Experimental (13C NMR) and theoretical (ab initio molecular orbital calculations) studies on the prototropic tautomerism of benzotriazole and some derivatives symmetrically substituted on the benzene ring

The prototropic tautomerism in anhydrous DMSO of benzotriazole and six derivatives symmetrically substituted on the benzene ring (5,6-dichloro, tetrachloro, 4,7-dibromo, tetrabromo, 5,6-dimethyl, and tetramethyl), was followed by both experimental (C-13 NMR and UV spectroscopy) and theoretical methods. In all of the analyzed systems, predominance of the asymmetric form, N(1)/N(3) protonated, was found. The rates of the N(1)-H <-> N(3)-H prototropic equilibrium, estimated by C-13 NMR techniques, were in the medium exchange regime of 300-3000 s(-1), and are correlated with the spectroscopically determined pK(a) values in aqueous medium, and the anionic forms are the putative rate-limiting intermediate states.