c t i o n o f t h e d e f e c t s a n d t h e n u c l e a r m o m e n t s o f t h e n u c l e i . I n a s i m p l e one p a r t i c l e a p p r o x i m a t i o n t h e i s o t r o p i c c o n s t a n t , a, is

4: ENDOR Spectroscopy in the Study of Defects in Solids

97

given by: al

where $(r) i s t h e wave f u n c t i o n o f t h e d e f e c t , $(rl) i t s a m p l i t u d e

at the site rl

of

a particular nucleus.

The a n i s o t r o p i c

tensor

e l e m e n t s a r e g i v e n by:

(3.7) where

r

is the

radius vector

from

the nuclear

s i t e of

(origin) where t h e o r i g i n i s s q u a r e d i n t h e i n t e g r a l of

concern equation

( 3 . 7 ) . Thus, t h e hf c o n s t a n t s are p r o p o r t i o n a l t o gI and t h e r e f o r e t h e i n t e r a c t i o n c o n s t a n t s o f d i f f e r e n t i s o t o p e s m u s t be i n t h e r a t i o of t h e i r r e s p e c t i v e g 1 f a c t o r s . T h i s w a s used f o r t h e i d e n t i f i c a t i o n of Tet d e f e c t s i n Si.8 I n s o l i d s t h e shf s t r u c t u r e of d e f e c t s is usually not, o r only p a r t l y , r e s o l v e d i n ESR. T h e r e a s o n i s n o t b e c a u s e i t i s t o o s m a l l , b u t b e c a u s e t h e ESR s p e c t r u m i s a s u p e r p o s i t i o n o f t o o m a n y s h f l i n e s w i t h s p l i t t i n g s , which are n o t very d i f f e r e n t from e a c h o t h e r . I n F i g u r e 5 t h i s i s s c h e m a t i c a l l y shown f o r a S = 112 d e f e c t and interactions with two s h e l l s of equivalent n u c l e i , each of which c o n t a i n s n1 a n d n 2 n u c l e i w i t h s p i n I1 a n d 1 2 , r e s p e c t i v e l y . b o t h e l e c t r o n Zeem a n l e v e l s t h e r e a r e N = ( 2 n l * 1 1 t 1 ) ( 2 n 2 * 1 2 t 1 )

For sub-

l e v e l s a n d t h e r e f o r e t h e r e a r e u p t o N ESR t r a n s i t i o n s . I f t h e r e i s no q u a d r u p o l e i n t e r a c t i o n , t h e r e a r e , transitions,

however,

only

f o u r ENDOR

two f o r e a c h s h e l l of e q u i v a l e n t n u c l e i , which f o l l o w s

f r o m t h e ENDOR s e l e c t i o n r u l e s a s N M R Taking i n t o a c c o u n t more i n t e r a c t i o n s ,

transitions

(see b e l o w ) .

then a n exponential increase

i n t h e n u m b e r o f s u p e r i m p o s e d ESR l i n e s r e s u l t s , w h i c h a r e n o l o n g e r r e s o l v e d . For s o l i d s t a t e d e f e c t s i n t e r a c t i o n s w i t h up t o t e n s h e l l s of n e i g h b o u r n u c l e i a r e n o t u n u s u a l .

W h a t t h e ESR s h o w s i s o n l y t h e

envelope of a l l t h e s h f t r a n s i t i o n s y i e l d i n g a n 'inhomogeneously' b r o a d e n e d ESR l i n e . F i g u r e 2 i s a n e x a m p l e o f t h i s :

t h e width of t h e

T e t ESR l i n e s i s d u e t o u n r e s o l v e d s h f i n t e r a c t i o n s w i t h many s h e l l s of

29Si

nuclei.

I n ENDOR t h e r e a r e o n l y t w o l i n e s f o r e a c h s h f

coupled nucleus with distinguishable shf I n ENDOR

there

is

therefore

i n t e r a c t i o n ( f o r S = 112).

an e x c e l l e n t r e s o l u t i o n of

shf

interactions.

4.1

4. Electron Nuclear Double Resonance (ENDOR) Stationary ENDOR - T h e r e a r e t w o m e t h o d s o f m e a s u r i n g ENDOR, i f

Electron Spin Resonance

98 one does not include s p i n echo techniques.

The d i s p e r s i o n method i s

s u i t e d f o r e s p e c i a l l y l o n g 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 s , TI. T h e o t h e r i s t h e s t a t i o n a r y ENDOR m e t h o d , the two. Seidel.".

w h i c h i s t h e m o r e v e r s a t i l e of

The f o r m e r w a s i n t r o d u c e d by F e h e r , l

latter

the

by

A q u a n t i t a t i v e d e s c r i p t i o n o f both methods is d i f f i c u l t ,

since f o r s o l i d state defects t h e s i t u a t i o n is so complicated with regard t o r e l a x a t i o n p a t h s a n d c o u p l i n g s t h a t a s y e t t h e r e i s no good g e n e r a l q u a n t i t a t i v e u n d e r s t a n d i n g o f

t h e ENDOR e f f e c t .

Since

t h e p u r p o s e o f t h i s C h a p t e r i s t o s h o w t h e a p p l i c a t i o n o f ENDOR t o

Superhyperfine structure shell I

shell I1

ry equivalent nuclei

n2 equivalent nuclei with Spin 1 2

with Spin 11

.+n21 :\n 2~ 1-n2I

n

hve 00..

M12= - P I ,

1

3

+nl Ill Figure

5

.-n212

-

:,n 212-1 - +n212

Electron Zeeman l e v e l scheme f o r an e l e c t r o n w i t h s h f c o u p l i n g t o several s h e l l s o f e q u i v a l e n t n u c l e i . The ESR l i n e i s an envelope over a l l s h f l i n e s , w h i c h u s u a l l y a r e n o t r e s o l v e d a n y more due t o t h e l a r g e number of superimposed l i n e s

99

4: ENDOR Spectroscopy in the Study of Defects in Solids

t h e d e t e r m i n a t i o n o f d e f e c t s t r u c t u r e s , o n l y t h e more commonly u s e d s t a t i o n a r y ENDOR i s d i s c u s s e d by means o f a s i m p l e w o r k i n g m o d e l t o i l l u s t r a t e the experiments carried out. F igu re 6 s h o w s f o r S = 1 / 2 and o n e n u c l e u s w i t h I

=

3/2 schemat-

i c a l l y t h e a l l o w e d ESR t r a n s i t i o n s w i t h r e s o l v e d s h f s t r u c t u r e . I n

TI

v[MHz]

Figure

6 Energy l e v e l d i a g r a m t o e x p l a i n s t a t i o n a r y e l e c t r o n n u c l e a r d o u b l e resonance (ENDOR) ( s e e t e x t )

100

Electron Spin Resonance

a n ENDOR e x p e r i m e n t o n e o f t h e a l l o w e d ESR t r a n s i t i o n s i s p a r t i a l l y saturated, t h a t i s one c h o o s e s a microwave power s u f f i c i e n t l y h i g h s o t h a t t h e t r a n s i t i o n s p r o b a b i l i t y , W M w = yB:,

is of t h e o r d e r of

o r l a r g e r t h a n t h e s p i n l a t t i c e r e l a x a t i o n r a t e , WREL = l / T 1 ( y i s t h e g y r o m a g n e t i c r a t i o o f t h e e l e c t r o n a n d B1 t h e m i c r o w a v e f i e l d amplitude).

If

t h i s i s t h e case t h e n

the spin population

of

the

l e v e l s c o n n e c t e d by t h e m i c r o w a v e t r a n s i t i o n s d e v i a t e s f r o m t h e Boltzmann e q u i l i b r i u m d i s t r i b u t i o n . becom e e q u a l l y p o p u l a t e d . disappearance of

I f WMW>>WREL, t h e n t h e s e l e v e l s

This r e s u l t s i n a decrease and u l t i m a t e l y

t h e observable microwave absorption,

s i n c e micro-

wave a b s o r p t i o n a n d e m i s s i o n p r o b a b i l i t i e s a r e e q u a l . T h e l e v e l s n o t c o n n e c t e d by t h e m i c r o w a v e t r a n s i t i o n s a r e n o t a f f e c t e d . e.g.,

Therefore,

t h e l e v e l p o p u l a t i o n m s = + 1 / 2 , m I = + 1 / 2 i s now i n v e r t e d w i t h

respect t o t h e l e v e l ms = +1/2,

m 1 = -1/2

( s e e F i g u r e 6). I f t h e s e

t w o l e v e l s a r e c o n n e c t e d by a n N M R t r a n s i t i o n , t h e l e v e l p o p u l a t i o n s can be e q u a l i s e d , l e v e l ms = +1/2,

which r e s u l t s i n a population d e c r e a s e of

ly) s a t u r a t e d t r a n s i t i o n ms = -1/2, +1/2.

the

m I = + 1 / 2 l e a d i n g t o d e s a t u r a t i o n of t h e ( p a r t i a l m I = +1/2 t o ms

This d e s a t u r a t i o n has been monitored.

=

+1/2,

mI =

It o c c u r s f o r two NMR

frequencies i n t h e example i n F i g u r e 6 s i n c e t h e NMR f r e q u e n c i e s f o r

ms

= +1/2

and ms

= -1/2

a r e d i f f e r e n t (see below).

Thus,

each

n u c l e u s g i v e s r i s e t o t w o ENDOR l i n e s ( f o r S = 1 / 2 ) . A c r o s s r e l a x a t i o n Tx ( s e e F i g u r e 6) a l l o w s t h e s t a t i o n a r y o b s e r v a t i o n o f

the

d e s a t u r a t i o n ( s t a t i o n a r y E N D 0 R ) . l 1 I f s e v e r a l n u c l e i w i t h t h e same o r similar i n t e r a c t i o n s are coupled t o t h e unpaired e l e c t r o n ,

then

t h e ESR p a t t e r n b e c o m e s c o m p l i c a t e d a n d t h e s h f s t r u c t u r e i s u s u a l l y not resolved.

I n ENDOR a l l n u c l e i w i t h t h e same i n t e r a c t i o n g i v e

r i s e o n l y t o t w o ( f o r S = 1 / 2 ) ENDOR l i n e s , w h i c h g r e a t l y e n h a n c e s

t h e r e s o l u t i o n . ENDOR l i n e s , l i k e N M R l i n e s , a r e t y p i c a l l y 10-100 kHz w i d e ,

a b o u t t h r e e o r d e r s of magnitude n a r r o w e r t h a n homogeneous

ESR l i n e s . T h u s i n ENDOR o n e u s e s t h e s e n s i t i v i t y e n h a n c e m e n t d u e t o a q u a n t u m s h i f t f r o m f r e q u e n c i e s o f t h e o r d e r o f MHz t o t h e m i c r o -

w a ve f r e q u e n c i e s o f t h e o r d e r o f GHz a n d t h e i n c r e a s e d r e s o l u t i o n pow e r d u e t o t h e s m a l l e r N M R l i n e w i d t h a n d t h e r e d u c t i o n i n t h e num be r o f l i n e s . F i g u r e 7 s h o w s a s a n e x a m p l e ENDOR l i n e s (ESR d e s a t u r a t i o n ) o f t h e n e a r e s t 1 9 F n e i g h b o u r s o f F(C1-) 1/2),

where

the

unpaired

s t r u c t u r e m o d e l o f BaFC1,

c e n t r e s i n SrFCl ( f o r ms =

e l e c t r o n o c c u p i e s a C1- v a c a n c y .

w h i c h h a s t h e same s t r u c t u r e ,

-

The

is contained

i n F i g u r e 18.

4.2

Analysis of ENDOR Spectra - I n g e n e r a l , t h e r e i s b e s i d e s t h e

101

4: ENDOR Spectroscopy in the Study of Defects in Solids

I I

40

42

44

46

1

48

50

52

54

FrequencyIMHz Figure 7 ENDOR l i n e s d u e t o t h e n e a r e s t 1 9 F n e i g h b o u r s of F(Cl-) c e n t r e s i n SrFCl shf i n t e r a c t i o n a quadrupole i n t e r a c t i o n i f I>1/2 €or t h e i n t e r a c t -

ing nuclei. T h i s i n t e r a c t i o n is due t o t h e i n t e r a c t i o n between t h e

electrical f i e l d g r a d i e n t a t t h e s i t e of a nucleus and its n u c l e a r quadrupole moment. T h e r e f o r e t h e s p i n H a m i l t o n i a n d e s c r i b i n g t h e

ENDOR s p e c t r a i s :

T h e s um r u n s over a l l n u c l e i i n t e r a c t i n g w i t h t h e u n p a i r e d e l e c t r o n . For s i m p l i c i t y i t is a s s u m e d t h a t g e i s i s o t r o p i c ( w h i c h i s generally not t h e case). Q i s t h e traceless quadrupole i n t e r a c t i o n tensor with the elements Qik =

eQ 21(2I - 1 )

2 1 aXiaXk

r =

o

(4.2)

w h e r e Q i s t h e q u a d r u p o l e mo me n t a n d V t h e e l e c t r i c a l p o t e n t i a l . T h e s p e c t r a are u s u a l l y a n a l y s e d i n terms of t h e q u a d r u p o l e i n t e r a c t i o n constants:

Electron Spin Resonance

102

T h e s e l e c t i o n r u l e f o r ENDOR t r a n s i t i o n s

Ams = 0 ,

,

l i k e NMR t r a n s i t i o n s ,

A m 1 = 21

is: (.4.4)

I f t h e s h f and q u a d r u p o l e i n t e r a c t i o n i s small compared t o t h e elect r o n Zeeman term,

then the quantisation of the electron spin is not

i n f l u e n c e d by t h e s e i n t e r a c t i o n s a n d t h e n u c l e i a r e i n d e p e n d e n t o f e a c h o t h e r . T h e y c a n b e t r e a t e d s e p a r a t e l y a n d t h e sum i n e q u a t i o n (4.1)

c a n be o m i t t e d .

I n p u r t u r b a t i o n t h e o r y of f i r s t o r d e r ,

t h a t is

with the conditions

with the following abbreviations: Wshf = a t b ( 3 c o s 2 8

where 8,

-

vn

(4.7)

6 a n d 8', 6 ' a r e t h e p o l a r a n g l e s o f B O i n t h e p r i n c i p a l

s h f and t h e quadrupole a x i s s y s t e m ,

where

1) t b ' s i n 2 8 c o s 2 6

respectively.

i s t h e Lamor f r e q u e n c y o f a f r e e n u c l e u s i n t h e m a g n e t i c

f i e l d Bo.

+

m9 = 1/2(mI

m'I)

(4.10)

where m q i s t h e a v e r a g e o f t h e t w o n u c l e a r s p i n q u a n t u m n u m b e r s , w h i c h a r e c o n n e c t e d by t h e t r a n s i t i o n . T h e b a s i c c o n c e p t s f o r t h e a n a l y s i s o f ENDOR s p e c t r a w i l l b e described assuming positions

of

that

equation

t h e ENDOR l i n e s .

(4.6) holds for

the

frequency

T h i s is i n g e n e r a l n o t t h e c a s e ,

s i n c e b o t h t h e a n i s o t r o p i c a n d q u a d r u p o l e i n t e r a c t i o n s c a n become t o o l a r g e w i t h r e s e c t t o t h e i s o t r o p i c term a n d g - a n i s o t r o p i e s

w e l l as t h e f i n e s t r u c t u r e i n t e r a c t i o n can influence t h e spectra.

as A

d e t a i l e d d i s c u s s i o n of t h e c o m p l i c a t i o n s i s beyond t h e s c o p e o f t h i s C h a p t e r . Some p a r t i c u l a r l y i m p o r t a n t c o n s e q u e n c e s o f h i g h e r o r d e r

4: ENDOR Spectroscopy in the Study of Defects in Solids

103

c o n t r i b u t i o n s t o t h e frequency p o s i t i o n s w i l l , however, be d i s c u s s e d below. F i g u r e 8 s h o w s a n d ENDOR s p e c t r u m o f i n t e r s t i t i a l a t o m i c h y d r o g e n i n K C 1 f o r B O / / ( l l O ) , t h e ESR s p e c t r u m o f w h i c h i s s h o w n i n F i g u r e 3 [ f o r BO//(lOO)].

T h e ENDOR s p e c t r u m c o n t a i n s t h e i n t e r a c t i o n w i t h

the n e a r e s t C1-neighbours.9 nature of

The i d e n t i f i c a t i o n of

the chemical

n u c l e i c a n b e a c h i e v e d i n v a r i o u s ways.

According t o

equation (4.6),

f o r S = 1/2 and no quadrupole i n t e r a c t i o n s ,

each

n u c l e u s g i v e s a p a i r o f l i n e s s e p a r a t e d by 2vn i f 1 / 2 W s h f > h v n a n d by Wshf

i f

hvn>l/2Wshf.

equation (4.9),

Since

vn c a n be c a l c u l a t e d a c c o r d i n g

to

t h e n u c l e i can be i d e n t i f i e d e i t h e r from t h e l i n e

p a i r s s e p a r a t e d by 2vn o r by s y m m e t r i c l i n e p a t t e r n s a b o u t vn.

V;faCl,J

I .

7.0

6.5

8.0

9.0 10.0

12.0

14.0

16.0 MHz 18.0

frequency Figure

8

ENDOR s p e c t r u m o f t h e n e a r e s t h a l o g e n n e i g h b o u r s o f i n t e r s t i t i a l atomic hydrogen c e n t r e s i n KCl .BO//. ( A f t e r r e f . 9 )

I f t h e r e are s e v e r a l magnetic i s o t o p e s p r e s e n t (such as 37Cl

35Cl

and

i n F i g u r e 8), t h e n t h e i r l i n e p o s i t i o n s m u s t be i n t h e r a t i o o f

t h e i r r e s p e c t i v e n u c l e a r m o m e n t s ( i f Q = 0). roughly r e f l e c t s t h e i s o t o p e abundance.

The l i n e i n t e n s i t y

Unfortunately,

t h e ENDOR

l i n e i n t e n s i t y is very l i t t l e understood q u a n t i t a t i v e l y i n s o l i d s d u e t o t h e many a n d c o m p l i c a t e d r e l a x a t i o n p a t h s . T h e r e f o r e , t h e t w o l i n e s a c c o r d i n g t o e q u a t i o n (4.6)

of e a c h n u c l e u s c a n n o t a l w a y s be

o b s e r v e d . i n p a r t i c u l a r f o r l o w f r e q u e n c i e s i n s t a t i o n a r y ENDOR. However,

o n s h i f t i n g t h e m a g n e t i c f i e l d t h r o u g h t h e ESR l i n e t h e

ENDOR l i n e p o s i t i o n s a r e a l s o s h i f t e d a c c o r d i n g t o e q u a t i o n s ( 4 . 6 ) a n d (4.9).

T h e ENDOR l i n e s h i f t i s d u e t o t h e s h i f t o f v n a n d i s

thus p r o p o r t i o n a l t o gI,

w h i c h is c h a r a c t e r i s t i c f o r a p a r t i c u l a r

n u c l e u s . I f t h e ESR l i n e w i d t h d o e s n o t a l l o w a b i g e n o u g h ENDOR l i n e s h i f t t o b e d e t e c t e d ( w h i c h d e p e n d s o n t h e ENDOR l i n e w i d t h ) ,

then

Electron Spin Resonance

104

o n e c a n e i t h e r d o a d d i t i o n a l e x p e r i m e n t s w i t h a d i f f e r e n t ESR b a n d ( e . g . , K-band o r Q-band) o r c h a n g e t h e r e s o n a n t f r e q u e n c y of t h e c a v i t y by i n s e r t i n g m a t e r i a l o f a s u i t a b l e d i e l e c t r i c c o n s t a n t a n d changing t h e microwave s o u r c e frequency a c c o r d i n g l y . I f t h e r e i s a q u a d r u p o l e s p l i t t i n g , t h e n e a c h ‘ h f ’ ENDOR l i n e i s s p l i t i n t o a c h a r a c t e r i s t i c m u l t i p l e t , e.g.,

f o r I = 3/2 i n t o a

t r i p l e t . T h i s is e a s i l y recognised i n Figure 8 as a t r i p l e t s t r u c t u r e o f e a c h o f t h e ENDOR l i n e s . In order t o determine t h e d e f e c t s t r u c t u r e and t h e i n t e r a c t i o n t h e d e p e n d e n c e o f t h e ENDOR l i n e p o s i t i o n s u p o n v a r i a t -

parameters,

ion of t h e magnetic f i e l d w i t h r e s p e c t t o t h e c r y s t a l o r i e n t a t i o n m u s t b e m e a s u r e d a n d a n a l y s e d . T h i s i s t h e m a j o r p r o b l e m i n a n ENDOR a n a l y s i s and t h e e s s e n t i a l t o o l f o r t h e determination of t h e d e f e c t structure. F i g u r e 9 (a)-(c) crystal,

show such a n a n g u l a r dependence f o r a c u b i c

such as an a l k a l i h a l i d e ,

calculated according t o equation

(4.6) f o r t h e f i r s t t h r e e neighbour s h e l l s of a d e f e c t on a lattice s i t e . T h e p a t t e r n s a r e c h a r a c t e r i s t i c f o r ( l o o ) , (110), a n d (111) ‘symmetry’ o f t h e n e i g h b o u r n u c l e i . F o r e a c h ms v a l u e s u c h a p a t t e r n i s o b s e r v e d . From t h e number o f s u c h p a t t e r n s , a c c o r d i n g t o e q u a t i o n

(4.6) one can i n f e r t h e e l e c t r o n s p i n of t h e d e f e c t and t h u s o f t e n its charge state. Each n u c l e u s h a s i t s own p r i n c i p a l a x i s s y s t e m f o r t h e s h f a n d quadrupole tensors.

Often,

mined by s y m m e t r y . a n a l y s i s of

t h e i r o r i e n t a t i o n i n a c r y s t a l is d e t e r -

Otherwise,

t h e y must be determined from t h e

t h e angular dependence of

t h e ENDOR s p e c t r a .

I f

the

d e f e c t c e n t r e ( i m p u r i t y ) and t h e r e s p e c t i v e n u c l e u s are i n a m i r r o r plane,

then two p r i n c i p a l axes must be i n t h i s m i r r o r plane.

If the

connection l i n e between t h e nucleus and t h e c e n t r e is a t h r e e f o l d o r higher symmetry a x i s ,

then t h e tensor is a x i a l l y symmetric with its

a x i s i n t h i s symmetry a x i s . I f t h e a n g u l a r p a t t e r n s are s e p a r a t e d i n frequency,

t h e y are e a s i l y recognised and t h e a n a l y s i s is f a i r l y

straightforward. F i g u r e 10 s h o w s s u c h a n a n g u l a r d e p e n d e n c e f o r s u b s t i t u t i o n a l T e + i n S i ( s e e F i g u r e 2 f o r t h e ESR s p e c t r u m ) .

as f o l l o w s :

In practice,

one proceeds

one assumes a c e n t r e model and t h e n c a l c u l a t e s t h e

expected a n g u l a r dependence a c c o r d i n g t o t h e a p p r o p r i a t e s p i n H a m i l t o n i a n making u s e of

t h e symmetry p r o p e r t i e s o f t h e assumed model.

Comparison w i t h t h e e x p e r i m e n t a l a n g u l a r dependence assumption t o b e

true or false.

then shows t h i s

T h e s y m m e t r y p a t t e r n s of

n e i g h b o u r c e l l s m ay n o t b e e a s i l y r e c o g n i s e d ,

the

i f the f i r s t order

s o l u t i o n o f t h e H a m i l t o n i a n is n o t s u f f i c i e n t . F o r t h e t e t r a h e d r a l

a+2b

3' .5-

a-ib

o- b

01

410011

10101

/

loo11

lolY''

Figare 9 Calculated ENDOR a n g u l a r dependence f o r a d e f e c t on a c a t i o n s u b s t i t u t i o n a l s i t e . ( a ) ( 1 0 0 ) neighbours. ( b ) ( 1 1 1 ) - n e i g h b o u r s , (c) (110)-neighbours

55

0

20

LO 60 Angle I Degrees

80

0

20

LO

60

80

Angle I Degrees

Figure 10 Angular dependence o f 29Si ENDOR l i n e s of Te' d e f e c t s i n Si f o r r o t a t i o n o f t h e magnetic f i e l d i n a ( 1 1 0 ) p l a n e . 0' = . The s o l i d l i n e s a r e t h e c a l c u l a t e d a n g u l a r d e p e n d e n c i e s w i t h t h e parameters of T a b l e 3. ( A f t e r r e f . 1 2 )

107

4: ENDOR Spectroscopy in the Study of Defects in Solids

symmetry o f a s u b s t i t u t i o n a l p o i n t d e f e c t i n a d i a m o n d l a t t i c e o r zincblende l a t t i c e , t h e s h f i n t e r a c t i o n s of n e a r e s t neighbours s h o u l d g i v e t h e s i m p l e p a t t e r n o f F i g u r e 11. I f

I = 3/2 f o r t h e

n e a r e s t n e i g h b o u r s a n d a small q u a d r u p o l e i n t e r a c t i o n is p r e s e n t , the pattern looks l i k e Figure ll(b), s p l i t t i n g of

where t h e quadrupole t r i p l e t

t h e ENDOR l i n e s i s e a s i l y r e c o g n i s e d .

However,

for

l a r g e r values of t h e quadrupole i n t e r a c t i o n constant, q, a f t e r diago n a l i s a t i o n of t h e s p i n Hamiltonian t h e p a t t e r n changes considerably [ F i g u r e l l ( c ) ] a n d i t s o r i g i n i s n o t so e a s i l y r e c o g n i s e d .

It is

o f t e n t h i s c o m p l i c a t i o n through quadrupole i n t e r a c t i o n s t h a t makes the analysis difficult. I n t h e case o f t h e d i a m o n d o r z i n c b l e n d e s t r u c t u r e ( s i l i c o n o r

111-V

compounds) t h e r e is t h e p a r t i c u l a r d i f f i c u l t y i n t h a t t h e

symmetry o f t h e s u b s t i t u t i o n a l s i t e a n d t e t r a h e d r a l i n t e r s t i t i a l

s i t e cannot be d i s t i n g u i s h e d as t o t h e symmetry t y p e of neighbour nuclei with respect

(loo),

t o both defects.

There are n e i g h b o u r s w i t h

(110), a n d (111) s y m m e t r y o f t h e i r t e n s o r s , o n l y t h e s e q u e n c e

i s d i f f e r e n t i n e a c h case. F r o m e x p e r i m e n t o n l y t h e s y m m e t r y t y p e o f nuclei c a n be d e t e r m i n e d , n o t t h e d i s t a n c e of t h e n u c l e i from t h e d e f e c t c o r e . Here t h e o r e t i c a l a r g u m e n t s a b o u t t h e s i t e o f t h e d e f e c t o r t h e c h a r a c t e r of t h e wave f u n c t i o n must b e t a k e n i n t o a c c o u n t before establishing a definite m ~ d e l . ~ * * ~ ~ I f t h e agreement between t h e c a l c u l a t e d angular dependence and t h e e x p e r i m e n t a l one i s good, t h e a n a l y s i s i s unambiguously c o r r e c t . T h e r e a r e many m o r e e x p e r i m e n t a l d a t a t h a n p a r a m e t e r s t o b e e x t r a c t e d f r o m them. A s a n e x a m p l e o f t h e r e s u l t o f s u c h a n a n a l y s i s , T a b l e

1 gives t h e shf and quadrupole i n t e r a c t i o n c o n s t a n t s of atomic h y d r o g e n o n a n i o n s i t e s i n KCl(H:,,

centres)."

The a n a l y s i s unam-

biguously t h e site f o r t h e hydrogen atom from t h e symmetry of t h e neighbour nuclei.

The s h f a n d q u a d r u p o l e i n t e r a c t i o n c o n s t a n t s c o u l d

b e d e t e r m i n e d down t o v a r y s m a l l i n t e r a c t i o n s w i t h h i g h p r e c i s i o n . The

spin

Hamiltonian

has

to

A n o t h e r e x a m p l e i s t h a t o f Te'

be

numerically.lO*l4 Here t h e s e q u e n c e o f

diagonalised

i n S i ( T a b l e 2).

S i neighbours is c l a s s i f i e d according t o t h e i r symmetry types. A s m e n t i o n e d a b o v e , o n l y t h e o r e t i c a l a r g u m e n t s l e a d f i n a l l y t o a n unamb i g u o u s a s s i g n m e n t o f t h e Te'

t o t h e s u b s t i t u t i o n a l site.13

I n a s t r a i g h t ENDOR a n a l y s i s o n l y t h e r e l a t i v e s i g n s o f t h e s h f constants a, b,

b' c a n b e d e t e r m i n e d a n d a l s o n o t t h e s i g n s o f t h e

quadrupole i n t e r a c t i o n constants with respect t o t h e shf i n t e r a c t i o n constants.

W i t h ENDOR-induced

ESR a n d DOUBLE-ENDOR

more can be

e x t r a c t e d from experiment about t h e s i g n s of t h e i n t e r a c t i o n s . I n s o l i d s a p a r t i c u l a r s i t u a t i o n i s met, i f t h e r e are s h f (and

108

Electron Spin Resonance

Table 1 Shf and quadrupole c o n s t a n t s of H;,,-centres She1 1

Cons t a n t

39KI

a

i n KCl ( i n Mz), T = 40 K*

253 219 198

b 9

35clII

a

57 312 -3 -88 -94

b b' 9 9'

a b

37 54 f45

9

4 11

a b b'

" 0

26.0'

43

0.2'

13.5O f 0.2'

4Q

*After ref. 10.

f

*39 f17

9 9'

Table 2 S h f i n t e r a c t i o n s of t h e Te' and 'S c e n t r e s w i t h 29Si*

Type

a/MHz

Si :Te* b/MHz

111

17.7 11.4 1.3

9.8 0.47 0.03

100

4.6 1.8

; (b) ENDOR-induced ESR spectrum for t h e 19F-ENDOR l i n e s a t 42.5 MHz - t h e s p e c t r u m i s due t o Fe3'1 ( c ) ENDOR-induced ESR s p e c t r u m for t h e 19F-ENDOR l i n e s a t 21.0 M H z - t h e s p e c t r u m i s due t o F-centres. ( A f t e r r e f . 45)

126

Electron Spin Resonance

each ENDOR l i n e c a n b e ' l a b e l l e d '

t o a particular defect,

i n the

case o f t h e s i m u l t a n e o u s p r e s e n c e o f s e v e r a l d e f e c t s t h i s c a n b e a tedious task, dependence.

e s p e c i a l l y i f one h a s t o f o l l o w a complicated angular

Therefore,

a method i s r e q u i r e d w i t h which t h e ENDOR

s p e c t r a o f d i f f e r e n t d e f e c t s c a n be m e a s u r e d s e p a r a t e l y , t h i s c a n be i n which two NMR f r e q u e n c i e s

d o n e by m e a s u r i n g a t r i p l e r e s o n a n c e ,

are a p p l i e d s i m u l t a n e o u s l y t o g e t h e r w i t h t h e microwaves. I n a n ENDOR e x p e r i m e n t t h e r f - i n d u c e d

NMR t r a n s i t i o n s between t h e

n u c l e a r Zeeman l e v e l s o f a n e i g h b o u r n u c l e u s c o u p l e d t o t h e u n p a i r e d e l e c t r o n by a s h f i n t e r a c t i o n c h a n g e s o m e w h a t t h e p o l a r i s a t i o n o f t h e e l e c t r o n s p i n i n t h e p a r t i a l l y s a t u r a t e d s i t u a t i o n . T h i s coupling between neighbour nucleus and unpaired e l e c t r o n is i n d i c a t e d s c h e m a t i c a l l y by a ' s p r i n g '

i n F i g u r e 22.

DOUBLE 0

.

.

.

I f s i m u l t a n e o u s l y a second

ENDOR .

.

.

Figure 22 Schematic representation of ENDOR and DOUBLE-ENDOR

NMR t r a n s i t i o n i s i n d u c e d w i t h a s e c o n d r f f r e q u e n c y a t a n o t h e r n u c l e u s c o u p l e d t o t h e same u n p a i r e d e l e c t r o n ,

then the induced

change i n t h e e l e c t r o n s p i n p o l a r i s a t i o n is d i f f e r e n t f r o m what i t would b e i f t h e f i r s t N M R t r a n s i t i o n d i d n o t o c c u r s i m u l t a n e o u s l y . Thus, t h e p o l a r i s a t i o n c h a n g e d u e t o t h e s e c o n d N M R t r a n s i t i o n i s dependent on t h e o c c u r r e n c e o f t h e f i r s t NMR t r a n s i t i o n . The t o t a l e l e c t r o n d e s a t u r a t i o n (ENDOR e f f e c t ) i s a f u n c t i o n o f t h e p r o d u c t o f t h e e f f e c t s o f t h e t w o N M R t r a n s i t i o n s . T h u s , when m o d u l a t i n g t h e two N M R t r a n s i t i o n s w i t h d i f f e r e n t f r e q u e n c i e s , u s i n g d o u b l e l o c k - i n

127

4: ENDOR Spectroscopy in the Study of Defects in Solids

techniques,

one can induce one p a r t i c u l a r

ENDOR t r a n s i t i o n o f

a

p a r t i c u l a r n e i g h b o u r n u c l e u s a n d m o n i t o r t h e c h a n g e i n t h e ENDOR s i g n a l h e i g h t a s a f u n c t i o n o f t h e s i m u l t a n e o u s s e c o n d ENDOR t r a n s ition.

One o b s e r v e s c h a n g e s i n t h e NMR l i n e i n t e n s i t y a s a f u n c t i o n

o f t h e s e c o n d f r e q u e n c y s w e p t r f s o u r c e , A c h a n g e i s o b s e r v e d when a second

ENDOR

transition

is

induced

at

a

nucleus

that

is

also

c o u p l e d t o t h e same e l e c t r o n , b u t n o t i f t h e n u c l e u s b e l o n g s t o a different

centre.

T h e ESR o f

saturated

i f

ESR

Therefore,

the

spectra

this of

d i f f e r e n t c e n t r e may a l s o be the

d i f f e r e n t centres overlap.

t h i s t r i p l e r e s o n a n c e e x p e r i m e n t c a n be u s e d t o s e p a r a t e

t h e ENDOR s p e c t r a o f d i f f e r e n t d e f e c t s i f t h e i r ESR s p e c t r a o v e r l a p . Such a s e p a r a t i o n m a y w e l l o t h e r w i s e be i m p o s s i b l e i f t h e c e n t r e s h a v e m a n y ENDOR l i n e s w i t h c o m p l i c a t e d a n g u l a r d e p e n d e n c i e s . However, t h e method f u n c t i o n s w e l l o n l y i f t h e n e i g h b o u r s have a h i g h

+?

1

-f--JL

1

--

2

ESR

1

/2-j !

1

+-

2

mS

mI

Figure 23 L e v e l s c h e m e t o e x p l a i n t h e s p e c i a l t r i p l e r e s o n a n c e e x p e r i m e n t s (DOUBLE-ENDOR)

Electron Spin Resonance

128

5

10

15

Frequency/ MHz

5

10

15

Frequency/ MHz

5

10

15

Frequency / M Hz Figure 24

( a ) Part o f t h e ENDOR spectrum o f F(C1-) and F(F-) c e n t r e s s im ultane o u s l y p r e s e n t i n BaFCli ( b ) DOUBLE-ENDOR s p e c t r u m o b t a i n e d f r o m s e t t i n g o n e f r e q u e n c y t o an ENDOR l i n e o f F(Cl-) c e n t r e s ( s e e ar r ow i n Figure 24) and sweeping t h e second r f frequency; ( c ) DOUBLE-ENDOR spectrum obtained from s e t t i n g on r f frequency t o an F(F-)-ENDOR l i n e ( s e e arrow i n Figure 24). ( A f t e r r e f . 44)

4: ENDOR Spectroscopy in the Study of Defects in Solids

129

a b u n d a n c e o f m a g n e t i c n u c l e i . I n S i , w h e r e o n l y 4.7% o f t h e n u c l e i

are magnetic (29Si), t h e p r o b a b i l i t y of having s i m u l t a n e o u s l y two m a g n e t i c n u c l e i a s d e f e c t n e i g h b o u r s i s t o o l o w t o p e r m i t DOUBLEENDOR e x p e r i men t S, The s i m p l e s t e x p e r i m e n t ,

-

t h e so c a l l e d ' s p e c i a l t r i p l e r e s o n a n c e '

( s p e c i a l DOUBLSENDOR) i s s c h e m a t i c a l l y s h o w n i n F i g u r e 2 3 f o r t h e s i m p l e case o f S

1/2,

t h e t r a n s i t i o n 'NMR1'

I = 1 / 2 . I f s t a t i o n a r y ENDOR i s m e a s u r e d f o r m I = 1 / 2 a n d -1/2, t h e n t h e

between mS = -1/2,

s i g n a l h e i g h t i s d e t e r m i n e d by t h e ESR t r a n s i t i o n p r o b a b i l i t y ( t h a t

i s by B f ) ,

t h a t Tx > T e ,

experiment),

time,

[ t h a t i s B:f (NMRl)] a n d It i s assumed (and a c o n d i t i o n f o r t h e

the NMRl t r a n s i t i o n probability

t h e c r o s s r e l a x a t i o n t i m e Tx.

the electron spin lattice relaxation

due t o t h e comparatively long n u c l e a r 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 Tn. I f t h e n a s e c o n d r f f r e q u e n c y i s a p p l i e d b e t w e e n t h e l e v e l s mS = 1 / 2 , m I = 1 / 2 a n d - 1 / 2

(NMR2) t h e n T n i s e f f e c t i v e l y s h o r t e n e d

b y t h i s t r a n s i t i o n a n d t h e r e f o r e Tx i s a l s o s h o r t e n e d , w h i c h r e s u l t s i n a n e n h a n c e m e n t o f t h e m o n i t o r e d ENDOR s i g n a l a t t h e f r e q u e n c y NMR1. I n t h e e x p e r i m e n t o n e i r r a d i a t e s w i t h t h e f i x e d ENDOR f r e q u e n c y N M R 1 , m o n i t o r s t h e ENDOR l i n e i n t e n s i t y o f t h e l i n e a t N M R 1 , while sweeping t h e second r f

frequency.

