When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamental vibrational motions that result in infrared absorption can be classified as stretching or bending vibrations.

Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the simultaneous vibration of two bonds elongates and contracts together. In contrast, asymmetric stretching involves one bond moving in one direction while another bond moves in the opposite direction.

Unlike stretching, bending vibrations do not occur along the bond line; instead, they change the bond angle. Bending vibrations can be divided into in-plane and out-of-plane bending. In-plane bending vibrations are categorized as scissoring (symmetric) and rocking (asymmetric), while out-of-plane bending is classified as twisting (symmetric) and wagging (asymmetric).

For nonlinear molecules, the total number of allowed fundamental vibrations is given by the formula 3n − 6, where n represents the number of atoms. In contrast, linear molecules like HBr allow only 3n − 5 fundamental vibrations. For HBr, only one type of vibration is possible.

In IR spectroscopy, the frequency corresponding to the excitation of bonds from the ground state to the lowest energy excited state is called fundamental absorption frequency. The integral multiples of the fundamental absorption frequencies are called overtones. Any physical vibration generates overtones in a molecule. The band produced due to the fusion of two vibrational frequencies, which is IR active, is called a combination band. In comparison, the band resulting from the difference between two bands is called a difference band. Additionally, fermi resonance refers to the coupled vibration created by the interaction of the fundamental band with the either overtone or combination band.