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核磁共振中自旋裂分或J偶合.doc

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核磁共振中自旋裂分或J偶合 Spin-spinsplittingorJcoupling Couplingin1HNMRspectra WehavediscussedhowthechemicalshiftofanNMRabsorptionisaffectedbythemagneticfieldBeproducedbythecirculationofneighboringelectrons.NowwewishtoexaminehowthemagneticfieldproducedbyneighboringnucleiBnaffectstheappearanceofthe1HNMRabsorption.Theeffectoccursthroughtheinteractionofnuclearspinswithbondingelectronspinsratherthanthroughspace.Let'sfirstconsidertheabsorptionofahydrogennucleuslabeledAwithonlyoneneighboringhydrogennucleusinavicinalpositionlabeledX.Let'salsoassumethatHAandHXhavesignificantlydifferentchemicalshifts. HXwillhaveapproximatelyequalprobabilityofexistingineitherthelowenergyalphastateorhighenergybetastate.Againbecauseofthesmallenergydifferencebetweenthelowandhighenergystates,thehighenergystateiseasilypopulatedfromthermalenergy.ForthosemoleculesinwhichHXexistsinthelowenergystate,abouthalfthemoleculesinthesample,itsmagneticfieldBnwillsubtractfromthemagneticfieldBo-BeandforthosemoleculesinwhichHXexistsinthehigherenergystate,againabouthalfthemolecules,itsmagneticfieldBnwilladdtoBo-Be. Note:whetherBnforaparticularspinstateaddstoorsubtractsfromBoisafunctionofthenumberofinterveningbonds;thisphenomenondoesn'tusuallyaffecttheappearanceofthesignalandwillnotbeexplainedherebutresultsfromthemechanismofcouplinginvolvinginteractionofnuclearspinswithelectronspins.Fortheexampleofvicinalcoupling(3interveningbonds),theBnfieldisnegativeforHXinthealphaspinstate;forgeminalcouplingBnispositiveforHXinthealphaspinstate.Geminalcouplingoccursbetweenprotonsofdifferentchemicalshiftbondedtothesamecarbon(2interveningbonds);itwillbediscussedlater. AsaconsequenceoftheBnfieldinavicinalsystem,atfixedexternalmagneticfieldBo,alowerfrequencywillberequiredtoachieveresonanceforthosemoleculeswhichhaveHXinthestatethanforthosemoleculeswhichhaveHXinthestate.TheNMRsignalforHAwillappearasatwolinepatternasshowninFigure16.WesaytheHXsplitstheabsorptionHAintoadoubletandthetwoprotonsarecoupledtoeachother.Theintensityofthetwolineswillbeequa