Theoretical Investigation on Mechanism and Kinetics of M2CAA Initiated by Cl Atom in the Atmosphere

Abstract

Methyl 2-chloroacetoacetate (M2CAA) is a class of volatile organic compound which finds their place in atmosphere by anthropogenic sources. The present study focused on the hydrogen abstraction reaction of M2CAA by Cl atom. The fate of these M2CAA is however poorly understood and scarcely taken into account in atmospheric chemistry modeling. The objective of this study was to explore the mechanistic and kinetic data relative to Cl oxidation in the atmospheric degradation of M2CAA. A comprehensive theoretical investigation was conducted to elucidate the mechanism, kinetics, and thermochemistry of the gas-phase reactions between Methyl 2-chloroacetoacetate (CH₃C(O)CHClC(O)OCH₃, M2CAA) and Chlorine atoms using the M06-2X functional. The most thermodynamically stable conformer of M2CAA was identified at ambient temperature. Three primary hydrogen abstraction pathways were characterized, each proceeding through the formation of a pre-reactive complex, indicating that the reactions follow an indirect hydrogen abstraction mechanism. Rate coefficients for these pathways were calculated for the first time over a temperature range of 250–450 K using Canonical Transition State Theory (CTST). Based on these kinetic results, the atmospheric lifetime of M2CAA was estimated to be approximately 1.85 days. From these results, it can be emphasize that the hydrogen abstraction from the -CHCl group is kinetically more advantageous than the other two reaction pathways i.e. abstraction from –CH3 and – COOCH3 group.