Fermi resonance

A Fermi resonance is the shifting of the energies and intensities of absorption bands in an infrared or Raman spectrum.

[1] The phenomenon was first explained by the Italian physicist Enrico Fermi.

[citation needed] Fermi resonance leads to two effects.

High-resolution IR spectra of most ketones reveal that the "carbonyl band" is split into a doublet.

This splitting arises from the mixing of νCO and the overtone of HCH bending modes.

Idealized appearance of a normal mode and an overtone before and after Fermi resonance. Beneath the idealized spectra are idealized energy-level schemes.