Theory of Infra Red Spectrometry:

You know that the bond between two atoms in a molecule is not rigid. It has elasticity and as a consequence the atoms in the molecule do not remain fixed in their positions. They are in a constant state of motion around their mean position. The molecule is said to be undergoing vibrational motion. An quantum mechanical treatment of the molecular vibrational motion shows that it is quantised. As the transitions between these quantised energy levels can be brought out by using EM radiation in the infra- red range it gives rise to an IR spectrum. As the excitation among the quantised vibrational energy levels of the molecules is involved, the IR spectroscopy is also known as vibrational spectroscopy. Now let us learn about the vibration motion of the molecules and origin of IR spectrum. We start with diatomic molecules.