On the azo/hydrazo equilibrium in Sudan I azo dye derivatives.

In this study, Raman, infrared, UV/vis, NMR, and single crystal X-ray diffraction spectroscopies are used to elucidate the tautomeric equilibrium of azo dyes derived from 1-phenyl-azo-2-naphthol (Sudan I). A new crystallographic structure is described for Sudan I, revealing the presence of intramolecular hydrogen bonds and supramolecular interactions, such as the unconventional C-H···O hydrogen bond type, π-stacking, and charge-dipole interactions. All of these weak intermolecular interactions play a role in the stability of the crystalline structure. Theoretical calculations are also reported for geometries, energy, and spectroscopic properties. The predicted spectra are in accordance with the experiments carried out in the solid state and in solution of dichloromethane, carbon tetrachloride, and chloroform, suggesting the hydrazo form as the preferable tautomer in gas and condensate phases for Sudan I and its derivatives.