Fermi resonance

A Fermi resonance is the shifting of the energies and intensities of absorption bands in an infrared or Raman spectrum. It is a consequence of quantum mechanical mixing. The phenomenon was explained by the Italian physicist Enrico Fermi.

Selection rules and occurrence

Two conditions must be satisfied for the occurrence of Fermi Resonance:

• the two vibrational state of a molecule transform according to the same irreducible representation of the molecular point group.
• The energies of the transitions (accidentally) have almost the same energy.

Since the normal modes of a molecule are generally of disparate energies, they do not mix. Thus, Fermi resonance most often occurs between normal and overtone modes, which are often nearly coincidental in energy.

Fermi resonance leads to two effects. First, the high energy mode shifts to higher energy and the low energy mode shifts to still lower energy. Second, the weaker mode gains intensity (becomes more allowed) and the more intense band decreases in intensity. The two transitions are describable as a linear combination of the parent modes. Fermi resonance does not really lead to additional bands in the spectrum.

Examples

Ketones

High resolution IR spectra of most ketones reveal that the "carbonyl band" is...
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