15N CP‐MAS NMR study of azo–hydrazone tautomerism of some Azo dyes

Abstract The 20.28 MHz 15N CP‐MAS NMR spectra of 15N doubly labelled 3‐methyl‐1‐phenylpyrazole‐4,5‐dione 4‐phenylhydrazone (1), 4‐hydroxyazobenzene (2), 2‐hydroxy‐5‐tert‐butylazobenzene (3) and 1‐phenylazo‐2‐naphthol (4) were measured and the temperature dependence of δ(15N) was followed. For 4, representing a mixture of the azo and hydrazone forms, the hydrazone content was calculated from the 15N chemical shifts of both nitrogen atoms at various temperatures. The two calculations gave identical results. In comparison with measurements in solution, the hydrazone content in 4 is slightly higher in the solid state. Thermodynamic data were calculated using the temperature dependence of In K (K = [hydrazone form]/[azo form]). Some hydrazone content was found in 2 in the solid state, in contrast to the measurements in solution.