Theoretical and In Situ FTIR Studies of the Atmospheric\nSink of Methyl Dichloroacetate by <sup>•</sup>OH Radicals and\nCl<sup>•</sup> Atoms: Kinetics, Product Distribution, and Mechanism

The\natmospheric degradation of methyl dichloroacetate can be initiated\nby <sup>•</sup>OH and Cl<sup>•</sup> radicals through\nH atom abstraction from the alkyl groups (Cl<sub>2</sub>HC–\nor –CH<sub>3</sub>) of the chloroester. Product yields for\nthe gas-phase reaction with <sup>•</sup>OH were determined\nexperimentally in a 480 L Pyrex glass atmospheric-simulation reactor\ncoupled to an in situ Fourier transform infrared (FTIR) spectrometer.\nIn addition to those results, we present in this paper a complete\ndegradation mechanism based on thermodynamic data obtained by identifying\nall critical points on the potential-energy surface for these reactions,\nemploying density functional calculations with the M06-2X and MN15\nhybrid exchange–correlation functionals and the aug-cc-pVTZ\nbasis sets. A conformational search for reactants and transition states\nwas performed. The energies of these conformers were later corrected\nat the CCSD(T,Full)-F12/complete basis set level by using the SVECV-f12\ncomposite method. The corrected energies were then used to obtain\nthe theoretical rate coefficients in a multiconformer approach. The\nglobal rate coefficient calculated for the reaction of methyl dichloroacetate\nwith <sup>•</sup>Cl atoms is (7.34 × 10<sup>–12</sup> cm<sup>3</sup> molecule<sup>–1</sup>·s<sup>–1</sup>), and the global rate coefficient calculated for the reaction with <sup>•</sup>OH radicals is (1.07 × 10<sup>–12</sup> cm<sup>3</sup> molecule<sup>–1</sup>·s<sup>–1</sup>). The identified products and their respective yield percentages\nfor the reaction of MDCA with <sup>•</sup>OH were Cl<sub>2</sub>CHCOOH (44 ± 3%), COCl<sub>2</sub> (43 ± 3%), and CO (41\n± 6%). The analysis of the mechanism suggests that formation\nof P1 (Cl<sub>2</sub>CO, phosgene) occurs mainly by abstraction from\nthe Cl<sub>2</sub>HC– group since the formation of P4 (Cl<sub>2</sub>CHC(O)OH, dichloroacetic acid) and P5 (CO, carbon monoxide)\nis more favorable in the path for abstraction from the –OCH<sub>3</sub> group. The multiconformer calculated rate constant values\nwere compared with the values obtained employing only the low-lying\nTSs and with our own previous experimental studies. Branching ratios\nfor the reaction with <sup>•</sup>Cl were compared to the experimental\nproduct yields.