Computational study on the kinetics of OH initiated oxidation of methyl difluoroacetate (CF2HCOOCH3)

Theoretical Investigation on the Mechanism and Kinetics of Atmospheric Reaction of Methyldichloroacetate with Hydroxyl Radical

The atmospheric reaction of methyldichloroacetate (MDCA) with OH radical is studied using electronic structure calculations. Five different pathways were considered for the initial reactions, which results in the formation of alkyl radical of MDCA along with H₂O, HOCl, and CH₃O^•. Among the five pathways studied, the α-carbon atom (-CHCl₂ site) H atom abstraction reaction, which leads to the formation of the alkyl radical intermediate ^•CCl₂C(O)OCH₃ (I1) is found to be more favorable with an energy barrier of 7.3 kcal/mol, and Cl-atom abstraction reaction is having high energy barrier of 21.3 kcal/mol at M06-2X/6-311++G(2df,2p) level. The calculated thermochemical parameters show that except Cl-atom abstraction channel the other initial reaction channels are highly exothermic. The rate constant is calculated for the initial H atom abstraction reactions using canonical variational transition state theory over the temperature range of 278 to 350 K. The Arrhenius plot shows positive temperature dependence for both the reactions. The results from the calculated thermochemical parameters and rate constants show that the formation of the alkyl radical intermediate (I1) is more favorable with the rate constant of 2.07 × 10^-13 cm³ molecule^-1 s^-1 at 298 K. The calculated atmospheric lifetime of MDCA is 28 days at normal atmospheric OH concentration. The results obtained from secondary reactions show that the major product formed from the oxidation chemistry of MDCA is methyl-2-chloro-2-oxoacetate (or) methyl oxalyl chloride.

Theoretical Investigations on the Mechanism and Kinetics of OH Radical Initiated Reactions of Monochloroacetic Acid

The oxidation mechanism of monochloroacetic acid (CH₂ClCOOH) by OH radical has been systematically investigated employing quantum mechanical methods coupled with kinetic calculation using canonical variational transition state theory. Three distinct transition states were identified for the titled reaction, two corresponding to the hydrogen atom abstraction and one corresponding to the chlorine atom abstraction. The rate constants of the titled reactions are computed over the temperature range 278-350 K, and the branching ratios calculated for the hydrogen atom abstraction from the -C(O)OH site and the -CH₂Cl site are 25 and 75%, respectively, at 298 K. The computed branching ratio indicates that the kinetically favorable reaction is the hydrogen atom abstraction from the -CH₂Cl site resulting in the formation of CHClC(O)OH radical, which further undergoes secondary reaction with O₂ and other atmospheric species. The calculated overall rate constant for the hydrogen atom abstraction reactions is in consistent with the reported experimental rate constant. The atmospheric lifetime of CH₂ClCOOH is found to be around 18 days.

Theoretical investigation on the atmospheric fate of CF3C(O)OCH 2O radical: alpha-ester rearrangement vs oxidation at 298 K

A theoretical study on the mechanism of the thermal decomposition of CF(3)C(O)OCH(2)O radical is presented for the first time. Geometry optimization and frequency calculations were performed at the MPWB1K/6-31 + G(d, p) level of theory and energetic information further refined by calculating the energy of the species using G2(MP2) theory. Three plausible decomposition pathways including α-ester rearrangement, reaction with O(2) and thermal decomposition (C-O bond scission) were considered in detail. Our results reveal that reaction with O(2) is the dominant path for the decomposition of CF(3)C(O)OCH(2)O radical in the atmosphere, involving the lowest energy barrier, which is in accord with experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid TFA makes a negligible contribution to decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways were evaluated using canonical transition state theory (CTST) at 298 K.