Dissociation Dynamics of CH3CHOO Criegee Intermediates in the Earth’s Atmosphere: Ab Initio and RRKM Study

Abstract

Criegee intermediates (CIs) are highly active molecules, which usually arise due to the ozonolysis of alkenes. They play an important role in many chemical reactions in both the lower and upper atmospheres of the Earth. Further dissociation products of CIs may interact with other atmospheric compounds to produce hydroxyl radicals, nitric and sulfuric acids, and other chemically active substances. In this work, we focus on one of the simplest and most easily formed Criegee intermediates, acetaldehyde oxide (CH₃CHOO), which exists in two possible forms: syn-CH₃CHOO and anti-CH₃CHOO, which differ in the orientation of the −OO group. Due to the high isomerization barrier between them, they are usually considered different isomers. In this work, we study the dissociation reactions of this molecule based on the assumption that the reaction can start from either syn-CH₃CHOO or anti-CH₃CHOO isomers. For that, we have determined the relative maximum and minimum energies as well as the main isomerization/dissociation reaction pathways based on the ab initio B3LYP/CCSD(T) calculations followed by the estimation of rate constants and product yields by the RRKM method. It was found that the main dissociation products in both cases are OH, CH₂CHO, and CH₃CO radicals, whereas the methyldioxirane decomposition products include methane, CO₂, and acetic acid. At high internal energy, a small number of O(¹D) atoms may be produced. The dissociation product yields of syn- and anti-isomers are generally quite different.