近代分析实验原理：第十三课-光谱

十三、光谱分析

（VibrationalSpectroscopy

forMolecularAnalysis）近代分析实验原理（Introductionofmodernanalyticalmethods）12Examinebothinorganicandorganicmaterials.metallicmaterialsstronglyreflectelectromagneticwavesFouriertransforminfraredspectroscopy]傅里叶变换红外光谱学(FTIR)

Ramanmicroscopymostcommonlyusedinteractionbetweenelectromagneticradiationandnuclearvibrationsinmoleculesmuchlongerwavelengths,intheorderof10−7m31.TheoreticalBackground1.1ElectromagneticRadiationEnergy,frequency,wavelengthandwavenumberrangesofelectromagneticwaves.Frequencyrangeofmolecularvibrationsisininfraredregionclosetovisiblelight.cm4severalhundredstothousandsintheorderofabout10−2–10−1eVthephotonenergyofX-rays,whichisintheorderof10,000eV.51.2OriginofMolecularVibrationsDiatomicmodelofmolecularvibration.Thecentreofgravitydoesnotchangeduringthestretchingvibration.forceconstantequilibriumdistancedisplacementaharmonicvibration6vibrationalquantumnumber7Molecularvibrations200to4000cm−1latticevibrations20–300cm−1notassensitivetotemperaturechangesensitivetotemperaturechangecanbedistinguishedthevibrationalspectroscopycoversawavenumberrangefrom200to4000cm−1.crystallinesolidsalsogeneratelatticevibrationsinawavenumberrangeofabout20–300cm−1.Couplingbetweenlatticeandmolecularvibrationscanoccurifthemolecularvibrationslieinsuchalowwavenumberrange.81.3PrinciplesofVibrationalSpectroscopyTypicalIRabsorptionspectrumofhexanal(醛).1.3.1InfraredAbsorptiondominatesfromv=0tov=1anindividualdeepvalleyrepresentsasinglevibrationbandthatcorrespondstoacertainmolecularvibrationfrequency.91.3.2RamanScatteringElasticandinelasticscatteringofincidentlightbymolecules.Rayleigh,elasticscattering;Stokesandanti-Stokes,inelasticscattering.100123e电子基态振动能级eeRayleigh散射eeeRaman散射Stocks线Anti-Stocks线温度升高概率大！11CCl4的拉曼光谱Stockslinesanti-StockeslinesRayleighscatteringΔν/cm-1Theintensityofanti-StokesscatteringissignificantlylowerthanthatofStokesscattering.12TypicalRamanspectrumofpolycrystallinegraphite.Ramanshift131.4NormalModeofMolecularVibrationsNormalmodesofvibrationsinaCO2molecule.wecan‘decompose’anybendingmodeandexpressitintermsofv2andv4,asfordecomposingavector.thetwobendingmodesareindistinguishableintermsoffrequency,becausetherelativemovementsofnucleiinthetwobendingmodesarephysicallyidentical.Thenormalmodevibrationofmoleculeshasthefollowingcharacteristics:14NormalmodesofvibrationsinaH2Omolecule.Bendinginthehorizontalplane,willgeneraterotationofthemolecule,whichisnotconsideredasavibration.15NumberofNormalVibrationModesNatomicnucleiinamolecule3Ndegreesoffreedomthreerelatedtotranslationofamoleculealongx,yorzdirectionthreedegreesoffreedomrelatedtorotateamolecule3N−6Foralinearmolecule3N−5ClassificationofNormalVibrationModesrigidbodytherotationaroundthebondaxisismeaningless16171.5InfraredandRamanActivityInfraredActivityavibrationmodemustcausealternationofdipolemomentinamolecule.thedipolemomentCO2moleculevibrations:dipolemoment(μ)asafunctionofvibrationdisplacement(q).infraredactiveonlysomecanbedetected18RamanActivityavibrationmodemustcausepolarizabilitychangesinamolecule.determinesthedeformabilityoftheelectroncloudofamoleculebyanexternalelectricfieldCO2moleculevibrations:(b)polarizability(α)asafunctionofvibrationdisplacement(q).19NormalmodesofH2Ovibrationsandchangesinpolarizabilityellipsoids.ν1,ν2andν3

