Table Of ContentF.Gerson,W.Huber
ElectronSpinResonance
SpectroscopyofOrganic
Radicals
ElectronSpinResonanceSpectroscopyofOrganicRadicals.FabianGerson,WalterHuber
Copyright82003WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
ISBN:3-527-30275-1
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Fabian Gerson, Walter Huber
Electron Spin Resonance Spectroscopy of
Organic Radicals
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v
Contents
Preface ix
AbbreviationsandSymbols xi
A GeneralPart 1
1 PhysicalFundamentalsofElectronSpinResonance 3
1.1 SpinandMagneticMomentofElectron 3
1.2 ZeemanSplittingandResonanceCondition 4
1.3 Spin-latticeRelaxation 6
1.4 Line-widthandLine-form 8
2 ParamagneticOrganicSpeciesandTheirGeneration 10
2.1 SpinMultiplicity 10
2.2 NeutralRadicals 13
2.3 RadicalIons 19
2.4 Triplets:Electron–ElectronMagneticInteraction 27
3 Electron–NuclearMagneticInteraction 37
3.1 NuclearMagnetism 37
3.2 HyperfineSplittingofESRSignal 39
4 SpinDensity,SpinPopulation,SpinPolarization,andSpinDelocalization 49
4.1 Concepts 49
4.2 pRadicals 56
4.3 sRadicals 75
4.4 TripletStates 79
4.5 CalculationsofSpinPopulations 80
5 Multiresonance 83
5.1 HistoricalNote 83
5.2 ENDOR 84
vi Contents
5.3 TRIPLEResonance 94
5.4 ELDOR 96
6 TakingandAnalyzingESRSpectra 97
6.1 Instrumentation 97
6.2 ge Factor 99
6.3 OptimalConditions 102
6.4 UnravellingHyperfinePattern 109
6.5 AssignmentandSignofCouplingConstants 127
6.6 IonPairing 141
6.7 IntramolecularDynamicProcesses 153
B SpecialPart 167
7 OrganicRadicalsCenteredonOne,Two,orThreeAtoms 169
7.1 C-,N-,andO-centeredRadicals 169
7.2 Si-,P-,andS-centeredRadicals 186
7.3 CC-,NN-,andOO-centeredRadicals 189
7.4 NO-andNO2-centeredRadicals 200
7.5 PO-,PP-,SO-,SS-,andSO2-centeredRadicals 208
8 ConjugatedHydrocarbonRadicals 210
8.1 TheoreticalIntroduction 210
8.2 OddAlternantRadicals 217
8.3 OddNonalternantRadicalsandRadicalDianions 224
8.4 EvenAlternantRadicalIons 229
8.5 EvenNonalternantRadicalIons 254
8.6 RadicalsandRadicalIonswithaPerturbedpPerimeter 261
8.7 RadicalIonsofPhanes 278
8.8 RadicalIonsofRadialenes 287
9 ConjugatedRadicalswithHeteroatoms 290
9.1 NeutralRadicals 290
9.2 RadicalAnionsofElectronAcceptors 302
9.3 RadicalCationsofElectronDonors 346
9.4 RadicalCationswithSpecialStructures 366
9.5 RadicalIonsofMulti-redoxSystems 372
10 SaturatedHydrocarbonRadicals 375
10.1 RadicalCationsofAlkanes 375
10.2 StructurallyModifiedRadicalCations 380
11 BiradicalsandTriplet-stateMolecules 386
11.1 Biradicals 386
Contents vii
11.2 MoleculesinPhotoexcitedTripletState 389
11.3 MoleculesinGroundorThermallyAccessibleTripletState 393
Appendices 405
A.1 NitroxylsasSpinLabelsandSpinAdducts 405
A.2 HyperfineSplittingbyAlkali-MetalNucleiinCounterionsofRadical
Anions 409
References 415
Index 447
ix
Preface
Several years ago, electron spin resonance (ESR) spectroscopy celebrated the 50th
anniversary of its discovery in 1944. Its application to organic radicals [1] under-
wentrapidexpansioninthefollowingthreedecades,withmanymonographsbeing
published between 1965 and 1978 [2–15]. Among them, a booklet by one of us,
entitled High-Resolution ESR Spectroscopy [6], concerned the multiline hyperfine
patterns of organic radicals in solution. The radicals discussed were mostly ions
readilygeneratedbyreductionoroxidationofaromaticcompounds.Thislimitation
permitted the number of pages to be kept low,and the comprehensible treatment
madethebookletattractivetoresearcherswithabackgroundinorganicchemistry.
