chemical-shift-化学位移.ppt

ChemicalShift ChemicalShift B0Forasamekindofnucleus the 0shouldbethesame ThemagneticfieldfeltbythenucleusisnotthesameasappliedfieldThee cloudthatsurroundsthenucleushasmotionandgeneratesamagneticmoment ThismagneticfieldaltersB0inthemicro environmentaroundthenucleusThiselectronicmodulationoftheB0fieldiscalledmagneticshielding B B0 1 ChemicalShift MagneticfieldatthenucleiisNOTequaltotheappliedfieldbecauseofshieldingordeshieldingofelectronsaroundthenuclei Variationoftheresonancefrequencywithshieldingiscalledthechemicalshift B B0 1 B0 Bloc B0 1 Exampleethanol ChemicalShiftScale B0 B0 1 2 B0 1 2 2 1 B0isconstant S R R S R 1 R S R R 1 2 0isconstant BS BR BR S R 1 R S R R 1 S R ppm 10 6 StandardReferences 0 Themostcommonreferencecompoundistetramethylsilane TMS TMSisusedasreferencebecauseitissolubleinmostorganicsolvents isinert volatile andhas12equivalent1Hsand4equivalent13Cs TMSisnotwatersoluble socan tbeusedinaqueousmedia thenuse2 2 dimethyl 2 silapentane 5 sulfonicacid DSS 0 015ppm Otherreferencescanbeused suchastheresidualsolventpeak dioxanefor13C Example Lotsofelectronsnearby Fewelectronsnearby Chemicalshiftsareinfluencedbytheelectronicenvironment Therefore theyarediagnosticforparticulartypesofmolecularstructures Thefollowingfigureindicatesaveragerangesofprotonchemicalshiftsfordifferenttypesofmolecules Differentfunctionalgroupsexperiencedifferenteffectivefield ScalesforDifferentNuclei Theshieldingofdifferentnucleidependsontheelectrondensityinitssurroundings sdia diamagnetic arisesfromthemagneticfieldopposingBofromtheelectronsimmediatelysurroundingthenucleus sorbitals spara paramagnetic generatedbyelectronsinporbitals aswellasbonds ItisinfavorofBo sl whichisduetoneighboringgroups anditcanaddorsubtractfromB0 dependingonthenatureofthegroupanditsspatialorientation slisthetermthatactuallymakesnucleiwithsimilarcharacteristicsinamoleculehavedifferentshieldings andthereforechemicalshifts Originsofs Bloc s sdia spara sl sparawilldominatebecausewehavemorepelectronshellsthatcanbecomeoccupied lowerenergythanporbitalsin1H 1H 13C 1Hhasonlya1sorbital sdiawilldominate Originsofs Bloc Theshieldingisdeterminedbytheelectrondensityandforanisolated1H itiscalculatedwiththeLambformula r r isverycomplicated Wehavetoconsiders p d etc atomicorbitals andwealsohavetoconsidermolecularorbitals Thisistherealmofquantummechanicalchemicalshiftcalculations farmorethanwhatwewanttoknow However mostoftheeffectscanbequalitativelydescribedbyusingsdiaands1forprotons rememberthatsparahaslittlecontributionsin1H Wewillstartwitheffects orcontributions tos1 Theseareknownasinductiveandconjugativeeffects andaswewillsee theiroriginliesinhowtheelectrondensityaroundthe1Hisaffectedbydifferentspeciesattachedtoit InductiveEffects Sincewelowertheelectrondensityaroundthenucleus itwillbecomedeshielded andthereforeitwillmovetowardslowerfields higherchemicalshift Forexample ifweconsiderhydrogenhalides we llseethatthemoreelectronegativethehalideis thedropinshieldingconstantsisinverselyproportionaltotheEofthehalide s HF s HCl s HBr s HI H 1s Anisolated1Hatom Lambformula 17 8ppmforsdia Now weadda CH3toit andgetmethane theelectroncloudonthe1H onanyofthe4 willbecomedeformed becausetheelectronegativity E ofthecarbonwillpullthe1selectronofthe1Htowardsit