Ch 12 - Infrared Spectroscopy and Mass Spectrometry

Ch 12 - Infrared Spectroscopy and Mass Spectrometry - I...

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12 fixed mIrror Infrared Spectroscopy and Mass Spectrometry 12-1 Introduction 508 saMple laseR calib±atiOn BEAM d²Tec³O´ One of the mosT importanT Tasks of oRganIc chemIstRy Is the deteRminatIon of oRgan±c sTrucTuRes. When an inTeresting compound is isolaTed from a naTural source, iTs struc- TuRe musT be completely deTeRmined befoRe a synThesIs is begun² Whenever we run a reaction³ we must determIne whetheR The product has The desiRed stRucture´ µhe stRuc¶ Ture of an unwanted pRoduct musT be known so The Reaction condiTions can be aLteRed To favoR the desiRed product· ¸n many cases, a compound can be idenTiFed by chemical means² We ¹ºnd the molecuLar formula by analyzing the elemenTaL composiTion and deteRmInIng the molecular weighT´ ¸f the compound has been chaRacTeRized befoRe, we can compare Its physicaL propertIes (meLTIng poInt, boiLIng poinT» eTc²) wIth The published values· Chemical TesTs can suggest The functionaL gRoups and naRRow the range of possIbLe sTructures before The physIcal pRopeRTIes aRe used to make an IdenTiFcation. µhese procedures aRe noT su¼cIenT³ howeveR³ foR complex compounds ThaT have neveR been synThesIzed and chaRacTeRIzed² µhey aRe aLso impRactIcaL with compounds thaT aRe dI½¾culT to obtaIn, because a ReLatIvely laRge sample Is Required to compLete the elementaL anaLysIs and all the funcTional gRoup TesTs´ We need anaLyTIcaL technIques that work with tiny samples and That do noT desTroy The sampLe² SpecTroscopIc Techniques often meet These RequIRements· Absorption spec- troscopy is the measuRemenT of The amount of LIght absoRbed by a compound as a func¶ Tion of The wavelength of LighT. ¿n geneRaL, a sample is IrRadiated by a LighT souRce, and The amounT of light TRansmitted at vaRious wavelengThs is measuRed by a detectoR and plotted on a gRaph· Unlike chemIcal Tests, most spectroscopic techniques aRe nondes- tructive; ThaT is, The sampLe Is noT destroyed´ Many dIÀerent kInds of spectRa can be measuRed wIth lItTLe oR no Loss of sample´ ¸n ThIs book, we coveR fouR spectroscopIc or reLaTeD Techniques ThaT serve as poweÁful Tools for stRucTuRe deTeRminatIon in organIc chemistRy: Infrared (IR) spectroscopy, covered In This chapTeR³ obseRves the vibraTIons of bonds and pRovIdes evidence of The functIonal groups pResent´ Mass spectrometry (MS)± also coveRed in this chapteR bombaRds moLecuLes wIth eLecTRons and breaks Them aparT. AnaLysis of The masses of the fRagments gIves the moleculaR weIghT³ possIbly the moleculaR formula³ anD cLues to The stRucture and funcTionaL groups´ Ãess Than a miLligRam of sampLe Is destRoyed in This analysis² Nuclear magnetic resonance (NMR) spectroscopy, coveRed In ChapteR 13, obseRves The chemical environments of The hydRogen atoms (oR the carbon aToms) and pRovIdes evIdence foR the sTructure of The alkyl gRoups and clues To The funcTionaL groups.
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12-2 The Electromagnetic SpectruM 509 Ultraviolet (UV) spectroscopy, covered in Chapter 15, observes eLectronic tran- sitions and provides inforMation on tHe e±ectronic bonding in tHe saMpLe.
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Ch 12 - Infrared Spectroscopy and Mass Spectrometry - I...

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