Thermochemical and Kinetics of CH3SH + H Reactions: The Sensitivity of Coupling the Low and High-Level Methodologies.

The reaction system formed by the methanethiol molecule (CH3SH) and a hydrogen atom was studied via three elementary reactions, two hydrogen abstractions and the C-S bond cleavage (CH3SH + H → CH3S + H2 (R1); → CH2SH + H2 (R2); → CH3 + H2S (R3)). The stable structures were optimized with various methodologies of the density functional theory and the MP2 method. Two minimum energy paths for each elementary reaction were built using the BB1K and MP2 methodologies, and the electronic properties on the reactants, products, and saddle points were improved with coupled cluster theory with single, double, and connected triple excitations (CCSD(T)) calculations. The sensitivity of coupling the low and high-level methods to calculate the thermochemical and rate constants were analyzed. The thermal rate constants were obtained by means of the improved canonical variational theory (ICVT) and the tunneling corrections were included with the small curvature tunneling (SCT) approach. Our results are in agreement with the previous experimental measurements and the calculated branching ratio for R1:R2:R3 is equal to 0.96:0:0.04, with kR1 = 9.64 × 10-13 cm3 molecule-1 s-1 at 298 K.

Thermochemical and kinetics studies of the CH3SH+S (3P) hydrogen abstraction and insertion reactions.

Sulfur-containing molecules have a significant impact on atmosphere and biosphere. In this work we studied, from the point of view of electronic structure and chemical kinetics methods, the elementary reactions between a methanethiol molecule and a sulfur atom leading to hydrogen abstraction C-S bond cleavage (CH(3)SH+S; R1:→ CH(3)S+SH; R2: → CH(2)SH+SH; R3:→ CH(3)+HS(2)). The geometrical structures of the reactants, products, and saddle points for the three reaction paths were optimized using the BB1K method with the aug-cc-pV(T+d)Z basis set. The thermochemical properties were improved using single point coupled-cluster (CCSD(T)) calculations on the BB1K geometries followed by extrapolation to the complete basis set (CBS) limit. This methodology was previously applied and has given accurate values of thermochemical and kinetics properties when compared to benchmark calculations and experimental data. For each reaction, the thermal rate constants were calculated using the improved canonical variational theory (ICVT) including the zero-curvature (ICVT/ZCT) and small-curvature (ICVT/SCT) tunneling corrections. For comparison, the overall ICVT/SCT reaction rate constant at 300 K obtained with single-point CCSD(T)/CBS calculations for the CH(3)SH+S reaction is approximately 1400 times lower than the isovalent CH(3)SH+O reaction, obtained with CVT/SCT. The reaction path involving the hydrogen abstraction from the thiol group is the most important reactive path in all temperatures.