chemical-shift-化学位移

ChemicalShiftChemicalShift•ω=γB0Forasamekindofnucleus,the0shouldbethesame!•Themagneticfieldfeltbythenucleusisnotthesameasappliedfield•Thee-cloudthatsurroundsthenucleushasmotionandgeneratesamagneticmoment.ThismagneticfieldaltersB0inthemicro-environmentaroundthenucleusThiselectronicmodulationoftheB0fieldiscalledmagneticshielding,•B=B0(1-)ChemicalShift•MagneticfieldatthenucleiisNOTequaltotheappliedfieldbecauseofshieldingordeshieldingofelectronsaroundthenuclei.•VariationoftheresonancefrequencywithshieldingiscalledthechemicalshiftB=B0(1-)=B0-Bloc=B0(1-)HO-CH2-CH3olowfieldhighfieldExampleethanolChemicalShiftScaleω=γB0ν=B0(1-)γ/2π△ν=B0(1-2)γ/2π1）B0isconstantδ=(νS-νR)/νR=(S-R)/(1-R)=(S-R)(R1)2)ν0isconstantδ=(BS-BR)/BR=(S-R)/(1-R)=(S-R)(R1)δ=(S-R)ppm(10-6)StandardReferencesH3CSiCH3CH3CH3=0Themostcommonreferencecompoundistetramethylsilane,TMSTMSisusedasreferencebecauseitissolubleinmostorganicsolvents,isinert,volatile,andhas12equivalent1Hsand4equivalent13Cs.•TMSisnotwatersoluble,socan’tbeusedinaqueousmedia,thenuse2,2-dimethyl-2-silapentane-5-sulfonicacid,DSS(=0.015ppm)•Otherreferencescanbeused,suchastheresidualsolventpeak,dioxanefor13C.ExampleLotsofelectronsnearbyFewelectronsnearbyChemicalshiftsareinfluencedbytheelectronicenvironment.Therefore,theyarediagnosticforparticulartypesofmolecularstructures.Thefollowingfigureindicatesaveragerangesofprotonchemicalshiftsfordifferenttypesofmolecules.DifferentfunctionalgroupsexperiencedifferenteffectivefieldScalesforDifferentNuclei.Theshieldingofdifferentnucleidependsontheelectrondensityinitssurroundings.•dia(diamagnetic):arisesfromthemagneticfieldopposingBofromtheelectronsimmediatelysurroundingthenucleus(sorbitals).•para(paramagnetic):generatedbyelectronsinporbitals(aswellasbonds…).ItisinfavorofBo.•l:whichisduetoneighboringgroups,anditcanaddorsubtractfromB0,dependingonthenatureofthegroupanditsspatialorientation.listhetermthatactuallymakesnucleiwithsimilarcharacteristicsinamoleculehavedifferentshieldings(andthereforechemicalshifts).Originsof(Bloc)=dia+para+lparawilldominatebecausewehavemorepelectronshellsthatcanbecomeoccupied(lowerenergythanporbitalsin1H…)1H13C1Hhasonlya1sorbital,diawilldominateOriginsof(Bloc)•Theshieldingisdeterminedbytheelectrondensityandforanisolated1H,itiscalculatedwiththeLambformula:•r(r)isverycomplicated:Wehavetoconsiders,p,d,etc.,atomicorbitals,andwealsohavetoconsidermolecularorbitals.•Thisistherealmofquantummechanicalchemicalshiftcalculations,farmorethanwhatwewanttoknow.•However,mostoftheeffectscanbequalitativelydescribedbyusingdiaand1forprotons(rememberthatparahaslittlecontributionsin1H).•Wewillstartwitheffects(orcontributions)to1.Theseareknownasinductiveandconjugativeeffects,andaswewillsee,theiroriginliesinhowtheelectrondensityaroundthe1Hisaffectedbydifferentspeciesattachedtoit.200()3eerrdrmrInductiveEffects•Sincewelowertheelectrondensityaroundthenucleus,itwillbecomedeshielded,andthereforeitwillmovetowardslowerfields(higherchemicalshift).•Forexample,ifweconsiderhydrogenhalides,we’llseethatthemoreelectronegativethehalideis,thedropinshieldingconstantsisinverselyproportionaltotheEofthehalide:(HF)(HCl)(HBr)(HI)H(1s)H(1s)C(sp3)•Anisolated1Hatom：Lambformula→17.8ppmfordia:•Now,weadda-CH3toit(andgetmethane),theelectroncloudonthe1H(onanyofthe4)willbecomedeformed,becausetheelectronegativity(E)ofthecarbonwillpullthe1selectronofthe1Htowardsit:InductiveEffects•Theinductiveeffectontheshieldingofthe1Hisnotlimitedtogroupsbondeddirectlytoit.Wehavetorememberthattheelectrondensityaroundthe1Hdependsonthemolecularorbitalsofthewholemolecule.•Theeffectsofelectronegativityare‘transmitted’throughmolecularorbitals(bonds):Ifwehaveaveryelectronegativeatombondtoacarbon,protonsbondedtothatcarbonwillhavetheir1selectronspulledawaymorethanifwedidnothavetheelectronegativegroup.So,forthemethaneserieswehave:H-CH3H-CH2IH-CH2BrH-CH2ClH-CH2F2.12.52.83.04.00.231.982.452.844.13EFurthermore,wedon’tneedaparticularlyelectronegativeatom.Ifwelengthenthecarbonchain,thedeshieldingwillalsoincrease.ConjugativeEffects•Nowwehaveanolefinicoraromaticprotonandwehaveasubstituentthatcanhavedifferentconjugativeeffects(+Mor–M).•1)EtheneandEWGsorEDGsassubstituents.Ifweconsidermethylvinylketone,thechemicalshiftsoftheolefinicprotonswillmovedownfieldconsiderably.Becausetheketone(anEWG)istakingelectronsawayfromthedoublebond,theelectrondensityaroundthe1Hwilldiminish:•Ifontheotherhandweconsidermethylvinylether,thesituationisreversed:Themethoxygroups(anEDG),willdonateelectronstothedoublebond,whichwillincreasetheelectrondensityaroundthe1Handthereforeincreasetheshielding:5.255.25OHHH6.356.376.12OHHH4.184.04EWG/EDG=ElectronWithdrawing/DonatingGroup•2)1Hsonsubstitutedaromaticsystems.Forexample,inanilinewehaveanEDG,whichhasa+Meffect.Sincewe’llhavemoreelectrondensityinthering,allprotonswillbemoreshieldedthantherespectiveprotonsinbenzene(7.24ppm).•Furthermore,ifwedrawresonantstructureswecanseethattheorthoandparapositionswillhavealargerelectrondensity.Therefore,proto