Oxidation reaction mechanism and kinetics between OH radicals and alkyl

Kinetic rate constants for the oxidation reactions of OH radicals with CH3SH (1), C2H5SH (2), n-C3H7SH (3) and iso-C3H7SH (4) under inert conditions (Ar) over the temperature range 252？430？K have been studied using the CBS-QB3 composite method. Kinetic rate constants under atmospheric pressure and in the fall-off regime have been estimated using transition state theory (TST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Comparison with experiment confirms that in the OH-addition pathways 1？4 leading to the related products, the first bimolecular reaction step has effective negative activation energies around ？2.61 to 3.70 kcal mol？1. Effective rate coefficients have been calculated according to a steady-state analysis of a two-step model reaction mechanism. As a result of the negative activation energies, pressures larger than 104？bar would be required to restore to some extent the validity of this approximation for all the channels. By comparison with experimental data, all our calculations for both the OH-addition and H-abstraction reaction pathways indicate that from a kinetic viewpoint and in line with the computed reaction energy barriers, the most favourable process is the OH-addition pathway to n-C3H7SH to yield the [n-C3H7SH？OH]？ species, whereas under thermodynamic control of the bimolecular reactions (R？SH+OH？), the most abundant product derived from the H-abstraction pathway will be the [n-C3H7？S？+H2O] species