Quantum-chemical considerations on the acidity of thiamin pyrophosphate and related systems

The H-D exchange reactions of 1,3-azolium cations have been studied by the semiempirical CNDO/2 method with optimization of all geometrical parameters, in order to explain the rate enhancement for the 1,3-thiazolium cations. The following results are obtained: (a) stabilization of a carbanion by the adjacent sulfur atom is not due to (d-p) conjugation; (b) the 1.3-thiazolium conjugate base is stabilized with respect to the other conjugate bases by the greater polarizability of sulfur; (c) the smaller amount of energy necessary for the 1.3-thiazolium cation, with respect to the other cations, to use the penultimate u MO gives an explanation for the unique rate enhancement.