InductiveEffects Theinductiveeffectontheshieldingofthe1Hisnotlimitedtogroupsbondeddirectlytoit Wehavetorememberthattheelectrondensityaroundthe1Hdependsonthemolecularorbitalsofthewholemolecule Theeffectsofelectronegativityare transmitted throughmolecularorbitals bonds Ifwehaveaveryelectronegativeatombondtoacarbon protonsbondedtothatcarbonwillhavetheir1selectronspulledawaymorethanifwedidnothavetheelectronegativegroup So forthemethaneserieswehave Furthermore wedon tneedaparticularlyelectronegativeatom Ifwelengthenthecarbonchain thedeshieldingwillalsoincrease ConjugativeEffects Nowwehaveanolefinicoraromaticprotonandwehaveasubstituentthatcanhavedifferentconjugativeeffects Mor M 1 EtheneandEWGsorEDGsassubstituents Ifweconsidermethylvinylketone thechemicalshiftsoftheolefinicprotonswillmovedownfieldconsiderably Becausetheketone anEWG istakingelectronsawayfromthedoublebond theelectrondensityaroundthe1Hwilldiminish Ifontheotherhandweconsidermethylvinylether thesituationisreversed Themethoxygroups anEDG willdonateelectronstothedoublebond whichwillincreasetheelectrondensityaroundthe1Handthereforeincreasetheshielding EWG EDG ElectronWithdrawing DonatingGroup 2 1Hsonsubstitutedaromaticsystems Forexample inanilinewehaveanEDG whichhasa Meffect Sincewe llhavemoreelectrondensityinthering allprotonswillbemoreshieldedthantherespectiveprotonsinbenzene 7 24ppm Furthermore ifwedrawresonantstructureswecanseethattheorthoandparapositionswillhavealargerelectrondensity Therefore protonsattachedtotheorthoorparacarbonswillbemoreshielded lowerchemicalshift ConjugativeEffects 7 24 Ontheotherhand nitrobenzene whichhasanEWG hasa Meffect Allcenterswillhavealowerelectrondensity buttheorthoandparapositionswillhaveaparticularlyloweredelectrondensity Allprotonsinnitrobenzenewillbemoredeshieldedthanbenzene Inparticular theeffectattheorthoandparapositionswillbethelargest ConjugativeEffects AnisotropicEffects CH3 CH3CH2 CH2CH CH0 885 311 48 Thechemicalshiftsofcompoundswithdoubleandtriplebondsandthoseofaromaticsystemscannotbesolelyexplainedonthebasisofshieldinganddeshieldingasinthecaseofaliphaticcompounds thelocaldiamagenticeffects Compoundswithdoubleandtriplebondsinadditiontodiamagneticshieldingexhibitmagneticanisotropyduetodelocalized electrons MagneticAnisotropy AnisotropicEffects Now wehaveamoleculelikeH YandYcanbeanatomorgroup ie C CH WhenweaddafieldB0 theelectronsaroundYwillgetmovementwhichcreatsmagneticmoment Y B0 Y B B0 2 3r3 2 3r3 B B0 AnisotropicEffects RingCurrentEffects Oneofthemostpronouncedeffectsarisingfrominducedmagneticmomentsinachemicalgroupareduetoaromaticrings Ifweconsidertheringcurrentofthering itwillgenerateamagneticfieldperpendiculartotheplaneofthering thatwillbeagainsttheexternalmagneticfield Aswesee thefieldlinesthroughtheringareagainstoftheexternalmagneticfield theinducedmagneticmomentwillopposetheeffectofBo butthe return lines whichgoontheoutsideofthering areinfavorofit Therefore wecansafelyassumethatprotonssittingontheplaneoftheringandthereaboutswillbedeshielded whilethoselyingontoporbelowtheringwillbeshielded i e higherfieldsandthereforelowerchemicalshifts RingCurrentEffects Example DeshieldinginAlkenes Downfield deshielded shiftofvinylhydrogens RingCurrentEffects Example ShieldinginAlkenes sphydrogensareshielded upfieldshift RingCurre