Theoretical study on the unimolecular dissociation of CH3SCH3 and CH3SCH2

We have studied the unimolecular dissociation reaction of CH3SCH3 (DMS) and CH3SCH2 radical theoretically. The structures of reactants, activated complexes, and products have been optimized at the MP2/6-311G(D,P) level. Energies have been derived from single point MP4SDTQ/6-311G(D,P) calculations at the MP2 geometries. The barrier height corrected for zero point energy for the unimolecular dissociation of DMS to CH3 and SCH3 in MP4SDTQ and CAS(2,2)MP2 calculations was found equal to 295.3 and 310.0 kJ mol(-1), respectively. The barrier height corrected for zero point energy for the dissociation reaction of CH3SCH2 radical to CH3 and SCH2 was calculated to be 135.5 kJ mol(-1) at the MP4SDTQ level of theory. DMS is a C-2v molecule with two C tops. The potential constants and barrier height for the torsional motion of methyl groups in DMS were also calculated. At the MP4SDTQ level of theory, the torsional barrier height for a methyl group was found to be 8.21 kJ mol(-1). Generalized transition state theory and RRKM method were employed to calculate the rate constants for the two reactions in the title in a temperature range of 300-3000 K. According to generalized transition state theory, we have found the Arrhenius parameters for the unimolecular dissociation reactions of DMS and CH3SCH2, k(1) = 5.3 x 10(15) exp(-318.8 kJ mol(-1)/RT) s(-1) and k(3) = 9.2 x 10(13) exp(-138.4 kJ mol(-1)/RT) s(-1), respectively. According to RRKM method, we have found the high-pressure limiting rate constant values: k(1) = 6.1 x 10(15) exp(-317.2 kJ mol(-1)/R7) s(-1) and k(3) = 4.4 x 10(13) exp(-138.0 kJ mol(-1)/RT) s(-1).