Kinetics of the Gas-Phase Reactions of syn - and anti -CH 3 CHOO Criegee Intermediate Conformers with SO 2 as a Function of Temperature and Pressure.

Kinetics of reactions between SO 2 and CH 3 CHOO Criegee intermediate conformers have been measured at temperatures between 242 and 353 K and pressures between 10 and 600 Torr using laser flash photolysis of CH 3 CHI 2 /O 2 /N 2 /SO 2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. The kinetics of syn -CH 3 CHOO + SO 2 are pressure-dependent and exhibit a negative temperature dependence, with the observed pressure dependence reconciling apparent discrepancies between previous measurements performed at ∼298 K. Results indicate a rate coefficient of (4.80 ± 0.46) × 10 -11 cm 3 s -1 for the reaction of syn -CH 3 CHOO with SO 2 at 298 K and 760 Torr. In contrast to the behavior of the syn -conformer, the kinetics of anti -CH 3 CHOO + SO 2 display no significant dependence on temperature or pressure over the ranges investigated, with a mean rate coefficient of (1.18 ± 0.21) × 10 -10 cm 3 s -1 over all conditions studied in this work. Results indicate that the reaction of syn -CH 3 CHOO with SO 2 competes with unimolecular decomposition and reaction with water vapor in areas with high SO 2 concentration and low humidity, particularly at lower temperatures.