Ab initio pressure-dependent reaction kinetics of methyl propanoate radicals

Abstract

The unimolecular dissociation and isomerization kinetics of the three methyl propanoate (MP) radicals, CH3CH2C(=O)OĊH2 (MP-m), CH3ĊHC(=O)OCH3 (MP-α), and ĊH2CH2C(=O)OCH3 (MP-β), are theoretically investigated using high-level ab initio methods and the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Stationary-point energies are obtained using the coupled cluster singles and doubles with perturbative triples correction (CCSD(T)), multi-reference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multi-reference averaged coupled pair functional (MRACPF2) theories. The isomerization barriers between the three radicals are predicted to be generally lower than the corresponding bond dissociation channels, leading to a strongly coupled reaction system in subsequent kinetics studies. The phenomenological temperature- and pressure-dependent rate coefficients are computed using the RRKM/ME theory over a temperature range of 500 to 2000 K and at a pressure range of 0.01 atm to the high-pressure limit, which are then fitted to modified Arrhenius expressions. The β-scission rate coefficients of MP-α to CH3CHC(=O) and CH3O[combining dot above] are predicted to be the smallest because of its highest activation energy among all studied unimolecular reactions channels. Analysis of branching fractions shows that both MP-m and MP-α radicals mainly decompose to the bimolecular products CH3CH2Ċ(=O) and H2CO, whereas the MP-β radical primarily decomposes via cleavage of a C-C bond to form C2H4 and CH3OĊ(=O). The isomerization channels dominate at low temperatures, the branching fractions of which decrease with increasing temperature and become very minor at about 2000 K. Our accurate rate coefficients and branching fractions help to illuminate the unique combustion properties of MP.

Contributions to improving small ester combustion chemistry: Theory, model and experiments

“…For the H-abstraction reactions from MP, the stationary point energies were then refined at the CCSD(T)/cc-pV X Z ( X = D,T,Q) level [26] . The energy extrapolation [27] to the complete basis set (CBS) limit using the energies with the cc-pVDZ, cc-pVTZ basis sets, referred to as CBS(D-T), are in good agreement with the corresponding CBS limit using the ccpVTZ and cc-pVQZ bases (CBS(T-Q)) for the reactions MP + H/O/OH/CH 3 with other methods used in [19] , the former CBS extrapolation was used in the kinetics calculations for all fiv e studied abstraction reaction systems. Rate constants for MP H-abstraction by H, CH 3 , and O were obtained using (E, J)-resolved TST-RRKM theory within the one-dimensional (1D) separable-hindered-rotor approximation for all torsions, where the rotational potential energy surface (PES) for the low-frequency torsional mode was obtained by a relaxed surface scan at the DFT-M08-HX/cc-pVTZ level.…”

“…The asymmetric Eckart approximation was employed to account for quantum tunneling in all kinetics calculations. Details of the calculations are available in [19,20] . Figure S1 and Table S1 schematically represent important reactions included in this work, while Fig.…”

“…Therefore, high-level ab-initio RRKM masterequation calculations using the VARIFLEX code [24] were performed to determine rate coefficients for H-abstractions from MP by H, CH 3 , OH, HO 2 , and O [20] and for subsequent unimolecular reactions of the three MP radicals produced from Habstraction [19] . The asymmetric Eckart approximation was employed to account for quantum tunneling in all kinetics calculations.…”

“…In the present study, based on the knowledge of high-level theoretical calculations of MP reactions [19,20] , our goal is to obtain new measurements of species distribution in low-pressure flames and flame speeds at elevated pressure, and to develop a new kinetics mechanism to gain detailed insights into the reaction kinetics of MP. Specifically, full speciation measurements of MP oxidation at 40 mbar were made at two different stoichiometries ( φ = 0.8 and 1.5) using lowpressure flat flames.…”

“…Furthermore, theoretical calculations were recently performed on the H-abstraction by OH [17] and H-abstraction reactions at the methoxy group of methyl esters [18] . Recent high-level quantum chemistry and kinetics computations have addressed the decomposition and isomerization of MP radicals [19] ; also, H-abstractions from MP by H/O/OH/CH 3 /HO 2 have been calculated [20] . These newly calculated reaction rates and their pressure dependences deviate significantly from those estimated from the similarity rules.…”