Suggestionsforwritingasecond,updatedversionhavebeenmaderepeatedlysince
then, but for various reasons, theywere notimplemented. Only recently, afterthe
author’sretirementin1997,wassuchaprojectenvisagedand,twoyearslater,also
tackled. It soon became obvious that supplementing the booklet with a few para-
graphs would not suffice to account for the important developments in the field
and, particularly, for the enormous amount of data accumulated in the literature
duringthelastthirdofthe20thcentury.Thus,analmostcompletelynewandmore
comprehensivevolumehadtobewritten,butwehavetriedtopreservethelucidity
ofitsmodestforerunner.
The term ESR has been retained throughout, even though the more extensive
term electron paramagnetic resonance (EPR) has been recommended. As argued
in Chapt. 2.1, this is because the magnetism of organic radicals is predominantly
due to the electron spin. Also retained has been a division of the contents into a
General Part A, serving as an introduction to the field, and a Special Part B, in
whichorganicradicalsareclassifiedandcharacterizedbytheirhyperfinedata.
Themostimportanttopicsaddedtothefirstversionareasfollows.
(1)Organicpradicals,bothchargedandneutral,aswellassradicals,havebeen
fully dealt with. (2) Biradicals and triplet molecules have also received consider-
ation.(3)Moreattentionhasbeengiventog factorsofradicals.(4)Theoriginand
e
consequences of g and hyperfine anisotropies have been described (thus the epi-
e
thet‘‘high-resolution’’isnolongerappropriate).(5)Newmethodsforgenerationof
radicals have been introduced, in particular those producing radical cations from
compounds with higher ionization energy, either by more efficient reagents in
solution or by X- or g-irradiation in solid matrices. (6) Multiresonance methods
x Preface
have been described, especially electron-nuclear double resonance (ENDOR) spec-
troscopy[12,15,16]anditsphysicalfundamentals.(7)Modernquantum-chemical
procedures for calculation of spin distribution in radicals, going beyond the p-
electronmodels,havebeenbrieflypresentedandtheirresultsforparticularradicals
are quoted. However, the theories underlying these procedures are outside the
scope of this monograph; the pertinent computer programs are readily available
andcaneasilybehandledbyexperimentalists.
Severalareasinthefield,whicharelessrelevanttoESRspectroscopyoforganic
radicalsandthushavenotbeendealtwith,arelistedbelow.
(1)Paramagneticspeciesinphysicsandbiology,likecolorcentersincrystalsand
radicalsproducedbyhigh-energyirradiationofbiologicalmaterial.(2)Chemistryof
radicalsassuch,althoughwehaveindicatedthroughouthowradicalsaregenerated
and, in many cases, into which secondary paramagnetic species they convert. (3)
Complexes of organic ligands with transition metals, because their structure
strongly differs from that of organic radicals and their hyperfine interactions are
dominated by those with the nuclei of heavy atoms. (4) Instrument conditions
other than those at constant waves (CW), namely the pulsed ESR and ENDOR
techniques.
A book illuminating the achievements in the ESR field appeared in 1997 [17].
Data relevant to radicals (g factors and hyperfine-coupling constants) have been
e
compiledintheLandolt–Bo¨rnsteinTablessince1965[18],andpublicationsonESR
spectroscopy have been reviewed in Chemical Society Special Reports since 1973
[19].
We thank our colleagues, Professors Alwyn G. Davies, London, Harry Kurreck,
Berlin, and Ffrancon Williams, Knoxville (Tennessee), and Ms. Marj Tiefert, San
Ramon (California), for critical reading the manuscript and suggesting improve-
ments. A constructive collaboration with Drs. Gudrun Walter, Karen Kriese, and
Romy Kirsten, and Mr. Hans-Jo¨rg Maier of Wiley-VCH, Weinhheim, is gratefully
acknowledged. Our special thanks are also due to Ms. Ruth Pfalzberger for the
skilfuldrawingsoftheFigures.
xi
Abbreviations and Symbols
ESR electronspinresonance
EPR electronparamagneticresonance
ENDOR electron-nucleardoubleresonance
ELDOR electron-electrondoubleresonance
TRIPLE electron-nuclear-nucleartripleresonance
NMR nuclearmagneticresonance
MW microwaves
RF radiofrequency
SLR spin-latticerelaxation
SSR spin-spinrelaxation
ZFS zero-fieldsplitting
AO atomicorbital
LCAO linearcombinationofAOs
MO molecularorbital
SOMO singlyoccupiedMO
HOMO highestoccupiedMO
LUMO lowestoccupiedMO
NHOMO nexthighestoccupiedMO
NLUMO nextlowestoccupiedMO
NBMO nonbondingMO
IE ionizationenergy
EA electronaffinity
UHF unrestrictedHartree-Fock
DODS differentorbitalsfordifferentspins
INDO intermediateneglectofdifferentialoverlap
MNDO modifiedneglectofdifferentialoverlap
AM1 Austin model 1 (reparametrized version of
MNDO)
DFT densityfunctionaltheory
ACN acetonitrile
DME 1,2-dimethoxyethane
