Direct and solvent-assisted keto-enol tautomerism and hydrogen-bonding interactions in 4-(m-chlorobenzylamino)-3-phenyl-4,5-dihydro-1H-1,2,4-triazol-5-one: a quantum-chemical study.

The tautomeric equilibrium of the title triazole compound was computationally analyzed at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory. The solvent effect was considered for three solvents (chloroform, methanol, and water). Two distinct mechanisms were applied: a direct intramolecular transfer using the polarizable continuum model (PCM) and a solvent-assisted mechanism. The calculations indicated that the keto form is more stable in all cases. It was found that the barrier heights for the tautomerization reaction are very high, indicating a relatively disfavored process. Although the barrier heights for solvent-assisted reactions are significantly lower than those for the unassisted tautomerization reaction, implying the importance of the superior catalytic effect of the solvents, monosolvation was not found to be sufficient for the reaction to occur. Finally, the two intermolecular hydrogen-bonding interactions in the crystal structure were investigated in the gas phase; according to the calculated energies and structural parameters, the order of stability is N3-H3···O1 > N1-H1···O1.