Abstract:

Criegee intermediates (CIs) are a reactive species that are products of alkene ozonolysis and contribute to the oxidative capacity of the atmosphere. The temperature-dependent kinetics of the reactions of the simplest and most abundant CI, CH₂OO, with a series of ketones and functionalized ketones was studied using laser flash photolysis transient absorption spectroscopy experiments and complementary quantum chemical calculations. The reactions proceed via a 1,3-dipolar cycloaddition mechanism, forming cyclic secondary ozonides (SOZs). The bimolecular rate constants for the reactions of CH₂OO with acetone, acetylacetone, biacetyl  and two hydroxyketones (hydroxyacetone and hydroxybutanone) were measured across the temperature range, 275–335 K. The α-dicarbonyl biacetyl reacts around 30 times faster than the model ketone acetone, while the hydroxyketones react twice as fast. All reactions studied show negative temperature dependencies, consistent with the submerged transition state barriers identified in the quantum chemistry calculations. Observed reactivity trends can be rationalized using frontier molecular orbital theory, in which electron-withdrawing substituents on the carbonyl decrease the energy of the π* orbital. A structure-activity relationship (SAR) was developed that quantitatively relates the experimental rate constants to the carbonyl orbital energies, evaluated using low-cost density functional theory methods.