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.
The mode coupling can be symmetric or asymmetric. Symmetric coupling takes place when two vibrational modes of same symmetry interact with each other. This generally causes splitting of the energy levels, one lower and other higher than the original energy levels. Asymmetric coupling arises when two modes of different symmetry interact with each other and results in shift of energy levels of both the modes.
Fermi resonance leads to two effects-
1. The high energy mode shifts to higher energy. The low energy mode shifts to still lower energy.
2. The weaker mode gains intensity (more allowed). The more intense band decreases in intensity.
One of the most common examples of Fermi resonance is the interaction between the stretching and bending modes of water molecules. The stretching and bending modes have almost similar energies, and the interaction between them leads to a splitting of the energy levels. This impacts not only the vibrational energy of water molecules but also their ability to absorb and emit radiation.
