Ab initio computational modeling on the tautomerism of monochalcogenocarboxylic acids CH3C(=O)XH (X = S, Se, and Te) in the polar and aprotic solution

Computational modeling by an ab initio molecular orbital method (HF and MP2) with the 6-311+G(d,p) and 6-311++G(2df,2pd) basis sets on the tautomerism of three monochalcogenocarboxylic acids CH3C(=O)XH (X = S, Se, and Te) in the polar and aprotic solution tetrahydrofuran (THF) was undertaken using the combined supramolecular/continuum method, in which CH3OCH3 was considered as a realistic model for THF in the supermolecule. Our calculations showed that the tautomerism involving CH3OCH3 not only gives the preference of the enol form CH3C(=X)OH (X = S, Se, and Te) in the tautomeric equilibrium, but also significantly lowers the tautomeric barriers by > 34 kJ/mol in THF solution. These results support the experiments of Kato and coworkers, in contrast with earlier Delaere et al.'s theoretical prediction for the tautomeric barrier of thioformic acid HC(=O)(SHO)-O-...(CH3)(2), in which the barrier is increased by similar to 6 kJ/mol relative to reactants complex because of the constraint in locating the tautomeric transition structure. (c) 2006 Wiley Periodicals, Inc.