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1、P1:JYSc01JWST025-RichardsSeptember27,201017:16Printer:Yettocome1GettingStarted1.1TheTechniqueThisbookisnotreallyintendedtogiveanin-deptheducationinallaspectsoftheNMReffect(therearenumerousexcellenttextsifyouwantmoreinformation)butwewilltrytodealwithsom
2、eofthemorepertinentones.The?rstthingtounderstandaboutNMRisjusthowinsensitiveitiscomparedwithmanyotheranalyticaltechniques.ThisisbecauseoftheoriginoftheNMRsignalitself.TheNMRsignalarisesfromaquantummechanicalpropertyofnucleicalled‘spin’.Inthetexthere,we
3、willusetheexampleofthehydrogennucleus(proton)asthisisthenucleusthatwewillbedealingwithmostly.Protonshavea‘spinquantumnumber’of1/.Inthiscase,whentheyareplaced2inamagnetic?eld,therearetwopossiblespinstatesthatthenucleuscanadoptandthereisanenergydifferenc
4、ebetweenthem(Figure1.1).Theenergydifferencebetweentheselevelsisverysmall,whichmeansthatthepopulationdifferenceisalsosmall.TheNMRsignalarisesfromthispopulationdifferenceandhencethesignalisalsoverysmall.Thereareseveralfactorswhichin?uencethepopulationdif
5、ferenceandtheseincludethenatureofthenucleus(its‘gyromagneticratio’)andthestrengthofthemagnetic?eldthattheyareplacedin.Theequationthatrelatesthesefactors(andtheonlyoneinthisbook)isshownhere:γ=GyromagneticratioγhBE=h=Planck’sconstant2πB=Magnetic?eldstre
6、ngthBecausethesensitivityofthetechniquegoesupwithmagnetic?eld,therehasbeenadrivetoincreasethestrengthofthemagnetstoimprovesensitivity.Unfortunately,thisimprovementhasbeenlinearsincethe?rstNMRmagnets(withafewkinkshereandthere).Thismeansthatinpercentaget
7、erms,thebene?tshavebecomesmallerasdevelopmenthascontinued.Butsensitivityhasnotbeentheonlyfactordrivingthesearchformorepowerfulmagnets.Youalsobene?tfromstretchingyourspectrumandreducingoverlapofsignalswhenyougotohigher?elds.Also,whenyouexamineallthefact
8、orsinvolvedinsignaltonoise,thedependenceon?eldistoEssentialPracticalNMRforOrganicChemistryS.A.RichardsandJ.C.Hollerton?2011JohnWiley&Sons,Ltd.ISBN:978-0-470-71092-0P1:JYSc01JWST025-RichardsSeptember27,201017:16Printer:Yettocome2Essentia
