A density functional theory (DFT) B3LYP method was employed to study the mechanism of the reaction of CH3S with HO2 at the 6-311＋＋G(d,p) basis sets. Geometries of the stationary points were completely optimized. The transition states were validated by the vibrational analysis and the internal reaction coordinate (IRC) calculations, which testified the authenticity of intermediates and transition states. Five feasible reaction pathways of this reaction have been studied. The results indicate that the main reaction pathway is the singlet spin pathway CH3S＋HO2→CH3SOOH (1P), which has no activation energy and is beneficial from the aspects of reaction kinetics and thermodynamics. For the triplet spin reaction pathway, CH3S＋HO2→COM11→TS1→COM12→CH3SH＋O2 (3P) is the main route, and the corresponding activation energy is 53.5 kJ/mol. This exothermic pathway has the lowest activation energy, which is beneficial from the aspects of reaction kinetics and thermodynamics.

Key words: methylthio radical, peroxyl radical, reaction mechanism, topological analysis of electronic density