Thermodynamic and kinetic analysis on the reaction of dimethyl sulfide radical with oxygen

Thermochemical properties of methylthiol-methylhydroperoxide, CH ₃SCH ₂OOH and its two terminal radicals resulting from the loss of hydrogen atoms, viz. CH ₃SCH ₂OOA· and CA·H ₂SCH ₂OOH, are important to understand the stability, reaction paths, and kinetics of the reactions of dimethyl sulfide and related sulfur-containing hydrocarbons in the atmosphere and the combustion processes. Thermochemical properties ΔH ^o _f, S(T) and C _p(T) and internal rotation potentials for reactants, intermediates, and transition states in the methylthiomethyl (CH ₃SCA·H ₂) radical + O ₂ reaction system are analyzed to evaluate reaction paths and kinetics versus temperature and pressure. Isodesmic reaction analysis is used to determine the enthalpies of formation (ΔH ^o _f298) using density functional B3LYP/6-311G(d,p) and complete basis set extrapolation CBS-QB3 computational methods. Entropy and heat capacities C _p(T) are determined using geometric parameters and vibration frequencies obtained at B3LYP/6-311G(d,p) level of calculation. ΔH _f ^o ₂₉₈ values estimated at the CBS-QB3 level (units: kcal mol ^-1) are CH ₃SCA·H ₂ (32.7), CH ₃SCH ₂OOH (-29.1), CH ₃SCH ₂OOA· (5.2), CA·H ₂SCH ₂OOH (11.5). Quantum Rice-Ramsperger-Kassel analysis is used to calculate energy-dependent rate constants, k(E), and master equation is used to account for collisional stabilization of peroxy adduct and the hydroperoxide isomer. The methylthiomethyl radical associates with O ₂ to form a methylthiomethyl peroxy radical with a well depth of 27.49 kcal mol ^-1. The peroxy radical can dissociate back to reactants, isomerize via hydrogen shift to form a hydroperoxide methyl radical CA·H ₂SCH ₂OOH, decompose via hydrogen transfer, and undergo subsequent low-energy beta scission to form CH ₃SC(=O)H and OH radical. The peroxy radical can also attack the sulfur atom via TS3 to form CH ₃S(=O) + CH ₂O product set. The CA·H ₂SCH ₂OOH isomer can decompose via an elimination transition state to CH2=S + CH2=O + OH, or through a four-member ring transition state to 1,3-Oxathietane + OH products.