核磁共振中自旋裂分或j偶合

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1、核磁共振中自旋裂分或J偶合Spin-spinsplittingorJcouplingCouplingin1HNMRspectraWehavediscussedhowthechemicalshiftofanNMRabsorptionisaffectedbythemagneticfieldBeproducedbythecirculationofneighboringelectrons.NowwewishtoexaminehowthemagneticfieldproducedbyneighboringnucleiBnaffectstheappearanceofthe1HNMRabsorpt

2、ion.Theeffectoccursthroughtheinteractionofnuclearspinswithbondingelectronspinsratherthanthroughspace.Let'sfirstconsidertheabsorptionofahydrogennucleuslabeledAwithonlyoneneighboringhydrogennucleusinavicinalpositionlabeledX.Let'salsoassumethatHAandHXhavesignificantlydifferentchemicalshifts.HXwill

3、haveapproximatelyequalprobabilityofexistingineitherthelowenergyalphastateorhighenergybetastate.Againbecauseofthesmallenergydifferencebetweenthelowandhighenergystates,thehighenergystateiseasilypopulatedfromthermalenergy.ForthosemoleculesinwhichHXexistsinthelowenergystate,abouthalfthemoleculesint

4、hesample,itsmagneticfieldBnwillsubtractfromthemagneticfieldBo-BeandforthosemoleculesinwhichHXexistsinthehigherenergystate,againabouthalfthemolecules,itsmagneticfieldBnwilladdtoBo-Be.Note:whetherBnforaparticularspinstateaddstoorsubtractsfromBoisafunctionofthenumberofinterveningbonds;thisphenomen

5、ondoesn'tusuallyaffecttheappearanceofthesignalandwillnotbeexplainedherebutresultsfromthemechanismofcouplinginvolvinginteractionofnuclearspinswithelectronspins.Fortheexampleofvicinalcoupling(3interveningbonds),theBnfieldisnegativeforHXinthealphaspinstate;forgeminalcouplingBnispositiveforHXinthea

6、lphaspinstate.Geminalcouplingoccursbetweenprotonsofdifferentchemicalshiftbondedtothesamecarbon(2interveningbonds);itwillbediscussedlater.AsaconsequenceoftheBnfieldinavicinalsystem,atfixedexternalmagneticfieldBo,alowerfrequencywillberequiredtoachieveresonanceforthosemoleculeswhichhaveHXinthestat

7、ethanforthosemoleculeswhichhaveHXinthestate.TheNMRsignalforHAwillappearasatwolinepatternasshowninFigure16.WesaytheHXsplitstheabsorptionHAintoadoubletandthetwoprotonsarecoupledtoeachother.Theintensityofthetwolineswillbeequalsinceth