essential practical nmr for organic chemistry

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1、P1:JYSc01JWST025-RichardsSeptember27,201017:16Printer:Yettocome1GettingStarted1.1TheTechniqueThisbookisnotreallyintendedtogiveanin-deptheducationinallaspectsoftheNMReffect(therearenumerousexcellenttextsifyouwantmoreinformation)butwewilltrytodealwithsomeofthemorep

2、ertinentones.ThefirstthingtounderstandaboutNMRisjusthowinsensitiveitiscomparedwithmanyotheranalyticaltechniques.ThisisbecauseoftheoriginoftheNMRsignalitself.TheNMRsignalarisesfromaquantummechanicalpropertyofnucleicalled‘spin’.Inthetexthere,wewillusetheexampleofthe

3、hydrogennucleus(proton)asthisisthenucleusthatwewillbedealingwithmostly.Protonshavea‘spinquantumnumber’of1/.Inthiscase,whentheyareplaced2inamagneticfield,therearetwopossiblespinstatesthatthenucleuscanadoptandthereisanenergydifferencebetweenthem(Figure1.1).Theenergy

4、differencebetweentheselevelsisverysmall,whichmeansthatthepopulationdifferenceisalsosmall.TheNMRsignalarisesfromthispopulationdifferenceandhencethesignalisalsoverysmall.Thereareseveralfactorswhichinfluencethepopulationdifferenceandtheseincludethenatureofthenucleus(

5、its‘gyromagneticratio’)andthestrengthofthemagneticfieldthattheyareplacedin.Theequationthatrelatesthesefactors(andtheonlyoneinthisbook)isshownhere:γ=GyromagneticratioγhB
E=h=Planck’sconstant2πB=MagneticfieldstrengthBecausethesensitivityofthetechniquegoesupwithmagnet

6、icfield,therehasbeenadrivetoincreasethestrengthofthemagnetstoimprovesensitivity.Unfortunately,thisimprovementhasbeenlinearsincethefirstNMRmagnets(withafewkinkshereandthere).Thismeansthatinpercentageterms,thebenefitshavebecomesmallerasdevelopmenthascontinued.Butsensi

7、tivityhasnotbeentheonlyfactordrivingthesearchformorepowerfulmagnets.Youalsobenefitfromstretchingyourspectrumandreducingoverlapofsignalswhenyougotohigherfields.Also,whenyouexamineallthefactorsinvolvedinsignaltonoise,thedependenceonfieldistoEssentialPracticalNMRforOrg

8、anicChemistryS.A.RichardsandJ.C.Hollerton©2011JohnWiley&Sons,Ltd.ISBN:978-0-470-71092-0P1:JYSc01JWST025-RichardsSeptember27,201017:16Printer:Yettocome2Essentia