Theoretical and In Situ FTIR Studies of the Atmospheric Sink of Methyl Dichloroacetate by •OH Radicals and Cl• Atoms: Kinetics, Product Distribution, and Mechanism

Abstract

The atmospheric degradation of methyl dichloroacetate can be initiated by ^•OH and Cl^• radicals through H atom abstraction from the alkyl groups (Cl₂HC– or –CH₃) of the chloroester. Product yields for the gas-phase reaction with ^•OH were determined experimentally in a 480 L Pyrex glass atmospheric-simulation reactor coupled to an in situ Fourier transform infrared (FTIR) spectrometer. In addition to those results, we present in this paper a complete degradation mechanism based on thermodynamic data obtained by identifying all critical points on the potential-energy surface for these reactions, employing density functional calculations with the M06-2X and MN15 hybrid exchange–correlation functionals and the aug-cc-pVTZ basis sets. A conformational search for reactants and transition states was performed. The energies of these conformers were later corrected at the CCSD(T,Full)-F12/complete basis set level by using the SVECV-f12 composite method. The corrected energies were then used to obtain the theoretical rate coefficients in a multiconformer approach. The global rate coefficient calculated for the reaction of methyl dichloroacetate with ^•Cl atoms is (7.34 × 10^–12 cm³ molecule^–1·s^–1), and the global rate coefficient calculated for the reaction with ^•OH radicals is (1.07 × 10^–12 cm³ molecule^–1·s^–1). The identified products and their respective yield percentages for the reaction of MDCA with ^•OH were Cl₂CHCOOH (44 ± 3%), COCl₂ (43 ± 3%), and CO (41 ± 6%). The analysis of the mechanism suggests that formation of P1 (Cl₂CO, phosgene) occurs mainly by abstraction from the Cl₂HC– group since the formation of P4 (Cl₂CHC(O)OH, dichloroacetic acid) and P5 (CO, carbon monoxide) is more favorable in the path for abstraction from the –OCH₃ group. The multiconformer calculated rate constant values were compared with the values obtained employing only the low-lying TSs and with our own previous experimental studies. Branching ratios for the reaction with ^•Cl were compared to the experimental product yields.