Unimolecular decomposition of CH3SH+:: an ab initio and RRKM study

The potential energy surface for the rearrangement and dissociation reactions of methanethiol radical cation (CH3SH+) and methylenesulfonium radical cation (CH2SH2+) 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 GAUSSIAN-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 CH2SH (+) and CH2S+. 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 CH3SH+ might not be efficient enough to ensure complete randomization of energy on the time scale of reaction. (C) 2000 Elsevier Science B.V. All rights reserved.