Temperature-Dependent Kinetics of the Reaction of the Criegee Intermediate CH2OO with Pyruvic Acid.

The kinetics of the reaction between formaldehyde oxide (CH2OO) and pyruvic acid (CH3COCOOH) has been investigated. Laser flash-photolysis-transient absorption spectroscopy measurements over the range T = 275-375 K were used to measure T-dependent bimolecular rate constants. The rate constants show non-Arrhenius behavior, initially increasing and subsequently decreasing with T. A maximum value of (21.4 ± 1.1) × 10-12 cm3 s-1 was observed at 335 K. At room temperature (296 K), the rate constant was measured to be (19.5 ± 1.2) × 10-12 cm3 s-1, which is in line with the value expected for reaction of CH2OO with a substituted carbonyl and significantly smaller than typical values for reactions with carboxylic acids. Various pathways have been characterized by ab initio calculations at the CBS-QB3 level for reaction of CH2OO with the two lowest-energy conformers of pyruvic acid, labeled Tc and Tt. Both conformers can undergo 1,3-dipolar cycloaddition reactions at either carbonyl group to form secondary ozonides via submerged barriers. The presence of an internal H-bond in the more stable Tc conformer has a significant impact on reactivity, inhibiting the alternative 1,2-addition/insertion and 1,4-addition reaction pathways involving the OH or COOH groups, respectively, that occur more readily for the less stable Tt conformer and form hydroperoxymethylpyruvate ester. The unusual T dependence of the measured rate constants is attributed to the increasing thermal population of the less stable Tt conformer at higher T, and the availability of additional reaction pathways.