IR spectrum:

An IR spectrum is a graph of the absorbed energy versus the wave number (υ). The wave number is the reciprocal of the wavelength (that is 1/λ) and is calculated in units of cm^-1. It is proportional to the frequency or energy of the radiation and thus the higher the wavenumber, the higher the energy. For instance, the absorption peak because of the stretching of an alkyne triple bond comes within the region 2100-2600 cm^-1. This corresponds to a higher energy as compared to the stretching absorption of an alkene double bond that is within the range 1620-1680 cm^-1.

The stretching vibration for a C-H bond takes place within the region 2853-2962 cm^-1, as compared to the stretching vibration of a C-O bond that takes place in the finger print region below 1500 cm^-1, demonstrating the effect of mass on vibrational frequency.

Most stretching vibrations take place wiin the region 3600-1000 cm^-1, whereas bending vibrations are restricted to the region below 1600 cm^-1. The common range for IR spectra is 4000-600 cm^-1.