Synthesis, structure and tautomerism of two benzothiazolyl azo derivatives of 2-naphthol: A crystallographic, NMR and computational study

Abstract The efficient synthesis, 1 H and 13 C NMR characterization, crystal and molecular structure, DFT calculations of 6-[(2-hydroxy-1-naphthyl)diazenyl]-2-methylbenzothiazole, as well as DFT calculations of 6-[(2-hydroxy-1-naphthyl)diazenyl]benzothiazole, are reported. Single-crystal X-ray diffraction data show that crystals of the azo dye derived from 2-methylbenzothiazole is principally composed of the hydrazone form. 2D NMR techniques (COSY, HMQC and HMBC) were used to assign 1 H and 13 C chemical shifts for each azo dye in DMSO and CHCl 3 solution. Azo–hydrazone tautomerism of the dyes was investigated jointly by 13 C NMR and DFT calculations. All three approaches show that in solution the equilibrium is only slightly shifted towards the hydrazone form. The very good agreement between the three differently obtained equilibrium constants suggest M06-2X/6-311+G(d,p)-SMD as the level of theory of choice for quantum-mechanical calculations of these and similar compounds in solution.