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**Molecular Spectroscopy Types of transitions:**

1) Electronic (UV-Vis-Near IR) 2) Vibrational (IR) 3) Rotational (microwave)

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**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.

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**Electronic Absorption Spectra**

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

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**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.

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**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.

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**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.

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**Polyatomic Electronic Transitions**

s orbitals – s p orbitals – p non-bonding orbitals - n Ingle and Crouch, Spectrochemical Analysis

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**Vibronic Transitions Selection Rules: DS = 0 Dv = 0, 1, 2, 3, …**

DJ = 1

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**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.

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**Vibronic transitions where the wavefunctions line up are favored.**

Ingle and Crouch, Spectrochemical Analysis

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**What do the intensity patterns tell us about the potential diagrams?**

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

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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.

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

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**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.

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**Shape of the electronic spectrum is determined by vibrational and rotational structure.**

Ingle and Crouch, Spectrochemical Analysis

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**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

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

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Assumptions Ingle and Crouch, Spectrochemical Analysis

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**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.

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**Absorbed/Emitted Colors**

Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

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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

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**Characteristic Electronic Transitions**

L mol-1 cm-1 Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

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**Characteristic Electronic Transitions**

L mol-1 cm-1 Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic Compounds

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**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.

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