InfraredSpectroscopyTransmissionVsATR红外光谱透射比.pptx
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1、OverviewIR spectroscopy theoryTransmission vs.Attenuated Total Reflectance(ATR)techniquesBased on study by:Based on study by:Koulis,Cynthia,et.al.Comparison of Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Transmission and Internal Reflection Infrared Spectra of Co
2、caine.Journal of Forensic Spectra of Cocaine.Journal of Forensic Sciences,2001.Sciences,2001.What is Spectroscopy?SpectroscopySpectroscopy-study of the interaction of atoms -study of the interaction of atoms and molecules with electromagnetic radiationand molecules with electromagnetic radiationProb
3、e the world at an atomic and molecular Probe the world at an atomic and molecular levellevelqualitative analysisqualitative analysis Bond lengthsBond lengths Bond Force ConstantsBond Force Constants Molecular structuresMolecular structuresquantitative analysis quantitative analysis Quantum StatesAbs
4、orption of electromagnetic radiation Absorption of electromagnetic radiation excitation to higher energy statesexcitation to higher energy stateshv=|Ehv=|E2 2 E E1 1|Electronic Vibrational RotationalElectronic Vibrational RotationalInfrared RadiationFrequencies-12800 to 10 cmFrequencies-12800 to 10
5、cm-1 -1 (cm(cm-1-1=wavenumbers)=wavenumbers)Divided into three Regions Divided into three Regions Near:12800 to 4000 cmNear:12800 to 4000 cm-1-1 Middle:4000 to 200 cmMiddle:4000 to 200 cm-1-1 Far:200 to 10 cmFar:200 to 10 cm-1-1Radiation of most use for spectroscopy falls in the mid-infrared region
6、between 4000 to 670 cm-150Infrared AbsorptionAbsorption of IR radiation Absorption of IR radiation excitation of excitation of vibrational and rotational energy levelsvibrational and rotational energy levelsnot sufficient energy for electronic excitationnot sufficient energy for electronic excitatio
7、nVibrational energy states are of greatest Vibrational energy states are of greatest importance for IR spectroscopyimportance for IR spectroscopyDynamic Dipole MomentAbsorption of IR radiation is only observed in Absorption of IR radiation is only observed in molecules with a dynamic dipole momentmo
8、lecules with a dynamic dipole momentAn induced dynamic dipole moment is An induced dynamic dipole moment is observed in polar moleculesobserved in polar molecules Magnitude of dipole moment depends on overlap of Magnitude of dipole moment depends on overlap of electron densities in moleculeelectron
9、densities in molecule Vibration results in change in internuclear distance Vibration results in change in internuclear distance and therefore fluctuations in magnitude of dipole and therefore fluctuations in magnitude of dipole Homonuclear diatomic molecules(e.g.HHomonuclear diatomic molecules(e.g.H
10、2 2,N,N2 2,OO2 2)dont absorb IR radiation,b/c of no dynamic)dont absorb IR radiation,b/c of no dynamic dipole moment dipole moment IR absorptionIF vibrational frequency of molecule=frequency IF vibrational frequency of molecule=frequency of radiation THEN energy is absorbedof radiation THEN energy i
11、s absorbed Results promotion to higher energy level and increase Results promotion to higher energy level and increase in amplitude of molecular vibrationin amplitude of molecular vibrationMolecular VibrationsMolecular Vibrational and Bending modesSingle absorption peak appears for each Single absor
12、ption peak appears for each mode with a dynamic dipole momentmode with a dynamic dipole momentInterpreting the SpectraPeaks are identified by:-Intensity-Shape-PositionCorrelation charts are used to aid in identifying functional group absorption bandsDetermining peak positionUsing the model of a simp
13、le harmonic oscillator the frequency vibration can be predicted Wavenumber=(2c)-1(k/)Reduced mass()Force constant(k)C-C C=C CC Qualitative AnalysisEnergy spectrum for each Energy spectrum for each chemical is uniquechemical is unique“Fingerprint”“Fingerprint”Functional groups can be Functional group
14、s can be identified identified Uncertainty lies in Uncertainty lies in overlapping of group overlapping of group frequenciesfrequenciesCatalogs of IR spectra of Catalogs of IR spectra of known compounds used known compounds used for final identificationfor final identificationMS and NMR for further
15、MS and NMR for further confirmationconfirmationInstrumental Setup:Transmission TechniqueIncident radiation is passed through sampleIncident radiation is passed through sampleRadiation of the right frequency will be absorbed Radiation of the right frequency will be absorbed while the rest is transmit
16、tedwhile the rest is transmittedDetector placed directly behind sample measures Detector placed directly behind sample measures the resulting radiationthe resulting radiationAbsorbance=-log TransmittanceAbsorbance=-log TransmittanceInstrumental Setup:Attenuated Total Reflectance(ATR)TechniqueIR radi
17、ation passes through IR radiation passes through(IRE-internal reflection(IRE-internal reflection element)crystal and hits element)crystal and hits sample at a 45 degree anglesample at a 45 degree angle IRE made of high refractive IRE made of high refractive index material(zinc selenide,index materia
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- InfraredSpectroscopyTransmissionVsATR 红外 光谱 透射
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