Solving the discrepancy between the direct and relative-rate determinations of unimolecular reaction kinetics of dimethyl-substituted Criegee intermediate (CH 3 ) 2 COO using a new photolytic precursor.

We have performed direct kinetic measurements of the thermal unimolecular reaction of (CH 3 ) 2 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 3 ) 2 CIBr), which photolysis at 213 nm in the presence of O 2 produces acetone oxide, (CH 3 ) 2 COO. The results show that the thermal unimolecular reaction is even more important main loss process of (CH 3 ) 2 COO in the atmosphere than direct kinetic studies have suggested hitherto. The current experiments show that the unimolecular reaction rate of (CH 3 ) 2 COO at 296 K and atmospheric pressure is 899 ± 42 s -1 . Probably more importantly, current measurements bring the direct and relative-rate measurements of thermal unimolecular reaction kinetics of (CH 3 ) 2 COO into quantitative agreement.