Theoretical investigation of thione-thiol tautomerism, intermolecular double proton transfer reaction and hydrogen bonding interactions in 4-ethyl-5-(2-hydroxyphenyl)-2H-1,2,4-triazole-3(4H)-thione

The thione-thiol tautomerism and intermolecular double proton transfer reaction for the title triazole compound were studied at the B3LYP level of theory using 6-311++G(d,p) basis function. The solvent effect on the proton transfer reactions was investigated in three solvents (chloroform, methanol and water) using the polarizable continuum model (PCM) approximation (direct solvent effect) and solvent-assisted mechanism. The results show that the thione-enol tautomer is the most stable isomer among the four possible tautomeric forms of the compound both in the gas phase and in solution phase. A very high tautomeric energy barrier is found for the thione-thiol tautomerism between the enol and keto forms of the compound both in the gas phase and in solution phase, indicating a quite disfavored process. The direct solvent effect is found to be sizable with increasing polarity of the solvents. Even though the presence of the solvent molecules significantly lowers the barrier of the proton transfer, it is not adequate for the reaction to occur. The energetic and thermodynamic parameters of the double proton transfer process show that the double proton exchange from thione-enol dimer to thiol-enol dimer is thermodynamically unfavored. However, the exchange from thiol-enol dimer to thione-enol dimer for the gas phase and methanol phase seems to be feasible with a low barrier height, and is supported by negative values in enthalpy and free energy changes. The intermolecular hydrogen bonding interactions were analyzed in the gas phase regarding their geometries and energies. The stability of the H-bonds comes in the order of O1H1⋯N1 > S1H2⋯N2 > N2H2⋯S1 > N2H2⋯N1.