4-Carboxyl-2,6-dinitrophenylazohydroxynaphthalenes tautomerism NMR re-explained

(1Z)-1-[2-(2-Nitrophenyl)hydrazinylidene]naphthalen-2(1H)-one (1), (4Z)-5-methyl-2-phenyl-4-(2-phenylhydrazinylidene)-2,4-dihydro-3H-pyrazol-3-one (2), (4Z)-5-methyl-4-[2-(2-nitrophenyl)hydrazinylidene]-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (3) and 3,5-dinitro-4-[(2Z)-2-(2-oxonaphthalen-1(2H)-ylidene)hydrazinyl]benzoic acid (4) were prepared and one- and two-dimensional 15N, 13C and 1H NMR spectra were measured and analysed. Results obtained were used in characterization of these compounds from viewpoint of azo-hydrazo tautomerism. All these compounds exist almost completely in appropriate hydrazo forms in dimethyl sulfoxide. Advantages of NMR spectroscopy application in determination of acidic proton positions and azo-hydrazo content determination are discussed.