An increase in wavenumber corresponds to an increase in energy. The mid IR (4000-400 cm-1) is most frequently used for functional group studies of most organic compounds and also called fundamental region which is further divided into two parts:
(a) The group frequency region [4000-1300 cm-1]
(b) The fingerprint region [1300-400 cm-1 ]
Infrared radiation is absorbed by organic molecules and converted into energy of molecular vibration. In IR spectroscopy, an organic molecule (or inorganic compounds) is exposed to infrared radiation. When radiant energy matchs the energy of specific molecular vibration, absorption occurs. A typical IR spectrum is shown in figure 1. The wave number, plotted on the X-axis, is proportional in energy (or wavenumber); therefore, the highest energy vibrations are on the left. The percent transmittance (%T) is plotted on the Y-axis.