When t h e t r a n s i t i o n NMR2 i s

i n d u c e d t h e ENDOR l i n e i n t e n s i t y N M R l i n c r e a s e s . T h e i n c r e a s e i s t h e DOUBLE-ENDOR

s i g n a l and is d e t e c t e d w i t h a double lock-in

technique.

I n t h e s t a t i o n a r y DOUBLE-ENDOR s p e c t r u m p o s i t i v e a n d n e g a t i v e s i g n a l s are observed.42s43 Negative s i g n a l s occur i f

t h e second NMR

f r e q u e n c y i s i n d u c e d b e t w e e n n u c l e a r s t a t e s b e l o n g i n g t o t h e same mS q u a n t u m n u m b e r . F i g u r e 2 4 ( c ) s h o w s t h e DOUBLE-ENDOR s p e c t r u m f o r t h e t w o F c e n t r e s i n BaFC1.

I n F i g u r e 24(a)

centres are superimposed;

t h e ENDOR l i n e s o f b o t h F

a f u l l a n a l y s i s was n o t p o s s i b l e .

In

F i g u r e 2 4 ( b ) t h e f i x e d E N D O R f r e q u e n c y N M R l was s e t t o o n e E N D O R l i n e b e l o n g i n g t o F(Cl-)

c e n t r e s a n d NMR2 was s w e p t b e t w e e n 1 a n d 9

MHz a n d t h e DOUBLE-ENDOR e f f e c t was r e c o r d e d .

I n F i g u r e 24(c) t h e

a n a l o g o u s e x p e r i m e n t w a s c a r r i e d o u t f o r a F(F-) DOUBLE-ENDOR

s p e c t r a show o n l y l i n e s due t o

c e n t r e s alone. lines,

ENDOR l i n e . B o t h

t h e F(Cl-)

of

F(F-)

E s p e c i a l l y a r o u n d 5 MHz b o t h c e n t r e s h a v e many ENDOR

which o t h e r w i s e c o u l d n o t have been separated.44

F i g u r e 1 2 ( a ) s h o w s a s a n o t h e r e x a m p l e t h e DOUBLPrENDOR s p e c t r u m measured f o r t h e Ga-vacancy

i n GaP f o r B O / / < l l l > . C o m p a r i s o n w i t h

Figure 12(b) demonstrates t h a t a l l t h e l i n e s measured i n s i n g l e ENDOR a l s o a p p e a r i n DOUBLE-ENDOR.

T h i s p r o v e s t h a t a l l ENDOR l i n e s

belong i n d e e d t o one d e f e c t and t h a t t h e v a c a n c y is n o t d i s t o r t e d w i t h t h e c o n s e q u e n c e t h a t t h e ENDOR s p e c t r u m m i g h t b e a s u p e r p o s i t i o n o f s e v e r a l v a c a n c y c o n f i g u r a t i o n s . T h e o c c u r r e n c e o f s o many

Electron Spin Resonance

130 ENDOR l i n e s ,

a t f i r s t unexpected f o r a simple t e t r a h e d r a l surround-

i n g o f f o u r e q u i v a l e n t 31P n e i g h b o u r s ,

is i n d e e d due t o a l a r g e and

h i t h e r t o undescribed e f f e c t o f s h f s t r u c t u r e of second order.16 DOUBLE-ENDOR i s a l s o v e r y i m p o r t a n t f o r a n a l y s i n g l o w s y m m e t r y The d e f e c t s are d i s t r i b u t e d o v e r s e v e r a l o r i e n t a t i o n s i n

defects.

t h e c r y s t a l . T h e ESR a n d ENDOR s p e c t r a o f t h e s e o r i e n t a t i o n s o v e r lap.

I n a sense each defect orientation is equivalent

defect species.

W i t h DOUBLE-ENDOR

t h e spectra of

t o a new

one particular

d e f e c t o r i e n t a t i o n c a n be measured s e p a r a t e l y , which g r e a t l y f a c i l i t a t e s t h e a n a l y s i s o r makes i t a l l p o s s i b l e . ation of 'no'

I n a recent investig-

c e n t r e s i n mAl203, which has very low symmetry ( t h a t is

(r

symmetry),

a d e f i n i t e a s s i g n m e n t o f t h e q u a d r u p o l e ENDOR l i n e s

t o t h e i r corresponding 'hf'

ENDOR l i n e s was o n l y p o s s i b l e a f t e r o n e

p a r t i c u l a r c e n t r e o r i e n t a t i o n c o u l d be m e a s u r e d ~ e p a r a t e l y . ~ ~

Optically detected ENDOR

6.

R e c e n t l y ENDOR c o u l d s u c c e s s f u l l y b e m e a s u r e d a l s o by o p t i c a l d e t e c t i o n using t h e o p t i c a l absorption of defects i n 111-V

semiconductors

a n d i n i o n i c c r y s t a l s . T h e o p t i c a l d e t e c t i o n o f ESR i s b a s e d o n t h e detection of microwave-induced

c h a n g e s of

the magnetic circular

d i c h r o i s m ( M C D ) o f t h e o p t i c a l a b s o r p t i o n a s was f i r s t s h o w n f o r F T h e MCD i s p r o p o r t i o n a l

centres i n a l k a l i halides.46 polarisation

of

the

ground

s t a t e Zeeman l e v e l s .

For

to the spin

a Kramer's

d o u b l e t w i t h S = 1 / 2 i t i s g i v e n by o+ - 0-

MCD = 1 / 2 a 0

n- - n + . -. -

o+

w h e r e a. 0-

are

+

0-

n-

+

(6.1)

n+

is t h e absorption c o n s t a n t of t h e unpolarised

the

cross sections for

l i g h t , '0 a n d l e f t and r i g h t p o l a r i s e d l i g h t ,

r e s p e c t i v e l y , a n d n- a n d n + a r e t h e o c c u p a t i o n n u m b e r s f o r t h e m s = i 1 / 2 s t a t e s . The o c c u p a t i o n d i f f e r e n c e o r s p i n p o l a r i s a t i o n c a n be decreased

by a

microwave

transition,

provided

t h e microwave

transition rate is of t h e o r d e r of o r l a r g e r t h a n t h e s p i n l a t t i c e r e l a x a t i o n r a t e 1/ T1 . o f t h e MCD,

T h e ESR t r a n s i t i o n t h u s r e s u l t s i n a d e c r e a s e

which is m ~ n i t o r e d . ~ ~

F i g u r e 25 s h o w s t h e l e v e l scheme f o r t h e case of S = 1 / 2 , a c e n t -

r a l n u c l e u s w i t h 1, = 3 / 2 a n d o n e l i g a n d n u c l e u s w i t h I = 3 / 2 (e.g., a s i m p l i f i e d m o d e l f o r t h e E L 2 + d e f e c t i n GaAs). I n t h e e x p e r i m e n t o n e s e t s t h e m a g n e t i c f i e l d o n t o a p a r t i c u l a r p o s i t i o n o f t h e ODESR l i n e , e.g.,

i n t o t h e f l a n k . ESR t r a n s i t i o n s m u s t o b e y t h e s e l e c t i o n

r u l e A m 1 = 0 , A m s = * l . T h u s , w h e n m e a s u r i n g t h e ESR i n o n e o f t h e f o u r l i n e s o f t h e 1, = 3 / 2 s y s t e m , t h e n a t m o s t a q u a r t e r o f t h e a l l

131

4: ENDOR Spectroscopy in the Study of Defects in Solids

ligand hf interaction

central hf interaction

t t

Figure 25 Level scheme t o i l l u s t r a t e t h e d e t e c t i o n of ENDOR v i a a r f and microwave-induced decrease of the MCD of the o p t i c a l absorption

s p i n p a c k e t s c a n b e i n v o l v e d , o n l y o n e q u a r t e r o f t h e MCD c a n b e d e c r e a s e d when s a t u r a t i n g t h e t r a n s i t i o n . H o w e v e r , i f e a c h o f t h e l i n e s i s i n h o m o g e n e o u s l y b r o a d e n e d by f u r t h e r s h f i n t e r a c t i o n s , t h e n o n l y a f r a c t i o n o f t h e d e c r e a s e s h o u l d o c c u r , s i n c e A ~ I =, 0~ m u s t b e o b e y e d f o r t h e l i g a n d I,.

Thus,

between

levels,

the

nuclear

Zeeman

upon i n d u c i n g NMR t r a n s i t i o n s one

can

include

more

mI,a

s u b s t a t e s i n t o t h e ESR p u m p i n g c y c l e a n d t h u s i n c r e a s e t h e e f f e c t o f d e c r e a s i n g t h e MCD.

T h e r e f o r e , t h e ENDOR t r a n s i t i o n s a r e d e t e c t e d a s

a f u r t h e r i n c r e a s e o f t h e ODESR. F i g u r e 2 6 s h o w s a s e c t i o n o f t h e ODENDOR l i n e s d u e t o t h e n e a r e s t 75As

neighbours of

t h e EL2+ d e f e c t i n a n a s - g r o w n

undoped G a A s

c r y s t a 1 . 6 * 4 7 * 4 8 T h e ENDOR l i n e s a r e a s s h a r p a n d n u m e r o u s a s i n c o n v e n t i o n a l ENDOR.

From t h e i r a n g u l a r dependence

the structure

m o d e l o f t h e EL2 d e f e c t s c o u l d b e d e r i v e d . A s i m i l a r s t u d y was r e c e n t l y m a d e f o r Pb'

and BaF2,

c e n t r e s i n CaF2, S r F 2

i n w h i c h t h e s t r u c t u r e was s h o w n t o b e a Pb'

s i t e s n e x t t o a n F-

on c a t i o n

v a c a n c y a l o n g < 1 1 1 > . T h e 1 9 F ENDOR l i n e s

m e a s u r e d i n t h e MCD a n d t h e i r a n g u l a r d e p e n d e n c e c l e a r l y s h o w e d t h e

132

Electron Spin Resonance

A 40

GO 80 100 f requencyl MHz

120

Figure 26 S e c t i o n o f t h e ODENDOR s p e c t r u m o f t h e ELZt d e f e c t s i n s . i . ' a s grown' GaAs. T = 1.5 K, w i t h Bo i n a ( 1 1 0 ) p l a n e . The l i n e s a r e d u e t o f i v e 7 5 A s neighbours od t h e &Gat, which form t h e core o f t h e ELZt defect. (After ref. 6) symmetry of t h e d e f e c t and i t s a t o m i s t i c s t r u c t u r e . 4 9 s 5 0 The s i g n a l to-noise

r a t i o o f t h e E N D O R l i n e s was v e r y g o o d , a s i t a l s o is f o r

EL2'. There a r e s e v e r a l a d v a n t a g e s f o r t h e o p t i c a l d e t e c t i o n o f ENDOR w i t h t h e M C D t e c h n i q u e . The s e n s i t i v i t y 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 h i g h e r t h a n i n c o n v e n t i o n a l ENDOR.

EL2'

defects i n semi-insulating

approximately

lo4 times

higher

GaAs, than

I n p a r t i c u l a r f o r t h e case o f t h e ENDOR s i g n a l i n t e n s i t y i s expected

from

simple

spin

statistics. T h i s i s n o t y e t understood. Furthermore, t h e method i s v e r y s e l e c t i v e , s i n c e o n l y o n e s i n g l e d e f e c t c a n be m e a s u r e d as l o n g

as i t s o p t i c a l a b s o r p t i o n d o e s n o t f u l l y o v e r l a p t h a t of a n o t h e r d e f e c t . T h i s was r e c e n t l y s u c c e s s f u l l y u s e d f o r FH(OH-)

KBr,

centres i n

which occur i n a b i s t a b l e c o n f i g u r a t i o n a t very low t e m p e r a t u r e

and t h e o p t i c a l a b s o r p t i o n bands of which d i f f e r o n l y s l i g h t l y on

133

4: ENDOR Spectroscopy in the Study of Defects in Solids

the

high

and

low

energy

A

side.51

further

advantage is

the

p o s s i b i l i t y o f c o r r e l a t i n g ENDOR s p e c t r a w i t h o p t i c a l a b s o r p t i o n b a n d s v i a a s o r t o f e x c i t a t i o n s p e c t r o s c o p y o f t h e ODENDOR l i n e s ( ' t a g g i n g ' ) .47*52 F i n a l l y , i t o p e n s up t h e p o s s i b i l i t y o f p e r f o r m i n g spatially

r e s o l v e d ESR a n d

which i s of

ENDOR e x p e r i m e n t s ,

high

a c t u a l i n t e r e s t i n current semiconductor research f o r characterising semiconductor wafers. A s a f i r s t example t h e d i s t r i b u t i o n o f t h e two c h a r g e s t a t e s o f t h e EL2 d e f e c t s , EL2'

a n d ELZO, were i n v e s t i g a t e d

a c r o s s a s e m i - i n s u l a t i n g GaAs w a f e r . 5 3 Only a v e r y observed

i n

systems.54

were p e r f o r m e d w h e r e ENDOR was

few experiments

luminescence

except

for

triplet

I n i o n i c c r y s t a l s i t was Tm'

states i n

i n CaF2,55

organic

a rare e a r t h

d e f e c t . A f e w o b s e r v a t i o n s were r e c o r d e d i n s e m i c o n d u c t o r s the donor

-

acceptor recombination luminescence.

observations i n amorphous

Si56,57

and

using

There have been

i n ZnSe,ZnSe.58

T h e ENDOR

l i n e s were o b s e r v e d a s a n e m i s s i o n i n c r e a s e u p o n i n d u c i n g N M R t r a n s m i s s i o n s by a r f a p p l i e d i n a s m a l l l o o p a t t a c h e d t o t h e s a m p l e . I n f o r example,

ZnSe,

t w o l i n e s d u e t o 67Zn a n d 7 7 S e were s e e n c e n t r e d

a t t h e f r e q u e n c i e s of t h e f r e e n u c l e i . The e m i s s i o n enhancement e f f e c t w a s o f t h e o r d e r o f 1%.T h e ENDOR l i n e s w e r e b r o a d , a b o u t a n o r d e r o f m a g n i t u d e b r o a d e r t h a n i n c o n v e n t i o n a l ENDOR. No a n g u l a r d e p e n d e n c e was r e p o r t e d . 5 8

U s i n g t i m e r e s o l v e d t e c h n i q u e s t h e ENDOR

o f l 1 5 1 n i n ZnO c o u l d b e o b s e r v e d . 5 9 The

observation

disadvantage.

of

ENDOR

via

emission

has

a

principal

I f t h e l i f e t i m e of a n e x c i t e d state giving rise t o t h e

emission is 1 s o r shorter,

t h e n t h e h o m o g e n e o u s ENDOR l i n e w i d t h i s

1 MHz o r m o r e . T h u s , o n e l o s e s a l l t h e d e t a i l e d i n f o r m a t i o n r e q u i r e d for the structure determination. Furthermore, t o be able t o s h i f t populations between

n u c l e a r Zeeman l e v e l s w i t h i n

the

radiative

l i f e t i m e one n e e d s v e r y h i g h NMR p r o b a b i l i t i e s r e q u i r i n g r f f i e l d s t r e n g t h s o f 10 mT o r m o r e ,

which a t low temperatures cause q u i t e a

t e c h n i c a l problem. 7.

Conclusions

ENDOR s p e c t r o s c o p y defects i n solids.

is a p o w e r f u l t o o l f o r t h e i n v e s t i g a t i o n o f The a v a i l a b i l i t y o f modern e x p e r i m e n t a l tech-

n i q u e s i n c l u d i n g t h e a p p l i c a t i o n o f computer a i d e d methods makes i t possible t o tackle d i f f i c u l t

p r o b l e m s of

i n t e r e s t i n materials

s c i e n c e . The d e v e l o p m e n t o f t h e s e m e t h o d s i s by no means c o m p l e t e . I n p a r t i c u l a r , i t is c o n c e i v a b l e t h a t t h e u s e o f methods l i k e e x p e r t s y s t e m s i n c o m p u t e r s c i e n c e ma y h e l p t o a n a l y s e d i f f i c u l t s p e c t r a . T h e o p t i c a l d e t e c t i o n o f ENDOR o p e n s u p q u i t e a n e w r a n g e o f a p p l i c -

134

Electron Spin Resonance

a t i o n s a n d s h o u l d b e d e v e l o p e d f u r t h e r . An e x t e n s i o n i n t o f u r t h e r i n f r a r e d would open up i n v e s t i g a t i o n s i n S i and t h e u s e of microo p t i c a l techniques could improve s p a t i a l r e s o l u t i o n

a n d p e r h a p s be

a p p l i e d t o t h i n l a y e r s . T he ENDOR d e t e c t i o n v i a e m i s s i o n h a s n o t y e t r e a l l y b e e n e x p l o r e d i n i n o r g a n i c s y s t e m s . Much r e m a i n s t o be done .

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33 H. S'dthe, P. S t u d z i n s k i , and J.-M. S p a e t h , Phys.Stat.Solid. (b), 1985, 130, 339. 34 P. Wagner, C. Holm, E. S i r t l , R. Oeder, and W. Z u l e h n e r , i n ' F e s t k o r p e r probleme: A d v a n c e s i n S o l i d S t a t e P h y s i c s , ' ed. P. G r o s s e , V i e w e g , Braunschweig, Vol. 24, 1984, p. 191. 35 J. M i c h e l , J.R. N i k l a s , J.-M. S p a e t h , and C. W e i n e r t , Phys.Rev.Lett., 1986, 57, 611. 36 S. Greulich-Weber, D i s s e r t a t i o n , Paderborn, 1987. Spaeth, t o be published. 37 S. Greulich-Weber, J.R. Niklas, and J.-M. 38 J.-M. S p a e t h and J.R. N i k l a s , i n 'R ecent Devel opme nts i n Condensed Matter physics 1,' ed. J.T. Devreese, Plenum, N e w York, 1981, p. 393. 39 J.R. N i k l a s and J.-M. S p a e t h , Phys.Stat.Solid. (b), 1980, 101, 221. S p a e t h , J.Phys. C, 1980, 13, 1745, 40 R.C. B a r k l i e , J.R. N i k l a s , and J.-M. 1757. 41 R.C. DuVarney, J.R. N i k l a s , and J.-M. S p a e t h , Phys.Stat.Solid. ( b ) , 1980, 97, 135. 42 B. B i e h l , M. P l a t o , and K. Mobius, J.Chem.Phys., 1975, 63, 3515. 43 N.S. Dalal and C.A. McDowell, Chem.Phys.Lett., 1970, 6, 617. 44 J.R. N i k l a s , R.U. Bauer, and J.-M. S p a e t h , Phys.Stat.Solid. (b), 1983, 119, 171. S p a e t h , Phys.Stat.Solid. (b), 1985, 45 R.C. DuVarney, J.R. N i k l a s , and J.-M. 128, 673. S. Pan, Phys.Rev. B, 1972, 6, 772. L.F. M o l l e n a u e r and 46 S p a e t h , Phys.Rev.Lett., 1984, 47 D.M. Hofmann, B.K. Meyer, F. Lohse, and J.-M. 53, 1187. 48 D.M. Hofmann, Di sse rt at i on, Paderborn, 1987. S p a e t h , Proc. 5 t h E u r o p h y s i c a l T o p i c a l 49 F. Lohse, M. F o c k e l e , and J.-M. Conf. , 'Lattice Defects i n I o n i c C ryst al s, Madrid, 1985. 50 M. F o c k e l e , F. Lohse, J.-M. S p a e t h , and R.H. Bartram, Proc . 1 1 t h I n t e r n a t . Conf. on Defects i n I n s u l a t i n g C ryst al s, Parma, I t a l y , 1988. S p a e t h , and F. L i t y , Proc . 1 1 t h I n t e r n a t . Conf. 51 M. J o r d a n , H. S o t h e , J.-M. on Defects i n I n s u l a t i n g C r y s t a l s , Parma, I t a l y , 1988. 52 F.J. A h l e r s , F. Lohse, J.-M. S p a e t h , and L.F. M o l l e n a u e r , Phys.Rev. B, 1983, 28, 1249. S p a e t h , and K. L a h n e r t , i n ' D e f e c t Recog53 M. Heinemann, B.K. Meyer, J.-M. n i t i o n and Image P r o c e s s i n g i n 1 1 1 - V Compounds 11,' ed. E.R. Weber, Else v i e r , Amsterdam, 1987. 54 K.P. D i n s e a n d C.J. Winscon, ' O p t i c a l l y d e t e c t e d ENDOR S p e c t r o s c o p y , ' i n ' T r i p l e t S t a t e ODMR Spectroscopy,' ed. R.H. Clarke, Wiley, N e w York, 1984. 55 G. S t r a u c h , Th. V e t t e r , and A. Winnacker, Phys.Lett., 1983, 34, 160. 56 Y.Sano, K. Morigatei, and I. Hirabayashi, Physica, 1983, 117A,118B, 923. 57 F. B o u l i t r o p , Phys.Rev. B, 1983, 28, 6192. 58 J.J. D a v i s , J.E. N i c h o l s , and R.P. B arnard, J.Phys. C, 1985, 18, L93. 59 D. B lo c k , A. Herv6, and R.T. Cox, Phys.Rev. B, 1982, 25, 6049.

5 Inorganic and Organometallic Radicals and Clusters prepared in a Rotating Cryostat by Metal Vapour Techniques BY J. A. HOWARD AND B. MILE Introduction It is a pleasure to write a synoptic review of our and other e.s.r. studies of the new chemistry of metals which is emerging from the use of high temperature metal vapourisation The sources in conjunction with cryogenic quench techniques. rapid disintegration of organic and inorganic substrates under the extreme conditions needed to produce most metal atoms and small clusters in the vapour phase (high temperatures, high light fluences, electric discharges) is obviated by letting the reactants encounter each other only in the surface layers of matrices held at low temperatures. The application is an important and large extension of the matrix isolation technique which has been so productive in free radical A wide range of evaporative sources are now available which enable even the most refractory materials to be vapourised and recondensed. Resistive heating of furnaces of Mo, W and Ta have been supplemented with E-gun evaporation’, laser ablation’ 11, cold cathodes12 and magnetron sputtering.13’14 The unbound metal atoms are completely coordinatively unsaturated and valence unsatisfied and hence usually have a much higher reactivity than partially saturated atoms in complexes and on surfaces and completely coordinated atoms in the lattice of the bulk solid. The source of their activity is the high energy deposited to break the bonds in the bulk solid, i.e. a highly positive AH; value. However, despite their high reactivity, the use of cryogenic conditions reveals subtle electronic and symmetry effects and a resulting high selectivity. 1.

’-’

Previous Reviews and Books E.s.r. has been a major spectroscopic technique complementing and supplementing other spectroscopic methods in elucidating the structure of metal substrate complexes and charting the course of reactions of metal atoms and clusters. References 15-17contain details of some of these studies. Previous reports in this series also contain information in this 1.1

136

5: Inorganic and Organometallic Radicals and Clusters

area.” In many cases e.s.r. provides the most definitive stoichiometric information which is often extremely difficult to extract by other methods. In combination with high level molecular orbital (M.O.) calculations it also gives plausible indications of the electronic and geometric structures of metal atom adducts and clusters. The field provides good examples of the fruitful interplay between e.s. r. and quantum chemistry and generates good cases for testing the various approximations that underlie even the most sophisticated M.O. calculations. In this report we give a detailed review of the application of e.s.r. in metal vapour chemistry using our own work on the rotating cryostat as the framework on which to build the extensive work of others: our own predilections inevitably surface. We review (i) matrix effects on the electronic structures of trapped atoms; (ii) small metal clusters and complexes; (iii) monoligand complexes of metal atoms with organic and inorganic substrates: (iv) di-, tri- and tetra-ligand complexes. 1.2

Experimental Procedures Both static and slow moving cryostats have been

employed, the former usually for matrix isolation spectroscopic studies and the latter for larger scale synthetic purposes. 1-4 In the static cryostats the reactants and matrices are condensed together on to the cold surface using high metal dilutions and slow deposition rates so as to minimize reactions and aggregation in the newly forming, molten, slushy surface layers. In the slowly rotating cryostats (ns, J. Chem. Soc., 1964, 4352. 18 P. A. Narayama, M. K. Bobanan, L. Kevan, V. F. Yudanov and Yu. D. Tsvetkov, J. Chem. Phvs., 1975, 63, 3365. 19 J. E. Bennett, B. Mile and A. Thcmas, J. Chem. SOC. A, 1967, 1393. 20 K. Ohno, I. T a k m a and J. Sohma, J. Chem. Phvs., 1972, 56, 1202. 21 D. M. Golden and T. R. Tuttle, J. Phvs. Chem., 1984, 88, 3781. 22 H. F. Hameka, G. W. Robinson and C. J. Marsden, J. Phvs. Chem., 1987, 91, 3150. 23 A. Hamaidia, A. Hachimi, J. Margerie and J. F. Hemidy, Phys. Stat. Solid EJ, 1986, 137, 47. 24 G. H. M o z and J. 0.Rubio, J. Phvs. C: Solid State Phvs., 1988, 21, 847. 25 S. F. J. Cox and M. C. R. s’ymms, Hvl3erfine Int., 1986, 2, 689. 26 R. F. Kiefl, S . R. Kreitzman, M. Celio, R. Keitel, G. M. Luke, J. H. Rrewer, D. R. Noakes, P. W. Percival, T. Matzuzaki and K. Nishiyama, Phvs. Rev., 1986, A34, 681, 27 M. Heming, E. Roduner, B. D. Patterson, W. Wermatt, J. Schneider, H. Bameler, H. Keller and I. M. Savic, Chem. P~Ys.Lett., 1986, 128, 100. 28 R. A. Zhitnikov and Yu. A. Dmitriev, Zh. Eks~.Teor. Fiz., 1987, 92, 1913. 29 D. M. Bartels, T. M. Chin, A. D. Trifunac and R. G. Lawler, Chem. Phvs. Lett., 1986, 123, 497. 30 D. M. Bartels, D. W. Werst and A. D. Trifunac, Chem. Phvs. Lett., 1987, w , 191. 31 K. Eiben and R. W. Fessenden, J. P ~ Y .S Chem., 1971, 75, 1186. 32 M. C. R. Symons, J. Chem. S o c . , Faradav Trans. 1, 1982, 78, 1953. 33 J. Bednarck and A. Plonka, J. Chem. Soc., Faradav Trans. 1, 1987, 83, 3725. 34 P. H. Kasai and D. MSeod, Jr., J. Am. Chem. Soc., 1975, 97, 5609. 35 J. A. Howard, B. Mile, J. S. Tse and H. Morris, J. Chm. Soc., Faraday Trans. 1, 1987, 83, 3701. 36 P. M. Jones and P. H. Kasai, J. Phvs. Chm., 1988, %, 1060, 37 T. Ichikawa, H. Yoshida, A. S. W. Li and L. Kevan, J. Am. Chem. Soc., 1984, 106,4324. 38 M. C. R. Symons, D. R. Russell, A. D. Stephens and G. W. Eastland, J. Chem. Soc., Faradav Trans. 1, 1986, 82, 2729. 39 M. T. O h , M. C. R. Symm and R. S. Eachus, Proc. R. Soc. Und. A, 1984, 392, 227. 40 Th. W s i g , J. R. N i k l a s and J.-M. Spaeth, CNst. Latt. Def. and Amorph. m.,1987, 16, 169.

A.

6: Inorganic and Organometallic Radicals

205

41 I. Heyndfxickx, E. GoaMerts, S. V. Nistor and D. schoemaker, Phw. Status Solidi B, 1986, 136, 69. 42 L. B. Knight, R. W. Martin and E. R. Davidson, J. Chem. Phvs., 1979, 71, 3991. 43 L. B. Knight, S. T. Cobranchi, B. W. Gregory and E. Earl, J. Chem. Phvs., 1987, 86, 3143. 44 L. B. Knight and W. Weltner, J. Chem. Phvs,, 1971, 55, 206. 45 L. B. Knight, B. W. Gregory, S. T. Cobranchi, D. Feller and E. R. Davidson, J. PJn. Chem. Soc., 1987, 109, 3521. 48 L. B. Knight and J. T. Petty, J. Chem. Phvs., 1988, 88, 481. 47 L. B. Knight, J. T. Petty, S. T. Cobranchi, D. Feller and E. R. Davidson, J. Chm. Phvs., 1988, 88, 3441. 48 M. C. R. Symms, "Chemical and Biochemical Aspects of Electron Spin Resonance Spectroscopy",Van N o s t r a n d Reinhold Co. Ltd., Wokingham/Berkshire, 1978. 49 L. B. Knight, A. Limn, S. T. Cobranchi, E. Earl, D. Feller and E. R. Davidson, J. Chem. Phvs., 1986, 85, 5457. 50 L. B. Knight, E. Earl, A. R. Ligon, D. P. Cobranchi, J. R. woodward, J. M. Bostick, E. R. DaVidson and D. Feller, J. Am. Chem. Soc., 1986, 108, 5065. 51 H. J. Kalinowski and L. C. S. do Carmo, J. Chem. Soc., Faradav Trans. 1, 1987, 83, 3709. 52 D. M. Lindsay and D. A. Garland, J. Phvs. Chem., 1987, 9l, 6158. 53 P. H. Kasai and P. M. Jones, J. Phvs. Chem., 1986, 90, 4239. 54 J-X. Wang and J. H. Lunsford, J. Phvs. Chem., 1986, 90,3890. 55 H. M. M h g , J. R. Johns and R. F. How=, LPhvs. Chem., 1988, 92, 1291. 56 A. Pmorelli, J. C. Evans and C. C. Rowlands, J. Chem. Soc., Faradav Trans. 1, 1986, 84, 1723. 57 5. B. Raynor, I. J. Rowland and M. C. R. Symons, J. Chem. S o c . , Dalton Trans., 1987, 421. 58 T. Clark, J. Am. Chem. Soc., 1988, 110, 1672. 59 J. Bednarck and A. Plonka, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 3737. 60 R. J. Van Zee, R. F. Ferrante and W. Weltner, Chem. Phvs. Lett., 1987, 139, 426. 61 CWesterling and A. Lund, Chem. Phvs. Lett., 1988, 147, 111. 62 K. V. Topchieva, S. E. Spiridonov and A. Y. bginov, J. Chm. Soc., C?-xw Ccmn., 1986, 636. 63 M. ZaMCk and K. Baberschke, Phvs. Rev. B: Condens. Matter, 1987, 36, 5756. 64 J. S. Lea and M. C. R. Symons, J. Chem. Soc., Faradav Trans. 1, 1988, 1181. 65 S. M. bugh and J. W. M5Mnald, Inom. Chem., 1987, 25, 2024. y and S. V. Bhat, Chem. Phvs. Lett., 1987, 133,455. 66 K. M 67 F. J. Callens, R. M. H. Verbeck, P. F. A. Matthys, L. Martens and E. R. Boesman, Bull. Soc. chim. Bels., 1987, 96, 165. 68 N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie, J. Chm. Soc.. Faradav Trans. 1, 1987, 83, 3717. 69 D. M. Bartels and R. G. Lawler, J. Chem. Phvs., 1987, &, 4843. 70 D. M. Stanbury, J. A. H o b , Z. H. Kafafi and J. L. Margrave, Chem. Phvs. Lett., 1986, 129, 181. 71 N. M. Atherton and R. D. S. Blackford, J. Chem. Soc., Faradav Trans. 2, 1987, 83, 569. 72 Y. R. Setchov, H. Bill and D. Invy, Chem. Phvs. Lett., 1987, 136, 57. 73 R. Caballol, J. A. Catala and J. M. Probkt, Chem. Phvs. Lett., 1986, 130, 278. 74 A. Hasegawa, S. Kamhaka , J. Wakabayashi, M. Hayashi and M. C. R. SYnrJns, J. Chem. Soc., Dalton Trans., 1984, 1667. 75 M. C. R. Symns, B. W. Wren, H. Mto, K. Toriyama and M. IWaSaki, Chem. Phvs. Lett., 1986, 127, 424. 76 M. C. R. Symns and R. Janes, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 383. 77 M. D. Sastry, A. G. I. Dalvi and M. L. Bamsal, J. Phvs. C: Solid State Phvs., 1987, 20, 1185.

a,

Electron Spin Resonance

78 R. A. J. Janssen, J. A. J. M. Kingma and H. M. Buck, J. Am. Chm. Soc., 1988, 110,3018. 79 F. J. OkRns, J. Chem. Phvs., 1987, 82, 6066. 80 T. Fukami, S, Akahoshi and K. Hukuda, J. Phw. Soc. J a m , 1988, 57, 255. 81 J. R. g Y m , J. chem. Phvs., 1987, 86, 6065. 82 M. C. R. Symons and S. P, Mishra, J. Chem. Soc., Dalton Trm., 1981, 2185. 83 P. J. Van Zee, G. R. Smith and W. Weltner, J. Am. Che4n. Soc., 1988, 609. 84 K. Ohta, M. Shiotani and J. Sohm, Chm. Phvs. Lett., 1987, 140, 148. 85 L. D. Kispert, K. G. Ezell and J. Joseph, Chm. Phm. Lett., 1987, 141, 206. 86 N. S. Ganghi, D. N. R. Rao a n d M . C. R. Symns, J. Chem. Soc., Faradav Trans. 1, 1986, 82, 2367. 87 A. A. Vasil'ev, V. N. Lisetskii, N. Kulikav and G. G. F. Savel'ev, Khh. Ws. Enm., 1987, 21, 189. 88 L. B. Knight, J. Steadman, P. K. Miller and J. A. Cleveland, J. Chm. Phvs., 1988, 88,2226. 89 L. B. Knight, K. D. Johamessen, D. C. Cobranchi, E. A. Earl, D. Feller and E. R. Davidson, J. Chm. Phvs., 1987, 82, 885. 90 J-T. YU and S. Y. ChOU, Mol. Phvs., 1987, 62, 971. 91 H. Qlandra, S. P. Mishra and M. C. R. Symons, Chem. Phvs. Lett., 1987, 134, 307. 92 J. N. Kirwan and B. P. Roberts, J. Chem. Soc.. Chm. Carm., 1988, 480. 93 V. Paul and B. P. Roberts, J. Chem. Soc., Chem. Comn., 1987, 1322. 94 J. R. Byberg, J. Chem. Phm., 1986, 84, 6204. 95 H. C. Chandra, D. N. R. Rao and M. C. R. Symons, J. Chem. Soc.. Dalton Trans., 1987, 729. 96 E. J. Friebele and D. L. Grisccm, Mater. Res. Soc. S ~ PPrm., 1986, 61, 319. 97 T. E. Tsai and D. L. Griscm, J. Non-Crvst. Solids, 1987, 91, 170. 98 R. B. bssoli and L. E. Halliburton, Phvs. Stat. Solidi B, 1986, 136, 709. 99 H. Kawazoe, M. Kohketsu, Y. Watanabe, K. Shibuya and K. Mta, Mater. Res. Soc. sVmP Proc., 1986, 339. 100 S. Pizzini, D. Narducci, D. Daverio, C. L. Mari, F. lbrazzoni and A. Gervasini, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 705. 101 T. Malccxn, J. Gorse and R. G. Koosev, J. Hish. Res. chromatoa., 1988, 11, 416. 102 M. Hgiwara, T. Wenma, Y. Chiha and M. Date, J. Phvs. Soc. J a m , 1988, 57, 741. f Int., 103 N, moo, K. c. Mishra, M. van ROSSU~and T. P. as, ~ w e rine 1987, 3, 701. 104 T. A. Claxton, A. Evans and M. C. R. Symons, J. Chem. Soc., Fara&y Trans. 2, 1986, 82, 2031. 105 H. Yokanichi and K. Wrigaki, J. Non-Cryst. Solids, 1987, 97, 67. 106 M. C. R. Symons, HvPerfine Int., 1984, 17, 771. 107 K. Nakazawa, H. Arai and S. Kohd, Appl. Phvs. Lett., 1987, 5.1, 1623. 108 K. Honda, T. Matsumori and T. I d , J. Non-Wst. Solids, 1987, 22. 109 R. D. Harris, J. L. Newton and G. D. Watkins, Phvs. Rev. B, 1987, 36, 1094. 110 D. Jousse, E. Bustarret, A. Deneauville and J. P. Stoquert, Phvs. Rev. B, 1986, 34, 7031. 111 D. Jousse, E. Bustarret, A. Deneauville and J. P. Stoquert, Phvs. Rev. B, 1986, $4, 7031 112 K. Thanke, A. Teschner and R. Saueer, Solid State C u m . , 1987, 61, 291. 113 P. Schultz and R. Messmer, Phys. Rev. B, 1986, 3,2532. 114 S. Y a M Z a k l , S. Kuroda, J. Isoya and K. Tanaka, J. Non-Crvst. Solids, 1987, 9,691. 115 T. Fujita, Y. Saitoh, N. Itoh and M. Kubota, J m . J. Awl. Phvs. 2, 1987, 26, L1116. 116 U. Kaufmann, J. Schneider and A. Rauber, Phvs. Lett., 1976, 29, 312.

m,

.

