IR Spectrum

A region of IR spectrum ranges from 600-4000 cm^-1 and the infrared spectrum can be broadly divided into two regions as explain below.

i) The region spanning from 3600 to 1200 cm^-1 is called the functional group region and includes the stretching vibrations or group frequencies of common functional groups. The absorptions within this region give information about the presence of features functional groups.

ii) The region that includes all frequencies below 1200 cm^-1 is called the fingerprint region and includes molecular vibrations, features of the whole molecule or large fragments of the molecule. This region is meaningful for confirming the identity of a particular molecule through comparing with the spectral absorptions of its authentic sample.

iii) In addition to the presence (or even the absence) of a particular functional group, an analysis of IR spectrum provides the following information:

a) Presence of hydrogen bonding within the molecule (3200-3600 cm^-1)

b) Geometry (cis or trans) in case of alkenes

c) Substitution pattern of aromatic compounds (690-850 cm^-1)

Table containing characteristic group frequencies of some important functional groups is being reproduced here for your reference.

Table:  Characteristic group frequencies of some important functional groups