Ab initio computational investigations on the gas-phase homodimerization and keto-enol tautomerism of the monochalcogenocarboxylic acids CH 3C(=O)XH (X = S, Se, Te)

Computational investigations on the gas-phase homodimerization and the keto-to-enol tautomerism of monochalcogenocarboxylic acids, CH ₃C(=O)XH (X = S, Se, Te), were performed using ab initio molecular orbital methods (HF and MP2) with the 6-311+G(d, p) and 6-311+G(2df, 2df) basis sets. Calculated results indicate that the dimerization enthalpy values, ΔH, for the enol-dimers, [CH₃C(=X)OH]₂ (X = S, Se, Te), are notably higher than those for the corresponding keto-dimers, [CH ₃C(=O)XH]₂ (X = S, Se, Te), while the ΔH values decrease as the electronegativity of chalcogen atom is lowered, for both the keto- and enol-dimers. It is found that the homodimerization of monochalcogenocarboxylic acids is thermodynamically unfavorable because the releasing dimerization heat cannot overcome the loss of entropy. This is contrasted with the case in the carboxylic acid, where the much higher dimerization enthalpy is responsible for the favorable dimerization in the gas phase. Our results also suggest that the tautomeric reactions of the monochalcogenocarboxylic acids in the gas phase may proceed by an eight-membered ring TS with intermolecular double proton transfer and lower tautomeric barrier, instead of by direct intramolecular proton transfer in monomer with a highly strained four-membered TS. The geometrical and energetic characteristics of the homodimers and tautomeric TSs are also further elucidated by NBO analysis. © 2011 World Scientific Publishing Company.