.

a,

s,

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207

117 A. huger, H. J. von Bardelken and J. C. Bourgoin, Phvs. Rev. B, 1987, 36, 5982. R- F. Kiefl, M. celio, T. L. Easlte, G. M. w ~ e s. , R. Kreitzman, J. H. Brewer, D. R. Noakes, E. J. Ansaldo and K. Nishiyama, Phw. Rev. Lett., 1987, 103, 1843. 119 S. F. J. Cox and M. C. R. Symons, Chem. Phvs. Lett., 1986, 126, 516. 120 F. W. Kleinhans and S. T. Barefoot, J. Biol. Chem., 1987, 262, 12452. 121 B. E. Britigan, M. S. Cohen and G. M. Fbsen, J. Leukocyte Biol., 1987, 41, 349. 122 E. G. Janzen, L. T. Jandrisits and D. L. Barber, Free Rad. Res. Cunn., 1987, 4, 115. 123 J. L. You, K. S. Butcher, A. Agostiano and F. K. Fong, pros PhotosYnth. 1987, 2, 345. 124 T. Ozawa and A. Hanaki, Biochm. BioPhvs. Res. Cunn., 1986, 136, 657. 125 C. L. Christmas, A. J. Carmichael, M. M. Mssoba and P. Riesz, Ultrasonics, 1987, 25, 31. 126 B. Lac. Lynn, H. D. Connor, R . P. Mason and R. G. T h m , Ml. Pharm., 1988, 33, 351. 127 K. Stolze, D. R. W i n g and R. P. Mason, J. Chem. Soc., Chem. Ccfnn., 1988, 268. 128 T. Ozawa and A. Hanaki, Bicchem. Biophvs. Res. Cunn., 1987, 142, 410. 129 B. K. Sinha, Blochim. Biophvs. Acta, 1987, 924, 261. 130 M. C. R. Symons, J. R. Mrton and K. F. Preston, h e r . Chm. S o c . , SVmD. Ser., 1987, 333, 169. 131 J. H. B. Chenier, C. A. Hampson, J. A. Howard and B. Mile, J. Phvs. Chm., 1988, 92, 2745. 132 P. H. Kasai and P. M. Jones, J. Phvs. Chem., 1985, 89, 1147. 133 R. H. Perez and D. S. Marynick, J. Chm. Phvs., 1987, 87, 4644. 134 J. A. Howard, R. Sutcliffe, C. A. Hampson and B. Mile, J. Phvs. Chem., 1986, 90, 4268. 135 J. R. brton, K. F. Preston, N. A. Cooley, H. C. Baird, P. J. Krusic and S. J. &Lain, J. Chem. Soc., Faradav Trans. 1, 1987, 83, 3535. 136 S. J. &Lain, P. J. Krusic, R. D. Farlec and J. Nicholson, unpublished results quoted in ref. 135. 137 R. N. Bagchi, A. M. Bond, R. Colton, D. L. L u s h and J. E. bir, J. Am. Chem. Soc., 1986, 108, 3352. 138 Phil. Trans. ROY. Soc. A, 1988, 324, 73-294. 139 J. Chem. Soc., Faraday Trans. 2, 1986, 82, 1089-1295. 140 J. M. Dyke, J. Chem. Soc., Faradav Trans. 2, 1987, 83, 69. 141 T. J. Sears, J. Chem. Soc., Faradav Trans. 2, 1987, 83, 111. 142 A. Carrington and R. A. Kennedy in "Gas Phase Ion C h d s t r y " , ed. H. T. Bowers, Academic Press, Vol. 3, 1984, p.393. 143 A. Carrington, I. R. McN& and C. A. Mntganerie, J. Chem. Phvs., 1987, 87, 3246. 144 L. Wolniewicz and J. D. Poll, Ml. Phvs., 1986, 59, 953. 145 P. Drausfeld and H. G. Wagner, Z. Phvs. Chem. (Mmichl,1987, 153, 89. 146 YU. R. Eedzhanyan, Y. M. Gershenzon, S . D. Il'in and 0. P. Kishkovich, Khim. Fiz., 1987, 6 , 338. 147 H. Zagogianni, A. Mellouki and G. Poulet, C.R. Acad. Sci. Ser. 2, 1987, 304, 573. 148 J . M . Brown, E. R. Ccmben, W. Bohle, D. Zeitz and W. Urban, Phvs. S c r . , 1987, 35, 144. 149 K. R. Leopold, K. M. Evenson, E. R. Comben and J. M. Brown, J. b l . Spec.,1987, 122, 440. 150 E. C. C. Vasconcellos, S. A. D a v i d s o n , J. M. Brown, K. R. Leopold and K. M. Evenson, J. Ml. Fmc ., 1987, 122, 242. 151 N. Seebass, J. Werner, W. Urban, E. R. Canker: dnd J. M. Brown, !!&L Phvs., 1987, 62, 161. 152 J. M. Brown and H. Uehara, J. Chm. Phvs., 1987, 87,880. 153 J. M. Brown, J. E. Schubert, R. J. Saykally and K. M. Evenson, J. b l . Spec., 1986, 120,421. 118

w.,

.

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Electron Spin Resonance

154 D. G. Hovde and R. J. Saykally, J. Chem. Phvs., 1987, 82, 4332. 155 D. Zeitz, U. Bohle, T. Nelis and W. Urban, a l . Phvs., 1987, 60, 263. 156 T. J. Sears, G . A. Takacs, C. J. Howard, R. L. Crownover, P. Helminger and F. C. de Lucia, J. M l . Spec., 1986, 118, 103. 157 K. Kawaguchi, T. Suzuki, S. Saito and E. Hirota, J. m t . Am. Soc., 1987, B4, 1203. Pahnke, S. H. Ashmrth and J. M. Brown, Chm. Phvs. Lett. , 1988, 142, 158 179. 159 R. A. Beaman, T. Nelson, P. S. Richards and D. W. Setser, J. Phvs. Chm., 1987, 91, 6090. 160 C. R. Brazier and P. F. Bernath, J. Chem. Phvs., 1988, 88, 2112.

7 Metal loproteins BY G. R. HANSON AND G. L. WILSON

1 Introduction

An increasing interest in the characterization of metal centres in metalloproteins is evident from the number of articles (310) presented in this review. In addition to an International Conference on Bioinorganic Chemistry, held at Noordwijkerhout, Netherlands in 1987l, metalloproteins have also featured at

several other meetings. 2-4 The characterization of metal centres in metalloproteins has relied on a combination of spectroscopic techniques rather than any single method. Some of the more common techniques include: electron

paramagnetic resonance (e.p.r.1, electron nuclear double resonance (ENDOR), electron spin echo envelope modulation (ESEEM)5, electronic absorption circular dichroism (CD), magnetic circular dichroism (MCD), extended X-ray absorption fine structure (EXAFS), nuclear magnetic resonance (NMR)

and Resonance Raman spectroscopy. 2 Copper Proteins

2.1

TYDe 1 C o w e r Proteins.-

Pulse

field

sweep

e.p.r.

(PFSEPR), has been developed to resolve hyperfine structure underlying inhomogeneously broadened e.p.r. lines. Both stellacyanin and the CuA site in cytochrome c oxidase have been examined and the results are similar to those obtained from ENDOR e ~ p e r i m e n t s . ~The electron self exchange rate of Pseudomonas a e r u g i n o s a azurin has been measured by 209

210

Electron Spin Resonance

combined fast f low/rapid-f reeze e .p. r . experiments on mixtures of 63Cu Resonance Raman, e.p.r. and and 65Cu azurin at pH 5.0 and 9.0.8 electrochemical studies, incorporating electronic spectra, have been performed on azurin isolated from A l c a l i g e n e s d e n i t r i f i c a n s . The results have been correlated with the refined crystal structure of this azurin and with corresponding data from other type 1 copper proteins. Pseudomonas a u r e o f a c i e n s truncates the respiratory reduction at the level of N20. A low molecular weight protein with properties similar to those of azurin and a nitrite reductase have been purified to homogeneity. Both of these proteins exhibit e.p.r. spectra which are typical of type 1 copper proteins. The reduced azurin was able to efficiently transfer electrons to the nitrite reductase producing N20 as a product. lo In another bacterium, Pseudomonas a e r u g i n o s a , a type 2 copper protein has been purified which inhibits the oxidation of azurin and cytochrome c551 catalyzed by nitrite reductase." 2 . 2 Type 2 C o m e r Proteins.- ESEEM experiments on the Cu2+ binding sites in porcine kidney and bovine serum amine oxidase have identified histidine and water as equatorial ligands, and axial coordination of water. Inequivalent 1 4 N magnetic coupling to the Cu2+ ion arising from the two nitrogen atoms in the imidazole ring was resolved in the spectrum. Anionic inhibitors displaced the axially An ESEEM comparison of the cocoordinated water molecule. l 2 l3 ordination sphere of copper in native bovine serum amine oxidase, containing two copper atoms per dimer molecule with a preparation in which half of the copper has been depleted revealed that both sites are similar. Displacement of ambient water around the Cu2+ ion occurs upon the addition of phenylhydrazine which is known to bind to the pyrroloquinoline. l3 Model Cu2+ pyrroloquinoline complexes have been examined by e.p.r.14 ENDOR studies of amine oxidase isolated from pig plasma are in agreemen# with the above ESEEM r e ~ u 1 t s . l ~An investigation of the reaction of diethyldithiocarbamate with bovine serum amine oxidase has been monitored with e . p . r.

Reduction of the pig kidney diammine oxidase cyanide complex with 1,5-diaminopentane causes a decrease in the intensity of the Cu2+ e.p.r. signal and the formation of an organic free radical signal.17 The coordination of three, or more likely four histidyl ligands to the Cu2+ site in dopamine p-hydroxylase, isolated from bovine adrenal medulla was established by ESEEM experiments. l8 Proton nuclear

7: Metalloproteins

21 1

magnetic relaxation measurements and e.p.r. studies of the cyanide and azide

complexes

of

dopamine

0-monooxygenase

indicated

that

two

molecules of water were bound to the active site copper ion. 19 Three distinct redox modifications of galactose oxidase have been prepared. The reduced enzyme is catalytically inactive and has an e.p.r. signal which is qualitatively similar to the native enzyme. Spin quantitation indicates the presence of a single Cu2+ per mol of protein. Catalytically active enzyme produced by oxidation of the native form has a reduced e.p.r. signal.2o Extremely acidic copper proteins, neurocupreins, isolated from bovine, rabbit, pig and sheep brains have been characterized by e.p.r. 21 Multifrequency e.p.r. and computer simulation studies of the second derivative solution spectra have identified the binding mode of histidine to Cu2+.22 Characterization of the binding of copper ions to cyclic dipeptides has been established through the analysis of the ligand nitrogen hyperf ine coupling.23 2.3 Multi-centered Cormer Proteins.- The two type 1 copper binding sites in ceruloplasmin were spectroscopically differentiated by the kinetic analysis of the e.p.r. spectra during the redox cycle.24

Ceruloplasmin has been purified from chicken plasma with a

copper content of 5.01 f 0.35 gatoms per molecule of which 50% was e.p.r. detectable. No type 2 copper signal was observed. Redox experiments have established that in the oxidized enzyme an interaction between the type 2 copper ion and the diamagnetic pair of copper ions occurs in such a way as to make the type 2 centre e.p.r. ~ n d e t e c t a b l e . ~The ~ binding of N3-, SCN- and OCN- to bovine ceruloplasmin was monitored by electronic absorption, CD and e.p.r. spectroscopy and revealed a reduction of type 1 copper or a conversion of type 1 to type 2 copper. No effect was observed with the binding of F- 26 Transfer of copper from Cu-thionein into apo-ceruloplasmin 27 was followed by e.p.r. spectroscopy.

.

Low temperature MCD, absorption and e.p.r.

spectroscopy were

employed in the characterization of the binding of N3- and F- to the different types of copper in laccase purified from Rhus v e r n i c i f e r a . F- was found to bind to the paramagnetic type 2 Cu2'

-

centre while

N3 formed a bridge between this site and the antiferromagnetically coupled type 3 Cu2+ centre.28 Removal of the type 2 Cu2+ centre from

212

Electron Spin Resonance

native laccase yields an enzyme containing a single type 1 and a coupled binuclear type 3 Cu2+ centre. Computer simulation of the gL region in the e.p.r. spectrum from the type 1 Cu2+ has identified ligand hyperfine coupling arising from two histidine nitrogens and two methylene protons from cysteine residues. 2 9 The binding of anions to deoxy- [Cu+Cu+l, half met- [Cu+Cu2+l and met- ICu2+Cu2+1 forms of laccase has been studied by electronic absorption, CD and e.p.r. spectroscopy. The met-azide complex gives rise to a dipole coupled triplet state e.p.r. spectrum.30 The effect of fluoride on the reduction of oxygen by laccase was monitored by rapid freeze e.p.r. under steady state conditions.31 Excretion of active laccase by sycamore cells, monitored by specific activity and e.p.r., closely 32 paralleled the amount of copper present in the culture medium. Multifrequency (35.1, 9-08, 2.54 GHz) e.p.r. studies of a derivative of laccase containing one mercury atom and three of copper have been reported.33 The e.p.r. signal of the mercury derivative and fluoride binding studies establish that a type 2 copper centre is present in a slightly rhombic site. The signal exhibits g-strain broadening, but at 2.54 GHz ligand hyperfine coupling from three nitrogen atoms can be resolved.

Reassessment of the stoichiometry of copper in a very pure solution of ascorbate oxidase has identified one type 1, two type 2 and four type 3 Cu2+ ions.34 The removal and reconstitution of the type 2 centre in cucumber ascorbate oxidase has been monitored with e . ~ . r . Azide ~~ binding studies have been performed on the native and copper depleted cucumber enzyme3' and ascorbate oxidase isolated from 36 Cucurbi ta pepo. Addition of nitrite to met-hemocyanin produces a binuclear pH dependent e.p.r. signal at g = 2 . The pKa for this process (6.50) indicates that the p-aquo bridging ligand, which can be replaced by nitrite, is deprotonated to form a p-hydroxo bridging ligand.37 Substitution of the antiferromagnetically spin-coupled Cu2+ pair in Limul us polyphemus hemocyanin by Co2+ involves supplementary binding to peripheral sites that contribute to the aggregation process in hemocyanin. E.p.r., optical absorption and magnetic susceptibility studies were employed in the characterization of these metal ion sites. 38

213

7: Metalloproteins

Nitrous oxide reductase has been isolated anaerobically from Paracoccus d e n i t r i f i c a n s 3 ' and aerobically from Rhodopseudomonas

.

s p h a e r o i d e s f sp. d e n i t r i f i c a n s . 40

E.p.r.

spectra of the former

enzyme are typical of a type 1 Cu2+ centre and is accounted for by 7 to 15% of the enzyme bound copper. The latter enzyme contains four gatoms of Cu2+, two gatoms of Zn2+ and one gatom of Ni2+. The e.p.r. spectra indicates the presence of type 2 copper centre(s1. Although the reactivity of nitric oxide towards a number of type 1, 2 and 3 copper enzymes has been investigated, binding of HN02 was only observed for types 1 and 3. Photodissociation of the copper-NO complex only occurred for type 1 enzymes. 41 3 . Iron Proteins

3.1 Non-Heme Iron Proteins.- Phenylalanine hydroxylase, an iron containing enzyme, catalyzes the hydroxylation of phenylalanine to tyrosine. Computer simulation of the e.p.r. spectrum established the presence of two high-spin Fe3+ centres in different environments. 42

In contrast, the enzyme from Chromobacterium v i o l a c e u m contains one Cu2+ ion per subunit. Computer simulation of the ESLEM spectrum from this enzyme has established that two histidyl imidazole groups are coordinated equatorially to the Cu2+ ion. " Mu1 tifrequency e .p. r . has shown that binding of the cofactor, 6,7-dimethyltetrahydropterin to 44 the Cu(I1) ion in this enzyme occurs through the N-5 position. E.p.r. spectroscopy of a pterin-dependent phenylalanine hydroxylase from Chromobacterium v i o l a c e u m suggests that it is a type 2 copper-containing protein. 4 5 T3.p.r. data and kinetic results indicate the formation of an enzyme-thiol complex during the aerobic reduction

of the enzyme by dithiothreitol. Computer simulations of the high-spin Fe3+ e.p.r. spectra arising from phenylalanine hydroxylase and transferrin have been employed in the determination of the fraction of iron contributing to each of the e.p.r. active sites.46 Hemerythrin (Hr) is a binuclear, non-heme iron protein. The binuclear Fe(I1) active site of deoxy-hemerythrin (deoxy-Hr), contains both six-coordinate and five-coordinate ferrous ions to which N3 , OCN- and F- can bind. The e.p.r. spectrum47 is consistent with antiferromagnetic coupling between the ferrous ions, where -J 2 12-38

Electron Spin Resonance

214

cm -1 , indicating a hydroxide bridge. The intensity of a g 1: 16 e.p.r. signal originating from the 14,+4> ground state in the deoxy-

n

N3--Hr

species has been

analyzed.

Using absorption, e.p.r.

and

Mijssbauer spectroscopies, two kinetically identifiable intermediates observed during the three stages of reduction of met- to deoxy-Hr have been characterized. 48 The properties of these intermediates are discussed and are used to propose a scheme for reduction of the iron site in Hr. The isolation and characterization of a met-Hr reduction system from erythrocytes of the sipunculan worm Phascolopsis gouldii has been reported.*' The sequence of electron flow in the system is: NADH to NADH-cytochrome b5 reductase to cytochrome b5 to met-Hr. E.p.r. spectroscopy demonstrates that a redox process involving formation of the intermediate semi-met-Hr occurs within intact erythrocytes as well as in the system of purified components. The rhombic g matrix of the e.p.r. signal in both cases resembles that of (semi-metIR-Hr, the form obtained by a one electron reduction of metHr. The chemical and spectroscopic consequences of allosteric interactions for ligand binding in sipunculid (Phascolopsis gouldii) 50 and brachiopod (Lingula reevii) hemerythrins have been investigated. E.p.r., CD, absorption, and Resonance Raman spectroscopies show no significant differences in the active site structures. The effects on redox kinetics caused by the change in reduction potential of p S2--met-Hr relative to the native met-Hr which has a p-0x0 bridge have been examined51 using a number of metal complexes and the heme proteins deoxy-myoglobin (deoxy-Mb) and cytochrome b5 as redox partners. A solution of met-Hr and deoxy-Mb exhibits the characteristic e.p.r. spectrum of (semi-metIR-Hr (g = 1.94, 1.87, 1.66) demonstrating reduction of met-Hr by deoxy-Mb. Nitric oxide adducts of deoxy-Hr gives rise to novel e.p.r. signals of deoxy-Hr-NO (deoxyHr-F-NO in the presence of fluoride) with g, = 2.77 (2.58) and g1 = 1.84 (1.80) due to antiferromagnetic coupling between Fe(I1) ( S = 2) and (FeNOl' ( S = 3/21 centres leading to an Seff = 1/2 ground state.52 Inclusion of a zero field splitting leads to a coupling constant, J 1 23 c m-', for deoxy-Hr-NO. Rubrerythrin, a new non-heme iron protein, has been found to contain two rubredoxin-like [FeSq] clusters and a hemerythrin-like binuclear iron cluster. Mossbauer and e.p.r. spectroscopy have been used in its characterization. Interestingly the midpoint potential for the CFeS41 cluster is + 230 f 10 mV. Its function is unknown at the present time.53

7: Metalloproteins

215

The properties of the Fe2+-Fe3+ derivative of red kidney bean purple acid phosphatase, and evidence for a binuclear Zn-Fe centre in the native enzyme, have been presented.54 The Fe2+-Fe3+ derivative has e.p.r. parameters characteristic of an antiferromagnetically coupled pair of high-spin ferrous and ferric ions with zero field splittings comparable to the exchange interaction. The native Zn-Fe enzyme gives two e.p.r. signals: one with rhombicity E/D = 0.25 and D/k 2 1.5 K with g 2 9.22 and 4.28 and another with E/D 2 0.14, D/k = 0.4 K giving rise to features at g = 8.53, 5.55 and 2.85. Anaerobic titrations of purple acid phosphatase from bovine spleen, monitored by optical and low temperature e.p.r. show that the conversion of the purple, enzymatically inactive form to the active pink form is a one electron process. The pink form exhibits a g'=1.77 e.p.r. spectrum due to a pH-dependent mixture of two rhombic species, with a pKa of 4.4. The temperature dependence of this signal is consistent with a weak antiferromagnetic coupling (-25 = 11 f 2 cm-') between Fe2+ ( S = 2 ) and Fe3+ ( S = 5/2) .55 Improved purification of purple

acid phosphatase

from sweet

potatoes has shown that the enzyme contains two moles of iron and insignificant quantities of manganese. Preliminary e.p.r. measurements show no evidence for a g' = 1.77 signal.56 The methane monoxygenase system from Methylococcus capsulatus consists of three proteins (A,B,C) each of which is essential for catalytic activity in vitro. An e.p.r. redox titration of protein A demonstrates the presence of an antiferromagnetically coupled binuclear high-spin Fe3+ cluster and a stable free radical. The lack of S2- and an absorption band at 406-410 nm suggests a similar structure to that found in h e m e r ~ t h r i n . ~These ~ studies have been extended to the methane oxygenase obtained from Methylococcus CR1-25 and Methylbacterium CRL-26. above results. 58

The EXAFS and e.p.r.

data confirm the

The optical absorption and e.p.r. spectra and phosphatase activity of a high molecular weight uteroferrin are almost identical to those of the purple uteroferrin which had been reduced by 2mercaptothanol. 59 Miissbauer , and the kinetics of spectral change and activity, clearly demonstrate that phosphate binds to reduced uterof errin forming a reduced uterof errin-phosphate complex.60 In

Electron Spin Resonance

216

contrast, Evan's susceptibility measurements at pH 4.9 indicate that the e.p.r. silent species generated upon phosphate addition to 61 pink uteroferrin is diamagnetic. Addition

of

molybdate,

a

tetrahedral

oxyanion

analogue

the

of

phosphate, to uteroferrin transforms the rhombic e.p.r. spectrum to one of axial symmetry. ESEEM and ENDOR studies show that a single molybdate anion is close to, and possibly a ligand of, the binuclear iron cluster. Accessibility of solvent to the binuclear iron cluster 62 has been established with these two techniques. An investigation of iron transport from either the purple or pink forms of native porcine uteroferrin to apo-transferrin in the presence or absence of mediators has shown that this is an unlikely physiological role for uteroferrin. 63 EXAFS in conjunction with e.p.r. has identified the presence of 64 a labile water molecule at the iron binding site in ovo-transferrin. Chemical modification of the uncoordinated histidine residues in transferrin has shown that a single uncoordinated histidine in each of the two iron binding domains kinetically stabilizes the protein against loss of iron.65 E.p.r. and electronic spectroscopy were used to demonstrate the lack of formation of Fe3+-transferrin-anion complexes. However the release of iron from the Fe3+-transf errinC032- complex was effected by the addition of anions. 66 Rat apo-transferrin binds both V4+ and V5+, forming 2:l metal complexes with the two major isotransferrins from rat plasma. Although the two transferrins have different physiological properties they exhibit identical V02+ e .p. r . spectra indicating similar metal ion binding sites in these proteins.67 35Cl and 14N NMR line width measurements of chloride, perchlorate and nitrate binding to transferrin, ultraviolet difference spectroscopy, equilibriumdialysis and ultraf iltration experiments confirm a 2 :1 complex between V5+ and apotransferrin and a weak interaction between V5+ and albumin. E.p.r. data reveal the presence of reducing agents in fresh serum which quantitatively converts the V5+ centres to V 0 2 + in the above proteins. 68 Characteristic changes upon binding Gd3+to transferrin from human serum in both the UV absorption of ligated tyrosines and the solvent proton longitudinal magnetic relaxation rates demonstrated a 2:l stoichiometry and pH dependent binding constants. The e.p.r.

217

7: Metalloproteins

spectrum consisted of resonances from specific (g = 4.96) and non specific Gd3+ complexes. 69 Lipoxygenase

is

a

non-heme

iron

(Fe2+) dioxygenase

which

catalyzes the dioxygenation of fatty acids containing a 12,42pentadiene system. Conversion of the native into the yellow enzyme (involving iron oxidation) was monitored by e.p.r. Magnetic susceptibility studies of the yellow enzyme and its anaerobic complex with linoleic acid has identified the presence of high-spin Fe2+ in the e.p.r. silent fraction.70 The existence of at least a single molecule of water coordinated to the high-spin Fe3+ ion in ferric soybean lipoxygenase I has been established by a comparison of the linewidths of the g component in normal and 170-enriched water. 71 Y The nitric oxide complex of exhibits an approximately axial sensitive to pH between 7 and 11.

ferrous soybean lipoxygenase I = 3/2 e.p.r.

spectrum which is Addition of ethanol abolished this

S

effect. Coordination of water to the iron was ruled out through H 2 1 7 0 exchange experiments. 72 E.p. r . and electronic absorption spectroscopies were employed in the characterization of a series of 4substituted catechols with ferric lipoxygenase I. At pH 7 , catechols with electron withdrawing groups formed stable Fe3+-catecho12complexes. In contrast, catechols with less electron withdrawing substituents reduced Fe3+ to Fe2+. These latter complexes provided an estimate of the Fe3+/Fe2+ reduction potential (+0.6 i 0.1 V) 73 Nordihydroguaiaretic acid an efficient inhibitor of lipoxygenase I was

.

shown by e.p.r., CD and fluorescence studies to reduce the catalytically active Fe3+ form to the inactive Fe2+ form. 74 Although phenidone is not a substrate for lipoxygenase I, e.p.r. studies showed that it readily reduced the catalytically active ferric enzyme to the ferrous state.75 A mechanism for the inactivation of lipoxygenase I by hexanal phenylhydrazone based on e.p.r., magnetic 76 susceptibility and inhibition studies has been proposed. E.p.r., optical absorption and stop flow kinetics have been used to characterize three lipoxygenase I1 product complexes (9R-hydroperoxyoctadecadienoic acid (9R-HPOD), 9 s and 13s-HPOD). A comparison of the results with those obtained from similar studies on lipoxygenase I suggest that the function of iron and the catalytic mechanism are similar for the two enzymes.77 Based on e.p.r. data, the iron in

218

Electron Spin Resonance

native lipoxygenase V from potato tubers appears in a high-spin Fe3+ 78 state with a pseudo-axial environment. Mossbauer and e.p.r. studies of catechol 1,a-dioxygenase from Pseudomonas putida and several of its complexes show that the iron centre is high-spin ferric in character in all of these complexes, including the enzyme-catechol complex and the steady state intermediate obtained from the reaction of the enzyme, pyrogallol and 02. These experiments show that the iron serves to activate the substrate rather than O2 .79 The role of the endogenous Ca2+ ion in horseradish peroxidase has been examined by proton NMR, e.p.r. and electronic absorption spectroscopy. In the calcium free enzyme the Fe3+ exists in thermal equilibrium between high- and low-spin states. These experiments show that Ca2+ acts in maintaining the protein structure in the heme environment and the spin state for the iron in favour of its enzymatic activity. 8o Tyrosine 3-monoxygenase (tyrosine hydroxylase) catalyzes the first and rate limiting step in the biosynthesis of dopamine, noradrenaline and adrenaline. l3.p.r. data has revealed a single axially symmetric high-spin ferric ion. Catecholate coordination to the iron was apparent from the electronic absorption spectrum. 81 Resonance Raman and e.p.r. studies have identified coordination of tyrosine to the high-spin non heme iron in 4-hydroxyphenylpyruvate dioxygenase isolated from Pseudomonas P. J. 874. 82 NADPH oxidase from neutrophils has been concentrated by immunoprecipitation and found to have a rhombic high-spin heme iron e.p.r. spectrum. 83 Nitrile hydratase isolated from Brevibacterium R312 or Pseudomonas chloroaphis is a new iron containing enzyme which catalyzes the hydration of aliphatic nitriles to the corresponding amides. Comparison of the e.p.r. spectrum of this enzyme with iron bleomycin complexes establishes that Fe3+ is present in a low-spin state. Axial coordination is provided by thiolate and aquo groups and substrate binding occurs through replacement of the water molecule. 8 4

7: Metalloproteins

219

D-Altronate hydratase and D-mannonate hydratase have been isolated from Escherichia c o l i and were activated upon addition of Fe2+ and a reducing agent. Preliminary e.p.r. studies of the native enzyme and enzyme-substrate complexes have been measured. 85 3.2 Heme Iron Proteins.- The e.p.r. spectra of myeloperoxidase (g = 6.88, 5.09, 1.96) and eosinophil peroxidase86 (g = 6.58, 5.28, 1.98) from rat bone marrow cells, although both characteristic of high-spin ferric heme iron in an environment of rhombic symmetry, differ slightly suggesting the microenvironment surrounding the heme iron is different in the two enzymes. Oxidized spinach catalase exhibits an e.p.r. spectrum consisting of two overlapping rhombic, high-spin ferriheme signals. 87 Optical spectral data and pyridine hemochromogen presence of a protoheme and a novel heme.

analyses

indicate

the

The heme site of thyroid peroxidase has been shown8* to be significantly different to that of lactoperoxidase from an e.p.r. study of the cyanide-, azide- and nitric oxide-treated enzyme and from native enzyme from porcine thyroids. Illumination of the peroxide compound of horseradish peroxidase (HRP-I) at low temperature causes partial conversion of the heme electronic structure from a ferryl-porphyrin radical species into a low-spin ferric state proposed to be a Fe3+-OH

Conversion

of HRP-I into the Fe3+-OH species is shown to be a two step process when followed by e.p.r. and MCD spectroscopy. Nitridomanganese (V) protoporphyrin IX, prepared by hypochlorite oxidation of the corresponding Mn3+ compound was inserted into apomyoglobin and apo-horseradish peroxidase and were subsequently characterized by electronic absorption, e.p.r. and Resonance Raman spectroscopies. 90 The peroxide compound of Mn3+ protoporphyrin IX and its complexes with ago-horseradish peroxidase and apo-cytochrome c peroxidase have been characterized by electronic absorption and e.p.r. spectroscopies. The e.p.r. spectra indicated a Mn4+ (S=3/2) site in the former enzyme while a Mn5+ (Sell centre was a potential candidate 91 in the latter system.

220

Electron Spin Resonance

Axial coordination of histidine to the heme site was established from e.p.r. studies of the nitrosyl adduct of ferrous lactoperoxidase. The effects of pH, buffer and the interconversion of this complex to the nitrite complex was also e ~ a m i n e d . ’ ~ Resonance Raman and e.p.r. data have been reported for the ferrous, ferric and ferryl lactoperoxidase derivatives. Examination of the nitrosyl and cyanide 93 derivatives of the ferrous enzyme are consistent with previous data. Two types of heme binding sites in prostaglandin H synthase (PGHS) were resolved by e.p.r. at 4.2 K during a titration of apoprotein with hemir~.’~ PGHS has one catalytically active heme per polypeptide which is in the high-spin ferric state as determined by e.p.r. The ferrous NO derivative has also been studied and the ligation of the catalytically active heme iron discussed. The peroxidase reaction catalyzed by PGHS generates a tyrosyl radical, detected by e.p.r., with a concomitant loss in the high-spin ferric heme ~ i g n a l . ’ ~ [Fe(IV)O(protoporphyrin 1x1 3 . . .Tyr+’ is suggested as the two electron oxidized state of the enzyme where the tyrosyl radical is weakly coupled to the ferryl heme.

Ovine PGHS was studied

~ the resting enzyme the by optical, MCD and e.p.r. s p e c t r o s ~ o p y . ~In distribution of heme between high- and low-spin forms is temperature dependent: 20% of the heme was low-spin at room temperature whereas 50% was low-spin at 12 K. Reaction of resting PGHS with substrate generated e.p.r. signals around g =: 2 that were accompanied by corresponding decreases in intensity of the signals from ferric heme (g = 3 and 6).

It is suggested the radical signals may reflect PGHS heme in the ferryl state complexed with a free radical derived from

hydroperoxide or fragments of hydroperoxide. Porphobilinogen oxygenase exhibits e.p.r. signals of high-spin ferric heme with a site symmetry distorted from axial (g = 6.00, 4.94) .97 A feature (g = 4.3) assigned to non-heme iron in a rhombic environment was also observed.

Upon cyanide addition, the high-spin

signal was replaced by a low-spin cyanide complex (g = 3.22) while in the presence of dithiothreitol, the low temperature e.p.r. spectrum is that of an unusual heme-thiolate complex (g = 2.37, 2.23, 1.93). Reduction of the enzyme generates an axial free radical signal (g = 2.07, 2.00). Low temperature e.p.r. and near infrared MCD spectra of low-spin bis-imidazole complexes of ferriheme are typical of proteins with bis-

7: Metalloproteins

22 1

histidine ligati~n.'~ The near infrared MCD spectra of low-spin bisbutylamine complexed ferriheme is clearly distinguishable from that of the imidazole complexes whereas the e.p.r. data is less definitive, demonstrating that near infrared MCD spectroscopy may be useful for determining the nature of the axial ligands of low-spin ferriheme proteins. E.p.r. studies" of N i trosomonas hydroxylamine oxidoreductase poised by redox potentiometry at known states of reduction has permitted tentative assignment of e.p.r. spectral features to specific heme centres and definition of which hemes respond to the redox poise of their neighbours. The first e.p.r. spectra from quintet ( S = 2) states of Mb have been reported"' for the usual B parallel to B1 case and also for B perpendicular to B1. The spectra of Mb and the photolysis product of the CO-adduct of Mb have been simulated using the parameters Do/k = 7 K, oD/k = 1.5 K, Eo/k = 1 K and aE/k = 0.25 K. The spectrum from deoxy-Mb arises from an excited doublet (D>O). Reactions of Erwinia chrysanthememi cytochrome b562, human hemoglobin (Hb) and whale skeletal Mb with triethylphosphine Au+ complexes gives rise to novel high-spin Fe3+ heme proteins as detected by e.p.r., electronic absorption and NMR spectroscopy."' The heme iron of Mb and horseradish peroxidase ( H R P ) has been The metal-substituted proteins substituted with M=O (M = Mo, W).lo2 are studied by visible absorption and e.p.r. spectroscopies in the M5+ oxidation states and by visible absorption spectroscopy in the M4+ oxidation state to study the differences in stabilities between the two high valent states and also substrate binding effects. Single-crystal e.p.r. studieslo3 of deoxy-Co2+-substituted hemoglobin reveals nonequivalence in the electronic structure of the prosthetic groups between two a(Co)-subunits within the deoxy-a(Co)2 0 (Fe) hybrid hemoglobin. ESEEM spectroscopy of Fe2+ nitrosylhemoglobin and of nitrosylferrous and oxy-cobaltous tetraphenylporphyrin complexes have been reported. The high-spin e.p.r. signals, assigned to cytochrome b595 of the Escherichia c o l i terminal oxidase cytochrome d complex, have been

222

Electron Spin Resonance

examined for various oxidized and reduced states of cytochrome d . lo5 From these results, electron flow in the cytochrome dcomplex proceeds in the order cytochrome b558 to cytochrome b S g 5to cytochrome d t o 02. The e.p.r.

signals of purified cytochrome b561 from E s c h e r i c h i a

c o l i at 4.2 K are remarkably weak when compared with those of other

cytochromes in E s c h e r i c h i a c o l i . This is ascribed to antiferromagnetic coupling between the two hemes in the molecule of cytochrome b561. The formation of nitrosyl derivatives upon the addition of nitrite to reduced cytochrome cdl from T h i o b a c i l l u s d e n i t r i f i c a n s has been studiedlo7 by optical, e.p.r.

and Mdssbauer spectroscopies as a

function of pH. The reduced heme dl binds NO under both alkaline and acidic conditions whereas NO binding to reduced heme c occurred only under acidic conditions. Optical and e.p.r. spectroscopy of ferric heme proteins of the porphyrin, 0x0-porphyrin and isobacteriochlorin classes has indicatedlo8 that nitrite reacts with these proteins at the heme iron while sulfite reacts only with proteins containing the isobacteriochlorin macrocycle. Reasons for the formation of substantial quantities of e.p.r. silent heme in the nitrate and sulfate adducts have been proposed. The e.p.r.

spectral features of the four hemes of D e s u l f o v i b r i o

v u l g a r i s Miyazaki have been identified, log and the midpoint potential

of each

(Em = -227, -287, -320, -366 mV) determined from potent-

iometric titration. A structural change of the environment of the heme with the most negative potential is observed during the first step of the reduction. The e.p.r.

spectrum of the ferricytochrome c complex (g = 1.54,

2.26, 3.02) differs from that of free ferricytochrome c (g = 1.25, 2.25, 3.04) at temperatures below 20 K,'l0 the spectral differences being due to ligand field changes within the heme group. E s c h e r i c h i a c o l i sulfite reductase contains an active centre (SiRO) comprising a siroheme (Fe3+, S=5/2) linked to an [Fe - S I 2+

cluster. ENDOR has been employed in the characterization of t4he 47Fe hyperfine interactions in the three distinct spectroscopic forms of the two electron reduced centre, SiR2-. For the g = 1.94 species, the ENDOR results are in agreement with those obtained from Mossbauer

223

7: Metalloproteins

studies, while those for the g = 2.29 are more anisotropic. The spectra of the g = 4.88 species are markedly different, with three or four highly anisotropic transitions being observed. Mossbauer , e.p.r. and optical studies of sulfite oxidase isolated from D e s u l f o -

v i b r i o b a c u l a t u s and D e s u l f o v i b r i o g i g a s identified an exchange coupled siroheme- [Fe4-S4] 2+ cluster. A one electron reduction of the enzyme in a hydrogen atmosphere with trace amounts of hydrogenase and methylviologen yields a siroheme in a high-spin ferrous ( S = 2 ) spin state. Similar sulf ite reductases isolated from Methanosarcina b a r k e r i and Desulfuromonas a c e t o x i d a n s have been characterized by e.p.r. and optical spectroscopy. 113 The saturation magnetizations of the sulfite and nitrite complexes of oxidized sulfite and nitrite reductases have identified e.p.r. silent S=1/2 spin states for the active site complexes. A possible explanation is the presence of a heterogeneous sample having a wide distribution of g-values "g-strain", resulting in a spectrum broadened beyond dete~ti0n.l'~ A comparative e.p.r. study of nitrite reductases isolated from E s c h e r i c h i a c o l i and W o l i n e l l a s u c c i n o g e n e s showed nearly identical signals attributable to both high- and lowspin ferric heme centres. Upon reduction the signals disappeared. '15 In a separate e.p.r. and Mossbauer study the observed e.p.r. signals were attributed to two isolated low-spin Fe3+ hemes and two exchange coupled heme chromophores . Exact solutions for the e.p.r.

g and A matrices in spin coupled

systems are given in terms of the parameters from the isolated spins for S1 = 1, 3/2, 2, 5/2 and S2 = l/Z.117 4 Iron Sulfur Proteins

Iron sulfur proteins are widely distributed in biological systems, being found in many microorganisms, in plants and in mammalian cells. Although electron transport appears to be the sole function of the majority of iron sulfur proteins, there are a few proteins which have a catalytic function, that is, where the substrate binds directly with the iron sulfur cluster. E.p.r. spectroscopy has played an important role in the investigation of these proteins. The present status and future potential of many iron sulfur proteins has been published in a book that emanated from the "International 117 Symposium on the Frontiers of Iron-Sulfur Protein Research".

