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

Abstract

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.

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