Kinetics of the Reactions of CH2OO with Acetone, α-Diketones, and β-Diketones

Rate constants for the reactions between the simplest Criegee intermediate, CH2OO, with acetone, the alpha-diketones biacetyl and acetylpropionyl, and the beta-diketones acetylacetone and 3,3-dimethyl-2,4-pentanedione have been measured at 295 K. CH2OO was produced photochemically in a flow reactor by 355 nm laser flash photolysis of diiodomethane in the presence of excess oxygen. Time-dependent concentrations were measured using broadband transient absorption spectroscopy, and the reaction kinetics was characterized under pseudo-first-order conditions. The bimolecular rate constant for the CH2OO + acetone reaction is measured to be (4.1 +/- 0.4) x 10(-13) cm(3) s(-1), consistent with previous measurements. The reactions of CH2OO with the beta-diketones acetylacetone and 3,3-dimethyl-2,5-pentanedione are found to have broadly similar rate constants of (6.6 +/- 0.7) x 10(-13) and (3.5 +/- 0.8) x 10(-13) cm(3) s(-1), respectively; these values may be cautiously considered as upper limits. In contrast, a-diketones react significantly faster, with rate constants of (1.45 +/- 0.18) x 10(-11) and (1.29 +/- 0.15) x 10(-11) cm(3) s(-1) measured for biacetyl and acetylpropionyl. The potential energy surfaces for these 1,3-dipolar cycloaddition reactions are characterized at the M06-2X/aug-cc-pVTZ and CBS-QB3 levels of theory and provide additional support to the observed experimental trends. The reactivity of carbonyl compounds with CH2OO is also interpreted by application of frontier molecular orbital theory and predicted using Hammett substituent constants. Finally, the results are compared with other kinetic studies of Criegee intermediate reactions with carbonyl compounds and discussed within the context of their atmospheric relevance.

Automated summary

The rate constants for the reactions between CH2OO and acetone, alpha-diketones, and beta-diketones were measured, showing faster reaction rates for alpha-diketones compared to beta-diketones with CH2OO. The results are consistent with previous measurements and provide insight into the reactivity of carbonyl compounds with Criegee intermediates.