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 (Cl 2 HC− or −CH 3 ) 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 cm 3 molecule −1 •s −1 ), and the global rate coefficient calculated for the reaction with • OH radicals is (1.07 × 10 −12 cm 3 molecule −1 •s −1 ). The identified products and their respective yield percentages for the reaction of MDCA with • OH were Cl 2 CHCOOH (44 ± 3%), COCl 2 (43 ± 3%), and CO (41 ± 6%). The analysis of the mechanism suggests that formation of P1 (Cl 2 CO, phosgene) occurs mainly by abstraction from the Cl 2 HC− group since the formation of P4 (Cl 2 CHC(O)OH, dichloroacetic acid) and P5 (CO, carbon monoxide) is more favorable in the path for abstraction from the −OCH 3 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.