A theoretical study of CH ₃ SH pyrolysis kinetics

The decomposition mechanism of methanethiol (CH₃SH) under pyrolysis conditions has been investigated using electronic structure calculations at the CCSD(T)-F12/cc-pVTQ∞ Z-F12//CCSD(T)-F12/cc-pVDZ-F12 level of theory for several potential energy surfaces such as CH₃SH + SH, CH₃SH + CH₃, CH₂S + SH, CH₂S + CH₃ and CS + SH. Pressure dependent reaction rates for the associated reactions have been deduced from energies, frequencies and geometries calculations by solving the master equation for each surface. Complex behavior is observed for these systems with very low to submerged energy barriers for the CH₂S + SH addition on the S atom, mainly due to multireference effects and van der Waals complex formation. The former has been explored in detail with CASPT2-F12/cc-pVTZ-F12 calculations up to an active space size of 13 electrons in 15 orbitals, and the latter was accounted for with phase space theory. Comparison with available experimental data shows that the resulting mechanism robustly predicts the abundance of the main species over the full temperature range.