Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH3)2COO

The Criegee intermediate acetone oxide, (CH 3 ) 2 COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O 2 and characterized by synchrotron photoionization mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO 2 was measured using photoionization mass spectrometry and pseudo-first-order methods to be (7.3 ± 0.5) × 10 –11 cm 3 s –1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10 –10 cm 3 s –1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of anti -CH 3 CHOO with SO 2, and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 and 100 Torr) in N 2 from cavity ring-down decay of the ultraviolet absorption of (CH 3 ) 2 COO yielded even larger rate coefficients, in the range (1.84 ± 0.12) × 10 –10 to (2.29 ± 0.08) × 10 –10 cm 3 s –1 . Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, k H / k D = (0.53 ± 0.06), for reactions with SO 2, which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD 3 ) 2 COO with NO 2 has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10 –12 cm 3 s –1 (measured with photoionization mass spectrometry), again similar to rate for the reaction of anti -CH 3 CHOO with NO 2 . Cavity ring-down measurements of the acetone oxide removal without added reagents display a combination of first- and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH 3 ) 2 COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s –1, is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH 3 ) 2 COO with SO 2 and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH 3 ) 2 COO with NO 2 and with SO 2 suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products.