The stretching frequency of the Azo-group in Formazans

Abstract

Infra-red spectra between 680 and 1600 cm^-1 are recorded for 10 formazans and five copper complexes derived therefrom. A common feature at c. 1450 cm^-1 in the formazans is removed by chelation ; it is therefore suggested that 1450 cm^-l is the stretching frequency of -N:N- as modified by conjugation and hydrogen-bonding in a system Ar.N:N.CAr:N-. Analogies with keto-enol systems indicate that azo- groups in simpler environments would absorb a" higher frequencies, so that of the two alternatives left open by Le Fèvre, O'Dwyer, and Werner (1953), namely, 1579 or 1406 cm^-1, the larger is to be preferred.