15N NMR study of azo-hydrazo tautomerism in some industrially produced azo dyes

15N NMR data (1J (15N, 1H) coupling constants and 15N chemical shifts) were used to characterize quantitatively azo-hydrazo tautomerism in some industrially produced azo dyes. Both 1J (15N, 1H) coupling constants and 15N chemical shifts were obtained at the natural abundance level of 15N using inverse detection of resonances. All three possible types of azo dye existence were found: (i) dyes existing completely in azo forms, (ii) dyes existing completely in hydrazo forms or (iii) dyes forming azo-hydrazo tautomeric equilibria.