The tautomerism in Sudan ill bis-azo dye has been analyzed using ab initio
Hartee-Fock [HF] and second-order Moller-Plesset perturbatim theory [MP2] a
nd density functional (B3LYP) methods. Gas-phase and solution calculations
were performed to investigate the solvent effect on the azo (OH) --> hydraz
one (NH) tautomeric equilibrium. The azo (OH) tautomer was found to be pref
erred in gas phase at the HF level of theory. The inclusion of the electron
ic correlation (MP2) shifted the equilibrium toward the hydrazone (NH) form
. The NH isomer was also found to be more favorable in the gas phase accord
ing 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 u
ltraviolet/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. (C) 2000 John Wiley & Sons, Inc.