Ab initio computational modeling on the tautomerism of monochalcogenocarboxylic acids CH 3 C(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(2 df ,2 pd ) basis sets on the tautomerism of three monochalcogenocarboxylic acids CH 3 C(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 CH 3 OCH 3 was considered as a realistic model for THF in the supermolecule. Our calculations showed that the tautomerism involving CH 3 OCH 3 not only gives the preference of the enol form CH 3 C(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)SH…O(CH 3 ) 2 , in which the barrier is increased by ∼6 kJ/mol relative to reactants complex because of the constraint in locating the tautomeric transition structure. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007