The fingerprint region:

For several organic molecules, there are a very large number of possible bond vibrations, and this number rises as the molecule becomes much more complex. The result of it is there are usually a large number of peaks observed such that the IR spectrum of one molecule is approximately certain to be different from that of another. The region in which most peaks occur is usually under 1500 cm^-1 and is termed as the fingerprint region. This region is specifically useful while comparing the spectrum of a test compound against the spectrum of a known compound. If the spectra are similar this is good proof that both compounds are identical.

Since the fingerprint region is usually complex with several peaks present, it is not feasible to assign the type of vibration associated with each peak unless a particular peak shows greater intensity over its neighbors or 'stands alone'. Absorptions for a number of functional groups like esters, nitro or sulfonate groups do occur within the fingerprint region and can be identified because of their position and intensity.