background to c-13 nmr and simple interpretation of spectra背景c-13 nmr光谱和简单的解释

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1、ThebackgroundtoC-13NMRspectroscopyNuclearmagneticresonanceisconcernedwiththemagneticpropertiesofcertainnuclei.Wewillcurrentlyfocusonthemagneticbehaviorofcarbon-13nuclei.Carbon-13nucleiaslittlemagnetsAbout1%ofallcarbonatomsaretheC-13isotope;therest(apartfromtinyamountsoft

2、heradioactiveC-14)isC-12.C-13NMRreliesonthemagneticpropertiesoftheC-13nuclei.Carbon-13nucleifallintoaclassknownas"spin½"nuclei(forreasonswhichdon'treallyneedtoconcernusatthisstage).TheeffectofthisisthataC-13nucleuscanbehaveasalittlemagnet.C-12nucleidon'thavethisproperty.

3、Ifyouhaveacompassneedle,itnormallylinesupwiththeEarth'smagneticfieldwiththenorth-seekingendpointingnorth.Provideditisn'tsealedinsomesortofcontainer,youcouldtwisttheneedlearoundwithyourfingerssothatitpointedsouth-liningitupopposedtotheEarth'smagneticfield.Itisveryunstable

4、opposedtotheEarth'sfield,andassoonasyouletitgoagain,itwillflipbacktoitsmorestablestate.BecauseaC-13nucleusbehaveslikealittlemagnet,itmeansthatitcanalsobealignedwithanexternalmagneticfieldoropposedtoit.Again,thealignmentwhereitisopposedtothefieldislessstable(atahigherener

5、gy).Itispossibletomakeitflipfromthemorestablealignmenttothelessstableonebysupplyingexactlytherightamountofenergy.Page

6、10Theenergyneededtomakethisflipdependsonthestrengthoftheexternalmagneticfieldused,butisusuallyintherangeofenergiesfoundinradiowaves-atfrequenciesofabout2

7、5-100MHz.(TheHawaiianMusicStationcanbefoundat105.1MHz!)It'spossibletodetectthisinteractionbetweentheradiowavesofjusttherightfrequencyandthecarbon-13nucleusasitflipsfromoneorientationtotheotherasapeakonagraph.Thisflippingofthecarbon-13nucleusfromonemagneticalignmenttotheo

8、therbytheradiowavesisknownastheresonancecondition.Theimportanceofthecarbon'senvironmentWhatwe'vesaidsofarwouldapplytoanisolatedcarbon-13nucleus,butrealcarbonatomsinrealbondshaveotherthingsaroundthem-especiallyelectrons.Theeffectoftheelectronsistocutdownthesizeoftheextern

9、almagneticfieldfeltbythecarbon-13nucleus.Supposeyouwereusingaradiofrequencyof25MHz,andyouadjustedthesiz