Energetic, structural and tautomeric analysis of 2-mercaptobenzimidazole

This paper reports an experimental and theoretical study about the energetic and structural characteristics of the 2-mercaptobenzimidazole (MBI) tautomeric forms (thione and thiol). The standard (pA degrees A = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, in the gaseous phase, for MBI was derived from its enthalpies of combustion and sublimation, obtained by rotating-bomb calorimetry and by the Knudsen effusion technique, respectively. The results are compared with the corresponding data for MBI thione/thiol tautomers calculated by the G3(MP2)//B3LYP approach that suggest the thione form as the preferred form of MBI in gaseous phase. Computationally, the molecular structures of both tautomers were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory. In addiction, the G3(MP2)//B3LYP tautomerization energy at 0 K for thione/thiol forms of MBI was calculated and also evidences that the tautomeric equilibrium favours the thione tautomer. The standard Gibbs energy of formation in crystalline and gaseous phases was also derived, allowing an analysis of the thermodynamic stability of MBI in comparison with other related compounds.