IR-active molecules exhibit two fundamental modes of vibration that result in IR absorption.

Stretching vibrations occur along the bond line and can change the bond length. Stretches can be symmetric or asymmetric.

Bending vibrations change the bond angle and are subdivided into in-plane or out-of-plane modes. Scissoring and rocking represent the former type, whereas wagging and twisting represent the latter.

The total number of fundamental vibrations allowed for a nonlinear molecule like ethanol containing n number of atoms is 3n − 6; however, only 3n − 5 fundamental vibrations are allowed for a linear molecule such as HCl.

A bond's stretching and bending vibrations occur at a characteristic fundamental absorption frequency and involve the excitation of bonds from the ground state to the lowest energy excited state.

Integral multiples of fundamental absorption frequencies are termed overtones.

The fusion of two IR active vibrational frequencies produces a combination band.

The coupled vibration generated by the interaction of the fundamental band with the overtone or combination band produces Fermi resonance.