areallRamanactive.NormalmodesofCO2vibrationsandchangesinpolarizabilityellipsoids.ν1isRamanactivebutν3,ν2andν4arenot.polarizabilityellipsoid20Insummary,wecanhighlightthefeaturesofinfrared-andRaman-activevibrations:Avibrationcanbeoneofthreecases:infrared-active,Raman-active,orbothinfrared-andRaman-active;Andinmoleculesthathaveacenterofsymmetry,infrared-andRaman-activevibrationsaremutuallyexclusive.Further,itishelpfulforustoknowthatvibrationsofionicbondsarestrongininfraredspectroscopy(forexampleO-H),andvibrationsofcovalentbondsarestronginRamanspectroscopy(forexampleC=C).212.FourierTransformInfraredSpectroscopy(FTIR)obtainaninfraredspectruminawholerangeofwavenumberssimultaneouslymuchhighersignal-to-noiseratio2.1WorkingPrinciplesOpticaldiagramofaMichelsoninterferometerinFTIRTheIRradiationfromtheFTIRsourceiscomposedofnumerouswavelengths.22Interferograms:(a)sumof(b)and(c);(b)interferogramoflightwithwavelength3λ;and(c)interferogramoflightwithwavelengthλ.232425Plotsof:(a)aninterferogram;and(b)aFouriertransformfromaninterferogramtoanIRspectrum.262.2InstrumentationInfraredLightSourceheatingsolidmaterialsNernstglowerGlobarBeam-Splitterreflectonehalfportionofinfraredlighttothemovingmirrorwhiletransmittingtherestinfraredtoafixedmirror.thinlayerofgermanium(Ge)betweentwopiecesofpotassiumbromide(KBr)oxidesofrare-earthelementssiliconcarbideHighlytemperaturesensitiveoperatedatthetemperaturewherethemaximumenergyofradiationisneartheshortwavelengthlimitoftheIRspectrumsubstratematerialsplitinfraredlightlowhumidityabsorbwatervaportransparenttoinfraredlightgoodmechanicalstrength27InfraredDetectorpyroelectriccrystaldeuteratedtriglycinesulfate(DTGS)InfraredradiationtemperaturechangeinDTGSChangesdielectricconstantCapacitancechangevoltagechangesemiconductordetector氘化三甘氨硫mercurycadmiumtelluride(MCT)碲镉汞10timesmoresensitiveneedstobecooledsaturateseasilyconvertinfraredlightsignalstoelectricsignalssimpleandinexpensivebutlesssensitivecausestheelectronstomigratefromthevalencebandtotheconductionbandCommonlytoliquidnitrogentemperature(−196◦C),282.3FourierTransformInfraredSpectraSinglebeamspectrumincludesbothspectrafromthesampleandbackground.polystyrene聚苯乙烯29(a)SinglebeamFTIRspectrumofbackground:asamplesinglebeamFTIRspectrumofpolystyrene(vibrationalbandsofpolystyrenearesuperimposedonbackgroundspectrum);(b)aplotofrawdetectorresponseversuswavenumberwithoutsample;and(c)finalFTIRspectrumofpolystyrenethatonlycontainsthevibrationbandsfromthepolystyrenesample(absorptionintensityisexpressedasthetransmittance).聚苯乙烯backgroundfromwatervaporandcarbondioxidetransmittancespectrum30FTIRspectrumwheretheabsorptionintensityisexpressedasabsorbance.Inthisspectrumtheabsorbancescaleisnormalizedtoarange0–1.Forquantitativeanalysis,anabsorbancespectrumshouldbeusedbecausethepeaksofatransmittancespectrumarenotlinearlyproportionaltoconcentration.31ExaminationTechniquesTransmittancehighsignal-to-noiseratiossuitableforsamplesinanyphasesDisadvantage:thethicknesslimitationmostcommonlySolidSamplePreparationthinfilmorpowdermakingKBrpelletsandmakingmullsLiquidandGasSamplePreparationdonotneedmuchpreparation32ReflectanceReflectanceexaminationtechniquesrefertomethodsforobtaininganinfraredspectrumbyreflectingIRradiationfromasolidorliquidsample.bulkandcoatingsampleswithoutdestructivepreparationmainadvantageAreflectancespectrumcanonlycontaintheinfraredsignalsfromthetop1to10μmofasolidsamplesurface.theinstrumentationmorecomplicatedandexpensive.332.4FourierTransformInfraredMicrospectroscopyOpticalpathsofFTIRmicroscopewithIRradiation:(a)transmittance;and(b)reflectance.M,mirror;C,Cassegrainlens.TheresolutionisprimarilydeterminedbythesizeofthefocusedIRbeamandprecisionofmotorizedstage.TheFTIRmicroscopehasanopticalsystemthatcaneasilyswitchbetweenvisiblelightobservationandinfraredlightspectroscopy.342.5ApplicationsMicrographofisolatedparticulatecontamination(AM)andcottonfiber(C).IRspectrumoftheisolatedparticle(lower)andIRspectrumofpolystyrene(upper).聚苯乙烯353.RamanMicroscopyOpticaldiagramofaRamanmicroscopecollectingaspectrumateachwavenumberseparatelycontinuous-wavelaserinthevisiblelightrangeorclosetotherangeblockthelaserlightenteringthedetectorsystem36LaserSourcegascontinuous-wavelasers（Ar+,Kr+AndHe–Ne）arangeofwavelengthsfilterPrefiltersSpectralresponseofanidealnotchprefilterinmicro-Ramanspectroscopytoremovetheelasticallyscatteredlightwithwavenumber.Thescatteredlightfromthemicroscopemustbepassedthroughspecialfiltersbeforereachingthespectralanalyzerinordertoremoveelasticallyscatteredlight.37DiffractionGratingPrincipleofopticalgratingdiffractionDetectorconvertsphotonsignalstoelectricsignalscharged-coupleddevice(CCD)multi-channelOnediffractiongratingcancoverarangeof1000wavenumbers.changeitsanglespectralresolutionof1cm−1383.1FluorescenceProblemColoredsamplesorimpuritiesinpolymersamplesmayabsorblaserradiationandre-emititasfluorescence.Theintensityoffluorescencecanbeasmuchas104timeshigherthanthatofRamanscatteredlight.1.Irradiatethesamplewithhigh-powerlaserbeamsforaprolongedtimetobleachouttheimpurityfluorescence;2.Changethewavelengthoflaserexcitationtoalongerwavelength(nearinfrared).Thechanceoffluorescenceisreducedbecausetheexcitationenergyofthelaserbeamislower;(butleadtotheintensityreduction)3.UseapulsedlasersourcetodiscriminateagainstfluorescencebecausethelifetimeofRamanscattering(10−12–10−10s)ismuchshorterthanthatoffluorescence(10−7–10−9s).Thus,anelectrongatecanbeusedtopreferentiallymeasuretheRama