Vibrational spectroscopy

Molecular vibrations

Molecules consist of atoms bound together by what are usually called chemical bonds. The nature of these bonds will be discussed more fully in chapter 2 and it is only necessary to note here that the bonds and the angles between them are not rigid. To a first approximation the force required to make a small change in the length of a bond, or a small change in the angle between two bonds, is proportional to the change produced; similarly, the torque required to twist one part of a molecule through a small angle with respect to the rest about a bond is approximately proportional to the angle of twist. The molecule thus consists of a set of coupled harmonic oscillators and if it is disturbed from its equilibrium state it will vibrate in such a way that the motion can be considered to be a superposition of a number of simple harmonic vibrations. In each of these so-called normal modes every atom in the molecule vibrates with the same frequency, and in the simplest molecules all atoms pass through their respective positions of zero displacement simultaneously.

There are three principal methods by which the vibrations may be studied: infrared and Raman spectroscopies and inelastic neutron scattering. The first two methods are available in very many laboratories, since the equipment required is relatively small and cheap.