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

2. Born – Oppenheimer Approximation
The wavefunction of a molecule is assumed to be the product of the electronic, vibrational and rotational wave functions.
The energy levels for each type of transition can be treated independently.
The total energy of the molecule is the sum of the electronic, vibrational and rotational energy.

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

4. Atomic Orbitals (AO) Combine to give Molecular Orbitals (MO)
Bond Order =
(# bonding e- - # antibonding e-) / 2
Total Spin Quantum # (S)
S = Ssi
Multiplicity = 2S + 1
Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry, Prentice Hall, Englewood Cliffs, NJ, 1991.

5. 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.

6. Selection Rules DS = 0 i.e. e- cannot change spin during transition.
This rule can be broken with heavier nuclei.
There are other selection rules for diatomics but DS = 0 is the only one valid for more complicated molecules.

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

8. Vibronic Transitions Selection Rules: DS = 0 Dv = 0, 1, 2, 3, …
DJ = 1

9. 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.

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

11. What do the intensity patterns tell us about the potential diagrams?
J. Michael Hollas, Modern Spectroscopy, John Wiley & Sons, New York, 1992.

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

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

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

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

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

17. Beer’s Law
Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

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

21. 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 affect 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

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

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

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