Temperature-Dependent Kinetics of the Reactions of the Criegee Intermediate CH2OO with Aliphatic Aldehydes.

Abstract

Criegee intermediates, formed by alkene ozonolysis in the troposphere, can react with volatile organic compounds (VOCs). The temperature-dependent kinetics of the reactions between the Criegee intermediate CH₂OO and three aliphatic aldehydes, RCHO where R = H, CH₃, and C₂H₅ (formaldehyde, acetaldehyde, and propionaldehyde, respectively), have been studied using a laser flash-photolysis transient absorption spectroscopy technique. The experimental measurements are supported by ab initio calculations at various composite levels of theory that characterize stationary points on the reaction potential and free energy surfaces. As with other reactions of CH₂OO with organic carbonyls, the mechanisms involve 1,3-dipolar cycloaddition at the C=O group, over submerged barriers, leading to the formation of 1,2,4-trioxolane secondary ozonides. The bimolecular rate constants of all three reactions decrease with increasing temperature over the range 275-335 K and are characterized by equations of Arrhenius form: k(T) = (7.1 ± 1.5) × 10^-14exp((1160 ± 60)/T), (8.9 ± 1.7) × 10^-15exp((1530 ± 60)/T), and (5.3 ± 1.3) × 10^-14exp((1210 ± 70)/T) cm³ s^-1 for HCHO, CH₃CHO, and C₂H₅CHO, respectively. Based on estimated concentrations of CH₂OO, the reactions with aldehydes are unlikely to play a significant role in the atmosphere.