Theoretical Analysis of the Kinetics of the Reaction of Methyl Mercaptan with Nitrogen Dioxide in the Gas Phase

In this study a theoretical analysis of the kinetics and thermochemistry of the gas phase reaction between methyl mercaptan and nitrogen dioxide is presented. The molecular properties (optimized geometries, vibrational levels, and rotational constants) of the reactants and products were derived from ab initio calculations. The relative total energies of the molecular structures taking part in the reaction kinetics were examined at the G3 level. The rate constants of the elementary steps and their dependence on temperature were evaluated using transition-state theory and a version of the statistical adiabatic channel model. The mechanism of the reaction CH 3 SH +NO 2 is complex and the first elementary step is related to hydrogen abstraction. The derived analytical expressions for the rate constants k 1a = 7.9×10 ―15 ×(T/300) 1.90 ×exp(―8190/T) and k 1b = 6.0×10 ―13 ×(T/300) 1.94 ×exp(―16290/T) cm 3 molecule ―1 s ―1 describe the kinetics of the hydrogen abstraction reactions CH 3 SH +NO 2 → CH 3 S + HNO 2 (1a) and CH 3 SH +NO 2 → CH 2 SH + HN0 2 (1 b), respectively, in the temperature range of200-500 K. The CH 3 S and CH 2 SH radicals formed can undergo subsequent radical-radical (2a-c) and radical-N0 2 (3a,b) recombination reactions with rates determined by the high-pressure limiting rate constant, expressed as k 2a,∞ (CH 3 S + CH 3 S) = 4.0×10 ―11 ×(T/300) 0.20 , k 2b,∞ (CH 3 S + CH 2 SH) = 1.2×10 ―10 ×(T/300) 0.15 , k 2c,∞ (CH 2 SH + CH 2 SH) = 6.7×10 ―11 ×(T/300) 0.19 , k 3a,∞ (CH 3 S+NO 2 )=1.3×10 ―11 (T/300) 0.19 , and k 3b,∞ (CH 2 SH + NO 2 ) = 2.6×10 ―11 ×(T/300) 0.23 cm 3 molecule s ―1 . Values of the calculated rate constants are in good agreement with available results of kinetic measurements.