The reaction mechanism of BrO with CH₃SH has been investigated by density function theory (DFT) and electronic density topological analysis method. Geometries of the stationary points on the potential energy surface have been optimized at B3LYP/6-311g (d, p) level. Vibration analysis and intrinsic reaction coordinate (IRC) calculation at the same level have been applied to validate the connection of the stationary points. The reaction barriers with zero point energy correction have also been calculated. The calculated results show that there are seven pathways on the reaction surface of BrO with CH₃SH, and those in the formations of CH₃S＋HOBr and CH₃SO＋HBr are dominant. The non-planar four-member-ring structure transition state (STS), which was firstly found in this paper, extended the concept of ring STS.

Key words: reaction mechanism, topological analysis of electronic density, structure transition state