13C NMR spectra of non‐labelled and 15N‐mono‐labelled azo dyes

Abstract 13C NMR spectra of four types of azo coupling products from benzenediazonium chloride have been measured and interpreted, viz. hydrazo compounds with an intramolecular hydrogen bond (3‐methyl‐1‐phenylpyrazole‐4,5‐dione 4‐phenylhydrazone), azo compounds without an intramolecular hydrogen bond (4‐hydroxyazobenzene), azo compounds with an intramolecular hydrogen bond (2‐hydroxy‐5‐tert‐butylazobenzene) and an equilibrium mixture of both the tautomers of 1‐phenylazo‐2‐naphthol. The absolute values of the J(15N13C) coupling constants have been determined by recording the spectra of the 15N isotopomers, and have been used, in some cases, for 13C signal assignment. A relationship has been found between the chemical shifts of the C‐1′ to C‐4′ carbons of the phenyl group (from the benzenediazonium ion) or the 1J(15N13C) coupling constant, and the composition of the tautomeric mixture.