Ab initio study of the unimolecular pyrolysis mechanisms of monothioformic acid

The mechanisms of the possible unimolecular reactions occurring during the pyrolysis of the four tautomers (two conformers of thiol- and two conformers of thiono-) of monothioformic acid have been proposed and investigated by ab initio methods with STO-3G and 6-31G⁷ basis sets. The effects of valence electron correlation were included by Møller-Plesset (MP) perturbation theory to the fourth order at the 6-31G⁷ level. Our best results of the activation energies are given by MP4/6-31G⁷//HF/ 6-31G⁷ plus scaled zero-point energy. The barrier heights of the dehydrogenation (via a four-centre transition state) and dehydrogensuphidation (via a three-centre transition state) of thiolformic acid pyrolysis are 67.47 and 67.09 kcal mol^?1 respectively. The s-cis thionformic acid is dehydrated via a three-centre transition state. The activation energy of the process (81.18 kcal mol^?1) is much higher than the activation energy of the dehydrogenation of the s-trans form (68.83 kcal mol^?1) which is dehydrogenated via a four-centre transition state. These results suggest that in thionformic acid pyrolysis, the dehyrogenation of the s-cis form is more favourable than the dehydration of the s-trans form.