Kinetics of the Gas-Phase Reactions of syn- and anti-CH₃CHOO Criegee Intermediate Conformers with SO₂ as a Function of Temperature and Pressure

Kinetics of reactions between SO₂ and CH₃CHOO 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 CH₃CHI₂/O₂/N₂/SO₂ gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. The kinetics of syn-CH₃CHOO + SO₂ 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 cm³ s^–1 for the reaction of syn-CH₃CHOO with SO₂ at 298 K and 760 Torr. In contrast to the behavior of the syn-conformer, the kinetics of anti-CH₃CHOO + SO₂ display no significant dependence on temperature or pressure over the ranges investigated, with a mean rate coefficient of (1.18 ± 0.21) × 10^–10 cm³ s^–1 over all conditions studied in this work. Results indicate that the reaction of syn-CH₃CHOO with SO₂ competes with unimolecular decomposition and reaction with water vapor in areas with high SO₂ concentration and low humidity, particularly at lower temperatures.