Christy L Hayne-UV-vis Absorption (Extinction) Spectroscopy.pptx

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1、UV-vis Absorption(Extinction)SpectroscopySingle-Beam or Double-BeamFixed l or DispersiveCommon:Source Tungsten Halogen Lamp(360-2000 nm)Sample Liquid In CuvetteDispersion Spectrograph w/Diffraction GratingDetector CCDBeers Law:A=ebcAssumptionsIngle and Crouch,Ingle and Crouch,Spectrochemical Analysi

2、sSpectrochemical AnalysisApparent Deviations from Beers LawNon-Zero InterceptImproper blank Improper blank measurement or measurement or correction.correction.Instrumental drift.Instrumental drift.Skoog,Hollar,Nieman,Skoog,Hollar,Nieman,Principles of Instrumental AnalysisPrinciples of Instrumental A

3、nalysis,Saunders,Saunders College Publishing,Philadelphia,1998.College Publishing,Philadelphia,1998.Apparent Deviations from Beers LawNon-Linear Calibration PlotChemical Equilibrium Chemical Equilibrium if multiple chemical forms of analyte if multiple chemical forms of analyte exist and only one ab

4、sorbsexist and only one absorbsOther Chemical Effects Other Chemical Effects solute/solvent interactions,solute/solvent interactions,solute/solute interactions,H bonding at high concentrationssolute/solute interactions,H bonding at high concentrationsUsing Polychromatic Radiation Using Polychromatic

5、 Radiation non-optimum wavelengths non-optimum wavelengths are still transmitted and detectedare still transmitted and detectedStray Light Stray Light causes measured transmittance to be larger causes measured transmittance to be larger than it should bethan it should beSkoog,Hollar,Nieman,Skoog,Hol

6、lar,Nieman,Principles of Instrumental AnalysisPrinciples of Instrumental Analysis,Saunders,Saunders College Publishing,Philadelphia,1998.College Publishing,Philadelphia,1998.Absorbed/Emitted ColorsPretsch/Buhlmann/Affolter/Badertscher,Pretsch/Buhlmann/Affolter/Badertscher,Structure Determination of

7、Organic CompoundsStructure Determination of Organic CompoundsChromophoresOften transitions are localized in specific bonds or Often transitions are localized in specific bonds or functional groups within a molecule.functional groups within a molecule.Group will have a characteristic Group will have

8、a characteristic l lmaxmax and and e e.Molecular structure or environment can influence Molecular structure or environment can influence l lmaxmax and and e e.Shift to longer Shift to longer l l bathochromic(red)shift.bathochromic(red)shift.Shift to shorter Shift to shorter l l hypsochromic(blue)shi

9、ft.hypsochromic(blue)shift.Increase in Increase in e e hyperchromic effect.hyperchromic effect.Decrease in Decrease in e e hypochromic effect.hypochromic effect.What effect does conjugation usually have?What effect does conjugation usually have?hyperchromic effect/bathochromic shiftCharacteristic El

10、ectronic TransitionsPretsch/Buhlmann/Affolter/Badertscher,Pretsch/Buhlmann/Affolter/Badertscher,Structure Determination of Organic CompoundsStructure Determination of Organic CompoundsL mol-1 cm-1Characteristic Electronic TransitionsPretsch/Buhlmann/Affolter/Badertscher,Pretsch/Buhlmann/Affolter/Bad

11、ertscher,Structure Determination of Organic CompoundsStructure Determination of Organic CompoundsL mol-1 cm-1AuxophoreDoes not absorbDoes not absorbInduces a bathochromic shift and hyperchromic effect when Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore(e.g.-OH,

12、-Br,-NHconjugated to a chromophore(e.g.-OH,-Br,-NH2 2).).Solvent EffectsHypsochromic shiftHypsochromic shift in n in n *transitions as solvent polarity*transitions as solvent polarity increases.Solvation stabilizes the nonbonding pair.increases.Solvation stabilizes the nonbonding pair.Bathochromic s

13、hiftBathochromic shift in in *transitions as solvent polarity*transitions as solvent polarity increases.Solvation stabilizes increases.Solvation stabilizes *,which is often more polar*,which is often more polar than than .Conjugated AlkenesWoodward-FieserRulesPretsch/Buhlmann/Affolter/Badertscher,Pr

14、etsch/Buhlmann/Affolter/Badertscher,Structure Determination of Organic CompoundsStructure Determination of Organic Compounds1,3-butadienebondingantibonding*Conjugated DienesUV Absorption of Conjugated AlkenesIncreasing conjugation gives:longer wavelength absorption more intense absorptione units=L m

15、ole-1 cm-1b b-Carotene11 double bondslmax =460 nm(e=139,000)Systems with More than 4 Double Bondslmax(nm)=114+5M+n(48.0-1.7n)16.5Rendo 10Rexon=number of conjugated double bondsM=number of alkyl or alkyl like substituents on the conjugated systemRendo=number of rings with endocyclic double bonds in theconjugated systemRexo=number of rings with exocyclic double bondsFieser-Kuhn RulesAre you getting the concept?Calculate the absorption maximum for lycopene: