Ab Initio Reaction Kinetics of CH₃OC(=O) and CH₂OC(=O)H Radicals

The dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonyl radical (CH₃OC(=O)) and (formyloxy)methyl radical (CH₂OC(=O)H) are investigated theoretically using high-level ab initio methods and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Geometries obtained at the hybrid density functional theory (DFT) and coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) levels of theory are found to be similar. We employ high-level ab initio wave function methods to refine the potential energy surface: CCSD(T), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled-pair functional (MRACPF2) theory. MRSDCI+DS and MRACPF2 capture the multiconfigurational character of transition states (TSs) and predict lower barrier heights than CCSD(T). The temperature- and pressure-dependent rate coefficients are computed using RRKM/ME theory in the temperature range 300-2500 K and a pressure range of 0.01 atm to the high-pressure limit, which are then fitted to modified Arrhenius expressions. Dissociation of CH₃OC(=O) to CH₃ and CO₂ is predicted to be much faster than dissociating to CH₃O and CO, consistent with its greater exothermicity. Isomerization between CH₃OC(=O) and CH₂OC(=O)H is predicted to be the slowest among the studied reactions and rarely happens even at high temperature and high pressure, suggesting the decomposition pathways of the two radicals are not strongly coupled. The predicted rate coefficients and branching fractions at finite pressures differ significantly from the corresponding high-pressure-limit results, especially at relatively high temperatures. Finally, because it is one of the most important CH₃O removal mechanisms under atmospheric conditions, the reaction kinetics of CH₃O + CO was also studied along the PES of CH₃OC(=O); the resulting kinetics predictions are in remarkable agreement with experiments.