What is the Difference Between Fermi Resonance and Overtones in IR Spectra?

Fermi resonance and overtones are both phenomena observed in infrared (IR) spectra, but they have distinct differences:

1. Fermi Resonance:

• It is the shifting of the energies and intensities of absorption bands in an IR or Raman spectrum.
• It occurs due to quantum-mechanical wavefunction mixing, as explained by Italian physicist Enrico Fermi.
• It requires that the two vibrational modes have the same irreducible representation and that their energies are nearly coincident.
• Fermi resonance leads to two effects: the high-energy mode shifts to higher energy, and the low-energy mode shifts to still lower energy. The weaker mode gains intensity (becomes more allowed), and the more intense band decreases in intensity.

1. Overtones:

• Overtones are spectral bands that occur in a vibrational spectrum upon the IR spectra.
• Overtones are the result of a transition with Δv > 1 from the ground state.
• Overtones generally have lower intensities than the fundamental transitions.
• Overtones decrease in probability as the number of quanta (Δv) increases.

In summary, Fermi resonance is a quantum-mechanical phenomenon that leads to shifts in energies and intensities of absorption bands, while overtones are spectral bands resulting from vibrational transitions with higher energies than the fundamental transitions.

Comparative Table: Fermi Resonance vs Overtones in IR Spectra

The main differences between Fermi resonance and overtones in IR spectra are as follows:

Fermi resonance helps explain and assign peaks within IR and Raman spectra that may not otherwise be accounted for, while overtones are used to explain spectral bands in vibrational spectra.