Methanethiol photolysis at 248 nm. Hydrogen atom yield and rate constant for the H + CH3SH reaction

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 k_R1 = 9.64 × 10^-13 cm³ molecule^-1 s^-1 at 298 K.

Combined Experimental and Master Equation Investigation of the Multiwell Reaction H + SO2

The temperature and pressure dependence of the rate coefficient for the reaction H + SO2 has been measured using a laser flash photolysis/laser-induced fluorescence technique, for 295 <or= T/K <or= 423 and for 3 <or= [He]/10(18) molecules cm(-3) <or= 23. Under these conditions, the reaction occurs exclusively to form HSO2. These data have been fitted to, and used to refine, a master equation (ME) model of the H + SO2 reaction system that includes the isomers HSO2 and HOSO and the high-temperature products OH + SO. The potential energy for the system is based on literature ab initio calculations. The three eigenvalues of lowest magnitude, obtained from solution of the ME, are the chemically significant eigenvalues that are related to the pressure and temperature dependent rate coefficients for the chemical system. The eigenvalues are generally well separated from each other and from the eigenvalues relating to collisional relaxation, except at high temperatures. The rate coefficients were extracted from the eigenpairs of the ME solution, using an analysis developed by Klippenstein and Miller (J. Phys. Chem. A 2002, 106, 9267) and an adaptation of an earlier analysis by Bartis and Widom (J. Chem. Phys. 1974, 60, 3474). The relationship between the temperature dependence of the chemically significant eigenvalues and combinations of rate coefficients was elucidated via an approximate analysis of the chemical rate equations and it was demonstrated that each eigenvalue relates primarily to passage over a single transition state. Some problems were encountered in equating ratios of rate coefficients for forward and reverse reactions to the equilibrium constant. This problem arises from the nonconservative nature of the reaction system; it was resolved by imposing orthogonality on the coupled rate equations of the chemical system, giving good agreement between rate coefficient ratios and equilibrium constants. The temperature and pressure dependences of the rate coefficients were parametrized according to Troe and modified Troe expressions. The key processes, from a combustion perspective, are the formation and dissociation of HOSO and the formation of OH + SO. Except at pressures >10(3) atm, the latter proceeds directly from H + SO2, via the energized states of HOSO. The derived rate coefficients rely heavily on measurements of the reverse reaction, OH + SO, which has only been determined at temperatures up to 700 K.