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Molecular Spectroscopy Types of transitions:

Molecular Spectroscopy Types of transitions:. 1) Electronic (UV-Vis-Near IR) 2) Vibrational (IR) 3) Rotational (microwave). Electronic Absorption Spectra. p → p *. Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.

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Molecular Spectroscopy Types of transitions:

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  1. Molecular SpectroscopyTypes of transitions: 1) Electronic (UV-Vis-Near IR)2) Vibrational (IR)3) Rotational (microwave)

  2. Electronic Absorption Spectra p→p* Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.

  3. Atomic Orbitals (AO) Combine to give Molecular Orbitals (MO) Bond Order = (# bonding e- - # antibonding e-) / 2 Total Spin Quantum Number (S) S = Ssi Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.

  4. Are you getting the concept? Calculate the bond order for CO and predict its stability. Assuming that the same set of molecular orbitals applies, suggest a diatomic that would be unstable.

  5. Are you getting the concept?

  6. Polyatomic Electronic Transitions s orbitals – s p orbitals – p non-bonding orbitals - n Ingle and Crouch, Spectrochemical Analysis

  7. * Common Chromophores: Alkenes * E lmax ~165 nm  ~15,000 

  8. * * n 170 nm 290 nm  = 100  = 10 allowed forbidden Common Chromophores:Carbonyl Compounds * E n 

  9. Possible transitions

  10. Frank – Condon Principle The electronic transition is fast (10-15 s) with respect to nuclear motions.  Transitions where the position and momentum of the nuclei don’t change are favored. J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

  11. Vibronic transitions where the wavefunctions line up are favored. Ingle and Crouch, Spectrochemical Analysis

  12. What do the intensity patterns tell us about the allowed transitions? J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

  13. Progressions Series of bands with the same v” (absorption) or v’ (emission). Bands are separated by v’ (for absorption) or v” (for emission). J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

  14. Progressions J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

  15. V ’ Sequences Dv is constant. Series of bands separated by (v’-v”). V ” More commonly observed in emission experiments. J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

  16. Shape of the electronic spectrum is determined by vibrational and rotational structure. Ingle and Crouch, Spectrochemical Analysis

  17. UV-vis Absorption (Extinction) Spectroscopy Single-Beam or Double-Beam Fixed l or Dispersive Common: Source – Tungsten Halogen Lamp (360-2000 nm) Sample – Liquid In Cuvette Dispersion – Spectrograph w/ Diffraction Grating Detector – CCD Beer’s Law: A = ebc

  18. Assumptions Ingle and Crouch, Spectrochemical Analysis

  19. Apparent Deviations from Beer’s Law Non-Zero Intercept Improper blank measurement or correction. Instrumental drift. Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

  20. Apparent Deviations from Beer’s Law Non-Linear Calibration Plot Chemical Equilibrium – if multiple chemical forms of analyte exist and only one absorbs Other Chemical Effects – solute/solvent interactions, solute/solute interactions, H bonding at high concentrations Using Polychromatic Radiation – non-optimum wavelengths are still transmitted and detected Stray Light – causes measured transmittance to be larger than it should be Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

  21. Absorbed/Emitted Colors Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

  22. Chromophores Often transitions are localized in specific bonds or functional groups within a molecule. Group will have a characteristic lmax and e. Molecular structure or environment can influence lmax and e. Shift to longer l bathochromic (red) shift. Shift to shorter l  hypsochromic (blue) shift. Increase in e  hyperchromic effect. Decrease in e  hypochromic effect. What effect does conjugation usually have? hyperchromic effect / bathochromic shift

  23. Characteristic Electronic Transitions L mol-1 cm-1 Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

  24. Characteristic Electronic Transitions L mol-1 cm-1 Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

  25. Auxophore Does not absorb Induces a bathochromic shift and hyperchromic effect when conjugated to a chromophore (e.g. -OH, -Br, -NH2). Solvent Effects Hypsochromic shiftin n  p* transitions as solvent polarity increases. Solvation stabilizes the nonbonding pair. Bathochromic shiftin p  p* transitions as solvent polarity increases. Solvation stabilizes p*, which is often more polar than p.

  26. Conjugated Alkenes Woodward-Fieser Rules Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

  27. * 1,3-butadiene antibonding bonding

  28. Conjugated Dienes

  29. UV Absorption of Conjugated Alkenes e units = L mole-1 cm-1 • Increasing conjugation gives: • longer wavelength absorption • more intense absorption

  30. b-Carotene 11 double bonds lmax = 460 nm (e = 139,000)

  31. Fieser-Kuhn Rules Systems with More than 4 Double Bonds lmax (nm) = 114 + 5M + n(48.0-1.7n) – 16.5Rendo – 10Rexo n = number of conjugated double bonds M = number of alkyl or alkyl like substituents on the conjugated system Rendo = number of rings with endocyclic double bonds in the conjugated system Rexo = number of rings with exocyclic double bonds

  32. Are you getting the concept? Calculate the absorption maximum for lycopene:

  33. Substituted Benzenes p6* p4* p5* p2 p3 p1 Symmetry and selection rules limit actual transitions: ~185 nm (e~60,000), “primary band” or “E band” ~204 nm (e~8,000), “second primary band” or “K band” ~256 nm (e~200), “secondary band” or “B band”

  34. Substituted Benzenes Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

  35. Polyaromatics

  36. Substituent Effects on Aromatic Absorption 256 nm band is sensitive to electron density of aromatic ring e units = L mole-1 cm-1 Electron density Red = highestGreen = moderate

  37. pH Effects on Aromatic Absorption Phenoxide ion electrostatic potential map e units = L mole-1 cm-1 Anilinium ion electrostatic potential map

  38. Aromatic Carbonyl Compounds Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

  39. Are you getting the concept? Predict the absorption lmax for cholesta-1,4-dien-3-one and the enol of 1,2-cyclopentanedione.

  40. Common Solvents Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

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