224

Electron Spin Resonance

[Fe-S41 (3+ 2+) C luster Containina Proteins.- A rubredoxin 4.1 exhibiting a high-spin Fe3+ e.p.r. signal, and a flavodoxin have been purified from D e s u l f o v i b r i o d e s u l f u r i c a n s Berre-Eau. temperature MCD study of oxidized rubredoxin from

A variable Clostridium

pasteurianum

has enabled estimates of the ground state g-values and the zero field splitting parameter to be made.12' A ligand field model shows that the differences between the spectroscopic properties of the reduced rubredoxin and desulforedoxin can be accounted for by a weak rhombic distortion. 12' 4.2

[Fe2-S2J (2+,1+) Cluster Containina Proteins.- Dihydroxy acid dehydratase catalyzes the dehydration and tautomerization of two naturally occurring 2,3-dihydroxycarboxylic acids to the corresponding 2,3-keto acids. The e.p.r. spectrum of the reduced enzyme consisted of two rhombic components (g = 1.99, 1.91, 1.82 and 2.04, 1.89, 1-82], the former resembling that of the reduced Reiske centre. Addition of substrate lead to changes in the e.p.r. spectrum suggesting that the iron-sulfur cluster has a catalytic role.122 The redox properties of the flavin and iron sulfur cluster in the flavoprotein of methane monoxygenase have been determined using e.p.r. spectroscopy. 123 4-Chlorophenylacetate 3,4 -dioxygenase system from Pseudomonas sp. CBS 3 consists of two protein components, an iron sulfur protein which functions as a dioxygenase and a reductase. Reduction of the reductase component with NADH yielded e.p.r. signals attributable to an [Fe2-S21 cluster and a f lavosemiguinone radical. 12' An angular overlap analysis of the Fe2+S4 centres in the gav

2

1.96 and gav = 1.91 classes of [Fe2-S21 proteins indicates that the Fe2+S4 chromophores are tetrahedrally compressed and the differences between the two classes can be explained by small variations in the angular and bonding parameters. 125 A second procedure for the simulation of g-strain broadened e.p.r. spectra has been applied to [Fe2-S21 proteins. The results demonstrated that the theoretical model which accounts for the variations in g-values also describes the conformational states of a given protein. 126 4.3 IFe3-S3,4J (3+'2+;1+' Cluster Containina Proteins. - The main spectral features of the e.p.r. spectrum arising from the 3Fe

225

7: Metalloproteins

clusters found in Azotobacter vinelandii ferredoxin (Fd) I and the trimeric Fd in Thermodesulfobacterium commune can be accounted for by including both the fine structure (D = 2 cm-') and exchange terms ( S in the calculation of the g matrix. Asymmetry in the lineshape can be accounted for by a distribution of the fine structure and exchange parameters

z -20 crn-')

Magnetic susceptibility measurements on both the oxidized and reduced forms of the [Fe3-S41 cluster in Fd I1 from Desulfovibrio gigas, obey the Curie law between 1.8 and 200 K and have yielded

values of -2JOx > 200 cm-'

and -2Jred > 40 cm-l-

The former value is

incompatible with that obtained from e.p.r. experiments.128 Mossbauer spectra of reduced Fd I1 indicate the presence of a single Fe3+ and a delocalized Fe2+-Fe3+ coupled pair in reduced Fd 11. X-band e.p.r. Oxidation of spectra reveal a A M = k 4 transition at low field.129 [Fe4-S412- with K3[Fe(CNI61 yields a 3-Fe cluster that has been characterized by e.p.r. and Mossbauer spectroscopy.130 4.4 Cluster Containina Proteins.- ~ . p . r . [Fe4-S4J spectroscopy was employed in the determination of the redox potentials (3+,2+;2+81+)

of the two Fe4-S4 clusters in Clostridium pasteurianium.

The midpoint

potentials were essentially identical (f10 mV) and independent of pH 131 (-412 ? 11 mv). Characterization

of

the

dimeric

pyruvate:flavodoxin

oxido-

reduc tase from K1 ebsiell a pneumoniae and C l o s tridium thermoace ti cum by chemical and e.p.r.spectroscopy has identified the existence of two [Fe - S ](2+f1+) clusters per subunit.132 E.p.r. was used to monitor 4 4 the electron transfer properties of the [Fe4-S41 (2+,1+) , [Fe2S2](2+r1+) and the flavin redox centres in a mutant form of fumarate reductase found in Escherichia c01i.133 The active form of aconitase has a diamagnetic IFe4-SqI2+ cluster in which a specific iron atom (FeA) acts as a substrate binding site. Binding of specifically labelled (170 and 13C) substrates, citrate, isocitrate and cis-aconitate to the reduced active paramagnetic [Fe4 S4]l+ cluster of the reduced active form was monitored by ENDOR. The results indicate that only the carboxyl at the C-2 position of the propane backbone in addition to H20 or OH- is strongly bound to FeA. 134 Lactyl dehydratase which catalyzes the dehydration of lactylCoA to acrlyl CoA consists of two enzymes, El and E2.

E.p.r.

and

226

Electron Spin Resonance

metal analyses were used to identify an [Fe3-S3/4] ( 3 + ' 2 + ; 2 + r 1 + ) and an unusual [Fe4-S4] (2+'1+) cluster in E2. Addition of substrate or inhibitors to E2 caused dramatic changes to the e.p.r.

spectrum of the

former cluster indicating that it was functioning as a substrate binding site Examination of the seven iron flavodoxins from Azotobacter vinelandii and Thermus thermoghilus by low temperature MCD, electronic absorption and e.p.r. has identified the presence of an [Fe3(3+,2+;2+,1+) and an [Fe4-S4] (2+'1+) cluster in each protein. s3/41 A rationalization for the absence of a AM = +4 transition in the former protein was provided. 136 Substitution of zinc into the fourth site of the [Fe3-S41(1+'0) cluster in Fd I1 from Desulfovibrio gigas has been characterized by e.p.r. and Mossbauer spectroscopies. The results indicate the presence of a single Fe3+ (S=5/2) site and a coupled Fe2+-Fe3+ pair in the reduced [ZnFe3-S41 cluster. 137

'+

5 Hydroaenase and Other Nickel Containina Enzymes Nicke1,a relatively abundant element, ca.8% of the earth's core and 0.01% of the earth's crust, is readily available to organisms by leaching of the most abundant from, Ni2+. A number of nickel enzymes have been purified, including jack bean urease, hydrogenases, methyl coenzyme M reductase and an acetyl CoA-cleaving CO dehydrogenase.

In

ureases the active site Ni(I1) ion has a catalytic role in the hydrolysis of urea. The latter enzymes function as biological hydrogenation, desulfurization and carbonylation catalysts. Several reviews on the current status of these enzymes have been published.138,139

5.1 Nickel Iron-Sulfur HYdroaenases.- Mossbauer evidence in conjunction with previous e.p.r. data has established that Desulfovibrio gigas hydrogenase contains one nickel centre , one magnetically isolated [Fe3-Sxl cluster and two [Feq-S41 2+ clusters. Conversion of

the [Fe3-Sxl to an [Fe4-Sql 2+ was not observed. Activation of the oxidized, 'unready' state of hydrogenase from Desulfovibrio gigas by hydrogen has been monitored by e.p.r. and optical absorption spectra. Loss of the Ni3+ (NiA) and the [Fe3-Sxl signals was accompanied by the appearance of a broad spectrum attributed to one or possibly both of

227

7: Metalloproteins

the [Feq-S4]'+ clusters. Involvement of a slow change in the coordination sphere of the Ni2+ ion during the activation process was inferred from the observation of a new Ni3+ (NiC) e.p.r. ~ i g n a 1 . l ~ ~ Enrichment of the D e s u l f o v i b r i o g i g a s hydrogenase with 61Ni showed that the e.p.r. signal attributed to NiC was in fact a nickel containing species and not an iron sulfur cluster. At pH 7.0 the midpoint potential was found to be -720 mV which was strongly pH dependent (-120 mV/pH unit). 142 A preparation of hydrogenase from W o l i n e l l i s u c c i n o g e n e s enriched with 3 3 S has been characterized by e .p.r. spectroscopy. Resolution of 33S superhyperfine coupling in the e.p.r. spectra arising from the Ni3+ site in the resting enzyme, and the Ni+ site before and after photodissociation of the nickel-hydrogen bond, indicates coordination of one sulfur ligand to the nickel ion.143 Exposure of reduced hydrogenase from Chromatium vino sum to CO resulted in two new e.p.r. detectable nickel species, one of which was sensitive to light. The similarity of the e.p.r. spectra arising from the illumination of the H2- and CO- reduced enzymes suggests that H2 and CO bind to nickel at the same site.144

A modification of the parent plasmid pHV15 to plasmids that direct a high expression of hydrogenase in E s c h e r i c h i a c o l i has produced an inactive hydrogenase. E.p.r. of the purified enzyme has identified the lack of the hydrogen binding active site and an iron sulfur cluster as the cause for inactivity.145 E.p.r. spectroscopy was used to characterize three selenium containing hydrogenases from three separate fractions (periplasmic, cytoplasmic and membrane bound) of D e s u l f o v i b r i o b a c u l a t u s . Spectra of partially and fully reduced samples indicated the presence of two different [Fe4-S4] (2+'1+) clusters and a signal reminiscent of the Ni C species.146 The temperature dependence ( 4 . 2 - 20 K) of the Ni+-HZ e.p.r. signal from Chromatium vino sum and M e t h a n o b a c t e r i u m t hermo a u t o t r o p h i c u m hydrogenases indicates spin coupling of the Ni+ centre to a paramagnet, possibly an [Fe4-S4] cluster. 147 Hydrogenase isolated from D e s u l f o v i b r i o a f r i c a n u s exhibits an e.p.r. signal typical of that obtained from [Fe3-Sx] clusters. No nickel signal was

'+

observed. Reduction of the enzyme yielded a signal which has been ascribed to either Ni+ or Ni3+H-. 14*

228

Electron Spin Resonance

5.2 Iron-Sulfur Hvdroaenases.- Two different hydrogenases have Hydrogenase I is been isolated from C l o s t r i d i u m p a s t e u r i a n u m W5. active in the oxidation of H2 while hydrogenase I1 is active in both the oxidation and evolution of H2. Mossbauer and e.p.r. studies of hydrogenase I show that the enzyme contains two diamagnetic [Feg-S412+ clusters and a cluster with 3-4 Fe atoms. ENDOR studies of the binding of 13C0 to hydrogenase I indicate that a single CO binds covalently to this latter centre. 148 Mossbauer experiments on the reduced and CO-bound oxidized states of the H cluster in hydrogenase I1 indicate the presence of an unusual diamagnetic (S=O) [Fe3-S41 cluster.150 Results from e.p.r., ENDOR and Mossbauer studies show two different iron sites upon CO binding. Site 1 has AFe = 18 MHz shifting to = 30 MHz upon CO binding and consisting of two Fe atoms and site 2 has AFe 2 7 MHz shifting to = 10 MHz and containing a single Fe. 13C ligand hyperfine coupling indicates that a single CO binds to this ~ 1 u s t e r . l ~The ~ H cluster of oxidized hydrogenase I1 is heterogeneous and exhibits three different e.p.r. signals, two of which are pH dependent. Reversible CO binding gives a single pH independent e.p.r. signal. Mechanisms for H2 activation by hydrogenases I and 11 are proposed on the basis of the midpoint potentials and were used to rationalize binding of C0.152 An e.p.r. investigation of reoxidized hydrogenase from D e s u l f o v i b r i o v u l g a r i s reveals an axially symmetric signal (g=2.06, 2.01)

which is overshadowed by a rhombic signal. In addition a signal at g=5.0 is observed.153 Binding of CO to this enzyme increased the intensity of the axially symmetric signal (0.94 spin/mol). 154 E.p.r. evid-2nce has been presented for the existence of a relatively high-potential regulatory centre, tentatively assigned as an [Fe2-S21 cluster, in the NAD-dependent hydrogenase from A l c a l i g e n e s e u t r o p h u s Zl .155 5.3 Nickel Enzymes.- Two forms of carbon monoxide dehydrogenase, purified from R h o d o s p i r i l l u m rubrum were shown to contain different amounts of nickel, iron, sulfide and zinc. Reduction of either form with CO or dithionite resulted in identical rhombic e.p.r. spectra.156 An inactive, Ni-deficient form of carbon monoxide dehydrogenase has also been purified from this photosynthetic bacterium. Comparison of the e.p.r. spectra from the apo- and native forms of the enzyme established that the two [Fe2-S21 clusters were intact.

229

7: Metalloproteins

Isotopic substitution of carbon monoxide dehydrogenase, from Methanosarcina t h e r m o p h i l a , with 61Ni, 57Fe or 13C0 resulted in a broadening of the e.p.r. spectra, indicating the presence of a Ni-FeC spin coupled complex. 157 Nickel EXAFS studies indicate a square pyramidal or distorted square planar geometry for the CO binding site in carbon monoxide dehydrogenase. These results have been compared with existing e.p.r. data.159 6 Molybdenum Enzymes

6.1 Molybdenum Iron Cluster Enzymes.- Nitrogenase consists of two proteins; a molybdenum iron (FeMo) protein, which contains the iron molybdenum cofactor (FeMoco), and a smaller iron (Fe) protein. Electrochemical titration16' of the S = 1/2 e.p.r. signal from Fe protein of A z o t o b a c t e r v i n e l a n d i i (Av2) and C l o s t r i d i u m p a s t e u r i a n u m (Cp2) indicates a one electron process in both cases. Further, the redox properties of the S = 3/2 form of Av2 are identical the

The number of to those of the S=1/2 form, as determined by e.p.r. electrons involved in the interconversion of the oxidized and semi reduced states of the MoFe protein of A z o t o b a c t e r v i n e l a n d i i (Avl) was found161 to be 1 by controlled potential coulometry and by double integration of the S = 3/2 e.p.r.

signal of the semi-reduced state.

FeMoco has always been extracted into the organic solvent Nmethylformamide. The first dimethylformamide and acetonitrile solutions of A z o t o b a c t e r v i n e l a n d i i FeMoco162 display the characteristic S = 3/2 e.p.r. signal, and the ability to reconstitute the inactive nitrogenase of the UW45 mutant of A z o t o b a c t e r v i n e l a n d i i . The g = 10.4, 5.8 and 5.4 excited state e.p.r.

signals observed

from thionine oxidised MoFe protein of A z o t o b a c t e r v i n e l a n d i i (Avl), 163 A z o t o b a c t e r chroococcum (Acl) and K l e b s i e l l a pneumoniae (Kpl), combined with the temperature dependence of the g = 10.4 peak area, are consistent with an S = 7/2 spin system, with spin Hamiltonian parameters D = -3.7 f 0.7 cm -1 , IEl = 0.16 f 0.01 cm-' and g = 2.0. Quantification relative to the S = 3/2 e.p.r. signal from dithionitereduced MoFe protein indicates one P cluster per FeMoco. Two models, one based on two and another on four P clusters per FeMo protein, have been discussed. Another model for the metal cluster composition of Avl, consisting of two e.p.r. centres, three P-clusters, and a single

Electron Spin Resonance

230

centre that is kinetically and spectroscopically distinct from the e.p.r. and P-cluster centres, has been proposed164 from an e.p.r., CD and visible-ultraviolet study of its redox reactions. Alternatively, from a Mossbauer study of Kp1165 which has 57Fe-enriched Pclusters and 56Fe-enriched FeMoco, it was concluded that the reduced MoFe protein contains two pairs of P-clusters: one pair containing one Fe2+, and three D-sites and the other one Fe2+, two D, and one S-site. The magnetization of the nitrogenous proteins of A z o t o b a c t e r v i n e l a n d i i has been examined166 using a commercial SQUID susceptometer to demonstrate the application of new techniques to the study of the magnetization of metalloproteins.

The principal values of the 57Fe hyperfine coupling matrix and its orientation relative to the g matrix (zero-field splitting matrix) have been determined167 for five distinct iron sites of the Avl FeMoco cluster by examining the variations in the frozen solution 57Fe ENDOR spectrum as the observing g value (static field) is moved across the e.p.r. envelope. 95M0 ENDOR168 of the FeMo protein from wild type and n i N mutant nitrogenase of K l e b s i e l l a pneumoniae reveals perturbations in the Mo site of the mutant cofactor that are not observable by e.p.r. or EXAFS studies. Component 1 of a nitrogenase isolated and purified from a tungsten resistant mutant (LM2) of A z o t o b a c t e r v i n e l a n d i i contains one atom each of molybdenum and tungsten.16’ The e.p.r. spectrum of this protein is composed of three signals, two of which originate from S = 3/2 spin states. One of these is from a conventional MoFe protein while the other is from a W-containing cofactor. 6.2 Mononuclear Oxomolvbdenum Enzymes.- Multifrequency (9 and 35 GHz) e.p.r. studies16’ of the Very Rapid and Rapid signals of xanthine oxidase, employing 95M0, 97M0, 33S and 170 isotope enrichments, were used to evaluate the various hyperfine couplings and angular relations between the principal axes of g and A, as well as the nuclear quadrupole interaction for 97M0. The results support the presence of an 0x0 ligand in the Rapid and of both an 0x0 and a sulfido ligand in the Very Rapid signal-giving species. Recent model studies171 have provided the strongest evidence to date for the Very Rapid, Rapid and Slow signal-giving species in xanthine oxidase being formulated as

23 1

7: Metalloproteins

[Mo5+OS(OR)3 , [Mo5+0(SH) (OR)] and [Mo5+O(OH)I centres respectively. The principle components of the 95M0 hyperfine matrix and its orientation relative to the g matrix have been determined172 for the xanthine oxidase Rapid Type 1 signal from multi-frequency (9.1, 3.6, 2.3 GHz) e.p.r studies. It has also been shown that the magnitude of the 170 superhyperfine coupling can be quite variable in [Mo5+0( 170R) ] species. The molybdenum iron sulfur protein from Desulfovibrio gigas exhibits e.p.r. signalslc13 reminiscent of the Slow and Rapid type 2 signals of xanthine oxidase and aldehyde oxidase. It appears this protein is a form of an aldehyde oxidase or dehydrogenase. Comparison of the Mo5+ e.p.r. signals and midpoint potentials of the Mo6+/Mo5+ and MO5+/Mo4+ redox couples of native formate dehydrogenase from Methanobacterium formicicum with those of cyanide treated enzyme indicates174 the mode of CN- inactivation is analogous to that established for the related enzyme xanthine oxidase. The organization of the metal components of the formate dehydrogenase-nitrate reductase chain of the membrane-bound respiratory particle complex of Pseudomonas aeruginosa was determined from a comparison of e.p.r. and MCD data from the intact particle175 with that from purified extracted The molybdenum of sigpurified formate dehydrogenase gives rise to two Mo5+ e.p.r. n a l lacking ~ ~ ~ proton ~ hyperf ine coupling when reduced by dithionite in the presence of excess formate. The spectroscopic evidence suggests the extracted enzyme is acting as dithionite as an electron donor to a [Feq-S4] donates electrons to molybdenum, producing a bound to the metal. In the intact particle175

a C02 reductase, with

cluster, which in turn M o 5 + species with CO, this molybdenum ion ii

at the low potential end of the chain and passes electrons to cytochrome b, which from e.p.r. and MCD studies176 is a bis-histidine coordinated heme with unusual steric restraint at the iron. The next component is a [Fe4-S4] cluster. The temperature dependence and quantitation of e.p.r.

signals

from a tungsten-containing formate d e h y d r ~ g e n a s e l ~suggests, ~ the presence of two [Fe2-S21 and two [Feq-S4J clusters per molecule. The possible origin of another signal assigned to W5+ is analyzed using ligand field theory, and the redox behaviour is considered with

Electron Spin Resonance

232 respect to possible ligation at the active site.

The oxidation reduction midpoint potentials for the molybdenum centre in assimilatory NADH:nitrate reductase from spinach (Spinacia are -8 olercea) , determined by monitoring the Mo5+ e.p.r. signal,"' and -42 mV for the Mo6+/Mo5+ and Mo5+/Mo4+ couples respectively. A structure function model of nitrate reductase from Chlorella vulgaris has been proposed18' from protease denaturation studies. A fragment having reduced methylvio1ogen:nitrate reductase activity and loss of NADH:nitrate

reductase and NADH:cytochrome

gives the same Mo5+ e.p.r.

c reductase activities

spectrum as is obtained from intact enzyme.

7 Vanadium Enzymes Anaerobic lysis of the vanadium containing blood cells from the marine tunicate Ascidia ceratodes have identified the presence of the pentaaquovanadyl ion in an environment of pH 1.1 to 1.4. 18' In contrast to these results, Brand et a1.182 have examined the vanadium containing blood cells from a variety of marine tunicates and shown that the intracellular pH is 6.5.

The e.p.r.

spectra of the cell

lysates and the whole animals indicate the source of the V02+ ion is not the pentaaquovanadyl ion. Similar results have been found for the vanadyl species in blood cells from Ascidia a h 0 d 0 r i . l ~ ~ A V5+ containing bromoperoxidase has been purified from the brown

alga Laminaria saccharina. E.p.r. was employed in the characterization of the dithionite reduced enzyme.184 An e.p.r. study of the reduced vanadium bromoperoxidase from the brown seaweed Ascophyllum nodosum has identified water as a ligand to the V02+ ion and an ionizable group (either histidine or aspartate/glutamate) with a pKa of 5 . 4 near the metal ion. The inability of hydrogen peroxide and bromide to oxidize the reduced enzyme shows that V4+ is not involved in catalysis. 185 E.p.r. and reconstitution experiments have shown that vanadium is essential for enzymatic activity with the vanadium present in the +5 redox state.186

The purification and properties of the vanadoprotein from the vanadium nitrogenase of Azotobacter chroococcum have been described 187 At low temperatures the dithionite-reduced protein showed

.

e.p.r.

signals at g = 5 . 6 , 4.35, 3.77 and 1.93, consistent with an S

233

7: Metalloproteins

= 3/2 ground state. An additional S = 1/2 centre gives rise to a

feature at g = 1.93. Iron vanadium cofactor extracts in N-methylformamide exhibit'l' e.p.r. signals near g= 4.5, 3.6 and 2.0 from a S = 3/2 centre. A nitrogenase of Azotobacter vinelandii capable of fixing nitrogen under Mo deficient conditions consists of two proteins (Avl' and Av2') .189'190 The larger protein Avl' contains V in place of M0.l" The e.p.r. spectrum of this protein differs from conventional component 1 in that it lacks the g = 3.6 resonance that arises from FeMoco, but contains an axial signal with g < 2 as well as inflections in the g = 4-6 region, possibly arising from an S=3/2 state. The second protein, Av2 ' is a [Fe4-S4] protein18' essentially identical to conventional Av2, both antigenically and by e.p.r. E.p.r. 190-192 studies of native and dithionite reduced Avl' indicate that the signals at g = 5.80 and 5.40 arise from an S = 3/2 spin system with ID1 = -0.74 cm -1 , while resonances at g=2.04 and 1.93 arise from an S = 1/2 spin system. MCDlg2 studies of Avl' confirm this interpreta-

tion and indicate an S=5/2 spin state for the thionine oxidized VFeS centre. 8 Cobalt Enzymes

Purification and some properties of the corrinoid-containing membrane protein from Methanobacterium thermoautotrophicum have been reported. lg3 A Co2+ e.p.r. signal is observed following reduction by 5 mM dithiothreitol, and at redox potentials between -50 mV and -350 mV. From the redox properties of the corrinoid membrane protein it is suggested that the Co may be reduced and reoxidized in vivo. The purificationof a corrinoid (5-methoxybenzimidazolylcobamide) Fe-S protein from Clostridium thermoaceticum, which acts as a methyl carrier in the synthesis of acetyl-CoA has been reported. E.p.r spectra of the reduced protein indicate the presence of a single [FeqS411+ cluster in two distinct (S-34) spin states. E.p.r. spectra of the Co2+ centre in the corrinoid indicate that the benzimidazole is 194 not coordinated.

Electron Spin Resonance

234

9 Manuanese Enzymes Ribonucletide reductase, a manganese containing enzyme exhibits an electronic absorption spectrum resembling Mn3+-containing pseudocatalase and of 0x0-bridged binuclear Mn3+ model complexes. Denaturation of the enzyme with trichloroacetic acid yielded a Mn2+ e.p.r. spectrum. 195 10 Paramaanetic Metal Substituted Enzymes

10.1 Substituted Zinc Enzymes.- The first volume of a series entitled "Progress in Inorganic Biochemistry", edited by H.B. Gray and I. Bertini has been published. This volume reviews the current status of a number of zinc metalloenzymes. lg6 The formation of ternary complexes between C-terminal products of peptide or ester hydrolysis or carboxylate inhibitor analogues and anions (azide, cyanate and thiocyanate) with cobalt carboxypeptidase have been characterized by electronic absorption, CD, MCD and e.p.r. spectroscopy.lg7 Distorted octahedral coordination spheres for both the catalytic and structural zinc ions of Bacillus cereus phospholipase C have been found from electronic absorption, CD, MCD and e.p.r. studies of the cobalt substituted enzyme. Rapid scanning stopped flow spectroscopy has identified three metallo-intermediates in the binding and hydrolysis of benzylpenicillin by Co2+ substituted Bacillus cereus p lactamase 11. E.p.r. studies indicate a five coordinate geometry for the Co2+ ion in the resting enzyme. lg9 E.p.r. studies of 63Cu2+ substituted glyoxylase I and its complexes with glutathione and glutathione derivatives indicate the native enzyme exists in two different conformations, involving one and two inequivalent equatorial histidine ligands respectively. 2oo At pH 6.0 alkaline phosphatase has been found to selectively bind copper in site A and cobalt in site B. The decrease in intensity of the room temperature Cu2+ e.p.r. spectrum upon the addition of Co2+ suggests a magnetic interaction between these two sites .201

235

7: Metalloproteins

The complex between NAD' and horse liver alcohol dehydrogenase exists in both acidic and alkaline forms. The transition between the two forms proceeds through several intermediates with an apparent pK

a

of 6.9. At pH values below 6.9 a ternary complex between NAD+, C1 and alcohol dehydrogenase has been shown to form. 202 Characterization of copper substituted horse liver alcohol dehydrogenase and its complexes with NADH and NAD+-pyrazole have identified a CuS2N(OH2) unit in the ligand free enzyme and a CuS2N2 unit in the ternary complex. A thermochromatic shift was only observed in the absorption spectrum of the copper substituted enzyme .203 Apparent dissociation constants have been determined for the binding of bivalent first row transition ions to the apo form of the iron activated alcohol dehydrogenase from Zymomonas mobilis. Spectroscopic characterization of the native Fe2+, Co2+ and Cu2+ derivatives indicate a six coordinate N4O2 coordination sphere. 204 Dietary copper has been shown to regulate superoxide dismutase (SOD) activity in the aortas of young developing chickens. In addition a copper deficiency suppresses superoxide dismutase activity without inhibiting synthesis or accumulation of the enzyme. 2 0 5 A comparison of proton NMR and optical absorption data with previous e.p.r. spectra for phosphate complexes with Co2Zn2-SOD or Co Co 2-SOD, reveal that the cobalt ion at the copper site is bound to three histidines and one phosphate group. His-61 which bridges the zinc and cobalt or the two cobalt ions dissociates upon binding of phosphate.206 The decrease in intensity of the Fe3+ e.p.r. signal upon addition of near stoichiometric amounts of H202 to Fe-SOD from Escbericbia coli can be attributed to the reduction of Fe3+.207 Stepwise incorporation of Cd2+ or Co2+ into apo-metallothionein from Cancer p a g u r a s as monitored by e.p.r., electronic absorption, CD and MCD, indicates that cluster formation occurs when more than four metal ions are bound. Prior to cluster formation, each metal ion has a tetrahedral tetra thiolate coordination sphere. 208 10.2 Other Metallosubstituted Enzymes.- Pulsed e.p.r. has been used to detect Mn2+-ligand hyperfine interactions in complexes with creatine kinase and in the Mn2+ metalloprotein concanavalin A. A 14N hyperfine interaction was observed in the spectrum of the transition state analogue

complex of

creatine kinase

(enzyme-Mn2+-ADP-NCS--

236

Electron Spin Resonance

creatine) indicating coordination of NCS- to Mn2+. Complexes prepared in 2H 0 gave strong signals due to weakly coupled 2H. In concanavalin 2 A, evidence for the coordination of histidine was obtained.209 In contrast to the mononuclear high-spin Mn2+ spectrum found for the complex between methyl a-D-mannopyranoside and Ca2+-Mn2+ concanavalin A, an eleven line spectrum is found for the Mn22+ enzyme saccharide complex. The spectrum was interpreted as arising from an antiferromagnetically coupled pair of Mn2+ ions (J = 1.8 cm-')

.210

The interaction of three Mn2+ substituted Ha-ras-encoded p21 proteins with guanosine-5'-phosphate (GDP) complexes has been monitored by e.p.r. Observation of 170 ligand hyperfine coupling from stereospecifically labelled GDP indicates that the j.3-phosphate is bound to Mn2+.211 E.p.r. studies indicate that the two isomers of L-methionine (R,S ) -sulfoximine interact differently with the Mn2+ ion in glutamine synthetase. Coordination of at least one water molecule to the Mn2+ ion is demonstrated from 170 enriched H20 experiments. ESEEM studies show that the imine nitrogen moiety of the S-isomer, but not the Risomer, interacts with the metal ion.212 An X-ray structure of staphylococcal nuclease from Escherichia coli indicates an octahedral coordination for the essential Ca2+, with Asp-21, Asp-40 and Thr-41. The Asp-40 codon was mutated to Gly40 on the gene that coded for this enzyme. A comparison of the Ca2+ and Mn2+ dissociation constants (as determined by e.p.r.1 for the enzyme and its complexes with 3'5'-pdTp and 5'-TMP indicates that this residue is involved in metal ion c ~ o r d i n a t i o n . ~ ' ~ Addition of diptheria toxin to Mn2+ (2:l) induced a 79% loss in intensity of the aguo Mn2+ e.p.r. signal indicating strong metal ion binding. Competition studies involving Mn2+ and Ca2+ indicate that digtheria toxin is a calcium binding protein. 213 An X-band e.p.r study of the binding of vanadyl ( V 0 2 + ) ions to calmodulin indicates a binding stoichiometry of 4 mol of V02+ per mol of protein. Two distinct binding sites for the vanadyl ion were identified from frozen solution spectra and metal competition studies with calcium.215

7: Metalloproteins

231

The relevant positions of the monovalent cation and the enzyme bound divalent cation in pyruvate kinase was probed by using the metal ions V02+ and T1+. Observation of a large 203'205Tl ligand hyperfine coupling indicates the two cations are quite close to each other, possibly linked through a bridging carboxylate group. 216 A similar experiment has been performed on S-adenosylmethionine synthetase which requires two monovalent cations for activity. 217 Two enzyme metal bound intermediates formed by the Co2+-activated ribulose-1,5-bisphosphate carboxylase/oxygenase enzyme have been studied by e.p.r. spectroscopy.218 The proposed e.p.r. detectable intermediates are proposed to be enzyme metal coordinated ribulose 1,5-bisphosphate and an enzyme metal coordinated enediolate anion of it, where binding of ribulose 1,5-bisphosphate occurs first.

A three site model for the binding of Cu2+ ions to yeast inorganic pyrophosphatase has been proposed on the basis of e.p.r. and measurements of the electron spin-lattice relaxation time. Magnetic interactions between the Cu2+ ion in the presence or absence of substrate were apparent from the e.p.r. ~pectra.~" 10.3 Extrinsic Metal Bindina Site in Bioloaical Molecules.- E.p.r. spectroscopy was employed in the characterization of the three distinct cation binding regions: Ca2+-Gd3+ site, a Cu2+-Zn2+ site and a V02+ site in a-lactalbumin. The lack of magnetic interactions between these sites indicates they are separated by at least 10 A. 220 E.p.r. and CD spectral titrations of Cu2+ (1 to 20 equivalents) (in the presence or absence of Fe3+-mesoporphyrin) with the histidine rich glycoprotein (HRG) from rabbit serum indicate two structurally distinct metal binding domains in which histidine is implicated in the 221 coordination sphere of the Cu2+ ion. E.p.r. studies indicate that 10 Mn2+ binding sites (five highand five medium- affinity sites) are present per bacteriorhodospin monomer from Halobacterium halobium.222 The interaction of cupric isonicotinohydrazide, an antiviral compound with calf thymus DNA was investigated by CD, H ' NMR and e.p.r. Gel electrophoresis established DNA cleavage upon incubation of the metal complex with DNA.223 Upon irradiation with 365 nm light,

238

Electron Spin Resonance

Cu2+-camptothecin significantly produced single and double strand

Loss of the e.p.r. signal during light scission of the DNA helix. activation indicates the formation of a Cu+ complex. Inhibition of

-

strand scission by catalase indicates that O2 is the reactive species. 224 Activation of Fe2+-bleomycin to Fe3+-bleomycin is inhibited by a large excess of DNA. E.p.r. studies indicated that in the presence of microsomes and NADPH this inhibition was reversed.

-

The role of O2 in this oxidation reaction was established using a preparation of xanthine oxidase 225

.

