What is Fermi Resonance ?

The phenomenon of Fermi resonance was initially suggested by Italian physicist, Enrico Fermi in 1930’s. It can take place in diatomic and polyatomic molecules although it is much more frequent in polyatomic molecules due to greater number of vibrational modes present.

Fermi resonance occurs when there are two or more vibrational modes (fundamental and overtones or combination bands) in a similar energy region in a molecule, causing interaction or coupling between the modes. This will result in vibrational energy changes of the molecule which can significantly influence physical and chemical properties of the molecule.

In order to understand Fermi resonance, we must understand the basic principles of molecular vibrations first. Every molecule has its own natural vibrational frequencies, which are determined by the masses of its atoms and the bonds strength between them. These natural frequencies are known as normal modes of vibration.

However, sometimes, two or more normal modes can have extremely similar energies causing overlap between their vibrational energy levels. This results in what is termed as coupling between the modes and is the root of the Fermi resonance. continue...

Key Points of Fermi Resonance

1. Similar Energies and Symmetry: Fermi resonance requires two vibrational modes (typically a fundamental mode and an overtone or combination band) with nearly identical energies (near-degenerate) and the same symmetry species in the molecule's point group.
2. Interaction of Modes: Involves anharmonic coupling between a fundamental vibration and an overtone or combination band, leading to mixing of their wavefunctions.
3. Frequency and Intensity Effects: Causes a splitting of vibrational frequencies (one shifts higher, one lower) and redistributes spectral intensities, often enhancing the intensity of weaker overtone or combination bands.
4. Spectral Anomalies: Explains unexpected features in IR and Raman spectra, such as unusual band intensities or positions, complicating vibrational assignments.
5. Environmental Sensitivity: Influenced by the molecular environment, including factors like hydrogen bonding, solvent effects, or structural changes, which can alter the strength of the coupling.