The Fingerprint Region

Infrared absorption bands between 1250cm−1 and 675cm−1 generally are associated with complex vibrational and rotational energy changes of the molecule as a whole and are quite characteristic of particular molecules. This part of the spectrum is often called the "fingerprint" region and is extremely useful for determining whether the samples are chemically identical. The spectra of 2-propanone and 2-butanone are seen to be very similar in the region 4000 cm−1 to 1250cm−1 but quite different from 1250 cm−1 to 675cm−1. The fingerprint region of the spectrum is individual enough so that if the infrared spectra of two samples are indistinguishable in the range of frequencies from 3600cm−1 to 675cm−1, it is highly probable that the two samples are of the same compound (or the same mixture of compounds).

Characteristic stretching and bending frequencies occur in the fingerprint region, but they are less useful for identifying functional groups, because they frequently overlap with other bands. This region is sufficiently complex that a complete analysis of the spectrum is seldom possible.