Tautomerism and Intramolecular Hydrogen Bonding in β-Thioxoketones. An ab initio Molecular Orbital Study on Monothiomalondialdehyde.

The energetics of tautomerism and intramolecular hydrogen bonding of monothiomalondialdehyde have been studied by fully optimizing with the 3–21G basis set the molecular geometry of several conformers followed by single-point calculations at a higher level of theory. At MP2/6-31G** level the z-enol tautomer is predicted to be 2.09 kJ mol–1 more stable than the z-enethiol tautomer, in agreement with experimental results, while the inclusion of zero-point vibrational energy pushes the equilibrium considerably towards the z-enethiol structure. The OHS hydrogen bond is ca. five times stronger than the SHO bond and appears to be a decisive factor in the stabilization of the enolic structure. The complete set of Δ(SCF6-31G**) ionization potentials (valence and core) is reported and the results are briefly discussed by reference to the effects of the hydrogen bonding on the relative stabilities of the tautomers in the ionized states.