Solving the discrepancy between the direct and relative-rate determinations of unimolecular reaction kinetics of dimethyl-substituted Criegee intermediate (CH)COO using a new photolytic precursor.

We have performed direct kinetic measurements of the thermal unimolecular reaction of (CH)COO in the temperature range 243-340 K and pressure range 5-350 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, 2-bromo-2-iodopropane ((CH)CIBr), which photolysis at 213 nm in the presence of O produces acetone oxide, (CH)COO. The results show that the thermal unimolecular reaction is even more important main loss process of (CH)COO in the atmosphere than direct kinetic studies have suggested hitherto. The current experiments show that the unimolecular reaction rate of (CH)COO at 296 K and atmospheric pressure is 899 ± 42 s. Probably more importantly, current measurements bring the direct and relative-rate measurements of thermal unimolecular reaction kinetics of (CH)COO into quantitative agreement.