CBS-QB3 + VTST Study of Methyl N-Methylcarbamate + OH Gas-Phase Reaction: Mechanism, Kinetics, and Branching Ratios.

Abstract

Different paths of reaction have been modeled, at high level of theory, accounting for the possible atmospheric fate of methyl N-methylcarbamate (MMC). The OH hydrogen abstractions from the methyl groups are predicted to account for almost 100% of the MMC + OH gas-phase reaction. The H abstraction from the methyl group at the N side of MMC was found to be the main path of reaction, with contributions to the overall reaction from 96% at 260 K to 89.2% at 400 K. Hydrogen abstractions from the other methyl group were identified as a secondary, but significant path. The Arrhenius activation energy, in the temperature range 260-400 K, was found to be close to zero, but slightly negative. The proposed value is (-0.10 ± 0.02) kcal/mol. The excellent agreement with the scarce experimental data available supports the reliability of the data reported here for the first time. Different IVTST-M-H/G schemes were tested for dynamic calculations. According to our results, including the Hessians and gradients, as few as four nonstationary points along the MEP is sufficient to achieve an excellent accuracy, provided that the data for these few points were obtained from high-quality levels of theory.