11 Mitochondria1 Enzymes A recent review226 on electron-transf erring proteins purified from two strictly aerobic, extremely thermophilic bacteria, Thermus t h e r m o p h i l u s and B a c i l l u s isolate PS3, covers many aspects of e.p.r. studies of the cytochromes c , caa3 (cytochrome oxidase) and 0 , and the Rieske proteins of these bacteria. The spin-lattice relaxation behaviour of the [Pe2-S21 proteins of S p i r u l i n a maxima, adrenal ferredoxin (gav = 1-96], Thermus t hermo -

p h i l u s Rieske protein and Pseudomonas p u t i d a benzene dioxygenase (gav 1.91) have been studied.227 From a comparison of the exchange coupling parameter, J, it was concluded that the structural factors which determine the value of the g matrix and the strength of the

=

antiferromagnetic exchange interactions are different. ESEEM and ENDOR spectra of the Reiske [Fe2-S2] centre from yeast mitochondria1 complex I11 indicate the irons are coordinated by at least one and probably two nitrogen ligands. 228 Evidence for two independent pathways of electron transfer in mitochondria1 NADH:Q oxidoreductase has been presented from a study of the pre-s teady-s tate kinetics of reduction229 and reoxidat ion230 of NADH:Q oxidoreductase by NADPH occurring in submitochondrial particles using a freeze-quench method. The extent of reduction of the iron sulfur clusters 1-4 was determined using the peak height of their respective e.p.r. features. The physicochemical properties of the iron sulfur clusters present in the NADH:ubiquinone oxidoreductase of P a r a c o c c u s d e n i t r i f i c a n s have been examined in the cytoplasmic membrane particles by redox potentiometry and e.p.r. spectroscopy. 231 Two binuclear and three tetranuclear e.p.r. detectable iron sulfur clusters (N-la, N-lb, N-2, N-3, N-4) were observed. The number and

239

7: Metalloproteins

type of iron sulfur clusters present in the NADH dehydrogenase of the mammalian respiratory chain have been studied232 by a combination of low temperature MCD and quantitative e.p.r. spectroscopies. The two methods gave concordant results showing the presence of one binuclear and three tetranuclear NADH-reducible iron sulfur clusters. A

phospholipid-dependent

interaction

between

2,5-dibromo-6-

methyl-3-isopropyl-~-benzoqu~none (DBMIB) and an iron sulfur protein in mitochondria1 ubiquinol-cytochrome c reductase has been studied233 using the change in the e.p.r. spectrum of the iron sulfur protein

observed upon addition of DBMIB to intact and dilipidated enzyme and to purified iron sulfur protein. 11.1 Succinate Dehvdroaenase.- Beef heart mitochondrial cytochrome b560 in a protein complex (QPs) that converts succinate dehydrogenase into succinate-ubiquinone reductase shows, two e.p.r. signals ( g = 3.07, 2.92) whereas cytochrome b560 in succinateubiquinone reductase exhibits only one e.p.r signal (g = 3.46). 234 When QPs is reconstituted with succinate dehydrogenase to form succinate-ubiquinone reductase, the g = 3.46 signal reappears at the This and other evidence shows expense of the g = 3.07 signal. cytochrome b560 to be physically associated with succinate dehydrogenase. 11.2 Cvtochrome 0xidase.- The electron spin relaxation rates of the two species of cytochrome a33+-azide found in the azide compound of bovine-heart cytochrome c oxidase were measured by progressive By taking into account the microwave saturation at T = 10 K.235 relaxation parameters for both gx and gz components of the cytochrome a3-azide g matrix, the angle between the gz components of the cytochrome a and cytochrome a3 g matrices was determined to be 0 and 18O, and the cytochrome a-cytochrome a3 spin-spin distance was found to be 19 k 8 A. Freezing fresh preparations of cytochrome c peroxidase (CCP) induces the reversible coordination of an internal strong field ligand to the penta-coordinated high-spin ferric heme iron to form a hexacoordinated low-spin compound (g = 2.70, 2.20, 1.78). 236 This is prevented by glycerol ( g = 6.4, 5.3, 1.97). Aging also results in hexa-coordination. Amino acid substitution of tryptophan 51 with phenylalaline produces a small structural perturbation in the symmetry

240

Electron Spin Resonance

of the ferric heme e.p.r. substitution

of

Trp-51

signals.237 by

a

It is also reported238 that

phenylalanine

residue

in

CCP

from

E s c h e r i c h i a c o l i results in an e.p.r.

spectrum that shows predominantly high-spin iron with a peak at g = 6.0 whereas normal enzyme exhibited a mixture of high- and low-spin e.p.r. signals. E.p.r. spectra of NO complexes of ferrous cytochrome P450 from bovine adrenocortical mitochondria are very similar to those of Pseudomonas p u t i d a rat liver microsome cytochrome P450, with rhombic

symmetry

(g

=

2.071, 2.001, 1.962) and AZ = 2.2 mT for 14N0 The changes observed on the addition of substrates are

discussed. The ENDOR signals of the CuA2+ centre have been240 compared from 2+ ( a 3 + . CuA

the following forms of cytochrome c oxidase: resting 3+

.

.

2+

a3 Cug 1 ; mixed valence, 2-electron-reduced CO-ligated oxidase ( a 3 + . CuA2+. a32+C0. CuB+); and a more completely reduced mixedvalence CO-ligated oxidase. The purification of a three-subunit (62, 39 and 20 kDa) ubiquinol-cytochrome c oxidoreductase complex from P a r a c o c c u s d e n i t r i f i c a n s has been reported. 241 The 20 kDa subunit is assumed to be a Rieske type iron sulfur protein on the basis of its molecular weight and the presence of an e.p.r. detectable signal typical of this iron sulfur protein in the three subunit complex. In addition, a thermodynamically stable ubisemiquinone radical is detectable by e.p.r. The micro-environment of the Rieske iron sulfur cluster in the cytochrome bcl complex changes in parallel with the redox state of the ubiquinone Incubation of beef heart cytochrome c oxidase at high pH induces a new copper e.p.r. signal with parameters markedly different from those obtained from copper centres which have undergone denaturation. 2 4 3 Spin quantitation establishes that the new signal does not arise from CuA and suggests that at high pH the magnetic coupling present at the cytochrome a3.CuB centre is lost. A significant proportion of cytochrome a3 maybe converted to a low-spin thiolate complex during this process. Cytochrome c oxidase, purified using a modified procedure, reacts in a single rapid phase with cyanide and lacks the g' = 12 e.p.r.

7: Metalloproteins

241

signal that is normally observed.244

This enzyme can be converted to a g' = 12 species by several methods.

Chemical and spectroscopic evidence for an e.p.r. silent, three electron reduced dioxygen intermediate at the dioxygen reduction site in the 428-580 nm component of oxygenated cytochrome c oxidase has been reported. 245 Cug couple.

The intermediate contains a ferry1 a3-Fe / cupric

EXAFS studies of copper in CuA-depleted, B-(hydroxymercuri)benzoate modified and native cytochrome c oxidase have been reported.246 The native and modified enzymes were characterized by e.p.r. spectroscopy. 12 Photosvnthetic Enzvmes. An 02-binding hemoprotein from the photosynthetic purple sulfur bacterium, C h r o m a t i u m v i n o s u m has been studied by MCD and e.p.r. 247 The e.p.r. spectrum of the oxidized protein exhibits features (g = 6, 2) characteristic of axially symmetric high-spin hemes, whereas in the presence of cyanide, one or more low-spin species are observed (g = 3.56, 2.31, 2.15, 1.98, 1-90). Resonance Raman and e.p.r. results identify248 histidine as an axial ligand in a high-spin, heme c-containing protein isolated from C h r o m a t i u m v i n o s u m . Cytochrome b6 in freshly prepared, active cytochrome b6-fcomplex from spinach chloroplasts shows a broad, low-spin e.p.r. signal at gz = 3.6.249 Up to 50% of the hemes of cytochrome b6 can be converted to high-spin

(g = 6) by inactivating treatments, or by isolating

cytochrome b6. This is accompanied by changes in the g signal of the Y Rieske iron sulfur centre. An additional unstable asymmetric lowspin e.p.r signal (g = 3.85) similar to cytochrome b in mitochondria1 or bacterial cytochrome bcl-complexes is also observed. 250 The inhibitors stigmatellin and 2-iod0-2',4',4'-trinitro-3-methyl-6-isopropyl diphenylether of the plastoquinol-plastocyanin oxidoreductase complex (cytochrome b6-fcomplex) alter the e.p.r. spectrum of the Rieske iron The results are considered in terms of binding sulfur centre. 251 domains for inhibitors in the cytochrome b6-f complex. From a stopped-flow and rapid-freeze e.p.r. study of the isolated cytochrome b f complex from spinach,252 a mechanism where the semi-

242

Electron Spin Resonance

quinone resulting from the reduction of the Rieske centre reduces a low potential cytochrome b563 was proposed. Measurements on the Rieske e.p.r. signal indicate that the binding of the inhibitor DBMIB to the cytochrome bf complex of pea (Pisum sativum) involves more than one polypeptide.253 The Rieske iron sulfur protein concentration was estimated using the size of the e.p.r. signal at g = 1.89 and the effect of DBMIB was monitored from the height of the modified Rieske e.p.r. signal at g = 1.95. The e.p.r. features (g = 1.81, 1.90) of the Reiske cluster of Rhodobacter capsulatus are significantly reduced in a nonphotosynthetic mutant (MT113) 254 MT113 lacks c-type cytochromes and the cytochrome bcl complex. The deletion

.

of the genes for cytochromes b and cl in strains MT-CBC1 and MT-GS18 also results in the absence of the Rieske cluster as shown by 255 e.p.r.

A comparative characterization of two heterocyst ferredoxins (Fd I and Fd 11) and a vegetative cell ferredoxin (Fd v) of Anabaena variabilis revealed256 that Fd I1 and Fd v are very similar while Fd I is significantly different. The rhombic e.p.r. signals for the reduced proteins (Fd I1 and Fd v, g = 1.88, 1.96, 2.05; Fd I, g = 1.9, 1.95, 2 . 0 4 5 5 ) are characteristic of a plant-type ferredoxin with an [Fe2-S21 (2+'

cluster.

E .p.r. and electronic spectral results257 for the ferricytochrome c' of the photosynthetic bacterium Rhodopseudomonas capsulata suggest that the ground state of heme Fe3+ at neutral pH consists of a quantum mechanical admixture of an intermediate spin and a high-spin state and that at pH 11.0 it is in a high-spin state. Chemotropically grown Rhodopseudomonas viridis contains little bacteriochlorophyll b and no detectable reaction centre .258 Evidence for the absence of the reaction centre-associated cytochrome c552-c558 peptide is the lack of an e.p.r. signal (g = 1.82) from the reduced reaction centre primary quinone-Fe2+ complex. Photochemical reaction centres from a strain of Rhodopseudomonas capsulatus that lack the light-harvesting complex known as B800-850, or LH 11, have

.

been isolated. 259 The observed e .p. r spectrum of the reduced primary quinone acceptor (g = 1.82, 1.69) arising from a quinone radical in close association with the high-spin ferrous ion of the reaction centre , is very similar to that seen in Rhodopseudomonas sphaeroides.

243

7: Metalloproteins

12.1 Photosvstem I.- Photosystem I (PS I) charge separation in a subchloroplast particle isolated from spinach was investigated by e.p.r. following graduated inactivation of the bound iron sulfur centres by urea-ferricyanide treatment. 260 During inactivation the disappearance of iron sulfur centres A, B and X (FA, FB, FX) correlates with the appearance of a spin-polarized triplet signal with ID1 = 2 7 9 ~ 1 0 -cm ~-' and ]El = 3 9 ~ 1 0 -cm-l. ~ Radiation inactivation studies on the PS I reaction centre from spinach ( S p i n e c i a o l e r a c e a ) and peas ( Pisum s a t i v u m ) have been The photochemical activity of PS I was measured at reported. 261 cryogenic temperatures using e.p.r. to demonstrate the photochemical oxidation of P,oo and the concomitant reduction of the iron sulfur centres. Centres A and B are located on different polypeptides with the electron carriers from P700 to centre X located on a separate polypeptide complex. Another view resulting from e.p.r. studies of FA, FB and F of a reaction particle containing P700 and F following X treatment of PS I particles with lithium dodecyl sulfate,562 is that FA and FB are located on a low molecular weight polypeptide and FX is shared between two high molecular weight polypeptides PS IAl and PS

The photosystem of the green photosynthetic green sulfur bacterium Chlorobium l i m i c o l a contains two early electron acceptors similar to photosystem I.263 One is an iron sulfur centre with gvalues of 1.94 and 1.88. The spinach PS I complex obtained after extraction of one of the two vitamin K1 molecules present p e r P7oo was fully active in transferring electrons as evidenced by the generation of the FA 264

e.p.r. signal (g = 2.05, 1.94, 1.86) after photoreduction at 15 K. This indicates that one vitamin K1 molecule is not required for these processes. Extraction of cartenoids, antenna chlorophylls and all vitamin K1 from PS I particles of spinach chloroplasts with diethyl and . ether eliminates the A1 e.p.r. signal but not the signals of A iron sulfur centres (FA, FB and FX) .265 Similar results are obtained The results strongly from pea ( P i s u m s a t i v u m ) PS I particles.266

-

suggest that A1 is vitamin K1 and that it functions to mediate electron transfer between A . and iron sulfur centres. Deuteration experiments267 strengthen this proposal and suggest A1 is an anionic semiquinone in a protic environment. In 2H20 the A1 signal is narrowed

Electron Spin Resonance

244

by a factor of 0.66 compared with the control. Deuteration also reversibly modified the relative extents of reduction of electron acceptors FA and FB. However, e.p.r. studies268 on light induced reduction of PS I iron sulfur centres, in different kinds of PS I reaction centres containing various amounts of vitamin K1, provides

-

evidence that direct electron transfer from A . to the iron sulfur centres is highly efficient at low temperature and is relatively independent of the presence of vitamin K1. The gx value of FX (1.77 in PS I particles) is sensitive to extraction of more than 50% of the chlorophyll, increasing to 1.80 in particles having about 5% chlorophy11.269 This suggests an interaction between FX and the surrounding chlorophyll a molecules. The temperature dependence of the spinlattice relaxation time of centre FX in PS I has been deduced270 from the relaxation broadening of the e.p.r. line above 10 K. The results indicate the presence of an excited state with an energy of 50 cm-'. The properties of this centre are explained in terms of a [Fe2-S2] cluster characterized by weak antiferromagnetic exchange interactions. Simulations of the EXAFS spectrum arising from the iron sulfur centres (FA, FB and FX) in photosystem I are consistent with either a mixture of [Fe2-S21 and [Fe4-S41 clusters or with unusually distorted [Fe4-S41 clusters. The FX centre is thus thought to arise from either the [Fe2-S21 or the distorted [Fe4-S4] clusters. 27 1 In a study of the PS I electron acceptors in pea chloroplasts,272 conditions for the generation of e.p.r. signals from FA and/or FB were determined. A comparison of the electron transfer characteristics of Fe2+ depleted reaction centres reconstituted with Fe2+, Mn2+, Co2+, Ni2+, Cu2+ and Zn2+, from Rhodopseudomonas sphaeroides with the native reaction centres indicate that neither Fe2+ nor any divalent ion is 273 required for rapid electron transfer from Q , to Q,. 12.2 Photosvstem 11.- Quantitative e.p.r. measurements show removal of the 17- and 23- kDa polypeptides by treatment with 2 M NaCl causes low potential cytochrome b559 to become fully oxidized during the course of dark a d a ~ t i o n . Oxidation ~~~ of the S1 state gives rise to the S 2 multiline e.p.r. signal, which arises from the Mn site of the 02-evolving centre. The inhibition of the 02-evolving enzyme of PS I1 by washing in NaC1, has been studied by monitoring the yield of

7: Metalloproteins

245'

the e.p.r. multiline signal of the S2 state.275 The results indicate that inhibition of S-state turnover occurs at the S3 to S transition. 0 In addition, reactivation by Sr2+ instead of Ca2+ induced a modified multiline signal membranes.

similar to that

observed

in NH3-treated

PS

I1

The magnetic properties of Mn in the photosynthetic 0 -evolving 2 complex have been interpreted in terms of a model consisting of an exchange coupled Mn tetramer, where both ferromagnetic and antiferromagnetic exchange occur simultaneously to give an S = 3/2 ground state and an S = 1/2 excited state.276 A study of the e.p.r. properties of the g = 2 multiline and g = 4.1 signals of the S2 state support this From a multifrequency (9.08, 9 . 2 5 , 9.46, 34.13 GHz) e.p.r. study278 it was concluded that the two signals arise from ground state doublets of paramagnetic species in separate S2 state PS I1 donors that are in a redox equilibrium with each other, and that the g = 4.1 signal originates from monomeric Mn4+. The g matrix of the multiline signal is isotropic (g = 1.982) with an excited spin multiplet at = 30 cm-l. In a study of the reactions of hydroxylamine with the electrondonor side of PS I1 in both 02-evolving and inactivated spinach PS I1 membrane preparations, low temperature e.p.r. was employed to monitor the oxidation state of the Mn complex in the 02-evolving centre and to detect the radical oxidation products of hydroxylamine. 279 The binding of hydroxylamine to the water oxidizing complex and the ferroquinone electron acceptor of PS I1 followed by the reversible loss of both of the S2-state e.p.r.

signals, titrates with

identical curves, suggesting a common chemical reactivity and, hence, origin for these signals. 280 The g = 4.1 e.p.r. signal has been assigned to Mn in the S2 state of the photosynthetic 02-evolving complex from an X-ray absorption edge study. 281 The results of X-ray K-edge and EXAFS studies of the Mn in the S1 and

S2

states of the 02-

evolving complex in P S I1 preparations from spinach have also been reported. 282 Two configurations of the S 2 state were studied: that characterized by the multiline e.p.r. signal and that characterized by the g = 4.1 signal.

The concentration of S1 in the 02-evolving complex has been estimated from the amplitude of the S2-state multiline e.p.r. signal From this and that could be generated by illumination at 200 K.283

246

Electron Spin Resonance

other e.p.r. information, the mechanism of oxidation of the is discussed.

So

state

The electron transfer resulting from illumination and dark storage of PS I1 has been studied using e.p.r. signals from several electron carriers. 284 A slow change which occurred during dark storage of PS I1 samples was detected using the power saturation characteristics of component D due to a rearrangement of the Mn complex during long term dark adaption. The e.p.r. signal IIs in 02-evolving S2 state PS I1 particles decays with a half-life time of about 5 days whereas no decay is observed in the Mn multiline signal, upon storage at 77 K.285 Signal

IIs recovers its original intensity upon dark adaption at 210 K accompanied by a decrease in the multiline intensity. Replacement of ‘H2O by 2H20 in 02-evolving PS I1 preparations causes an increased resolution of the fine structure of the S2 state multiline e.p.r. spectrum indicating that protons bind at or near the Mn complex. 286 The decay kinetics for the S2 and S 3 states have been measured in the presence of an external electron acceptor.287 The temperature dependence of the individual S-state transitions has been measured in single flash experiments in which the multiline e.p.r. signal has been used as a spectroscopic probe. Evidence for an association between a 33 kDa extrinsic membrane protein, manganese and photosynthetic evolution has been presented288 from an examination of the release of Mn, the loss of 0 evolution and the loss of the multiline e.p.r. signal during the 2 titration of the release of three peripheral membrane proteins (17, 23, 33 kDa) from PS I1 membranes. The formation and flash dependent oscillation of the multiline e.p.r. signal in an 02-evolving PS I1 preparation lacking these proteins suggests289 these proteins are not necessary for multiline signal formation and that complete advancement through the S-states can occur in their absence when sufficient C1is present. An EXAFS study2” shows that the Mn complex is largely unaffected by removal of the 33, 2 4 and 16 kDa extrinsic proteins, and thus these proteins do not provide ligands to Mn. The structure of the Mn complex is disrupted upon depletion of half the Mn.

247

7: Metalloproteins

A chlorophyll-protein complex, capable of photochemical water oxidation and consisting of only one extrinsic protein of 33 kDa in addition to six intrinsic proteins of the PS I1 reaction centre, has been isolated.2g1 The multiline e.p.r. signal ascribable to the S2state was elicited in this protein complex by illumination at 200 K, as in membrane preparations.

Binding of NH3 to the S2 state of the 02-evolving complex of PS I1 causes a structural change in the Mn site that is detectable with

low-temperature e.p.r. spectroscopy.292 The average hyperf ine spacing in the S2 state multiline spectrum changes from 87.5 G in untreated samples to 67.5 G.

Conversely amines other than NH3 appear not to

bind to the Mn site in the S2 state presumably because of steric factors. 293 These results, along with the observation that O2 evolution activity is inhibited in the presence of NH4C1, 291 indicates that the e.p.r. detectable Mn site functions as the substrate-binding site of the 02-evolving complex. The multiline e.p.r.

signals arising from Mn in the S2 state of

the 0 -evolving system of spinach and the cyanobacterium A n a c y s t i s 2 n i d u l a n s have very similar properties and are affected identically by NH3 suggest’irig that the system is highly conserved.294 The role of chloride in the Mn containing 02-evolving complex of PS I1 has been studied295 by observing the amplitude of the multiline e.p.r. signal as a function of C1- concentration or when C1- is replaced by Br- or F-. Various structural possibilities for the Mn complex, which would account for the observed fine structure of the multiline e.p.r. spectrum are discussed. After excitation by continuous light at 200 K or a single flash excitation at 273 K, C1-depleted particles do not show the e.p.r. multiline signal associated with the S2 state, but only showed the broad signal at g = 4.1 signal after constant illumination. 296 However, upon addition of chloride after the flash, the multiline signal was developed in the dark. The effect of chloride on paramagnetic coupling of Mn in the 02-evolving complex of CaC12 washed PS I1 preparations has been examined using Q297 band e.p.r. An e.p.r. signal of hexa-aquo Mn was observed after acidification of PS I1 membranes and purified PS I1 reaction complex.298

248

Electron Spin Resonance

The Mn donor complex in the S1 and S2 states and the iron-quinone acceptor complex (Q-Fe2+) in 02-evolving PS I1 preparations from a thermophilic cyanobacterium, Synechococcus sp., have been studied with X-ray absorption spectroscopy and e.p.r. 299 Illumination of these preparations at 220-240 K results in formation of a multiline e.p.r. signal very similar to the Mn S2 species observed in spinach PS 11, together with g = 1.8 and 1.9 signals similar to the Fe2+-QA- acceptor signals of spinach PS 11. An e.p.r. study of the low fluorescent mutant, LF1, of the green algae Scenedesmus obliguus, shows300 that stable photoinduced charge separation occurs at low temperature forming the characteristic e.p.r. signals from QA-Fe2+, the primary semiquinone-iron complex. In the wild type bacterium the S2 Mn multiline signal is generated. The results suggest LF1 has a functional reaction centre which lacks the redox active Mn of the 02-evolving enzyme. E.p.r. signals from components functioning on the electron donor side of PS I1 have been monitored in PS I1 membranes isolated from spinach chloroplasts after treatment with trypsin at pH 6 . 0 and 7.L301

A low-temperature e.p.r. study of a purified 02-evolving PS I1 reaction centre core preparation has revealed that cytochrome bSs9 has been converted to its low potential form.302 Characterization303 of fatty acid inhibition of secondary electron transport in PS I1 at room and cryogenic temperatures, demonstrate that linolenic acid, and related fatty acid analogs, abolish the production, either chemically or photochemically, of the e.p.r. signal (Q--Fe2+ 1 associated with the bound quinone acceptor,

-

A new e.p.r. signal from Rhodospirllum rubrum chromatophores is attributed3" to QB-Fe2+, the semiquinone-iron complex of the secondary quinone electron acceptor. The QB-Fe2+ signal has two main features at g = 1.93 and 1.82 which have different microwave power, temperature and pH dependencies. The ferrous ion associated with the electron acceptors in PS I1 can be oxidized by the unstable semiquinone form of certain high-

249

7: Metalloproteins

potential quinones which are used as electron acceptors. 305 These reactions are detected by monitoring e.p.r. signals (g = 7.9, 5.3) arising from Fe3+. Optical and e.p.r. spectroscopies show Fe2+ can also be oxidized to Fe3+ in PS I1 particles (g = 8.0, 5.6) under oxidizing conditions at 10 K, 306 and in 02-evolving thylakoid membranes (g = 8.0) through photoreduction-induced oxidation in the presence of exogenous quinones acting through the Q, binding site. 307 It is suggested306f307 that the iron transfers electrons from Q, to Q,, being reduced to Fe2+ by QA- and oxidized to Fe3+ by Q,. A number of inhibitors have been shown to modify the Fe3+ e.p.r. spectrum, 305'308 leading to the suggestion307 that the Fe3+ spectrum is a new and sensitive probe of the contact points at which molecules bind to the Q, binding site. Evidence for two QA-like quinone binding sites in the reaction of the redox centre of Rhodospirllum viridis has come from a study3'' properties of Q and the pheophytin intermediary electron acceptor (I) A in chromatophores treated with o-phenanthroline to displace Q,, and in purified reaction centres, using the iron-split e.p.r. signals of QA- and I-. After substitution of Cu2+ into the reaction centre diquinone electron acceptor complex of Rhodospirll urn spectrum of axial symmetry (g Al = 0.0019 cm-')

sphaeroides, a typical Cu2+ = 0.0203 cm-'; g1 = 2.05, I = 2 - 1 9 0 A ll is observable310 in dark-adapted samples. All of

the peaks of the Cu2+ spectrum exhibit ligand hyperfine splitting due A,-") = 0.0017 to coupling from four nitrogens (A (N) = 0.00145 cm-', -1 cm 1 most likely from four histidine ligands to Cu in the Fe binding site. All spectral features were satisfactorily simulated.

n

Acknow1edqements.- We wish to think Dr, encouragement and proof reading this manuscript.

John Pilbrow for GLW acknowleges a

Research Fellowship from the Australian Research Council. References 1 2

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158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 , 188 189 190 191 192

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-

-

255

7: Metalloproteins

193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231

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-

m,

256

Electron Spin Resonance

232

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233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258

259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275

a,

7: Metalloproteins

276 277 278 279 280 281 282 283 284

257

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285 286 287 288 289

KKawamori, J.-I. Satoh, T. Inui and K. Satoh, FEBS Lett., 1987, 217, 134. J.H.A. Nugent, Biwhim. BiODhyS. Acta, 1987, 893, 184. S. Styring and A.W. Rutherford, Biochim. BiophYS. Acta, 1988, 933, 378. D. Hunziker, D.A. Ambramowicz, R. Damoder and G.C. Dismukes, Biochim. BioPhvs. Acta, 1987, 890, 6. S. Styring, M. Miyao and A.W. Rutherford, Biochim. Biophvs. Acta, 1987, 890,

32.

290 291 292 293 294

J.L. Cole, V.K. Yachandra, A.E. McDermott, R.D. Guiles, R.D. Britt, S.L. Dexheimer, K. Sauer and M.P. Klein, Biochemistry, 1987, 26, 5967. Y. Yamada, X.-S. Tang, S. Itoh and K. Satoh, Biochim. BioDhys. Acta, 1987, 891, 129. c. Beck, J.C. de Paula and G.W. Brudvig, J. Am. Chem. SOC., 1986, 108,4018. W.F. Beck and G.W. Brudvig, Biochemistry, 1986, 25, 6479.

R. Aasa, L.-E. Andrhasson, G. Lagenfelt and T. VanngArd, FEBS Lett., 1987, 221, 245.

295

K.Yachandra, R.D.

Guiles, K. Sauer and M.P. Klein, Biochim. Biophvs. Acta,

1986, 850, 333.

296 297 298 299 300 301 302 303 304 305 306 307 308 309 310

T. Ono, J.L. Zimmermann, Y. Inoue and A.W. Rutherford, Biochim. Biophvs. Acta, 1986, 851, 193. G. Mavankal, D.C. McCain and T.M. Bricker, FEBS Lett., 1986, 202, 235. D.F. Ghanotakis and C.F. Yocum, FEBS Lett., 1986, 197, 244. A.E. McDermott, Y.K. Yachandra, R.D. Guiles, J.L. Cole, S.L. Dexheimer, R.D. Britt, K. Sauer and M.P. Klein, Biochemistry, 1988, 21, 4021. A.W. Rutherford, M. Seibert and J.G. Metz, Bochim. BioDhvs. Acta, 1988, 932, 171. M. Volker, G. Renger and A.W. Rutherford, Biochim. Biophys. Acta, 1986, 851, 424. D.F. Ghanotakis, D.M. Demetriou and C.F. Yocum, Biochim. Biophvs. Acta, 1987, 891, 15. J.T. Warden and K. Csatorday, Biochim. Biophys. Acta, 1987, 890, 215. C. Beijer and A.W. Rutherford, Biochim. Biophvs. Acta, 1987, 890, 169. J.-L. Zimmermann and A.W. Rutherford, Biochim. Biophvs. Acta, 1986, 851, 416. S. Itoh, X.-S. Tang and K. Satoh, FEBS Lett., 1986, 205, 275. V. Petrouleas and B.A. Diner, Biochim. Biophvs. Acta, 1987, 893, 126. B.A. Diner and V. Petrouleas, Biochim. BioDhvs. Acta, 1987, 893, 138. M.C.W. Evans, Biochim. Biophvs. Acta, 1987, 894, 524. S.K. Buchanan and G.C. Dismukes, Biochemistry, 1987, 26, 5049.

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8 Complexes of Paramagnetic Metals with Paramagnetic Ligands BY SANDRA S . EATON AND GARETH R. EATON 1.

Introduction

When a paramagnetic metal binds to a paramagnetic ligand, the analysis of the EPR spectrum requires consideration of the electronelectron spin-spin interaction. These are the complexes that are the focus of this chapter. Comprehensive reviews of metal-nitroxyl complexes have covered the periods up to mid-1977l and mid-1977 to mid-1986.’ These extensive reviews include EPR, structural, and magnetic susceptibility data and citations of related reviews. Selected examples from these earlier periods are included in the present review, but the emphasis here is on recent papers and primarily those that include EPR data. Literature is covered through mid-1988. Structural and magnetic susceptibility data are included as needed to provide perspective on the EPR data, or lack thereof. Section 2 includes complexes of spin-labeled ligands in which coordination is via an atom or atoms other than the nitroxyl oxygen. Coordination through the nitroxyl oxygen is discussed in section 3. Semiquinone complexes are reviewed in section 4 . When referring to magnetic susceptibility data we have attempted to report all values of J in + + terms of the Hamiltonian H = JS1.S2. Analysis of fluid solution spectra gives the magnitude of J, but not the sign. Recent reviews have discussed applications of spin-labeled ligands in analytical chemistry, the spin-spin interaction Hamiltonian for weakly interacting dimers, and long-range intramolecular exchange interactions. Analogous to the situation with organic molecules containing two paramagnetic centers, where the description of the spin system can vary from diradical to triplet depending on the strength of the spin-spin interaction, metal complexes of paramagnetic ligands can range from cases that are best described as two weakly interacting spins to ones in which the interaction is strong enough that the best description is a net spin for the molecule. In the former case, the EPR spectra are AB (or more complicated) second-order splitting patterns that can be analyzed by analogy with NMR AB, 258

8: Complexes of Paramagnetic Metals with Paramagnetic Ligands

259

etc., patterns. In the latter case the issue can be whether the net spin is localized primarily on one center or the other. The complexes described in section 2 span the range of behavior from two weakly interacting spins to a net spin for the molecule. All of the complexes described in sections 3 and 4 fall in the second category. Two caveats need to be provided to the novice reader of the literature in this area. 1) When spectra are obtained on magnetically concentrated samples intermolecular spin-spin interaction may be more important than intramolecular spin-spin interaction. 2) When a paper reports that the room temperature fluid solution EPR spectrum of a complex is an unperturbed threeline nitroxyl spectrum,6f7 two tests should be applied before the conclusion is reached that there is little or no metal-nitroxyl interaction. a) The double integral of the signal should correspond to 100% of the sample. It is easy to miss a broad signal from a complex with metal-nitroxyl interaction in the presence of a small amount of a sharp nitroxyl signal due to impurity, decomposition, or dissociation. b) Rapid relaxation of the metal unpaired electron can decouple the metal and nitroxyl spins so spectra for complexes of metals such as Co(II), Ni(II), and Fe(II1) that have short electron spin relaxation times should be run at lower temperatures before reaching a conclusion concerning the magnitude of the spin-spin interaction. Many papers do not provide sufficient quantitative and temperature-dependent information for proper analysis.

2. Complexes with Spin-Labeled Ligands This section is subdivided according to the metal spin since both the types of spectra observed and the analysis of the spectra depend on the spin of the metal in the absence of interaction.

1/2 - Most of the reported examples of metalnitroxyl complexes involve S = 1/2 metals, especially Cu(I1). Weak interaction between two S = 1/2 centers causes each of the EPR transitions to split into two lines. As the magnitude of the interaction becomes large relative to the g-value difference between the two interacting centers and relative to the hyperfine splittings, the g and a values approach averages of those observed in the absence of spin-spin interaction. F o r example, if there is

--2.1 Metals with 5

260

Electron Spin Resonance

one nitroxyl per metal the g value is 0.5*(gM + gNO) and the hyperfine splittings are one-half of the values observed for the isolated paramagnetic centers. AB splitting patterns have been obtained for interaction The dependence of the magnitude through 8-12 bonds and up to 14 i.5 of the electron-electron exchange interaction on molecular structure and bonding compels interpretation in terms of orbital overlap. Thus values of the coupling constant J are measures of the extent of electron spin delocalization. A s more information is obtained regarding the efficiency of electron-electron exchange through various bonding pathways, the magnitude of J can be used to probe the orbital structure of complex molecules and to analyze conformational changes. Papers related to the computation of these types of spectra include discussions of: the use of the relative intensity of the half-field transition to obtain interspin distances, 8 r diagonalization calculations of spectra of three interacting spins,lo perturbation calculations of frozen solution spectra of spin-labeled metal complexes with S = 1/2,11 and the spin-orbit contribution to the zero field splitting tensor.l2 2.1.1 Copper(I1) - Values of J have been obtained from AB patterns in fluid solution for a series of nitroxyl-labeled copper porphyrins and salicylaldimines. l3-I8 The electron-electron interaction was dependent on the atoms in the linkage between the copper and the nitroxyl and on the conformation of the linkage. Analysis of the orientation-dependent spin-spin splitting in single crystals of Z~I(TPP)(THF)~ doped with spin-labeled copper porphyrins permitted separation of the isotropic exchange and anisotropic dipolar contributions to the spin-spin interaction. There was no correlation between the interspin distance and the value of J. A series of spin-labeled pyridines coordinated to C ~ ( h f a c )were ~ used to examine the dependence of J in fluid solution on the metalnitroxyl linkage and on the nitroxyl ring size.22r23 If the value of J is just large enough to give averaged copper and nitroxyl g values at X-band, resolved AB patterns can be obtained at Q-band.24 Averaged g and A values have been reported for copper(I1) complexes of imidazoline n i t r o x ~ l s ~and ~ - nitroxyl ~~ derivatives of ,8semicarbazone,29 dithiocarbamate,30f 31 salicylaldimines,32-34

8: Complexes of Paramagnetic Metals with Paramagnetic Ligands

26 1

diketonates,35-38 glycine,39 40 glycylglycine,41 and acetic acid42. Analysis of the frozen solution spectra of C U ( ~ )gave ~ a zero field splitting D = 129 G.38 Use of the point-dipole approximation leads to a copper-nitroxyl distance of 6.0 i.38 In Cu( 2)2py2 D = 70 to 75 G which corresponds to a copper-nitroxyl distance of 7 i.42 Zero field splittings 2D = 364 to 375 G were obtained from frozen solution spectra of C U ( ~ ) ~Simulations . ~ ~ of the frozen solution spectra of Cu( 4)2 and copper complexes of related imidazoline nitroxyls gave D values between 280 and 410 G, but the relative orientations of the zero-field splitting and hyperfine tensors were not known.25f28

1

2

3

4

When 5 was coordinated to C ~ ( h f a c ) the ~ copper-nitroxyl interspin distance was estimated to be 3.64 i.43 The short distance was attributed to extensive spin delocalization into the pyridine ring.43 Frozen solution EPR spectra of Cu( 6 ) ( 5) were analyzed at five microwave frequencies.44 The temperature dependence of the intensity of the half-field transition gave J = 12.3 cm-’. The copper-nitroxyl interspin distance determined from the relative intensity of the half-field transitions and by simulation of the spectra was 3.36 i. The short distance was attributed to dimer formation in which the nitroxyl oxygen from one complex interacted with the copper ion from a second complex.44 These values of the interspin distance are based on the assumption that there is negligible anisotropic exchange contribution to the zero field splitting, which may not be valid at this short an interspin distance. 44 Further studies are needed to determine whether there are structural differences between these two complexes with similar ligands.

