Infra-red spectra of compunds containing the Azo-group

Infra-red absorption spectra, over the rock-salt region, are reported for 43 substances believed to contain the -N=N- group. In the " double bond " region two common frequencies are noted, at 1406 and 1579 cm-1 (with standard deviations of ±14 and ± 9 cm-1 respectively). Since these may arise wholly or partly from vibrations of the aromatic or heterocyclic nuclei adjacent to the azo-linkages, the conclusion is drawn that N=N stretching modes, if active in absorption, are contributors to either or both of the bands at the points mentioned.