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《核磁經(jīng)典教程 Understanding NMR Spectroscopy Keeler_ch2》由會員上傳分享，免費在線閱讀，更多相關內(nèi)容在行業(yè)資料-天天文庫。
1、2NMRandenergylevelsThepicturethatweusetounderstandmostkindsofspectroscopyisthatmoleculeshaveasetofenergylevelsandthatthelinesweseeinspectraareduetotransitionsbetweentheseenergylevels.SuchatransitioncanbeE2causedbyaphotonoflightwhosefrequency,ν,isrelatedtotheenergygap,E,betweenthetwolevels
2、accordingto:hν=EE21νE=hνspectrumE1wherehisauniversalconstantknownasPlanck’sconstant.ForthecaseenergylevelsshowninFig.2.1,E=E2?E1.InNMRspectroscopywetendnottousethisapproachofthinkingaboutFig.2.1Alineinthespectrumenergylevelsandthetransitionsbetweenthem.Rather,weusedifferentrulesisassocia
3、tedwithatransitionforworkingouttheappearanceofmultipletsandsoon.However,itisuse-betweentwoenergylevels.ful,especiallyforunderstandingmorecomplexexperiments,tothinkabouthowthefamiliarNMRspectraweseearerelatedtoenergylevels.Tostartwithwewilllookattheenergylevelsofjustonespinandthemmoveonquic
4、klytolookattwoandthreecoupledspins.Insuchspinsystems,astheyareknown,wewillseethatinprinciplethereareothertransitions,calledmultiplequantumtransitions,whichcantakeplace.SuchtransitionsarenotobservedinsimpleNMRspectra,butwecandetectthemindirectlyusingtwo-dimensionalexperiments;thereare,aswes
5、hallsee,importantapplicationsofsuchmultiplequantumtransitions.Finally,wewilllookatstronglycoupledspectra.Thesearespectrainwhichthesimplerulesusedtoconstructmultipletsnolongerapplybecausetheshiftdifferencesbetweenthespinshavebecomesmallcomparedtothecouplings.Themostfamiliareffectofstrongcou
6、plingisthe“roo?ng”or“tilting”ofmultiplets.Wewillseehowsuchspectracanbeanalysedinsomesimplecases.2.1Frequencyandenergy:sortingouttheunitsNMRspectroscopiststendtousesomeratherunusualunits,andsoweneedtoknowabouttheseandhowtoconvertfromonetoanotherifwearenottogetintoamuddle.ChemicalshiftsItisf
7、oundtoaverygoodapproximationthatthefrequenciesatwhichNMRabsorptions(lines)occurscalelinearlywiththemagnetic?eldstrength.So,ifthelinefromTMScomesoutononespectrometerat400MHz,doublingthemagnetic?eldwillresultinitcomingoutat800MHz.IfwewantedtoquotetheNMRfrequency