Electron Spin Resonance

262

5

6

2.1.2 Silver(I1) - AB patterns with values of J between 6 and 1000 G were observed in fluid solution for spin-labeled silver(I1) p o r ~ h y r i n s .Comparison ~~ with analogous vanadyl and copper( 11) complexes showed that the magnitude of the metal-nitroxyl exchange interaction increased in the order vanadyl < copper(I1) < silver( 11), which parallels other indications of the extent of delocalization of the metal unpaired electron spin into the porphyrin orbitals. Doped single crystal spectra of a spin-labeled silver porphyrin were used to separate the isotropic exchange and anisotropic dipolar contributions to the spin-spin interaction.46 Powder spectra of silver porphyrins in frozen solution and immobilized on imbiber beads showed that values of J were in good agreement with data from fluid solution, which indicated that the molecular conformations were substantially unchanged by immobilization.47 2.1.3 Vanadyl - Resolved AB patterns were observed in fluid solution for a series of spin-labeled pyridines coordinated to VO(hfac)2 and V O ( t f a ~ ) ~ .Different ~ ~ , ~ ~ trends in the values of J for copper and vanadyl complexes of the same ligands are consistent with the symmetries of the metal orbitals containing the unpaired electron. Single crystal EPR spectra were obtained f o r a spin-labeled vanadyl porphyrin doped into Zn( TPP ) ( THF)2. 48 Imbiber beads were used to prevent aggregation and to obtain immobilized EPR spectra of spinlabeled vanadyl complexes. 4 9 r 50 J = 0.014 cm-’ was obtained for a vanadyl complex of spin-labeled EDTA.’l Averaged g and a values were reported for a vanadyl complex of a spin-labeled P - d i k e t ~ n a t e . ~ ~

-2.1.4 Low spin Cobalt(I1) - Values of J have been obtained from the AB splitting patterns for spin-labeled pyridines coordinated to cobalt( 11) porphyrins. 52r53 The dependence of the value of J on the metal-nitroxyl linkage was similar to that observed for the same ligands coordinated to Cu( hfac)2, which is consistent with orbital

8: Complexes of Paramagnetic Metals with Paramagnetic Ligands

263

symmetry arguments.

~2.1.5 Low spin Iron(II1) - Resolved spin-spin splitting was observed in frozen solution spectra of bis(imidazo1e) and bis(1methylimidazole) adducts of four spin-labeled iron porphyrins. 54 Values of J ranged from 0.05 to 0.28 cm-'. A second conformation of the spin-labeled iron(II1) porphyrins exhibited weaker spin-spin interaction. Double integration of fluid solution spectra showed that the broad signals were due only to the conformation with weaker spin-spin interaction. Previously reported spectra of spin labels bound to cytochrome P450 have been interpreted.54

2.2 Metal 5

1 ,nickel(I1) - Intramolecular interaction of a nitroxyl with paramagnetic nickel(I1) can cause severe broadening of the fluid solution EPR spectra.55*56 Broadened and shifted spectra were obtained in frozen solution for nickel(I1) complexes of spinlabeled x a n t h a t e ~and ~ ~ spin-labeled EDTA.51 The absence of an EPR spectrum for nickel(I1) complexes of imidazoline nitroxyls in fluid solution or powders was attributed to strong spin-spin interaction and the rapid Ni( 11) electron spin relaxation rate.25-27 -~ 2.3 Metal g = 3/2,

chromium(III), h.s. cobalt(I1) - EPR spectra have been obtained f o r a series of spin-labeled pyridines coordinated to Cr( TPP)Cl. 57 In fluid solution weak spin-spin interaction causes broadening of the nitroxyl signal and stronger interaction results in resolved splitting. Resolved spin-spin splitting was observed in frozen solution. The values of J were 1/2 to 1/3 the values observed for the vanadyl (S=1/2) complexes of the same ligands, consistent with expectations f o r an S = 3/2 metal. The absence of an EPR spectrum for cobalt(I1) complexes of imidazoline nitroxyls in fluid solution or powders was attributed to strong spin-spin interaction and the rapid cobalt(I1) electron spin relaxation rate.25-27

5 5/2, manganese(II), h.s. iron(II1) - Values of J ranging from 1 4 ~ 1 0 -to ~ > ~ O O X ~ Ocm--'~ were obtained by computer simulation of spectra for spin-labeled pyridines coordinated to Mn(hfac)2 in fluid solution.58 These values of J are about an order of magnitude smaller than for the same ligands coordinated to

2.4 __ Metal _

264

Electron Spin Resoiiunce

vanadyl ion ( S = 1/2). In frozen solution spectra of two spinlabeled pyridines coordinated to Mn(hfac)2 with J - 0, well-resolved dipolar splitting of the nitroxyl signal was observed.59 Severely broadened lines were obtained in the frozen solution spectra of spin-labeled Mn(I1) complexes with stronger exchange interaction.5 1 ' 59 For spin-labeled iron(II1) porphyrins in fluid solution weak iron nitroxyl interaction had a greater impact on the nitroxyl T2 than on The impact of the metal on the nitroxyl relaxation decreased as the distance between the metal and the nitroxyl increased, which is consistent with predominantly dipolar interaction. The effect of Fe(II1) was greater than that of Mn(II1). Stronger iron-nitroxyl interaction causes severe broadening of the nitroxyl EPR spectrum (linewidths of several hundred gauss) and g value shifts.62 The absence of an EPR spectrum for the Fe(II1) tris complex of a spin-labeled dithiocarbamate in fluid solution was attributed to strong electron-electron spin-spin interaction. 63 In frozen solution spectra of spin-labeled high spin iron(II1) porphyrin halides, resolved spin-spin splitting was observed at 8 K. 64 A s the temperature was increased the iron signals broadened, and the splitting of the nitroxyl signal collapsed due to the increasing rate of relaxation of the iron unpaired electrons. The temperatures at which the splitting collapsed depended on the iron zero field splitting, which is consistent with the expectation that the iron relaxation rate increases with zero field splitting and temperature. A value of J = 0.013 cm-I was obtained by simulation of resolved spin-spin splitting in frozen solution spectra of the iron( I11 ) complex of spin-labeled EDTA. 51

Ti.6ot6i

2.5 Metal 2 7/2, gadolium(II1) - The fluid and frozen solution spectra of a Gd(II1) complex of spin-labeled E D T A had severely Nitroxyl-nitroxyl spin-spin broadened nitroxyl signals.51 interaction was observed in Gd(II1) complexes of multiply spinlabeled polycarboxlic acid ligands.65 3 . Complexes with Nitroxyl Radicals Coordinated via the Nitroxyl Oxygen 3.1 Vanadium

-

When the nitroxyl oxygen of TEMPO is coordinated to

8: Complexes of Paramagnetic Metals with Paramagnetic Ligands

265

V O ( h f a ~ )the ~ antiferromagnetic exchange is sufficiently strong that an EPR spectrum was not observed.66 The value of J was estimated to be >700 cm-'. 66 The equilibrium constant for adduct formation was 4~10~'M-1.66 The loss of the nitroxyl and vanadyl EPR signals when complex is formed was used to estimate equilibrium constants >lo6 and 4x102 M-l for coordination of a dinitroxyl to VO(hfacI2 and VO( tfac)2, respectively, in dichloromethane solution.67

TEUPO

PROXYL

NIT-R

3.2 Manganese - Two moles of TEMPO or PROXYL bind to M n ( h f a ~ )to ~ form a six-coordinate trans adduct, Mn(hfac)2L2, in which antiferromagnetic coupling gives an s = 3/2 ground state.68169 The value of J was 158 cm-' f o r L = TEMPO and 210 cm-' for L = PROXYL.68 Analysis of single crystal EPR spectra at 4.2 K at X-band and 140 K at Q-band gave D = 0.63 cm-l, E/D = 0.115 f o r L = TEMPO and D = 0.65 cm-l, E/D = 0.18 for L = PROXYL.6g At temperatures 5140 K only the S = 3/2 ground state is populated. At room temperature additional features in the EPR spectra were assigned to the lower S = 5/2 excited state.69 The trans complex Mn(hfa~)~(NIT-ph)~exhibits strong antiferromagnetic coupling to give an S = 3/2 ground state.70 The polycrystalline EPR spectra were similar to those observed for the TEMPO and PROXYL analogs and gave D "0.6 cm-'. The magnetic susceptibility of the cis complex Mn(hfa~)~(N1T-M@)~also indicated strong antiferromagnetic coupling, but the polycrystalline EPR spectra gave only a single line with a peak-to-peak width of 480 G at both X-band and Q-band." Mn(hfac)2(NIT-R) complexes with R = i-Pr, Et, Me, and Ph have been isolated as infinite chains with intrachain anti-ferromagnetic The coupling constants ranging from 330 to 208 CITI-'.~' polycrystalline EPR spectra were single broad lines with linewidths that decreased as J increased as expected for exchange-narrowed signals." In a single crystal of Mn(hfa~)~(NIT-iPr)the linewidth of the EPR signal was orientation dependent. In a cyclic hexamer,

266

Electron Spin Resonance

[Mn(hfaC)2(NIT-Ph)]6, the manganese and nitroxyl unpaired electrons were antiferromagnetically coupled to give a ground state with S = 12.72 The single crystal EPR spectrum exhibited broad signals for AM = 1 and AM = 2 transitions. The orientation-dependence of the linewidth of the AM = 1 transition was interpreted in terms of a model developed for iron clusters. Coordination of a series of nitroxyls to MnI'Iporphyrin perchlorate via the nitroxyl oxygen gave a strongly 2 The. frozen ~ ~ antiferromagnetically coupled species with S = ~ solution EPR spectra had g,, = 2 and gL = 4 with AMn = 84 G.

-3.3 Cobalt - Magnetic susceptibility and polycrystalline EPR spectra indicate that the ground state of CO(DTBN)~B~~ is S = 1/2 due to strong antiferromagnetic coupling ( J > 200 cm-' ) between high spin Co(I1) and two nitroxyls.74175 A vector coupling model was used to determine the g values for the Co(1I) from the observed g values in the spin-coupled EPR spectrum.74 Co( hfac 12( PROXYL l2 is isostructural with Mn( hfac)2( PROXYL)268 and exhibits antiferromagnetic coupling to give an S = 1/2 ground state.76 Antiferromagnetic coupling also was observed in the dimer [C~(hfac),(NIT-Et)l~.~~

DTBN

-3.4 Nickel - Ni(hfa~)~(PRoxYL)~ is isostructural with the analogous Mn( I1 ) and Co( I1) complexes.68I 76 Antif erromagnetic coupling gives an S = 0 ground state. Antiferromagnetic coupling also was observed for [ Ni ( hfac ) ( NIT-Et ) 3 2. 77 3.5 Copper - C U ( ~ ~ ~ ~ ) ~ ( ~ - O H - T Eadopts M P O ) a chain structure with the nitroxyl oxygen bound to one copper and the hydroxyl oxygen bound to a second copper.78 The copper interaction with the axially coordinated nitroxyl is ferromagnetic with J = -13 cm-1.79 The interaction between the nitroxyl and the copper to which the hydroxyl oxygen is coordinated is antiferromagnetic with J = 5.2 x lom2 cm-1.80 The components of the g and D matrices were obtained from single crystal and powder EPR spectra.79

8: Complexes of Paramagnetic Metals with Paramagnetic Ligands

267

b' rOH-TWO or T W O L In Cu(l)2 in the solid state each ligand binds bidentate to one copper and the nitroxyl oxygen is bound to a second copper.81 The copper coordination is distorted octahedral with two bidentate ligands in the equatorial plane and two axially coordinated nitroxyl oxygens. There is ferromagnetic coupling between the copper and the two directly bonded nitroxyls with J = -21.2 cm-1.82 Single crystal EPR spectra showed a single broad line that was orientation and frequency dependent. Sources of the broadening and mechanisms of exchange narrowing were discussed.82 c~(hfac)~(NIT-Ph)~ i s monomeric and shows weak ferromagnetic between the copper(I1) and two axially coupling (-J 600 ~ m " . '~ r '18 In high dielectric constant solvents Fe"I(SQ)(cat)(bidentate nitrogenous base), Q = 9,lOphenanthrenequinone, undergoes intramolecular electron transfer to form FeI1( SQ)2( bidentate nitrogenous base) .ll' EPR spectra were not reported f o r these complexes, consistent with the even-electron I

ground states.

-4.5 Cobalt -

The magnetic susceptibility data for Co(SQ)2py2, Q

=

2 72

Electron Spin Resonance

9,10-phenanthrenequinone, were interpreted as indicating weak (3.6 cm-') antiferromagnetic interaction between Co( 11 ) and the semiquinones.120 However, the recent reinterpretation121 of the data for the analogous Ni(I1) complex (see below) suggests that there may also be another interpretation of the Co(I1) data.

- Analysis of magnetic susceptibility data for Ni(SQ)2py2, Q = 9,10-phenanthrenequinone, and Ni(SQ)2bipyridine, Q = 3,5-di-tert-butyl-o-benzoquinone, suggested weak ( < 1 cm-' ) antiferromagnetic coupling between the metal and the semiquinone. 120 The crystal structure of Ni(SQ)2py2, Q = g,lO-phenanthrenequinone, shows pair-wise association involving H - x interaction between semiquinones coordinated to two different nickel ions.lZo It was. subsequently suggested that the magnetic susceptibility data were consistent with strong coupling between the two interacting semiquinone ligands to give an essentially diamagnetic unit and strong ferromagnetic coupling between Ni(I1) and the isolated semiquinone ligand.12' The orthogonality of the orbitals containing the unpaired electrons in Ni(CTH)(SQ)+, CTH = dl-5,7,7,12,14,14-hexamethyl1,4,8,11-tetraazacyclotetradecane, Q = 3,5-di-tert-butyl-obenzoquinone, resulted in strong ferromagnetic coupling with J > 400 cm-'. 12' When the anion was hexaf luorophosphate, the following parameters were obtained by analysis of the +1/2 transitions of the S = 3/2 manifold, which were observed in the single crystal E P R spectra: g = g = 2.17 and E/D = 0.293. The magnetic susceptibility data were fit with g = 2.22, D = 2.54 cm-l, and E/D = 0.3O.l2l

4.6 _ _ Nickel _

4.7 Copper - The unpaired electrons in Cu(SQ)(cat)-, Q = 3,5,-ditert-butyl-o-benzoquinone,are antiferromagnetically coupled, and an E P R spectrum was not observed.lZ2 Cu( SQ)2, Q = 3,5-di-tert-butyl-obenzoquinone, crystallizes as a dimer.123 Analysis of the magnetic susceptibility data indicated strong antiferromagnetic coupling within the CU(SQ)~unit to give a net S = 1/2.124 The two units of the dimer are weakly antiferromagnetically coupled with J = -7.9 cm-'. The E P R spectra are typical of weakly coupled copper In [Cu(di-2-pyridylamine)(SQ)]+, Q = 3,5-di-tertbutyl-o-benzoquinone, the two unpaired electrons are strongly ferromagnetically coupled due to strict orthogonality of the

8: Complexes of Paramagnetic Metals wiih Paramagnetic Ligands

273

orbitals that contain the unpaired electron on the metal and the ligand.I2* The EPR spectrum is very broad and typical of coupled systems with a triplet ground state.12* Acknowledgment This work was supported in part by NIH grant GM21156. Abbreviations The following abbreviations are used .in the text: EDTA, ethylenediamine tetraacetic acid anion; hfac, hexafluoroacetylacetonate anion; py, pyridine; tfac, trifluoro-acetylacetonate anion; TPP, tetraphenylporphyrin dianion. Additional abbreviations accompany structural formulas.

Electron Spin Resonance

214

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278

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Author Index

I n t h i s i n d e x t h e number g i v e n i n p a r e n t h e s i s is t h e C h a p t e r number o f t h e c i t a t i o n and t h i s is followed by t h e r e f e r e n c e number o r numbers of t h e r e l e v a n t c i t a t i o n s within t h a t Chapter. Aasa, R . (3) 58; (7) 77, 218, 278, 294 Abeles, R.H. (7) 135 Abragam, A. (4) 3 Acampora, L.A. (3) 200 Aceti, D.J. ( 7 ) 158 Adams, J.F. ( 1 ) 19 Adams, M.W. ( 7 ) 131, 149-152 Adrian, F.J. ( 5 ) 36 Agostiano, A. (6) 123 A g o s t i n e l l i , E. (7) 13,

16 Ahlers, F.J. ( 4 ) 52 Ahmad, F.F. (1) 22 Ahsan, M.Q. ( 3 ) 53 Ainscough, E.W. ( 7 ) 9 Aisen, P. ( 7 ) 61-63 Ajo, D. ( 3 ) 128 Akahoshi, S. ( 6 ) 80 Akiyama, K. (2) 116 Aknazarova, T.N. ( 2 ) 8 9 Albano, E. (1) 17, 21, 24 Albracht, S.P.J. ( 7 ) 143, 144, 147, 193, 229, 230 Albright, T.A. (5) 101 Alcock, N.W. (3) 191, 192 Alesbury, C.K. (5) 87 Alexandrova, T.A. (2) 71 Aliev, 1.1. ( 2 ) 67 Allen, G.W. (3) 200 Allendorf, M.D. ( 7 ) 28 Alybakov, A.A. (3) 79 Amachi, K. ( 2 ) 66 Ambramowicz, D.A. (7) 288 Ammeter, J . H . ( 5 ) 58 Amorelli, A. (6) 56 Andersen, P. ( 3 ) 142 Anderson, O.P. (3) 119; ( 8 ) 24, 78 Andersson, K.K. (7) 81 Andrea, R.R. (3) 184

Andreasson, L. -E. ( 7 ) 252, 294 Anfimov, B.N. (2) 69 Anisimov, D.A. (2) 44, 108 Anni, H. ( 7 ) 236 Anpo, M. (2) 23 Anraku, Y. (7) 105, 106 Ansaldo, E.J. (6) 118 A n t a n a i t i s , B.C. ( 7 ) 63, 210 Antholine, W.E. (7) 22, 33, 44; ( 8 ) , 1 0 0 Antisiferova, L.I. ( 2 ) 91 Antony, A. (7) 223 Aoyagi, M. ( 2 ) 58 Arai, H. ( 6 ) 107 Arakawa, M. (3) 246, 256 Arena, G. (7) 23 A r g i t i s , P. (3) 159 Arratia-Perez, R. ( 2 ) 45 Arrington, C.A. ( 2 ) 35 Asaji, T. (3) 70 Asakura, T. (2) 92 Ashe, A . J . (3) 179 Ashwell, G . J . (3) 200 Ashworth, S.H. (6) 158 Askerov, I.M. (3) 264 Aslanov, G.K. (3) 264 Assali, L.V.C. ( 2 ) 17 Astashkin, A.V. (6) 16 Asthana, S. (8) 29 Astheimer, H. (8) 39 Ata, M. (2) 58 Atherton, N.M. (3) 162; (6) 68, 71 Auld, D.S. (7) 197, 199 Auling, G. ( 7 ) 195 A v e r i l l , B.A. (7) 55, 56 Avigliano, L. ( 7 ) 34 Ayant, Y. (2) 114 Azevedo, L. (3) 144

279

Azzoni, C.B.

(3) 69

Babaeva, L.I. ( 8 ) 34 Baberschke, K. (6) 6 3 Bach, S.B.H. ( 5 ) 6 3 Badalyan, A.G. (3) 172 Baeva, G.N. ( 3 ) 4 Bagchi, R.N. (6) 137 Baglioni, P. ( 2 ) 79, 80, 83 B a i , G.R. (3) 33 Baird, H.C. (6) 135 Baker, E.N. (7) 9 Baker, G . J . (7) 15 Baker, G.M. ( 7 ) 243, 244 Baker, J.E. (1) 2 Bakker, P.T.A. (7) 229, 230 Bakshi, E. ( 7 ) 204 Baldas, J. (3) 173 Bales, B.L. (2) 47 B a l l e s t e r , M.J. (3) 91 Ballivet-Tkatchenko, D. ( 3 ) 186 Bally, T. (2) 111 Balquiere, C. (8) 40 Bamsal, M.L. (6) 77 Banci, L. (2) 10; ( 7 ) 201, 206 Banerjee, A. (7) 219 B a n i s t e r , A . J . (2) 33 Bank, J. (7) 7 , 240 Bannou, S. (2) 23 Barashkova, 1.1. (2) 69 Barata, B. (7) 173 Barber, D.L. (6) 122 Barber, M.J. (7) 158, 174, 179, 180 Barefoot, S.T. ( 6 ) 120 B a r k l i e , R.C. (4) 40 Barnard, R.P. ( 4 ) 58

Author Index

280 Barone, V. (2) 30 B a r t e l s , D.M. (2) 120; (6) 29, 30, 69 Bartram, R.H. (2) 21; (4) 50 Basch, H. (2) 15 Bashkin, J.S. (3) 60 B a s os i , R. (7) 22, 214 B a s t i n , F.N. (1) 19 Bates, C.A. (3) 243 Bates, G.W. (7) 66 B a t es , J.K. (5) 12 B a t t i o n i , J.P. (7) 75 Bauer, R.U. (4) 44 Baumann, C.A. (5) 40, 41 Baumbach, G.A. (7) 59 Baumeler, H. (6) 27 B a us chl i cher, C.W. (2) 14 Bazer, F.W. (7) 59 Beaman, R.A. (6) 159 Bear, J.L. (3) 52, 53, 54 Beardwood, P. (3) 56, 219 Bechtold, P.S. (5) 53, 54 Beck, J.L. (7) 54 Beck, W. (3) 57; (7) 276, 279, 292; (8) 75 Beck, W.F. (7) 293 Beckert, D. (2) 117 Beckwith, A.J. (2) 134, 135 Beckwith, A.L.J. (1) 19 Bednarck, J. (6) 33, 59 Bedzhanyan, Yu.R. (6) 146 B e el er , F. (4) 13 B ef ani , 0. (7) 13, 16 Behling, T. (3) 174 Behner, T. (3) 247 Beijer, C. (7) 304; (8) 104 B ei nert , H. (7) 134 B e l e r i n t e r , E. (2) 88 B e l f or d, R.L. (5) 28 Belousov, Yu.A. (3) 187 Benabe, J. (7) 215 Benard, M. (2) 25 Bencini, C. (8) 79 B e ndal l , D.S. (7) 253 Bender, C . J . (3) 59 Benecky, M.J. (7) 149, 151 B e n e l l i , C. (7) 125; (8) 69, 74, 79, 80, 82, 90, 94, 121 Benkovic, S.J. ( 7 ) 42-45 B ennet t , D.E. (7) 120, 136 Bennett, J.E. (5) 19, 118; (6) 19 Benosman, H. (7) 109 B er ezi n, I.V. (7) 155 Bergstrgm, J. (7) 252 B e r l i e r , Y. (7) 146

B e r l i n e r , L.J. (7) 220 B e r l i n g e r , W. (3) 257 Bernath, P.F. (6) 160 Berne, B.J. (6) 9 B e r r y , M.J. (7) 89 Berson, J . A . (2) 109 Bersuker, G . I . (3) 43 B e r t i n i , I. (2) 10; (7) 196, 197, 201, 206 B e r t r a n d , P. (2) 129; (7) 109, 121, 126, 127, 227, 270 Beyer, L. (3) 145 Bezhanova, L.S. (3) 26 Bhat, S.V. (3) 265; (5) 40, 41; (6) 66 B h a t t a c h a r y a , S. (3) 203 Bianchi, A. (3) 91 B i a s i , F. (1) 21 B i a t r y , B. (7) 75 B i c k n e l l , R. (7) 197-199 B i e h l , B. (4) 42 B i e h l , R. (4) 18 B i e l e c k i , K. (3) 87 B i g g i n s, J. (7) 268 B i l ' k i s , 1.1. (2) 44 B i l l , H. (6) 72 B i l l i n g , S. (1) 24 Bingham, A.G. (7) 9 Blackborow, J . R . (5) 3 B l a c k f o r d , R.D.S. (6) 71 Blackmore, R.S. (7) 116 Blahova, M. (3) 135 Blake, A . J . (3) 234, 235 Blandamer, M.J. (6) 17 B l a q u i e r e , C. (8) 39, 41 Bleaney, B. (4) 3 Bligny, R. (7) 32 Block, D. (4) 59 Bloom, L.M. (7) 42 Boas, J.F. (3) 173 Boate, R. (5) 24 Bocian, D.F. (7) 93 Bock, H. (2) 32 BEhme, H. (7) 256 BGscher, R. (3) 254; (7) 143 Boesman, E.R. (6) 67 BGttcher, R. (3) 230 Bogdanov, A.P. (3) 38 Bogomolova, L.D. (3) 104 B o g u sl a v sk i i , E.G. (8) 25, 28 Bohle, U. (6) 148, 155 B o l d t , N.J. (7) 93 Bologa, O.A. (3) 64 Bolognesi, M. (7) 34 Bombard, S. (7) 76 Bonam, D. (7) 156, 157 Bond, A.M. (6) 137 Bondybey, V.E. (5) 11 Bonnemann, H. (5) 108

Bonneviot, L. (3) 5 Bonnyman, J. (3) 173 Bonomo, R.P. (7) 23 Bonvoisin, J.J. (7) 114, 128 Boon, K. (7) 185 Borg, D.C. (1) 4 B o s s o l i , R.B. (6) 98 Bo s tic k , J.M. (6) 50 B o t t i n , H. (7) 272 B o u l i t r o p , F. (4) 57 Bourgoin, J . C . (6) 117 Boussac, A . (7) 275 Bowers, V.A. (5) 37 Bowman, M.K. (6) 18 Boyd, D.C. (3) 139 Boymel, P.M. (8) 11, 53 Braden, G.A. (8) 14 Bradley, F.C. (7) 82 Brand&, R. (7) 218 Brand, S.G. (7) 182 Bray, R.C. (1) 1; (7) 170, 173 Bray, T.M. (1) 26 B r a z i e r , C.R. (6) 160 Breit, G. (5) 25 Brewer, C.F. (7) 210 Brewer, J . H . (6) 26 B r i c k e r , T.M. (7) 297 B r i e r e , R. (5) 109; (8) 37 B r i t i g a n , B.E. (6) 121 B r i t t , R.D. (7) 271, 281, 282, 290, 299 B r i t t a i n , T. (7) 116 Brodie, A.M. (7) 9 Bromba, M.U.A. (4) 25, .~ 27, 28 Brooks, B.R. (5) 97 Brown, I.M. (2) 68 Brown, J.M. (6) 148- 53 , 158 Brown, R.D., I11 (7) 210 Bruck, M. (7) 171 Brudvig, G.W. (3) 57 (7) 274, 276, 279, 292, 293 Brumby, S. (2) 134, 135 Bubnov, N.N. (2) 107; (3) 112, 116-118; (8) 97, 109, 112, 114 Buchanan, R.M. (3) 61; (8) 95, 106, 107, 110, 113, 120 Buchanan, S.K. (7) 310 Buck, A.J. (5) 20 Buck, H.M. (6) 78 Bucklep, C.D. (2) 119, 125 Buckmaster, H.A. (3) 11 Buddensiek, D. (2) 34 Bu e ttn e r , G.R. (1) 8

28 1

Author Index

Bult, A . (3) 90 Bunce, N.J. (2) 2 Bunting, R.K. (3) 233 Burch, M.K. (7) 221 Burdett, J.K. (5) 101 Burman, S. (7) 55 Burns, D.T. (3) 22 Burstyn, J.N. (3) 273 Busch, D.H. (3) 191, 192 Bustarret, E. (6) 110, 111

Butcher, K.S. (6) 123 Butler, A.R. (3) 188 Buwalda, P.L. (2) 82 Byberg, J.R. (6) 81, 94 Byfleet, C.P. ( 5 ) 26 Bykov, I.P. (3) 268 Caballol, R. (6) 73 Caccavale, F. (3) 104 Calabrese, J.C. (8) 123 Calabrese, L. (7) 24, 25 Calle, P. (2) 28 Callens, F.J. (6) 67 Calsg-Harrison, A.H. (3) 188

Calvo, R. (3) 229 Cameron, J.H. (3) 191 Cammack, R. (7) 57, 101, 133, 141, 142, 227

Camoretti-Mercado, B. (7) 97

Candida, B.L. (2) 100 Caneschi, A. (8) 70-72, 77, 84, 85, 89, 91, 94

Canters, G.W. (7) 8 Carbonaro, M. (7) 24, 25 Carlin, R.L. (8) 80 Carlson, R.M.K. (7) 181 Carmichael, A.J. (6) 125 Carnegie, D.W., jun. (8) 80

Carrington, A. (6) 142, 143

Case, E.E. (7) 169, 189, 190, 192

Casella, L. (7) 36 Casida, M.E. (5) 97 Cass, M.E. (8) 107, 108 Cassoux, P. (3) 232 Castellani, M.P. (3) 248 Catala, J.A. (6) 73 Catterall, R. (6) 12 Cebe, P. (2) 74 Celio, M. (6) 26, 118 Chachaty, C. (2) 77 Chakravorty, A. (3) 203, 263

Chan, S.I. (7) 93, 245, 246

Chand, P. (3) 75

Chandar, P. (2) 78 Chandra, H. (2) 43; (6) 91, 95

Chandra, L. (3) 89 Chang, C.A. (2) 59 Chang, H.-R. (3) 60 Chang, S.-S. (5) 21 Chapman, S. (7) 168 Chasteen, N.D. (7) 65, 67, 68, 210

Chatt, J. (5) 78 Chau, L.K. (3) 54 Chavan, M.Y. (3) 53, 54, 192

Che, M. (3) 5 Chekalov, A.K. (2) 107 Chen, D. (3) 160 Chen, J.-J. (3) 237 Chenier, J.H.B. (5) 67, 68, 72, 74, 88, 94, 110, 121, 125; (6) 131

Chiba, T. (7) 83 Chignell, C.F. (1) 23 Chiha, Y. (6) 102 Chin, T.M. (6) 29 Chmielewski, P. (2) 52 Choh, S.H. (3) 270 Chong, D.P. (5) 26 Chopard, C. (7) 76 Chottard, J.-C. (7) 76, 78

Chou, S.Y. (3) 86; (6) 90 Christmas, C.L. (6) 4, 125

Christou, G. (3) 60, 156 Ciriolo, M.R. (7) 225 Claflin, J. (8) 113 Clark, H.C. (3) 113 Clark, T. (2) 36; (6) 58 Claxton, T.A. (6) 104 Cleary, D.A. (3) 262 Cleveland, J.A. (6) 88 Cline, J.F. (7) 111 Closs, G.L. (2) 126 Cobranchi, D.C. (6) 50, 89

Cobranchi, S.T. (2) 35; (5) 11; (6) 43, 45, 47, 49 Cocciaro, R. (2) 83 Cochran, E.L. (5) 37

Coddington, A. (7) 175-177

Coffman, R.E. (8) 9, 10, 44

Cohen, M.S. (6) 121 Colaneri, M.J. (2) 8 1 Cole, J.L. (7) 281, 282, 290, 299

Cole, S.T. (7) 133 Collison, D. (3) 156, 157, 171

Colton, R. (6) 137 Coltrain, B.K. (3) 191 Comben, E.R. (6) 148, 149, 151

Condon, C. (7) 133 Connelly, N.G. (3) 108, 231

Conner, K.A. (3) 181 Connor, H.D. (1) 29, 30; (6) 126

Conradi, E. (3) 218 Constable, E.C. (3) 220, 221

Cook, M. (2) 18; (3) 226 Cooley, N.A. (6) 135 Cooper, S.R. (8) 111 Cooperman, B.S. (7) 219 Cooray, L.S. (2) 93 Cordischi, D. (2) 133 Coremanns, J.M.C.C. (7) 193

Corradi, G. (2) 21 Cot;, C.E. (7) 207 Cotton, F.A. (3) 141 Cousins, M.J. (1) 19 Cowan, D.L. (1) 22 Cox, R.T. (4) 59 Cox, S.F.J. (6) 25, 119 Cradock, S. (5) 9 Cramer, S.P. (7) 58, 159 Crossley, C. (1) 27 Crownover, R.L. (6) 156 Csatorday, K. (7) 303 Cucurou, C. (7) 75 Cui, X. (3) 169 Czechowski, M.H. (7) 140, 154

Dahl, L.F. (3) 147 Dahlin, S. (7) 8 Dahmane, H. (5) 57, 85 Dai, S. (2) 111 Dalal, D.P. (8) 42 Dalal, N.S. (4) 43 Daldal, F. (7) 254, 255 Dalgarno, B.G. (3) 22 Dalichaouch, Y. (3) 99 Dalton, H. (7) 57 Dalvi, A.G.I. (6) 77 Dameron, C.T. (7) 205 Damoder, R. (7) 288; (8) 19-21,

46, 48

Dance, J.M. (3) 101, 102, 130

Daniel, M.F. (3) 226 Darcha, M. (3) 243 Das, M. (3) 48, 258 Das, T.P. (2) 20; (6) 103 Dasoler, W. (6) 5 Date, M. (6) 102 Daverio, D. (6) 100

Author Index

282

Davidovich, R.L. (3) 83 Davidson, E. ( 5 ) 11; ( 6 ) 42, 45, 47, 49, 50, 89; ( 7 ) 254 Davidson, M.W. (7) 247, 248 Davidson, S.A. (6) 150 Davies, M.J. (1) 13 Davis, E.A. (2) 94 Davis, E.R. ( l j 16 Davis, J.C. (7) 55 Davis, J.J. ( 4 ) 58 Day, E.P. (7) 114, 128, 166 De, D.K. ( 3 ) 44, 66 DeArmond, M.K. ( 3 ) 107 Deaton, J.C. (7) 178 de Boer, E. ( 7 ) 41, 184-186 Debrunner, P. (7) 52, 48, 55, 60, 100; (8) 115 Debus, R . J . ( 7 ) 273 Dehnicke, K. (3) 212 Dei, A. (8) 121 Dei, L. ( 2 ) 83 Deistung, J. (7) 173 de Jersey, J. ( 7 ) 54 De Lange, W.G.J. ( 3 ) 184 De L e y , M. (7) 37 De Loth, P. ( 3 ) 232 de Lucia, F.C. (6) 156 Demetriou, C. (7) 284 Demetriou, D.M. (7) 302 Demuynk, J. (2) 25 Deneauville, A. (6) 110, 111 de Paula, J.C. ( 7 ) 274, 276, 292 DerVartanian, D.V. ( 7 ) 107, 112, 115, 140, 154, 159 Deryer, J.-L. (7) 85 Desai, P.R. (7) 18 Dettlaff-Weglikowska, U. ( 3 ) 134 Dewar, M.J.S. ( 5 ) 77 Dexheimer, S.L. (7) 271, 281, 282, 290, 299 Deycard, S. (2) 106 Dianzani, M.U. ( 1 ) 21 Dickman, M.H. (8) 68, 69, 76, 86, 88 Diegruber, H. (3) 193 Dietz, R. ( 7 ) 95 Dietzsch, W. (3) 223 Dikanov, S.A. ( 3 ) 6 ; (6) 16 D i k s h i t , S.K. (3) 183 DiLella, D.P. (5) 49 Diner, B.A. ( 7 ) 307, 308 Dinse, K.P. (4) 54 Dismukes, G.C. (7) 280,

288, 310 Divakar, S. ( 7 ) 223 Dmitriev, Yu.A. (6) 28 Doadrido, J . C . (3) 218 Dobryakov, S.N. (2) 130 do Carmo, L.C.S. ( 6 ) 51 Doedens, R . J . (8) 68, 69, 76, 86-88 DGring, M. (3) 215 Doi, K. ( 7 ) 61-63 Donnerberg, H.J. (3) 250 Dooley, D.M. ( 7 ) 12, 17, 207 Dorr, R.G. (7) 240 Dossing, A. (3) 142 Douce, R. ( 7 ) 32 Dovbii, E.V. ( 2 ) 73 Dowerah, D. (7) 171 Downs, H.H. (8) 106, 110 Downs, P.E. ( 1 ) 18 Drago, R.S. ( 8 ) 66 Drake, J.E. ( 3 ) 93 Drausfeld, P. (6) 145 D r e w , M.G.B. ( 3 ) 136, 137 Drumheller, J.E. (3) 239, 240, 260 Du, M.-L. (3) 30 Dubinskii, A.A. (2) 53 Dubois, B. (3) 102 DuBose, C.M. ( 1 ) 25, 28 Duling, D.R. (6) 127 Dumorit, M.F. ( 6 ) 3 Duzach, M. (7) 222 Duncanson, L.A. (5) 78 Dunham, W.R. (7) 153 Dunn, A . J . (3) 226 Dunn, J.L. (3) 243 Dunsch, L. (2) 70 Durasov, V.B. (8) 25 Durfor, C.N. ( 7 ) 178 Dutov, M.D. ( 3 ) 213 DuVarney, R.C. ( 4 ) 41, 45 Dyke, J.M. (6) 140 Dzeda, M.F. (7) 190 Dzyuba, S.A. (2) 46, 48 Eachus, R.S. ( 6 ) 39 Eads, C.D. (7) 212 Eady, R.R. ( 7 ) 163, 187, 188 E a r l , E. ( 6 ) 43, 49, 50, 89 E a s l t e , T.L. ( 6 ) 118 Eastland, G.W. (6) 38 Eastman, M.P. (2) 59 Easwaran, K.R.K. (7) 223 Eaton, G.R. (2) 6 ; ( 3 ) 24, 119-125; (8) 1, 2, 5, 8, 11, 13-23, 42, 45-51, 53-62, 64, 67, 73

