有机化学中的光谱学第6版英语红外部分.docx

有机化学中的光谱学第6版英语红外部分.docx

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有机化学中的光谱学第6版英语红外部分

Infraredspectra

Introduction

Theinfraredspectraoforganiccompoundsareassociatedwithtransitionsbetweenvibrationalenergylevel.MolecularvibrationsmaybedetectedandmeasuredeitherinaninfraredspectrumorindirectlyinaRamanspectrum.Themostusefulvibrations,fromthepointofviewoftheorganicchemist,occurinthenarrowerrangeof2.5-16μm.Thepositionofanabsorptionbandinthespectrummaybeexpressedinmicrons,butstandardpracticeusesafrequencyscaleintheformofwavenumbers,whicharethereciprocalsofthewavelength,cm-1.Theusefulrangeoftheinfraredforanorganicchemistisbetween4000cm-1atthehigh-frequencyendand625cm-1atthelowfrequencyend.

Manyfunctiongroupshavevibrationfrequencies,characteristicofthatfunctionalgroup,withinwell-definedregionsoftherange;thesearesummarisedinCharts1-4attheendofthischapter,withmoredetailinthetablesofdatathatfollow.becausemanyfunctionalgroupscanbeidentifiedbytheircharacteristicvibrationfrequencies,theinfraredspectrumisthesimplest,mostrapid,andoftenmostmostreliablemeansforidentifyingthefunctionalgroups.

Equation2.1,whichisderivedfromthemodelofamassmvibratingatfrequencyvontheendofafixedspring,isusefulinunderstandingtherangeofvaluesofthevibrationalfrequenciesofvariouskindsofbonds.

Wherekisameasureofthestrengthofspring.However,inchemicalbonds,oneendofthe“spring”（bond）isnotfixed,butrathertherearetwomass（m1andm2）involvedandeachisabletomove.themofEq.2.1ismowdeterminedbytherelationshipinEq.2.2

Ifoneofthemasses（say,m1）isinfinitelylarge1/m1isthenzero,andtherelevantmassmforEq.2.1issimplythatofm2--makingitanalogoustothecasewhereoneendofthe“spring”isfixed.

Simplesubstitutionsofmassesintheseequationsallowustounderstandthatwithotherthingsbeingequal:

（1）C-HbondswillhavehigherstretchingfrequenciesthanC-Cbonds,whichinturnarelikelytobehigherthanC-halogenbonds;

（2）O-HbondswillhavehigherstretchingfrequenciesthanO-Dbonds;and（3）,sincekincreaseswithincreasingbondorder,therelativestretchingfrequenciesofcarbon-carbonbondslieintheorder:

Thesegeneralisationsareuseful,andEqs.2.1and2.2allowanincreasedunderstandingoftheempiricaldatathataresubsequentlypresentedinthischapter.Youmayoftenbeabletoextendtheuseofthemodelinawaythatwillmakeiteasiertounderstandthetrendsthatareobserved.However,becauseoftheothervariablesthatinfluencevibrationalfrequencies,theequationsshouldbetakenasnomorethanafrequentlyusefulguide.

2.2Preparetionofsamplesandexaminationinaninfraredspectrometer

Olderspectrometersusedasourceofinfraredlightwhichhadbeensplitintotwobeamsofequalintensity.Onlyoneofthesewaspassedthroughthesample,andthedifferenceinintensitiesofthetwobeamswasthenplottedasafunctionofwavenumber.Usingthisoldtechnology,ascantypicallytookabout10minutes.MostspectrometersinusetodayuseaFouriertransformmethod,andthespectraarecalledFouriertransforminfrared（FTIR）spectra.Asourceofinfraredlight,emittingradiationthroughoutthewholefrequencyrangeoftheinstrument,typically4600-400cm-1,isagaindividedintotwobeamsofequalintensity.Eitheronebeamispassedthroughthesample,orbotharepassed,butonebeamismadetotraversealongerpaththantheother.Recombinationofthetwobeamsproducesaninterferencepatternthatisthesunofalltheinterferencepatternscreatedbyeachwavelengthinthebeam.Bysystematicallychangingthedifferenceintothepaths,theinterferencepatternschangetoproduceadetectedsignalvaryingwithopticalpathdifference,asmodifiedbytheselectiveabsorptionbythesampleofsomefrequencies.Thispatternisknownastheinterferogram,andlooksnothinglikeaspectrum.HoweverFouriertransformationoftheinterferogram,usingacomputerbuiltintotheinstrument,convertsitintoaplotofabsorptionagainstwavenumberjustlikethatfromtheoldermethod.ThereareseveraladvantagestoFTIRovertheoldmethod,andfewwholespectrumismeasuredinatmostafewseconds.Becauseitisnotnecessarytoscaneachwavenumbersuccessively,thewholespectrumismeasuredinatmostafewseconds.Becauseitisnotdependentuponaslitandaprismorgrating,highresolutioninFTIRiseasiertoobtainwithoutsacrificingsensitivity.FTIRisespeciallyusefulforexaminingsmallsamples（severalscanscanbeaddedtogether）andfortakingthespectrumofcompoundsproducedonlyforashortperiodintheoutflowofachromatograph.Finally,thedigitalforminwhichthedataarehandledinthecomputerallowsforadjustmentandrefinement.Forexample,bysubtractingthebackgroundabsorptionofthemediuminwhichthespectrumwastaken,orbysubtractingthespectrumofaknownimpurityfromthatofaknownimpurityfromthatofamixturetorevealthespectrumofthepurecomponent.However,thewayinwhichinfraredspectraaretakendoesnotaffecttheirappearance.TheolderspectraandFTIRspectralookverysimilar,andolderspectraintheliteraturearestillvaluableforcomparison.Compoundsmaybeexaminedinthevapourphase,aspureliquids,insolution,andinthesolidstate.

