DFT and MP2 study of low barrier proton transfer in hydrazide schiff base tautomers via water bridges and in the gas.

MP2 and DFT studies were performed on the tautomers of N'-ethylideneacetohydrazide in different environments including gas phase, continuum solvent and microhydrated environment. The ground electronic state structures of the tautomers were optimized at the MP2 and B3LYP levels of theory using 6-311++G(d,p), separately. The optimized geometries of the transition states of different tautomerism processes, which occur through the proton transfer (PT) reaction, were determined using the QST3 approach at the same levels of theory. The same stability order as T1Z[Symbol in text] T2ZE[Symbol: see text] T2ZZ[Symbol in text] T2EE[Symbol in text] T2EZ[Symbol in text] T6[Symbol in text] T4E was found for the tautomers in the gas phase, continuum solvent and microhydrated environment for both B3LYP and MP2 levels of theory. In addition, the variations of the Gibbs free energies of tautomeric processes, the activation Gibbs free energies of the forward and reverse tautomeric processes with the polarity of the solvent (in continuum solvent model) and the number of water molecules (in microhydrated environment) were investigated. It was found that the reverse tautomeric process is more favorable in all considered environments thermodynamically and kinetically. In addition, it was shown that the rate constants of the reverse and forward considered tautomeric processes decrease with the solvent polarity in the continuum solvent model and the process becomes more difficult than the gas phase. The opposite trend is seen in the microhydrated environment.