Ab Initio Unimolecular Reaction Kinetics of CH₂C(=O)OCH₃ and CH₃C(=O)OCH₂ Radicals

The unimolecular dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonylmethyl (CH₂C(=O)OCH₃) and acetyloxylmethyl (CH₃C(=O)OCH₂) are theoretically investigated using high-level ab initio methods and the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Potential energy surfaces (PESs) are obtained using coupled cluster singles and doubles with perturbative triples correction (CCSD(T)), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled pair functional (MRACPF2) theory. The transition states exhibit high T1 diagnostics in coupled cluster calculations, suggesting the need for a multireference correlated wave function treatment. MRSDCI+DS and MRACPF2 capture their multiconfigurational character well, yielding lower barrier heights than CCSD(T) for these reactions. The rate coefficients are computed using the RRKM/ME theory over a 500-2500 K temperature range and at a pressure range of 0.01 atm to the high-pressure limit. The temperature- and pressure-dependent rate coefficients are given in modified Arrhenius expressions. The β-scission of CH₂C(=O)OCH₃ is predicted to have a much higher barrier than the corresponding isomerization reaction and the β-scission of CH₃C(=O)OCH₂. Consequently, the rate coefficients for β-scission of CH₂C(=O)OCH₃ are the smallest among the three reactions and the isomerization followed by decomposition to CH₃C(=O) and HCHO is the dominant reaction pathway for CH₂C(=O)OCH₃. Both radicals CH₂C(=O)OCH₃ and CH₃C(=O)OCH₂ are predicted to mainly decompose to CH₃C(=O) + HCHO rather than to the bimolecular product CH₂C(=O) + CH₃O. A newly developed MA combustion mechanism, using our theoretical rate coefficients for the MA-related reactions, predicts combustion properties in good agreement with available experimental data.