Quantum chemical investigation of intramolecular thione-thiol tautomerism of 1,2,4-triazole-3-thione and its disubstituted derivatives.

The one step intramolecular thione-thiol tautomerism of 1,2,4-triazole-3-thione and its disubstituted derivatives has been studied through the use of electronic structure methods. Due to the absence of experimental data for the parent molecule of 1,2,4-triazole-3-thione the structure and energetics of aforementioned tautomers were derived using various basis sets and levels including HF, B3LYP, and MP2 methods. The gas phase results show that in all different levels of theory the most stable tautomer is the thione form. It has also been revealed that B3LYP/6-31G(d,p) level is quite well suited and reliable to investigate these kinds of tautomerism. To account the influence of substituents on the mentioned tautomerization, the tautomerism and conformational properties as well as vibrational analysis of 20 halophenyl and isopyridyl derivatives were investigated using B3LYP/6-31G(d,p) calculations. In all cases the calculations indicate that substituents have no considerable effects on relative stabilities and energy barriers for the thione-thiol proton transfer and the thione forms are the predominant species in the gas phase. In order to figure out the relative stabilities of the species involved in the tautomerism, geometrical and natural bond orbital (NBO) analyses have been employed. It has also been shown that the computed vibrational frequencies of tautomers with different scaling factors could be used to interpret the vibrational frequencies in IR spectrum of similar species.