Eaton, S.S. (2) 6; ( 3 ) 24, 119-125; ( 8 ) 1, 2, 5, 8, 11, 13-24, 42, 45-51, 53-62, 64, 67, 73 Ebisu, H. ( 3 ) 246, 256 Echegoyen, L. (7) 215 Edwards, P.G. (3) 155, 174 Edwards, P.P. ( 6 ) 15 Egorov, A.M. (7) 155 Eiben, K. ( 6 ) 31 Eidsness, M.K. (7) 159 Eisenberg, R. ( 2 ) 104 E l b e r s , G. (3) 247 El-Dissouky, A. ( 3 ) 202 E l l i s , W.R. (7) 9 El-Naqadi, M. (3) 243 Elschenbroich, C. ( 3 ) 177-179 El-Sonbati, A.Z. (3) 202 Emad, M. ( 7 ) 50 Emberson, R.M. (8) 115 Emptage, M.H. ( 7 ) 122, 134 Enemark, J.H. (3) 171; ( 7 ) 171 Engberts, J.B.F.N. (2) 82 Engelhardt, L.M. (3) 153 Engels, B. ( 2 ) 26 Ennen, H. (4) 8 Ergoshin, V . I . (2) 37 Eriksson, L.E.G. (7) 200 Ermakova, E.A. ( 2 ) 37 E r n s t , R.R. (6) 8 E r n s t , S. (3) 109, 111 Ershov, V.V. (3) 117; ( 8 ) 109 (8) 36, 38, Espie, J.-C. 81 E s p o s i t o , G. (2) 8 4 E s t l e , T.L. ( 4 ) 5 E t t i n g e r , M.J. (7) 31 E u l e r , W.B. (7) 117 Evans, A. (6) 104 Evans, D.F. ( 3 ) 189 Evans, J.C. (6) 56 Evans, M.C.W. (7) 266, 267, 309 Evans, R.W. (7) 64 Evenson, K.M. (6) 149, 150, 153 Evmiridis, N.P. (3) 271 E x e l l , K.G. ( 2 ) 39; (6) 85 F a b b r i z z i , L. (3) 133 F a b r e t t i , A.C. (3) 216 F a i r h u r s t , S.A. ( 2 ) 33; (3) 162; (5) 132 Fan, C. (7) 7, 240

Aufhor Index

Farcas, S.I. (3) 87 Farchione, F. (7) 171 Farlec, R.D. (6) 136 Fasman, A.B. (3) 4 Fauque, G. (7) 112, 113, 119, 146

Faus, J. (3) 91 Febbraro, S. (3) 255 Fee, J.A. (7) 136, 226, 227

Feher, G. (4) 1; (7) 273 Feiler, U. (7) 263 Feinstein-Jaffe, I. (3) 150

Feiters, M.C. (7) 70, 77 Felix, C.C. (1) 2 Feller, D. (6) 45, 47, 49, 50, 89

Fendrick, C.M. (3) 192 Feng, K. (2) 19 Feng, X. (3) 141 Fenton, N.D. (3) 251 Ferguson, G. (3) 113 Fernbndez, V. (3) 218;

283

Franzi, R. (2) 29 Freedman, J.H. (7) 210 Freidlina, R.Kh. (1) 31 Freiha, B. (2) 59 Freitas, J.A., jun. (3) 103

Fricke, R. (2) 24 Friebel, C. (3) 135, 212 Friebele, E.J. (6) 96 Frigo, T.B. (2) 105 Fritzer, H.P. (3) 219 Frolov, E.N. (8) 63 Froncisz, W. (7) 22 Frydman, R.B. (7) 97 Fuchs, G. (7) 193 Fujii, K. (1) 20 Fujii, T. (2) 23 Fujimoto, M. (3) 77 Fujita, E. (3) 272 Fujita, T. (6) 115 Fukami, T. (6) 80 Fukuda, M. (2) 85 Funakoshi, K. (7) 224

(7) 141, 142

Ferrante, R.F. (6) 60 Ferry, J.G. (7) 158, 174 Fessenden, R.W. (5) 80, 117; (6) 31

Fessner, W.D. (3) 177 Feuchtwang, T.E. (4) 15 Feuerstein, J. (7) 211 Ficor, F. (1) 20 Fielding, L. (3) 120, 124; (8) 54, 62, 64 Fikar, R. (3) 217 Findsen, E.W. (7) 248 Finkelstein, E. (1) 6 Fishel, L.A. (7) 238 Fisk, Z. (3) 99 Fitzgerald, B.J. ( 8 ) 116 Flanagan, H.L. (3) 18, 19 Flatmark, T. (7) 8 1 Flint, C.D. (3) 219 Flint, D.H. (7) 122 Flockhart, B.D. (3) 22 Fockele, M. (4) 49, 50 Fogel'zang, A.E. (3) 213 Foley, A.A. (7) 66 Follmann, H. (7) 195 Foner, S.N. (5) 37 Fong, F.K. (6) 123 Fong, K.-L. (1) 15 Font-Altaba, M. (3) 232 Fontijn, R.D. (7) 143, 144, 147 Foote, N. (7) 89 Forbes, M.D.E. (2) 126 Forget, N. (7) 148 Foxman, B.M. (3) 200 Francis, A.H. (3) 262 Frank, P. (7) 181

Gennaro, A.M. (3) 229 Gennett, T. (3) 138 Geoffroy, M. (2) 29 George, G.N. (7) 58, 170 Gerbeleu, N.V. (3) 64 Gerfen, G.J. (3) 18 Gerloch, M. (3) 251 Gershenzon, Y.M. (6) 146 Gervasini, A. (6) 100 Getz, D. (3) 41 Ghanotakis, D.F. (7) 298, 302

Gheller, S.F. (7) 161 Gibson, J.F. (3) 2, 56, 219; (7) 29

Giglio, E. (2) 84 Gilpin, R.K. (2) 88 Girerd, J.-J. (7) 76, 78, 129

Giroud, A.-M. (8) 37 Giroud-Godquin, A.M. (2) 77

Glaunsinger, W.S. (6) 13 Glidewell, C. (3) 185, 188

Gade, S. (2) 13 Gadsby, P.M.A. (7) 89, 98, 116, 175-177

Gaffney, B.J. (7) 42, 46 Gahan, B. (3) 157 Gaillard, J. (2) 77; (3) 148; (7) 32

Gaizer, F. (3) 227 Galey, J.-B. (7) 76 Galizi, M.D. (1) 29 Galliano, N. (7) 119 Gampp, H. (3) 10 Gan, F. (3) 100 Ganghi, N.S. (6) 86 Garcia-Espana, E. (3) 91 Garland, D.A. (5) 30, 63, 64; (6) 52

Garner, A. (1) 24 Garret, R.C. (7) 64 Gasanov, R.G. (1) 3 1 Gatteschi, D. (3) 126, 128; (7) 125; (8) 69-72, 74, 77, 79, 80, 82-85, 90, 91, 94, 121 Gatti, G. (7) 34 Gaul, D.F. (7) 247, 248 Gayda, J.-P. (2) 129; (7) 109, 121, 126, 127, 148, 227, 270 Geib, S.J. (3) 248 Geiger, W.E. (3) 108, 138, 231 Gelfand, L.S. (8) 93 Gelles, J. (7) 246 Gelmini, L. (3) 154 Gemperle, C. (6) 8

Glinchuk, M.D. (3) 268 Glogowski, M.W. (3) 192 Godfrey, C. (7) 175-177 Godfrey, M.R. (7) 168 Goel, A.B. (3) 113 Goff, H.M. (7) 88, 92 Goher, M.A.S. (3) 211 Golbeck, J.H. (7) 260, 262

Gold, A.A. (3) 226 Golden, D.Y. (6) 21 Goldfarb, D. (3) 16, 17, 196

Goldstein, A.S. (3) 144 Golic, L. (3) 223 Gomes, V.M.S. (2) 17 Gondo, Y. (2) 58 Good, M. (7) 208 Goodgame, D.M.L. (3) 170 Goodin, D.B. (7) 237 Goodman, G. (7) 235 Goody, R.S. (7) 211 Goovaerts, E. (6) 41 Gordon, J. (2) 94 Gordon, N.R. (8) 108 Gormal, C. (7) 188 Gorren, A.C.F. (7) 41 Gorse, J. (6) 101 Goslar, J. (3) 214 Gotzmann, D.J. (7) 10 Gould, R.O. (3) 234, 235 Grachev, V.G. (3) 245 Grady, J.K. (7) 65, 67, 68

Graham, W.R.M. (3) 161 Grampp, G. (2) 99, 103 Grand, A. (8) 84, 89

Author Index

284 Gray, H. (7) 196, 9 Gray, K.A. (7) 87 Greene, D.L. (8) 107 Greenwood, C. (7) 89, 116, 175-177 Gregory, B.W. (2) 35; (5) 11; (6) 43, 45 G r e i n e r , S. (2) 104 Greulich-Weber, S. (4) 7, 23, 36, 37 G r i b n a u , M.C.M. (2) 131; (6) 3; (8) 12 G r i g o r y a n , N.A. (7) 21 G r i g o r y a n t s , V.M. (2) 44 Griller, D. (2) 109 Grimmeiss, H.G. (4) 8 G r i n b e r g , O.Ya. (2) 53 Griscom, D.L. (6) 96, 97 Groeneveld, C.M. (7) 8 G r o o t v e l d t , M.C. (7) 101 G r o s s , R. (3) 62, 182 Gruen, D.M. (5) 12 Gu, S. (3) 169 Guangzhi, X. (8) 6 Gubanova, V.A. (3) 79 G u i g l i a r e l l i , B. (7) 127, 270 G u i l e s , R.D. (7) 271, 281, 282, 290, 295, 299 G u l l o t t i , M. (7) 36 Gunnarson, 0. (4) 13 Gupta, D.C. (3) 180 Gupta, H.K. (3) 183 Gupta, N.S. (3) 89 Gupta, R. (7) 6 1 Guy, S.C. (6) 15 Gwak, S.H. (7) 233 Gyor, M. (3) 47, 227 Haaker, H. (7) 163 Haase, W. (7) 38; (8) 39 Haasnoot, J.G. (3) 92 Haavik, J. (7) 81 Hachimi, A. (6) 23 H a c k e t t , P.A. (5) 91 H a f i d , S. (8) 55 Hafmann, D.M. (4) 47 Hage, J. (4) 16 Hagen, W.R. (7) 145, 153, 163 Hagiwara, M. (6) 102 Hales, B.J. (7) 169, 189-192 H a l e y , P.E. (7) 234 Hall, P. (1) 19 H a l l i b u r t o n , L.E. (2) 16; (6) 98 H a l p e r i n , W.P. (3) 151

(6) 22 Hamizar, E.M. (3) 262 Hampson, C.A. (5) 31, 67, 68, 121, 122, 125, 128; (6) 131, 134 Han, C.S. (3) 270 Han, S. (3) 267 Hanack, M. (3) 195 Hanaki, A. (6) 124, 128 Hanck, K.W. (3) 107 Handley, J. (3) 243 Hanlan, L.A. (5) 131 Hanson, G. (3) 131; (7) 135, 198 Hansson, 0. (3) 58; (7) 278 Haquske, G. (7) 254 Hara, H. (3) 20 Harmalker, S. (8) 122 Harris, E.A. (3) 105 Harris, E.D. (7) 205 Harris, F.L. (2) 47 Harris, R.D. (6) 109 H a r r i s o n , M.R. (6) 15 Hart, A. (3) 14 Hartl, F. (3) 114 Hartmann, H.-J. (7) 27 Harvey, R.D. (2) 75 Hasegawa, A. (2) 43; (6) 74 H a s n a i n , S.S. (7) 64 H a s s e l b a c h , E. (2) 111 Hasumi, M. (3) 94 Hata-Tanaka, A. (7) 83, 105 H a t c h i k i a n , E.C. (7) 141, 142, 148 Hauge, R.H. (5) 30, 8 1 Hauptman, Z.V. (2) 33 Hauska, G. (7) 249, 250, 263 Hawkins, C.J. (7) 182 Hawkins, M. (7) 187 H a y a s h i , M. (6) 74 Hebden, J . A . (5) 26 Heder, G. (4) 10, 14 Hedewy, S. (3) 269 H e f l e r , S.K. (7) 56 Heimbrook, L.A. (5) 11 Hein, D.H. (7) 40 Heinemann, M. (4) 53 Helminger, P. (6) 156 Hemdrickson, D.N. (8) 119 Hemidy, J.F. (6) 23 Heming, H. (3) 205 Heming, M. (6) 27 Henderson, T. (2) 104 H e n d r i c h , M.P. (7) 100 H e n d r i c k s o n , D.N. (3) 60; (8) 106, 115, 116, 118, 120 H e r o l d , B.J. (2) 100

Herrmann, W.A. (3) 138 Herve, A. (4) 59 Heuer, W.B. (3) 201 Hewson, G.J. (3) 162 Heynderickx, I. (6) 41 Hideg, K. (3) 122; (8) 57 H i h i r a , T. (3) 96 H i n c h c l i f f e , A . J . (5) 9 H i n e s , J.L. (3) 233 H i r a b a y a s h i , I. (4) 56 Hirasawa, M. (7) 87 Hirata, J. (7) 183 H i r o t a , E. (6) 157 H i r o t a , N. (2) 118 H i r o u c h i , M. (7) 86 H i s t e d , M. (5) 31, 104, 105, 130 Hitchman, M.A. (3) 40, 42, 228 H i w a t a s h i , A. (7) 90, 239 Hlouskova, Z. (2) 6 1 Hodgson, K.O. (7) 181 H o e n t z s c h , Ch. (4) 22, 29 Hoffman, B.M. (3) 151, 152, 201; (7) 111, 134, 149, 151, 166-168, 228 Hoffman, R. (5) 103 Hoffmann, S.K. (3) 214, 269; (8) 91 Hofmann, D.M. (4) 6, 48 H o l l o c h e r , T.C. (7) 39 Holloway, C.E. (7) 68 Holm, C. (4) 34 Holm, E. (4) 8 Holme, J . A . (6) 70 Holmes, J . M . (3) 220, 221 H o l m q u i s t , B. (7) 135, 198 Honda, K. (6) 108 H o d r e , N. (7) 133 Hooper, A.B. (7) 99 Hore, P . J . (2) 125 Hori, H. (7) 90, 91, 103, 239 H o r i , Y. (2) 112 Hosokawa, M. (7) 86 H o t t a , T. (7) 90 Hovde, R . J . (6) 154 Howard, C.J. (6) 156 Howard, J . A . (5) 14, 17, 20, 29, 31, 45-48, 57, 60-62, 65, 67, 68, 72, 74, 75, 83-85, 88, 89, 94, 100, 104, 105, 110, 114, 116, 121, 122, 125-130; (6) 35, 131, 134 Howe, R.F. (6) 55 Hoyer, E. (3) 145 H r a b a n s k i , R. (3) 259 Hsu, C.C. (2) 59 HSU, S.-C.P. (6) 13

Author Index

Huang, S.J. (3) 86 Huang, Y. (2) 19 Hubbard, J.A.M. (7) 266 Huber, H. (5) 42 Huber, J.R. (5) 131 Hudson, A. (2) 1; (6) 1 Huffman, J.C. (3) 60, 156 Hughes, L. (2) 106 Hukuda, K. (6) 80 Hulse, J.E. (5) 52 Hunter, D.A. (2) 125 Hunziker, D. (7) 288 Hurst, J.K. (2) 8 1 Hursthouse, M.B. (3) 140, 155, 174, 175, 236

Hussain, B. (3) 175, 236 Hussain, H.A. (2) 96 Hutterman, J. (3) 7 Hutton, S.L. (3) 239, 240 Huynh, B.-H. (7) 53, 107, 112

Huynh, B.H. (7) 140, 154 Hwang, T. (5) 115 Hyde, J.S. (7) 22; (8) 100

Hyde, T.I. (3) 234, 235 Ibers, J.A. (3) 152 Ichikawa, T. (6) 6, 37 Ichikawa, Y. (7) 90, 239 Iijima, T. (2) 62-66, 72 Iitaka, Y. (3) 190 Ikeda-Saito, M. (7) 9 1 Ikegami, I. (7) 265, 268, 269

Ikegami, Y. (2) 115, 116 Ikenoue, T. (2) 116 Ikeya, M. (3) 13, 15 Ikorskii, V.N. (8) 25, 92 Il'in, S.D. (6) 146 Iliopoulos, P. (3) 224 Imamura, T. (2) 9 Impellizzeri, G. (7) 23 Ingold, K.U. (2) 106, 110 Inoue, Y. (7) 296 Inui, T. (7) 285 Ioffe, N.T. (2) 97, 98 Ishigure, K. (6) 11 Ishihara, S. (1) 20 Ishii, M. (2) 41 Ishizu, K. (3) 272; ( 8 ) 52

Isoya, J. (6) 114 Ito, S. (8) 103 Itoh, N. (6) 115 Itoh, S. (7) 14, 105, 265, 269, 291, 306

Ivanov, A.V. (3) 38, 225;

285

Iwai, K. (3) 95 Iwaki, M. (7) 265 Iwasaki, H. (7) 257 Iwasaki, M. (2) 132; (6) 7, 75

Izatt, R.M. (7) 23 Izmi, T. (6) 108 Jabro, M.N. (7) 88 Jachkin, V.A. (3) 104 Jackson, R.A. (2) 138 Jacobs, D.L. (7) 162 Jaenicke, W. (2) 103 Jagannathan, R. (3) 266 Jahn, H.A. (5) 43 Jakob, B. (3) 45 Jandrisits, L.T. (6) 122 Janes, R. (6) 76 Jani, M.G. (2) 16 Janick, P.A. (7) 111 Jansen, S. (3) 200 Janson, K. (7) 218 Janssen, R.A.J. (6) 78 Janszky, J. (2) 21 Janzen, E. (1) 5, 16, 25-27; (3) 113; (4) 8; (6) 122 Jen, C.K. (5) 37 Jepsen, 0. (4) 13 Jerzak, S. (3) 77 Jeunet, A. (2) 57 Jezierski, A. (2) 52 Jha, N.K. (3) 89 Jin. R. (3) 169 Jinchin, G. (8) 6 Johannessen, K.D. (6) 89 John, J. (7) 211 Johns, J.R. (6) 55 Johnson, I.L. (3) 185 Johnson, K.M. (2) 33 Johnson, M.K. (7) 120, 136, 192, 232 Joly, H.A. (5) 72, 105, 114 Jones, M.T. (3) 200 Jones, P.M. (5) 66, 69, 70, 73, 99, 123, 124; (6) 36, 53, 132 Jones, S.E. (8) 117, 122 Jones, T.F. (7) 207 Jordan, M. (4) 51 Jordan, S.J. (1) 33, 34 Jordanov, J. (3) 148 Joseph, J. (6) 85 Journaux, Y. (3) 146 Jousse, D. (6) 110, 111 Joy, A.M. (3) 170 Joy, J. (2) 39

(8) 31

Ivanova, N.K. (2) 89 Ivanova, T.A. (3) 81

Kabachnik, M.1.

(3) 116-118; (8) 97, 109, 112, 114 Kadam, R.M. (3) 72 Kaden, T.A. (3) 132, 133 Kadish, K.M. (3) 52-54 K&npf, C. (7) 258 Kafafi, Z.H. (5) 81; (6) 70 Kagoshima, S. (3) 94 Kahn, D. (3) 146 Kahn, 0. (3) 92; (8) 124 Kai, A. (3) 15 Kaim, W. (3) 62, 109-111, 182; (8) 98 Kaise, M. (2) 122 Kakinuma, K. (7) 83 Kalasknik, A.T. (2) 73 Kalbitzer, H.R. (3) 8; (7) 211 Kalinowski, H.J. (6) 51 Kalyanaraman, B. (1) 2, 9, 23; (8) 101, 102 Kamalian, M.G. (7) 11 Kaminaka, S. (6) 74 Kanba, S. (3) 95 Kanoda, K. (3) 94 Kanters, R. (2) 131 Kappl, R. (3) 7 Karapetian, A.V. (7) 11 Karas, J.L. (7) 207 Karayannis, N.M. (8) 93 Kariya, K. (7) 86 Karmazin, A.A. (3) 268 Karsanov, I.V. (3) 116 Karthein, R. (7) 94, 95 Kasai, P.H. (5) 23, 38, 39, 66, 69, 70, 73, 76, 82, 90, 93, 99, 113, 123, 124; (6) 34, 36, 53, 132 Kashiwabara, H. (2) 112 Kasturi, A. (2) 88 Katsuda, H. (3) 95 Kaufmann, U. (4) 17; (6) 116 Kawagoe, T. (3) 94 Kawaguchi, K. (6) 157 Kawamori, A. (7) 285 Kawazoe, H. (6) 99 Kawazura, H. (7) 19 Kayushin, L.P. (2) 130 Kazanskii, V.B. (3) 4 Keenan, T.R. (3) 217 Keijzers, C.P. (2) 131, 133; (6) 3; (8) 4, 12 Keitel, R. (6) 26 Keller, H. (6) 27 Kemal, C. (7) 74 Kemp, T.J. (3) 55 Kennedy, B.J. (3) 88 Kennedy, M.C. (7) 134 Kennedy, R.A. (6) 142

286

Kent, T.A. (7) 79, 130, 166

Kernisant, K. (5) 56 Kessel, S.L. (8) 106, 115 Kessissoglou, D.P. (3) 59 Ketcham, C.M. (7) 59 Kettler, U. (5) 53, 54 Kevan, L. (2) 79-81; (3) 3, 16, 17, 196, 204-206; (6) 18, 37 Khamkar, M.Z. (3) 225 Khramtsov, V.V. (2) 102 Khrapkovskii, G.M. (2) 37 Kiefl, R.F. (6) 26, 118 Kikuchi, H. (1) 20 Kikuchi, 0. (2) 41 Kim, L. (7) 215 Kim, S.S. (2) 74 Kim, Y. (2) 105 Kimura, E. (3) 190 King, M.D. (2) 86 Kingma, J.A.J.M. (6) 78 Kiosse, G.A. (3) 46, 64 Kirillov, V.I. (3) 21 Kirk, M.L. (3) 59 Kirmse, R. (3) 145, 223 Kirste, B. (2) 54-56, 136 Kirwan, J.N. (6) 92 Kiselev, V.N. (3) 43 Kishkovich, O.P. (6) 146 Kispert, L.D. (2) 39, 93; (6) 85 Kita, K. (7) 106 Klabunde, K.J. (5) 2 Klein, M.P. (7) 271, 281, 282, 290, 295, 299 Kleinhans, F.W. (6) 120 Klempner, D. (2) 75 Kline, E.S. (5) 8 1 Knaff, D.B. (7) 87, 247, 248, 258 Knapp, S. (3) 217 Knecht, K.T. (1) 32 Knight, L.B. (2) 35; (5) 11; (6) 42-47, 43, 50, 88, 89 Knispel, R. (2) 13 Knowles, P.F. (7) 15 Knunyants, I.L. (3) 112 Kobayashi, T. (3) 20, 95 Koch, J. (3) 178 Kohler, K. (3) 223 K&hler, R . (3) 145 Koenig, S.H. (7) 69, 210 Koepke, B. (2) 34 Kohd, S. (6) 107 Kohketsu, M. (6) 99 Kohlmann, S. (3) 110, 111 Kojima, K. (3) 96 Kojima, M. (3) 191 Kokoszka, G.F. (3) 144 Kolosova, T.A. (3) 187

Author Index

Komarov, A.M. (2) 130 Kon, H. (7) 110 Konaka, R. (2) 127 Konata, R. (2) 128 Kondo, S. (2) 50 Kony, M. (7) 172 Koosev, R.G. (2) 60;

49, 51, 52

(6) 101

Kopitsya, N.I. (2) 89; (8) 30, 31, 63

Korczak, S.Z. (3) 73, 74, 261, 267

Korybut-Daszkiewicz, B. (3) 191

Korytowski, W. (8) 101 Kostina, N.V. (2) 91 Kotake, Y. (2) 5 1 Koudelka, G.B. (7) 31 Kovarskii, A.L. (2) 67 Kowal, A.T. (7) 232 Kozlowski, H. (7) 203 Krajewski, T. (3) 167 Krasil'nikova, N.A. (3) 104

Krasser, W. (5) 53, 54 Kratsmar-Smogrovich, J. (3) 135

Kraut, J. (7) 238 Kreilick, R.W. (2) 104; (8) 43

Kreitzman, S.R. (6) 26, 118

Krenn, G.E. (7) 184 Kristofzski, J. (7) 171 KrGger, A. (7) 143 Kroeger, M. (8) 66 Kroker, J. (3) 179 Kronek, P.M.H. (7) 10 Kruczynski, Z. (3) 87 Kr{se-Wol ters, K M (7) 145, 153

Kumar, A. (7) 212 Kumar, K. (3) 84 Kundig, E.P. (5) 42, 131 Kunin, A.J. (2) 104 Kuroda, S. (6) 114 Kurono, M. (7) 80 Kurtz, D.M., jun. (7) 48,

..

Krupinski-Olsen, R. (7) 74

Krusic, P.J. (5) 109; (6) 135, 136

Krylova, I.D. (2) 89 Kubat-Martin, K.A. (3) 147

Kubota, M. (6) 115 Kubow, S. (1) 26 Kubozono, Y. (2) 58 Kucherov, A.V. (3) 163, 164, 222

Kuchta, R.D. (7) 135 Kudabaev, K. (3) 79 Kudryavtsev, G.B. (3) 208 Kuechler, T.C. (8) 66, 78 Kuila, D. (7) 226, 227 Kula, T. (7) 231 Kulikov, A.V. (2) 95 Kulikov, N. (6) 87 Kulmacz., R.J. (7) ~. , 96 -~

Kuryavyi, V.G. (3) 83 Kushnaryov, V. (7) 214 Kusters, J. (2) 75 Kuwahara, J. (7) 84, 224 Kuwata, K. (2) 51 Kwan, L. (3) 228 Kwiatkowski, E. (3) 134 Labadz, A.F. (3) 243 LaCagnin, L.B. (1) 30 Ladik, J. (2) 27 Lagenfelt, G. (7) 294 Laguta, V.V. (3) 268 Lai, A. (3) 18 Lai, C.-S. (7) 214 Lai, E.K. (1) 15, 16, 25, 27, 28 Lamb, J.D. (7) 23 Landrath, K.D. (3) 165 Lane, G.A. (3) 231 Lang, G. (3) 273 Langhoff, S.R. (2) 14 Langosch, D.J. (7) 189 Larionov, S.V. (8) 25-28, 92 Larsen, S. (3) 142, 115 Latour, J.M. (8) 82 Laugier, J. (8) 38, 70, 72, 77, 81, 83-85, 89, 90, 91, 94 Lawler, R.G. (2) 120; (6) 29, 69 Lawrence, S.A. (3) 150 Lea, J.S. (6) 64 Lebedev, Ya.S. (2) 53 Leblanc, J.-P. (7) 78 Lederer, F. (7) 76 Lee, B.W. (3) 99 Lee, C.W. (3) 50 Lee, D. (2) 113; (3) 50 Lee, E. (7) 86 Lee, J.C. (3) 54 Lee, S. (3) 151 Leeuwenkamp, O.R. (3) 90 Lefebvre, R. (5) 27 LeGall, J. (7) 53, 107, 112, 113, 115, 119, 140, 146, 154, 173 Lehmann, G. (3) 165, 166, 241, 247, 253, 254

Leigh, J.S., jun. (7) 209, 235

Leite, J.R. (2) 1 7

Author Index

Lemal, D.M. (5) 112 Lemire, B.D. (7) 133 Leon, L.E. (8) 117 Leont'ev, A.Y. (3) 46 Leopold, K.R. (6) 149, 150

Lespinat, P.A. (7) 146 Leung, W.P. (3) 153 Levanon, H. (2) 7 Levason, W. (3) 207 Levin, I.W. (7) 110 Levin, V.P. (2) 37 Levstein, P.R. (3) 229 Leyh, T.S. (7) 217 Li, A.S.W. (6) 37 Li, P.M. (7) 246, 274 Li, S.W. (8) 65 Li, X. (3) 59 Liang, S. (2) 35 Ligon, A.R. (6) 49, 50 Lilienthal, H.R. (7) 210 Lillich, T. (7) 231 Lin, W.K. (3) 192 Lin, X.Q. (3) 53 Lindahl, P.A. (7) 166, 194

Lindgren, M. (2) 38;

287 (3) 150

Lough, S.M. (6) 65; (7) 162

Louis-Flamberg, P. (7) 74 Lovy, D. (6) 72 Lowe, D.J. (7) 188 Lozovoi, V.V. (2) 44 Lubitz, W. (3) 193 Luchinat, C. (2) 10; (7) 196, 197, 201, 206

Luckhurst, G.R. (2) 49 Ludden, P.W. (7) 156, 157 Luders, K. (2) 137 Luety, F. (4) 51 Luji, 2. (8) 6 Lukat, G.S. (7) 88, 92 Luke, G.M. (6) 26, 118 Lukin, S.N. (3) 245 Lund, A. (2) 38; (6) 6 1 Lunsford, J.H. (6) 54 Luscombe, D.L. (6) 137 Lusztyk, J. (2) 106 Lyfar, D.L. (3) 97 Lynch, M.W. (8) 116, 119, 120

Lynn, B.LaC. (6) 126 Lyons, D.M. (7) 162

(6) 10

Lindley, P.F. (7) 64 Lindsay, D.M. (5) 30, 44, 55, 56, 64, 93; (6) 52 Lindstedt, S. (7) 82 Lingens, F. (7) 124 Lino, A.R. (7) 112, 113 Lipscomb, J.D. (7) 82 Lisetskii, V.N. (6) 87 Lisichkin, G.V. (3) 208 Liu, C. (5) 115 Liu, H. (3) 100 Liu, L.-M. (3) 52 Liu, M.-C. (7) 107, 115 Liu, M.-Y. (7) 115 Liu, S.C. (2) 27 Livanov, L.D. (2) 37 Livingston, H. (5) 35 Ljungdahl, L.G. (7) 159 Lloret, F. (3) 91 LoBrutto, R. (7) 209, 212, 219, 228 Lockau, W. (7) 263 Lockett, C.J. (7) 284 Loehnert, K. (4) 53 Loehr, T.M. (7) 9, 52, 55 Loginov, A.Y. (6) 62 Lohse, F. (4) 47, 49, 50, 52 Longley, C.J. (3) 175 Lontie, R. (7) 37 Lord, E.M. (7) 67 Lord, K.A. (7) 216 Lorzsch, J . (7) 38 Lott, K.A.K. (1) 17;

Mabbs, F.E. (3) 156, 157, 171

Mac&skov;, L. (3) 129 McCain, D.C. (7) 297 McCay, P.B. (1) 15, 16, 18, 25, 27, 28

McClung, R.E.D. (2) 113 McCracken, J. (7) 12, 13, 17, 18, 43, 62, 104

McCullough, E.A. (2) 3 1 McDermott, A.E. (7) 262, 271, 281, 282, 290, 299

McDonald, J.W. (6) 65; (7) 163

McDowell, C.A. (4) 43; (5) 26

McGarvey, B.R. (3) 93; (5) 131

McGavin, D.G. (2) 12 McGimpsey, W.G. ( 5 ) 14 McGrath, A.C. (3) 88 McGregor, R. (2) 62-66 McGregor, S.D. (5) 112 McGuirl, M.A. (7) 17 McIntosh, D.F. (5) 79 McKelvey, R.D. (2) 3 McKenna, M.C. (7) 157 McLachlan, K.A. (2) 4 McLain, S.J. (6) 135, 136 McLauchlan, K.A. (2) 119, 125

McLean, P.A. (7) 165, 168 McLeod, D., jun. (5) 23,

76, 113; (6) 34

McMillin, D.R. (7) 33 McNab, I.R. (6) 143 McQueen, R.C.S. (3) 220 McVicar, W.K. (3) 138 Mader, C.E. (2) 40 Magliozzo, R.S. (7) 104, 225

Maheshwari, S. (8) 29 Mairanovskii, V.G. (2) 97, 98

Makinen, M.W. (3) 9 Makino, Y. (2) 72 Maksimova, T.I. (3) 172 Malcom, T. (6) 101 Malik, N.A. (3) 233 Malik, W.V. (3) 180 Malinovskii, T.I. (3) 64 Malkin, R. (7) 251, 264 Malmstr&u, B.G. (7) 77 Malovichko, G.I. (3) 245 Maltempo, M.M. (3) 24 Malysheva, N.A. (3) 117; (8) 109

Malyshkin, A.P. (3) 38 Mamedova, P.Sh. (8) 32 Mamedova, Yu.G. (8) 34 Mann, K.R. (3) 139 Mannervik, B. (7) 200 Mansfield, R.W. (7) 266, 267

Mansuy, D. (7) 75, 76 Manthey, J.A. (7) 93 Maple, B. (3) 99 Maples, K.R. (1) 33, 34 Marchesini, A. (7) 36 Maret, W. (7) 196, 202, 203

Margerie, J. (6) 23 Margrave, J.L. (5) 30, 81; (6) 70

Mari, A. (3) 232 Mari, C.L. (6) 100 Markham, G.D. (7) 217 Markus, A. (7) 124 Marrs, B.L. (7) 254 Marsden, C.J. (6) 22 Marsh, D. (2) 86 Martens, L. (6) 67 Martin, J.J. (2) 16 Martin, R.W. (6) 42 Martin, V.V. (2) 69 Martinez-Maldonado, M. (7) 215

Martini, G. (2) 87 Martinsen, J. (3) 152 Martyna, G. (6) 9 Maruani, J. (5) 27 Marupov, R. (2) 91 Marynick, D.S. (2) 45; (6) 133

Mason, R.P. (1) 10-12,

288 14, 23, 29, 30, 32-34; (6) 126, 127 Massa, W. (3) 179 Massol, M. (8) 39 Masters, C. (5) 107 Mather, M.W. (7) 226 Mathis, P. (7) 270, 272 Matsch, P.A. (3) 139 Matsumori, T. (6) 108 Matsuura, K. (2) 51; (7) 105 Mattar, S.M. (3) 25, 210 Matthys, P.F.A. (6) 67 Matusz, M. (3) 141 Matzuzaki, T. (6) 26 Mauger, A. (6) 117 Mauk, A.G. (7) 237 Mauro, J.M. (7) 238 Maurya, R.C. (3) 180 Mauterer, L.A. (7) 190 Mavankal, G. (7) 297 May, H.D. (7) 174 Mazii, G.A. (2) 73 Meade, T.J. (3) 191 Medzhidov, A.A. (8) 32, 33 Meier, P.F. (5) 30 Meinhardt, S.W. (7) 231, 242; (8) 105 Mekhrabov, A.O. (3) 264 Melekhov, V.I. (2) 108 Mell, H. (7) 143 Mellouki, A. (6) 147 Melnik, M. (3) 129 Men, A . N . (3) 43 Messmer, B.D. (5) 79 Messmer, R . (6) 113 Metz, J.G. (7) 300 Meyer, B.K. (4) 6, 47, 53 Michalik, J. (3) 205 Michalski, W.P. (7) 40 Michel, J. (4) 35 Michibata, H. (7) 183 Mikaelyan, M.V. (7) 21 Miki, T. (3) 15 Miksztal, A.R. (3) 273 Mikulski, C.M. (8) 93 Mil'chenko, D.V. (3) 208 Milaeva, E.R. (8) 96 Mildvan, A.S. (7) 213 Mile, B. (5) 7, 14, 17, 19, 20, 29, 31, 45-48, 57, 60-62, 65, 67, 68, 72, 74, 75, 83-85, 88, 89, 94, 100, 104, 105, 110, 114, 116, 118, 121, 122, 125-130; (6) 19, 35, 131, 134 Miller, A.-F. (7) 274 Miller, P.K. (6) 88 Miller, R . J . (7) 43 Miller, R.W. (7) 187

Author Index

Miller, W.G. (2) 94 Minachev, Kh.M. (3) 4 Ming, H. (3) 238; (6) 55 Minge, J. (3) 78, 167 Mironova, G.N. (8) 26 Mishra, K.C. (2) 20; (6) 103

Mishra, S.P. (6) 82, 91 Misra, H.P. (1) 27 Misra, S.K. (2) 5; (3) 12, 73, 74, 82, 84, 252, 261, 267

Mita, S. (2) 50 Mita, T. (3) 190 Mitchell, S.A. (5) 16, 9 1 Mitsuda, K. (2) 92 Mixa, M.M. (3) 139 Miyao, M. (7) 289 Miyasaka, T. (3) 272 Mizoguchi, T. (3) 94 Mock, W.L. (5) 111 Mgbius, K. (4) 18, 42; (6) 2

Mtkeler, R. (3) 193 Mognaschi, E.R. (3) 69 Mohammed, A.W. (3) 42 Mohan, M. (3) 89 Moir, J.E. (6) 137 Moisseev, D.P. (3) 97 Molin, Yu.N. (2) 44, 108 Mollenauer, L.F. (4) 46, 52

Mondovi, B. (7) 13, 16 Monnanni, R. (7) 206 Montagna, L. (3) 133 Montgomerie, C.A. (6) 143 Moore, D.W. (5) 95 Moralev, V.M. (2) 48 Morgn, M. (3) 218 Moratal, J.M. (3) 91 Morazzoni, F. (6) 100 More, C. (2) 129; (7) 126, 127

More, J.K. (8) 23, 53, 58 More, K.M. (3) 119-124; (8) 8, 11, 13, 23, 24, 45-51, 57-62, 64, 73 Morgan, T.V. (7) Morgan, W.T. (7)

15-21, 54, 160 221 (7) 33 105 56 41

Morie-Bebel, M.M. Morigaki, K. (6) Morigatei, K. (4) Morihashi, K. (2) Morio, M. (1) 20 Morishima, I. (7) 80, 102 Morningstar, J.E. (7) 190-192,

232

Morokuma, K. (5) 98; (6) 11

Morpurgo, L. (7) 13, 16, 34

Morris, H. (5) 31, 89, 96, 104, 130; (6) 35

Mortenson, L.E. (7) 149-151, 160 (5) 24, 92, 126, 127, 129, 132; (6) 130, 135 Morton, T.A. (7) 159 Mosina, L.V. (3) 85 Moskovits, M. (5) 49, 50, 52 Mossoba, M.M. (6) 125 Motevalli, M. (3) 140, 174, 236 Mottley, C. (1) 12, 14 Motuz, A.A. (3) 97 Mougenot, P. (2) 25 Moura, I. (7) 53, 112, 113, 119, 128, 129, 137, 140, 146 Moura, 3.3.G. (7) 53, 112, 113, 119, 128, 129, 137, 140, 146, 173 MroziGski, J. (3) 129 Muller, E. (3) 215 Mcller, K.A. (3) 76, 244, 257 Mhler, K.T. (2) 104 Muller, U. (3) 218 Munck, E. (7) 79, 129, 137, 150, 165, 166, 194 Munz, X. (3) 195 Muessig, T. (6) 40 Muhoberac, B.B. (7) 221 Mukai, K. (8) 52 Mukherjee, R. (3) 203 Mul, P. (7) 147 Mulliez, E. (7) 78 Munoz, G.H. (6) 24 Murakami, H. (7) 106 Murphy, B.P. (3) 136, 137 Murray, K.S. (3) 88, 224; (7) 204 Murthy, K. (6) 66 Musaev, A.M. (8) 32 Musci, G. (7) 25, 220 Muta, K. (6) 99 Muto, H. (6) 75

Morton, J . R .

