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

The 20.28 MHz 15 N CP‐MAS NMR spectra of 15 N 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( 15 N) was followed. For 4, representing a mixture of the azo and hydrazone forms, the hydrazone content was calculated from the 15 N 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.