Unimolecular decomposition of CH 3SH +: an ab initio and RRKM study

Abstract

The potential energy surface for the rearrangement and dissociation reactions of methanethiol radical cation (CH 3SH +) and methylenesulfonium radical cation (CH 2SH 2 +) was investigated by ab initio calculations. The geometries of the species involved in these processes were optimized at the MP2 and QCISD levels of theory, using the 6-311G(d,p) basis set. Single point energies were also obtained at the QCISD(T) level of theory. Vibrational frequencies were computed by the MP2 method and used to evaluate zero-point energies. Standard enthalpies of reaction at 0 K calculated in this study are compared with corresponding values determined experimentally and predicted by the G aussian-2 (G2) theory. Rice–Ramsperger–Kassel–Marcus (RRKM) calculations were performed to evaluate microcanonical rate constants k( E) for reaction channels leading to formation of CH 2SH + and CH 2S +. The computed branching ratios for these species differ significantly from those obtained by charge exchange and photoelectron–photoion experiments, indicating that energy flow between the electronic and vibrational modes of CH 3SH + might not be efficient enough to ensure complete randomization of energy on the time scale of reaction.