Inthevapourphase.Thevapourisintroducedintoacell,usuallyabout10cmlong,whichcanthenbeplaceddirectlyinthepathofoneoftheinfraredbeams.Theendwallsofthecellareusuallymadeofsodiumchloride,whichistransparenttoinfraredintheusualrange.Mostorganiccompoundshavetoolowavapourpressureforthisphasetobeuseful.

Asaliquid.Adropoftheliquidissqueezedbetweenflatplatesofsodiumchloride（transparentthroughthe4000-625cm-1region）.Thisisthesimplestofallproduces.Alternatively,ifthesampleoftheliquidisnotsuitablefordispensingasadrop,asolutioninavolatileanddrysolventmaybedepositeddirectlyontothesurfaceofasodiumchlorideplate,andthesolventallowedtoevaporateinadryatmospheretoleaveathinfilm.

Insolution.Thecompoundisdissolvedtogive,typically,a1-5%solutionincarbontetrachlorideor,foritsbettersolventproperties,alcohol-freechloroform.Thissolutionisintroducedintoacell,0.1-1mmthick,madeofsodiumchloride.Asecondcellofequalthickness,butcontainingpuresolvent,isplacedinthepathoftheotherbeamofthespectormeterinorderthatsolventabsorptionsshouldbebalanced.Spectratakeninsuchdilutesolutionsinnon-polarsolventsaregenerallythemostdesirable,becausetheyarenormallybetterresolvedthanspectratakenonsolids,andalsobecauseintermolecularforces,whichareespeciallystronginthecrystallinestate,areminimised.Ontheotherhand,manycompoundsarenotsolubleinnon-polarsolvents,andallsolventsabsorbintheinfrared;whenthesolventabsorptionexceedsabout65%oftheincidentlight,usefulspectracannotbeobtainedbecauseinsufficientlightistransmittedtoworkthedetectionmechanismefficiently.Carbontetrachlorideandchloroform,fortunately,absorbover65%oftheincidentlightonlyinthoseregion（Fig.2.1）whichareoflittleinterestindiagnosis.Othersolvents,ofcourse,maybeusedbuttheareasofusefulnessineachcaseshouldbecheckedbeforehand,takingaccountofthesizeofthecellbeingused.Inrarecasesaqueoussolventsareuseful;specialcalciumfluoridecellsarethenused.

Inthesolidstate.About1mgofasolidisfinelygroundinasmallagatemortarwithadropofaliquidhydrocarbon（NujolKaydol）or，ifC-Hvibrationaretobeexamined，withhexachlorobutadiene.Themullisthenpressedbetweenhighlypolishedflatplatesofsodiumchloride.Alternatively，thesolid，oftenmuchlessthan1mg，isgroundwith10-100timesitsbulkofpurepotassiumbromideandthemixturepressedintoadiscusingamouldandahydraulicpress.TheuseofKBreliminatestheproblem（usuallynottroublesome）ofbandsfromthemullingagentandtends，onthewhole，togiveratheralmostalwaysappears（seeFig.2.7）.Solidsmayalsobedeposited，eitherfromameltor，aswithliquidsdescribedabove，byevaporationfromasolutiondirectlyontothesurfaceofasodiumchlorideplate，withasacrifice，usuallysmall，fromscatteringoffacrystallinesurface.Becauseofintermolecularinteractions，bandpositionsinsolidstatespectraareoffendifferentfromthoseofthecorrespondingsolutionspectra.Thisisparticularlytrueofthosefunctionalgroupswhichtakepartinhydrogenbonding.Ontheotherhand，thenumberofresolvelinesisoftengreaterinsolidstatespectra，sothatcomparisonofthespectraof，forexample，syntheticandnaturalsamplesinordertodetermineidentifyisbestdoneinthesolidstate.Thisisonlytrue，ofcourse，whenthesamecrystallinemodificationisinuse;racemic，syntheticmaterial，forexample，shouldbecomparedwithenantiomericallypure，naturematerialinsolution.

2.3ExaminationinaRamanspectrometer

Ramanspectraaregenerallytakenoninstrumentsusinglasersources，andthequantityofmaterialneededisnowoftheorderofafewmg.Aliquidoraconcentratedsolutionisirradiatedwithmonochromaticlight，andthescatteredlightisexaminedbyaspectometerusingphotoelectricdetection.Mostofthescatteredlightconsistsoftheparentlineproducedbyabsorptionandre-emission.Muchweakerlines，whichconstitutetheRamanspectrum，occuratlowerandhigherenergyandarecausedbyabsorptionandre-emissionoflightcoupledwithvibrationalexcitationordecay，respectively.ThedifferenceinfrequencybetweentheparentlineandtheRamanlineisthefrequencyofth