HIGH-LEVEL AB-INITIO CALCULATIONS ON THE INTRAMOLECULAR HYDROGEN-BONDIN THIOMALONALDEHYDE

High-level ab initio calculations, in the framework of the G2(MP2) the ory, have been carried out on the different tautomers of thiomalonalde hyde (TMA). These calculations are compared with those obtained using density functional theory methods, namely B3LYP, with extended basis s ets, In general the enethiol tautomers of TMA are 5-10 kcal/mol more s table than the corresponding enol analogues, with the only exception b eing the Z-enol (E1) and the Z-enethioi (T1) hydrogen-bunded species, which are the global minima of both series. At the G2(MP2) level both tautomers are nearly degenerate, the enethiol T1 being 0.2 kcal/mol mo re stable than the enol E1. Electron correlation effects stabilize pre ferentially the enol form, while the ZPE corrections go in the opposit e direction, due essentially to the differences between S-H and O-H st retching frequencies. As a consequence, when the hydrogen atom involve d in the intramolecular hydrogen bond (IHB) of both tautomers is repla ced by deuterium, the stability order is reversed and El is predicted to be more stable than T1. An analysis of these IHBs in terms of the t opological characteristics of the electron charge density and of the s hifts of the S-H and O-H vibrational frequencies reveals that the HE i n Ea is much stronger than in T1. The existence of this IHB results in an increase of the electron delocalization which enhances the stabili ty of tautomer E1. At the G2(MP2) level two open-chain rotamers, namel y T4 and T7, are predicted to be within an energy gap smaller than 0.5 kcal/mol with respect to the global minimum. The use of continuum and discrete-continuum models indicates that both open-chain enethiols an d enols are significantly stabilized by solute-solvent interactions, a nd they should predominate in aqueous solution. B3LYP/6-311+G(3df,2p) relative stabilities are in excellent agreement with G2(MP2) values.