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1、Electron Spin Resonance (ESR) Spectroscopyappliedtospecieshavingoneormoreunpairedelectrons:free radicals,biradicals,other triplet states,transition metal compoundsspecieshavingoneunpairedelectronhastwoelectronspinenergylevels:E = gm mBBoMs selectionruleDMs=1=D DE = gm mBBog:proportionalityconstant,2
2、.00232forfreeelectron1.992.01forradicals1.43.0fortransitionmetalcompoundsinisotropicsystems(gas,liquidorsolutionoflowviscosity,solidsiteswithsphericalorcubicenvironment),gisindependentoffielddirectionmB:Bohrmagneton9.274x10-24JT-1forelectronMS:electronspinquantumnumber+1/2or1/21Bo:externalmagneticfi
3、eldcommonly0.341.24T=correspondingfrequency9.5(X-band)35(Q-band)GHztheelectroninteractswithaneighboringnuclearmagneticdipole,theenergylevelsbecome:E = gm mBBoMS + am mBMSmImI:nuclearspinquantumnumberfortheneighboringnucleusa:hyperfinecouplingconstantenergylevelsandtransitionsforasingleunpairedelectr
4、oninanexternalmagneticfieldwith no couplingcoupling to one nucleus with spin 1/22spin-lattice relaxation:microwaveradiationtransferredfromthespinsystemtoitssurroundingslongrelaxationtime=decreaseinsignalintensityshortrelaxationtime=resonancelinesbecomewidetypicalESRspectrometeraradiationsource(klyst
5、ron)asamplechamberbetweenthepolesofamagnetadetectionandrecordersystemESRspectrum(a)absorptioncurve(b)first-derivativespectrum standard:DPPH(diphenylpicrylhydrazylradical)g=2.0036,pitchg=2.0028Bstdgsample = gstd Bsampleforfield-sweep,lowerfield(left-hand)thanstandard,highergvalue3hyperfinecouplingini
6、sotropicsystemsinteractionsbetweenelectronandnuclearspinmagneticmoments=finestructureinESRspectrumcouplingsariseintwoways:(i)directdipole-dipoleinteraction(ii)FermicontactinteractioncouplingpatternsinESRaredeterminedbythesamerulesthatapplytoNMRcouplingtonucleiwithspin1/2aremorefrequentlyobservedhype
7、rfinecouplingconstantgmBMHzorcm-1hyperfinesplittingconstantAgaussormillitelsladependsontheunpairedelectronspindensityatthenucleusinquestionisrelatedtothecontributiontotheatomofthemolecularorbitalcontainingtheunpairedelectronunpairedelectroncanpolarizethepairedspinsinanadjacentsbond=thereisunpairedel
8、ectronspindensityatbothnuclei4Ex.1C6H6-couplingtoall6Hatomstheelectronisdelocalizedoverall6CatomsEx.2pyrazineradicalanion (a)couplingto214Nnuclei(1:2:3:2:1quintet),andsplitby4Hatomsfurtherinto1:4:6:4:1quintet(b)Na+salt,furthersplittinginto1:1:1:1quartet5Ex.3BH4-+C(CH3)3BH3-+HC(CH3)3Ex.4NBut+S(=NBut)
9、2+Me2SiCl2SSiMeNButg=2.005A(N)=0.45mT6Ex.5S(=NBut)2-g=2.0071A(N)=0.515mTEx.6(MeO)3PBH27Ex.7CrIII(porphyrin)Clthepatternsofhyperfinesplittingsprovidedirectinformationaboutthenumbersandtypesofspinningnucleicoupledtotheelectronsthemagnitudesofthehyperfinecouplingsindicatetheextenttowhichtheunpairedelec
10、tronsaredelocalized,gvaluesshowwhetherunpairedelectronsarebasedontransitionmetalatomsoronadjacentligands.8zero-fieldsplittingintheabsenceofmagneticfield,2S+1energystatessplitdependsonthestructureofsample,spin-orbitcouplingtheappearanceofmorethanoneline(S1/2)finestructure-inprinciple,2Stransitionscan
11、occur,theirseparationsrepresentingtheextentofzero-fieldsplitting9anisotropicsystemssolids,frozensolutions,radicalspreparedbyirradiationofcrystallinematerials,radicaltrappedinhostmatrices,paramagneticpointdefectinsinglecrystalsforsystemswithsphericalorcubicsymmetrygfactorsforsystemswithlowersymmetry,
12、g=gandg=gxx,gyy,gzzESRabsorptionlineshapesshowdistinctiveenvelopesystemwithanaxisofsymmetrynosymmetry10Ex.8Li+13CO2-inCO2matrixlarge13Candsmall7Li(I=3/2)hyperfinesplittingEx.9HMn(CO)5/solidKrmatrixat77KhuMn(CO)5A(55Mn)=6.5mTA(55Mn)=3.5mTA(83Kr)=0.4mT11transitionmetalcomplexesthenumberofdelectronshig
13、horlowspincomplexconsequenceofJahn-Tellerdistortionzero-fieldsplittingandKramersdegeneracyESRspectraofsecondandthirdrowtransitionmetalcomplexesareoftenhardtoobserved,however,rare-earthmetalcomplexesgiveclear,usefulspectrashortspin-latticerelaxationtimes=broadspectrallineslowtemperatureexperimentswil
14、lbeneededtoobservespectraEx.10d3system12trans-Cr(pyridine)4Cl2+(a)frozensolutioninDMF/H2O/MeOH(b)intransRh(pyridine)4Cl2Cl6H2OpowderEx.11d6systemlow-spindiamagneticOhtetragonalhigh-spin5D5T25B2shortrelaxationtimes=broadresonanceslargezero-fieldsplittings=noresonanceobserved13Ex.12d9systemCuII(TPP)complex(frozensolutioninCCl3H)Cu(acac)2frozensolution14multipleresonanceENDOR(electron-nucleardoubleresonance)Ex.13Ti(C8H8)(C5H5)intoluene(frozensolution)(a)ESRspectrum(b)1HENDORspectrum151617