Na, Y. (3) 169 Nad, V.Yu. (3) 106 Nagao, Y. (3) 272 Nagasawa, T. (7) 84 Nagy, V.Yu. (8) 3 Nakahara, A. (7) 26, 35 Nakamura, D. (3) 70 Nakayama, M. (2) 41 Nakazawa, K. (6) 107 Nakhmetov, S.M. (3) 264 Nalbandyan, R.M. (7) 11, 21

Author Index

Nam, N.-H. (7) 76 Nantoi, O.G. (3) 64 Narayama, P.A. (6) 18 Narayana, M. (3) 204 Narducci, D. (6) 100 Nastainczyk, W. (7) 94, 95 Neimark, E.I. (2) 22 Neissel, W. (3) 219 Nelis, T. (6) 155 Nelsen, S.F. (2) 36, 111 Nelson, J. (3) 136, 137 Nelson, M.J. (7) 71-73, 167, 168 Nelson, S.F. (2) 105 Nelson, S.M. (3) 136, 137 Nelson, T. (6) 159 Nepven, F. (3) 149; (8) 39 Newton, J.L. (6) 109 Newton, W.E. (7) 161 Nicholas, D.J.D. (7) 40 Nichols, J . E . (4) 58 Nicholson, J. (6) 136 Nickel, B. (2) 57; (3) 186 Nieves, J. (7) 215 N i k l a s , J . R . (4) 6, 7, 10, 12, 16, 17, 21-24, 30, 35, 37-41, 44, 45; (6) 40 Nikonov, A.M. (2) 90 Nikonovo, S.I. (2) 90 Niloges, M.J. (5) 28 N i l s s o n , P. (7) 218 Nishida, Y. (3) 63 Nishiyama, K. (6) 26, 118 N i s t o r , S.V. (6) 41 N i t s c h e , S. (2) 111 Nitschke, W. (7) 249, 250, 263 NiviGre, V. (7) 148 Noakes, D.R. (6) 26, 118 Nocek, J.M. (7) 52 Noguchi, M. (7) 244 Noguchi, T. (1) 15 N o r r i s , G.E. (7) 9 N o r r i s , J . R . (2) 126 Notton, B.A. (7) 179 Novais, H.M. (2) 100 Nugent, J.H.A. (7) 253, 261, 266, 267, 284, 286 Nunome, K. (2) 132; (6) 7 Oaks, A. (7) 180 Obata, A. (7) 19 Oberhausen, K.J. (3) 61 Obi, K. (2) 9 Odermatt, W. (6) 27 Oeder, R. (4) 34 Ogasawara, M. (6) 10

289

Ogawa, M.Y. (3) 151, 152 Ogiyama, S. (8) 52 O'Hara, P.B. (7) 69 Ohbayashi, K. (3) 96 Ohbu, K. (2) 85 Ohnishi, T. (7) 228, 231, 242; (8) 105 Ohno, K. (6) 20 Ohta, K. (6) 84 Okada, K. (8) 52 Okamura, M.Y. (7) 273 Okazaki, M. (2) 127, 128 Okhlobystin, 0.Yu. (8) 96 Okida, M. (1) 20 O l i n g e r , G.N. (1) 2 Oliva, C. (6) 68 O l i v e r , E.J. (6) 68 O l m , M.T. (6) 39 Olson, J.S. (7) 248 Omoto, M. (2) 75 Ondrias, M.R. (7) 248 Ono, T. (7) 296 Orme-Johnson, W.H. (7) 132, 139, 165-168, 209 O s e r o f f , S.B. (3) 99 Oshiro, Y. (7) 14 O s k a m , A. (3) 184 Osman, M. (3) 214 Otiko, G. (7) 101 O t t a v i a n i , M.F. (2) 87 Otto, P. (2) 27 Ovcharenko, V . I . (8) 25-28, 92 O v e r n e l l , J. (7) 208 Owens, F.J. (6) 79 Owens, K. (2) 109 Ozarowski, A. (3) 93 Ozawa, T. (6) 124, 128 Ozin, G.A. (3) 25; (5) 16, 42, 79, 106, 131 Padros, E. (7) 222 Padula, F. (3) 144 Pahnke, R. (6) 158 Pake, G.E. (4) 5 P a l , A.K. (3) 48, 68, 258 P a l , S. (3) 263 P a l l a n z a , G. (7) 36 Palmer, G. (7) 96, 228, 243, 244, 247, 248 Palmer, S.M. (3) 152 P a l o c i o s , M.S. (3) 130 Pan, S. (4) 46 P a , W.-H. (7) 159 Pandeya, K.B. (7) 15 Panz, T. (7) 214 Papaconstantinou, E. (3) 159 Papadopoulos, N . J . (7) 18

Papaef thymiou , V. (7) 129, 165 P a p a s e r g i o , R . I . (3) 153 Papkov, S.P. (2) 73 P a q u e t t e , L. (2) 35 P a r d i , L. (8) 70, 84, 121 P a r n i s , J.M. (5) 106 P a r r e t t , K.G. (7) 262 P a r r y , D.L. (7) 182 Pasenkiewicz-Gierula, M. (7) 22, 44 Passmore, J. (2) 33 P a t e l , R.N. (7) 58, 123 P a t i l , D.S. (7) 57, 140-142, 154 P a t r i n a , L.A. (8) 25, 27 P a t t e r s o n , B.D. (6) 27 P a u l , V. (60 93 Pavel, N.V. (2) 84 Pavlikov, V.V. (8) 35 Payne, W . J . (7) 107, 115 Pearce, L.E. (2) 94 Pearce, L.L. (7) 48, 51 Peck, H.D., jun. (7) 107, 112, 113, 115, 140, 146, 154 Pecoraro, V.L. (3) 59 Pedersen, E. (3) 142 Pedersen, J . A . (8) 99 P e i , Y. (3) 146 Peisach, J. (7) 12, 13, 17, 18, 43, 62, 97, 104, 210, 225 P e l l e r , D. (5) 11 Pember, S. (7) 43-45 P e r c i v a l , P.W. (6) 26 Perez, R.H. (6) 133 Perez-Reyes, E. (1) 23 P e r k i n s , M.J. (1) 7 Pervukhina, N.V. (8) 92 P e t e r s o n , J. (7) 114, 128 P e t e r s s o n , L. (7) 70, 81 P e t r o u l e a s , V. (7) 307, 308 P e t r o v , R.R. (7) 155 P e t r u k h i n , O.M. (3) 106; (8) 3 P e t t y , J.T. (6) 46, 47 Peyerimhoff, S.D. (2) 26 Pezeshk, A . (8) 9, 10, 44 P h i l p o t , R.M. (1) 23 P i e r p o n t , C.G. (3) 115; (8) 95, 106-108, 110, 113, 116, 120 Pilbrow, J . R . (3) 1, 39, 131, 143, 194; (7) 172 P i l l a i , S.M. (5) 102 P i n h a l , N.M. (3) 197 P i n t a r , A. (7) 36 P i z z i n i , S. (6) 100 P l a t , H. (7) 184, 186 P l a t o , M. (4) 42

Author Index

290

Plesch, G. (3) 135 Plonka, A. (6) 33, 59 Plowman, J.R. (7) 9 Plug, C.M. (3) 90 Plummer, J.L. (1) 19 Podberezskaya, N.V. (8) 92

Poe, A.J. (5) 42 Poggi, A. (3) 133 Poli, R. (3) 141 Poll, J.D. (6) 144 Poluektov, O.G. (2) 53 Ponomarev, D.V. (3) 208 Poole, C.P., jun. (4) 19 Popkova, V.Ya. (3) 112 Popov, V.I. ( 2 ) 48 Popov, V.O. (7) 155 Porai-Koshits, M.A. (3) 213

Porter, L.C. (8) 68, 69, 76, 86, 87

Portier, J. (3) 101, 102 Postnikov, V.S. (3) 21 Potenza, J.A. (3) 217 Poulet, G. (6) 147 Power, W.J. (5) 79 Poyer, J.L. (1) 16, 18, 25

Prabhumirashi, L.S. (3) 70

Prasad, R.S. (3) 89 Premovic, P.I. (8) 102 Preston, K.F. (2) 33; (5) 24, 45, 46, 92, 126, 127, 129, 132; (6) 130, 135 Preti, C. (3) 216 Prickril, B. ( 7 ) 53, 140, 146 Prince, R.C. (7) 58, 99, 123, 131, 160, 254, 255, 259 Prins, R. (7) 130; (8) 124 Prinzbach, H. (3) 177 Probkt, J.M. (6) 73 Prokof'ev, A.I. (2) 107; (3) 116-118; (8) 96, 97, 109, 112, 114 Prokof'eva, T.I. (3) 117; (8) 109 Prudon, M.K. (3) 148 Puett, D. (7) 215 Pyrz, J.W. (7) 60, 79 Pytlewski, L.L. (8) 93

Qin, X.Z. (2) 111 Que, L., jun. (7) 60, 79, 82

Rabi, 1.1. (5) 25 Radhakrishna, S. (3) 71, 158

Raetzsch, M. (2) 70 Ragsdale, S.W. (7) 159, 194

Ramasseul, R. (8) 36, 38, 81

Ramsay, R.R. (7) 232 Rao, D.N.R. (6) 86, 95 Rao, J.L. (3) 249 Rao, N.U.M. (8) 65 Rao, U.R.K. (3) 72 Rasanen, M. (5) 11 Rassat, A. (2) 57; (3) 186; (8) 36-38,

81

Raston, C.L. (3) 153 Rath, J.K. (3) 71 Rauber, A. (6) 116 Rauckman, E.J. (1) 6 Raven, S.J. (3) 108, 231 Ravindvannathan, M. (5) 102

Rawlings, S.J. (3) 143 Raymond, K.N. (8) 111 Rayner, J.B. (7) 15 Raynor, D.M. (5) 9 1 Raynor, J.B. (6) 57 Recker, R. (3) 241 Reddy, M.V.V.S. (2) 29 Redhardt, A. (6) 5 Reed, G.H. (7) 209, 216, 220

Reedijk, J. (3) 92; (8) 124

Reem, R.C. (7) 47, 50 Rehorek, D. (3) 55 Reibenspies, J.H. (3) 119; (8) 24

Reijerse, E.J. (2) 131, 133; (6) 3

Reineck, S.R. (5) 13 Reinen, D. (3) 40, 45 Reinhamunar, B. (7) 8 Reinke, L.A. (1) 28 Remme, S. (3) 241, 247 Renger, G. (7) 301 Rex, G.C. (2) 76 Rey, P. (5) 109; (8) 36-38, 70-72, 77, 81-85, 89-91, 94 Rheingold, A.L. (3) 248 Ribas, J. (3) 232 Richards, P.S. (6) 159 Richardson, D.E. (7) 50, 59 Richardson, J.F. (3) 6 1 Richardson, P.F. (8) 43 Richardson, T.H. (7) 187 Richman, K.W. (2) 31 Richter, R. (3) 145 Rieger, P.H. (3) 231

Riesen, A. (3) 132 Riesz, P. (6) 4, 125 Rigaud, J.-L. (7) 222 Rigaud, M. (7) 78 Rijkhoff, M. (3) 184 Riley, M.J. (3) 40, 42 Ritschl, F. (2) 24 Rittmeyer, P. (2) 32 Rizzarelli, E. (7) 23 Robbing, A.P. (7) 180 Roberts, B.P. (6) 92, 93 Roberts, R.M. (7) 59 Robinson, G.W. (6) 22 Robson, R.L. (7) 187 Rockenbauer, A. (3) 47, 227

Rodgers, K.R. (7) 88, 92 Roduner, E. (6) 27 Roe, A.L. (7) 82 Roe, J.A. (3) 273 Roelofs, Y.B.M. (7) 144 Rohlfing, E.A. (5) 51 Romanelli, M. (2) 87; (3) 16

Rosa, J. (3) 172 Rosen, G.M. (1) 6; (6) 121

Rosenfeldt, J. (7) 97 Rossi, A.R. (2) 21 Roth, H.K. (2) 121 Rowland, I.J. (6) 57 Rowlands, C.C. (6) 56 Rozantsev, E.G. (8) 32, 34, 35

Ruangsuttinarupap, S. (3) 212

Rubenacker, G.N. (3) 239 Rubezhov, A.Z. (8) 96 Rubins, R.S. (3) 239, 240, 260

Rubio, J.O. (6) 24 Rudinskaya, G.V. (2) 73 Rudowicz, C. (2) 11; (3) 27-29

Ruegger, H. (3) 113 Ruf, H.H. (7) 94, 95, 124 Rundgren, M. (7) 82 Rusnak, F.M. (7) 150 Russell, D.R. (6) 38 Russo, N. (2) 30 Rutherford, A.W. (7) 275, 277, 283, 287, 289, 296, 300, 301, 304, 305; (8) 104 Ryabchenko, S.M. (3) 97

Sadler, P.J. (7) 101 Sage, J.T. (7) 52, 55, 60 Sahoo, N. (2) 20; (6) 103 Sairam, T.N. (3) 265 Saito, S. (6) 157

Author Index

Saitoh, Y. (6) 115 Sakabe, Y. (3) 95 Sakai, S. (5) 98 Sakata, S. (2) 127, 128 Sakurai, H. (3) 272; (7) 183

Sakurai, T. (7) 14, 26, 35

Salagram, M. (3) 158 Sales, K. (3) 14 Salling, C.T. (3) 99 Sanchez, A. (2) 28 Sanders-Loehr, J. (7) 52, 55

Sandman, D.J. (3) 200 Sandreczki, T.C. (2) 68 Sano, Y. (4) 56 Sarbasov, K. (8) 112 Sarkar, A.B. (3) 93 Sarkar, S. (3) 199 Sarna, T. (8) 101 Sasaki, H. (8) 52 Sass, A. (3) 4 Sastry, M.D. (3) 72; (6) 77

Sato, K. (8) 103 Satoh, J.-I. (7) 285 Satoh, K. (7) 285, 291, 306

Satyanarayana, N. (3) 158 Saueer, R. (6) 112 Sauer, K. (7) 271, 281, 282, 290, 295, 299

Savage, P.D. (3) 175, 236 Savas, J.C. (7) 58 Savel'ev, G.G.F. (6) 87 Savic, I.M. (6) 27 Savini, I. (7) 34 Sawada, S. (7) 35 Sawant, B.M. (8) 8, 11, 14, 15, 22, 53, 67

Sawyer, D.T. (8) 117, 122 Saykally, R.J. (6) 153, 154

Sayo, H. (7) 86 Schzffer, A. (7) 197, 199 Schastnev, P.V. (2) 44 Schechinger, T. (7) 27 Scheffler, M. (4) 13 Schiess, P. (3) 178 Schindewolf, U. (6) 14 Schirmer, 0. (4) 8; (3) 250

Schlick, S. (2) 75, 76 Schlosnagle, D.C. (5) 58 Schmidt, J. (2) 8 Schneider, G. (2) 117 Schneider, J. (3) 177, 178; (4) 8, 17; (6) 27, 116 Schneider, R. (3) 132 Schoemaker, D. (6) 41

29 1

Scholes, C.P. (7) 7, 240 Schrautemeier, B. (7) 256 Schroeder, M. (3) 234, 235

Schubert, J.E. (6) 153 Schugar, H.J. (3) 217 Schultz, F.A. (7) 161 Schultz, H. (7) 193 Schultz, P. (6) 113 Schultz, S. (3) 99 Schumacher, K.L. (2) 47 Schumann, W. (2) 70 Schweiger, A. (4) 2; (6) 8

Schweizer, D. (7) 124 Scopes, R.K. (7) 204 Scott, R.A. (7) 159 Scozzafava, A. (7) 206 Sealy, R.C. (8) 100-102 Sears, T.J. (6) 141, 156 Seebass, N. (6) 151 Seez, M. (7) 124 Seibert, M. (7) 300 Seidel, H. (4) 11, 20 Seigneuret, M. (7) 222 Sellin, S. (7) 200 Semenev, E.V. (8) 63 Semikopnyi, A.I. (8) 30, 63

Sen, G.-Y. (3) 30 Sermon, P.A. (3) 150 Serpersu, E.H. (7) 213 Serushkin, V.V. (3) 213 Sessoli, R. (8) 71, 72, 77, 9 1

Setchov, Y.R. (6) 72 Sitif, P. (7) 268, 270, 272

Setoyama, K. (2) 92 Setser, D.W. (6) 159 Shaefer, H.F., I11 (5) 97 Shaevitz, B.A. (3) 273 Shaikh, S.A. (3) 72 Shand, M.A. (3) 55 Shanina, B.D. (2) 22 Shapiro, A.B. (2) 71; (8) 32, 35

Shapiro, E.S. (3) 4 Sharrock, P. (8) 40, 41 Sharsheev, K. (3) 79 Shaw, G. (7) 133 Shaw, R.W. (7) 87, 247, 258

Shchegoleva, L.N. (2) 44 Shen, D. (8) 7 Shepherd, R.A. (3) 161 Shi, Z.G. (2) 31 Shibuya, K. (6) 99 Shida, T. (2) 108 Shields, L. (6) 17 Shiemke, A.K. (7) 52 Shiga, T. (2) 127, 128

Shimada, S. (2) 112 Shionoya, M. (3) 190 Shiotani, M. (6) 84 Shiraishi, H. (6) 11 Shiro, Y. (7) 80, 102 Shklyeav, A.A. (8) 28 Shohoji, M. (2) 100 Short, R.L. (3) 150, 155 Shorter, A.L. (7) 74 Shorthill, W.B. (8) 106 Shortle, D. (7) 213 Shrivastava, K.N. (3) 82, 98, 239, 240

Shroyer, A.L.W. (8) 22 Shteingarts, V.D. (2) 44 Shukla, R. (3) 180 Shul'ga, A.A. (3) 38 Shuler, R.H. (5) 117 Shvengler, F.A. (2) 89; (8) 30, 31, 63 (3) 4 (3) 255 (3) 144 ( 7 ) 108, 111, 114 Siegfried, L.C. (3) 133 Sieler, J. (3) 145 Siero, C. (2) 28 Siew, P.Y. (3) 113 Sikdar, R. (3) 68 Silver, B.L. (3) 41 Simard, B. (5) 91 Simpkin, D. (7) 228 Sinclair, G.R. (3) 131 Sinditskii, V.P. (3) 213 Singel, D.J. (2) 8; (3) 18, 19 Singer, T.P. (7) 232 Singh, H.B. (8) 29 Singh, R. (7) 171 Sinha, B.K. (6) 129 Siragusa, G. (3) 69 Sirtl, E. (4) 8, 34 Sivaraja, M. (7) 280 Sivaram, S. (5) 102 Skelton, B.W. (3) 88 Skorobogaty, A. (3) 143

Shvets, V.A. Siebert, D. Siedle, A.R. Siegel, L.M.

Skripnichenko, L.N. (8) 35

Skryantz, J.S. (8) 93 Slappendel, S. (7) 70 Slater, T.F. (1) 17, 21, 24

Slichter, C.P. (4) 4 Slinkin, A.A. (3) 163, 164, 222

Smadja, W. (5) 120 Smith, B.E. (7) 163, 188 Smith, G.R. (6) 83 Smith, M. (7) 237 Smith, P. (2) 40; (8) 56 Smith, R.E. (8) 14

Author Index

292

Smith, S.B. (7) 207 Smith, S.W. (2) 49 Smith, T.D. (3) 39, 131, 143, 194

Smithers, G.W. (7) 209 Smyth, J.F. (3) 99 Snow, L.D. (2) 111 Snow, M.R. (7) 172 Snyder, S.W. (7) 39 So, H. (2) 101; (3) 49, 50

Soethe, H. (4) 33, 5 1 Sofen, S.R. (8) 111 Sohmo, J. (6) 20, 84 Sokol, V.I. (3) 213 Solans, X. (3) 232 Solodovnikov, S.P. (2) 107; (3) 112, 116-118; (8) 97, 109, 112, 114 Solomon, E.I. (7) 28, 30, 47, 50, 178 Solomonson, L.P. (7) 179, 180 Solozhenkin, P.M. (2) 89; (3) 225; (8) 30, 31, 63 Somasundaran, P. (2) 78 Someno, K. (2) 122 Sarensen, P.E. (3) 188 Sosnovsky, G. (8) 65 Spaeth, J.-M. (3) 65, 67; (4) 6, 7, 9, 10, 12, 16, 17, 22, 23, 29-33, 35, 37-41, 44, 45, 47, 49-53; (6) 40 Spangenberg, B. (3) 178 Spence, J.T. (7) 171, 172 Spencer, B. (3) 147 Spicer, M.D. (3) 207

Spira-Solomon, D.J. (7) 28, 30

Spiridonov, S.E. (6) 62 Spirin, A.I. (3) 21 Sreeramachandra Prasad, L. (3) 242 Sridhar, R. (1) 27 Stach, J. (3) 223 Stam, P. (2) 133; (6) 3 Stanbury, D.M. (6) 70 Stanton, J.L. (3) 152 Stark, J.C. (3) 200 Stavropoulos, P. (3) 140, 174, 236

Steadman, J. (6) 88 Steenken, S. (2) 100 Stein, U. (2) 32 Stemp, E.D.A. (3) 24 Stephan, D.W. (3) 154 Stephens, A.D. (6) 38 Stephens, P.J. (7) 157 Steren, C.A. (3) 229 Stevens, D.G. (2) 4

Stier, A. (1) 17 Stockis, A. (5) 103 Stoke, K. (6) 127 Stoquert, J.P. (6) 110, 111

Strauch, G. (4) 55 Streib, W.E. (3) 60 Strom, U. (3) 103 Studzinski, P. (3) 65, 67; (4) 30-33

Stufkens, D.J. (3) 184 Styring, S. (7) 283, 287, 289

Subra, R. (2) 30; (8) 89 Subramanian, S. (3) 242 Such, K.P. (3) 253 Sugiura, Y. (7) 84, 224 Suhara, M. (3) 20 Sun, A.Y. (1) 22 Sundholm, F. (2) 49 Surerus, K.K. (7) 137 Sutcliffe, L.H. (2) 33 Sutcliffe, R. (5) 29, 47, 48, 57, 60-62, 65, 74, 75, 83-85, 121, 122, 126, 127; (6) 134 Sutton, S.L. (3) 260 Suzuki, S. (2) 23; (7) 14, 35, 257 Suzuki, T. (6) 157; (7) 224 Swartz, H.M. (1) 3 Sweeney, W.V. (7) 136 Swepston, P.N. (3) 201 Syage, J.A. (2) 123, 124 Symons, M.C.R. (1) 17; (2) 43; (3) 233; (5) 86, 87; (6) 12, 17, 25, 32, 38, 39, 48, 57, 64, 74-76, 82, 86, 91, 95, 104, 106, 119, 130 Szab&Pl&nka, T. (3) 47, 227

Tachikawa, H. (6) 10 Tagiev, B.G. (3) 264 Tahon, J.P. (7) 37 Tait, C.D. (3) 107 Tajima, K. (8) 52 Takacs, G.A. (6) 156 Takahashi, T. (3) 94 Takakuwa, S. (7) 257 Takeda, M. (7) 102 Takemura, I. (6) 20 Takeuchi, H. (3) 246, 256 Tan, S.L. (7) 209 Tanaka, H. (7) 19 Tanaka, K. (6) 114 Tang, J. (2) 19 Tang, X.4. (7) 291, 306; (8) 103

Tassi, L. (3) 216 Taylor, H. (7) 7 Taylor, K.V. (5) 49 Teasley, M.F. (2) 111 Teixeira, M. (7) 140, 146 Teller, E. (5) 43 Telser, J. (7) 134, 149, 151, 228

Telser, T. (6) 14 Temperley, J. (3) 156 Teo, B.K. (7) 168 Terech, P. (2) 77 Terlesky, K.C. (7) 158 Tero-Kubuta, S. (2) 116 Teschner, A. (6) 112 Thang, D.-C. (7) 76 Thanke, K. (6) 112 Theury, P. (3) 92 Thevand, A. (2) 114 Thiers, A.H. (2) 131 Thomas, A. (5) 19; (6) 19 Thomas, R. (3) 192 Thompson, C.P. (7) 65 Thompson, D.H.P. (2) 8 1 Thompson, G.A. (5) 44, 55, 56

Thompson, H.J. (7) 67 Thompson, J.S. (8) 123, 124

Thompson-Colon, J.A. (2) 105

Thomson, A.J. (3) 226; (7) 89, 98, 116, 175-177 Thurman, R.G. (1) 29, 30; (6) 126 Tiekink, E.R.T. (7) 172 Timakov, I.A. (8) 33 Tino, J. (2) 6 1 Tirant, M. (3) 39, 131, 194 Tolles, W.M. (5) 95 Tolpygo, S.K. (3) 97 Tomasi, A. (1) 17, 21, 24 Tomietto, M. (5) 72 Tominaga, K. (2) 118 Tooze, R.P. (3) 236 Topchieva, K.V. (6) 62 Tordo. P. (2) 114 Toriyama, K . . (2) 132; (6) 7, 75 Torosyan, D. (3) 26 Toscano, M. (2) 30 Tosi, G. (3) 216 Trammell, G.T. (5) 35 Trau, D.C. (3) 103 Trenary, M. (5) 97 Trifunac, A.D. (6) 29, 30 Trogler, W.C. (3) 248 Tromp, M.G.M. (7) 184, 186 True, A.E. (3) 201;

Author Index (7) 167, 168

Trumpower, B.L. (7) 241, 242; (8) 105

Tsai, A.-L. (7) 96, 228 Tsai, T.E. (6) 97 Tsay, F.D. (2) 74 Tse, J.S. (5) 57, 59, 60, 71, 75, 89, 96, 110, 116; (6) 35 Tse, P. (7) 204 Tsubaki, M. (7) 90, 239 Tsukada, M. (2) 92 Tsukerblat, B.S. (3) 43 Tsvetkov, Y.D. (3) 6 Tsvetkov, Yu.D. (2) 46, 48; (6) 16, 18

Tuchagues, J.-P.M. (8) 118

Tumanskii, B.L. (3) 112, 118; (8) 112, 114

Turner, N. (7) 173 Turro, N.J. (2) 78 Tuttle, T.R. (6) 2 1 Twiss, P. (3) 153 Udagawa, M. (3) 96 Udpa, K.N. (3) 199 Ueda, Y. (4) 17 Uehara, H. (6) 152 Uhlig, E. (3) 215 Unden, G. (7) 143 Undeutsch, B. (3) 215 Upreti, G.C. (2) 5; (3) 75, 252

Urban, W. (6) 148, 151, 155

Usachev, A.E. (3) 8 1 Ushida, K. (2) 108 Utecht, R.E. (7) 49, 51 Utkin, I.B. (7) 155

293

Van Zee, R.J. (5) 15, 40, 41, 63; (6) 60, 83 Vanqgard, T. (3) 58 Vannini, V. (1) 21 Vanturini, E.L. (3) 144 VaPGk, M. (7) 208

Vasconcellos, E.C.C. (6) 150

Vasil'ev, A.A. (6) 87 Vasserman, A.M. (2) 69, 71

Vassilikou-Dova, A.B. (3) 165, 166

Vasson, A. (3) 23, 243 Vasudevachari, M.B. (7) 223

Veeger, C. (7) 145, 153 Veldink, G.A. (7) 77 Venkateswarlu, K.S.

178

(3) 72

Venters, R.A. (7) 167 Verbeck, R.M.H. (6) 67 Veselov, A.V. (2) 108 Vess, T.M. (3) 107 Vetter, Th. (4) 55 Vicente, R. (3) 232 Videau, J.J. (3) 101, 102 Vier, D.C. (3) 99 Viezzoli, M.S. (7) 201 Villafranca, J.J. (7) 18, 43-45,

212, 238

Vincent, J.S. (7) 110 Vincent-Vaucquelin, J. (3) 186

Vinckier, C. (7) 37 Vithal, M. (3) 266 Vlcek, A., jun. (3) 114 Vliegenthart, J.F.G. (7) 70, 77

Volker, M. (7) 301 Vol'eva, V.B. (3) 117; (8) 109, 112

Volodarskii, L.B. (2) 69; Vanng&rd, T. (7) 252, 278, 294

Vainer, L.M. (2) 102 Valenti, J.J. (5) 5 1 Valentine, J.V. (3) 273 Valentine, M. (8) 119 Valigi, M. (2) 133 Vallee, B.L. (7) 197 van Berkel-Arts, A .

(8) 26

von Bardelben, H.J. (6) 117

Voordouw, G. (7) 145 Voronkova, V.K. (3) 85 Voss, J. (2) 34 Vreugdenhil, W. (3) 92 Vugman, N.V. (3) 197, 198

(7) 145, 153

Van der Graaf, T. (3) 184 Van der Linden, J.G.M. (3) 148

van der Zwaan, J.W. (7) 143, 144, 147

Van Hoof, D. (7) 37 van Kooyk, Y. (7) 186 Van Rossum, M. (2) 20; (6) 103

Walsh, C.T. (7) 139 Walton, J.C. (2) 110 Walton, R.A. (3) 181 Walz, L. (8) 39 Wang, C. (3) 267 Wang, J.-X. (6) 54 Wang, Y. (5) 55; (7) 201 Wang, Z.-C. (7) 164 Wani, B.N. (3) 72 Waplak, S. (3) 167 Ward, B. (5) 19 Warden, J.T. (7) 260, 303 Ware, D.C. (8) 111 Wassink, H. (7) 163 Watanabe, T. (5) 76 Watanabe, Y. (6) 99 Waterman, K.C. (2) 78 Watkins, G.D. (6) 109 Watt, G.D. (7) 162, 164,

Wagner, H.G. (6) 145 Wagner, P. (4) 8, 34 Wahl, R.C. (7) 132 Wakabayashi, J. (6) 74 Walch, S.P. (2) 14 Waleed, A.A. (2) 96 Waley, S.G. (7) 199 Wallqvist, A. (6) 9 Wallrafen, F. (3) 241

Wayner, D.D.M. (2) 109 Webb, J. (5) 14 Weber, E. (4) 23 Weber, J. (2) 29 Wedd, A.G. (7) 171, 172, 204

Weddle, C.C. (1) 18 Weil, J.A. (5) 22 Weiner, J.H. (7) 133 Weinert, C. (4) 35 Wells, G.B. (3) 9 Weltner, W. (5) 15, 40, 41, 63; (6) 44, 60, 83

Wemura, T. (6) 102 Wendel, H. (2) 27 Werbelow, L. (2) 114 Werner, J. (6) 151 Werst, D.W. (6) 30 Wertz, D.W. (3) 107 Weser, U. (7) 27 West, D.X. (3) 233 Westerling, J. (6) 61 Westrum, E.F., jun. (5) 21

Weterings, J.P. (7) 130 Wetherbee, P.J. (7) 178 Wever, R. (7) 41, 184-186 Whangbo, M.H. (5) 101 White, A.H. (3) 88, 153 White, C.T. (2) 18 Whittaker, J.W. (7) 20 Whittaker, M.M. (7) 20 Widom, J. (7) 79 Wilkerson, J.O. (7) 114 Wilkinson, G. (3) 140, 174, 175, 236

Williams, F. (2) 111 Willing, A. (7) 195 Willing, W. (3) 218 Wills, A.R. (3) 155 Wilson, G.L. (7) 171, 172 Wilson, T.M. (2) 16

Author Index

294

Windsch, W. (3) 77, 230 Winnacker, A. ( 4 ) 55 Winscom, C.J. (3) 193; (4 ) 54 W i t t , S.N. (7) 245 Witte, F.M. ( 2 ) 82 Witters, R. (7) 37 Wittinghofer, A. ( 7 ) 211 Woerner, R. (4) 8 Wojtowicz, W. (3) 78 Wolf, C.R. (1) 23 Wolniewicz, L. ( 6 ) 144 Wong, C.S. (3) 113 Wood, D. (3) 189 Woodland, M.P. (7) 57 Woodward, J . R . (6) 50 Wren, B.W. (6) 75 Wrest, M. (7) 134 Wright, R.B. (5) 12 Wrigley, S.K. (7) 29 Wu, B.W. (7) 274 Wu, J. ( 2 ) 19 Wuensch, M. (3) 179 Wuu, S.K. (2) 93 Wylie, D.M. (6) 68 Wynn, R.M. (7) 258 Xavier, A.V. (7) 53, 112, 113, 119, 146 Xia, Y.-M. (7) 48, 52 Xiong, Q. (3) 33 XU, J.-X. (7) 234 Yablokov, Yu.V. (3) 46, 81, 8 5 Yachandra, V.K. (7) 271, 281, 282, 290, 295, 299 Yagfarov, M.S. (3) 85 Yagi, T. (7) 109, 231 Yakovleva, I.V. (2) 71

Yamada, H. (7) 84 Yamada, Y. (7) 291 Yamase, T. (3) 51 Yamauchi, S. ( 2 ) 118 Yamazaki, S. (6) 114 Yampol'skaya, M.A. (3) 85 Yang, A.-S. (7) 46 Yang, F.D. (7) 233 Yang, X. (7) 241, 242; (8) 105 Yates, P.C. (3) 137 Yeh, J.Y. (3) 168 Yocum, C.F. (7) 298, 302 Yokomichii H. (6) 105 Yonetake, K. (2) 66 Yonetani, T. (7) 91, 103, 236 Yoshida, H. ( 6 ) 37 Yoshida, T. (7) 226 Yoshimizu, H. (2) 92 Yoshimura, T. (7) 257 You, J.L. (6) 123 Young, C.G. (3) 171 Young, D. (5) 3 Young, L.J. (7) 108, 114 Youvan, D.C. (7) 259 Youxian, D. (8) 6 Yu, C.-A. ( 7 ) 233, 234 Yu, J.-T. (3) 86, 168; (6) 90 Yu, L. (7) 233, 234 Yu, N.-T. (7) 90 Yu, W.L. (3) 31, 32, 249 Yu, Y. (3) 160 Yuan, C. (3) 160 Yudanov, V.F. (6) 18; (8) 28 Yudanova, E.I. (2) 95 Yudowsky, M. (3) 151 Yuzefovich, Yu.L. (2) 98

Zagogianni, H. (6) 147 Zanchini, C. (3) 128; (7) 125; (8) 69, 70, 72, 74, 79, 82, 83, 90 Zannini, P. (3) 216 Zanobi, A. (2) 84 Zapart, M.B. (3) 80 Zapart, W. (3) 80 Zaripova, N.B. (2) 37 Z e i t z , D. (6) 148, 155 Zeldes, H. (5) 35 Zeppezauer, M. (7) 196, 202 Zerner, B. (7) 54 Zhang, X. (3) 217 Zhao, K. (3) 176 Zhao, L. (8) 7 Zhao, M.G. (3) 30, 31, 32, 33, 34, 237 Zhdanov, A.A. (2) 107 Zheng, A. (3) 176 Zheng, W.-C. (3) 36 Zhitnikov, R.A. (6) 28 Zhou, C. (3) 160 Zhou, Y. (3) 34, 35, 37, 209 Zhu, L. (8) 7 Zhu, T.P. (3) 53 Ziatdinov, A.M. (3) 83 Zie gle r, H. (4) 25-28 Zimmermann, J.-L. (7) 277, 296, 305 Zinchenko, Z.A. (2) 89 Zirino, T. (7) 55 Zolotov, Yu.A. (3) 106; (8) 3 Zomack, M. (6) 63 Zulehner, W. (4) 34 Zuluaga, J. (2) 28 Zumft, W.G. (7) 10 Zund, A. (6) 10