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 <i>T</i> = 275-375 K were used to measure <i>T</i>-dependent bimolecular rate constants. The rate constants show non-Arrhenius behavior, initially increasing and subsequently decreasing with <i>T</i>. 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 <i>T</i> dependence of the measured rate constants is attributed to the increasing thermal population of the less stable Tt conformer at higher <i>T</i>, and the availability of additional reaction pathways.