Quantum mechanical investigation of the tautomerism in the azo dye Sudan III

The tautomerism in Sudan III bis-azo dye has been analyzed using ab initio Hartee–Fock [HF] and second-order Møller–Plesset perturbatim theory [MP2] and density functional (B3LYP) methods. Gas-phase and solution calculations were performed to investigate the solvent effect on the azo (OH)→hydrazone (NH) tautomeric equilibrium. The azo (OH) tautomer was found to be preferred in gas phase at the HF level of theory. The inclusion of the electronic correlation (MP2) shifted the equilibrium toward the hydrazone (NH) form. The NH isomer was also found to be more favorable in the gas phase according to the B3LYP results. In solution the equilibrium is shifted toward the NH tautomer as the dielectric constant of the medium increases. The energy barrier for the intramolecular proton transference was calculated and the value found suggested a strong hydrogen bond. The B3LYP and MP2 activation Gibbs free energies were very close and much lower than the HF value. The ultraviolet/visible electronic spectra for the minima and transition state (TS) structures were calculated and compared with the experimental data. The theoretical band positions obtained considering the TS geometry were found to best agree with the experimental data.