Theoretical investigation of the atmospheric chemistry of methyl difluoroacetate: reaction with Cl atoms and fate of alkoxy radical at 298 K

Mechanism and kinetics of the oxidation of dimethyl carbonate by hydroxyl radical in the atmosphere

The mechanism and kinetics for the reaction of dimethyl carbonate (DMC) with OH radical have been studied by using quantum chemical methods. Four reaction pathways were identified for the initial reaction. In the first two pathways, hydrogen atom abstraction is taking place and alkyl radical intermediate is formed with the energy barrier of 6.4 and 7.9 kcal/mol. In the third pathway, OH addition reaction to the carbonyl carbon (C2) atom of DMC and intermediate, I2, is formed with an energy barrier of 11.9 kcal/mol. In the fourth pathway, along with CH₃O^●, methyl hydrogen carbonate is formed. For this C-O bond breaking and O-H addition reaction, the energy barrier is 27 kcal/mol. The calculated enthalpy and Gibbs energy values show that the studied initial reactions are exothermic and exoergic except the OH addition reaction. For the initial reactions, the rate constants were calculated by using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) correction over the temperature range of 278-1200 K. At 298 K, the calculated rate coefficient for the in-plane and out-of-plane hydrogen atom abstraction reaction pathway is 2.30 × 10^-13 and 0.02 × 10^-13 cm³ molecule^-1 s^-1. Further, the reaction between alkyl radical intermediate formed from the first pathway and O₂ is studied. The reaction of alkyl peroxy radical intermediate with atmospheric oxidants, HO₂, NO, and NO₂ is also studied. It was found that the formic (methyl carbonic) anhydride is the end product formed from the atmospheric oxidation and secondary reactions of DMC.

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.