Near-infrared spectra Fermi resonance

Another phenomenon which adds to the complexity of an infrared spectrum is known as Fermi resonance. Fermi resonance is the result of an interaction of a fundamental vibration with an overtone or combination band which has nearly the same frequency. It is a consequence of quantum mechanical mixing. The phenomenon was explained by the Italian-American physicist Enrico Fermi. The result is the appearance of two bands equally displaced on both sides of the predicted interacting frequencies. The intensities of the two bands resulting from this interaction are not ordinarily the same. An example of Fermi resonance is the doublet centered at 1,765 cm for benzoyl chloride. 

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