Theoretical and Kinetic Study of the Hydrogen Atom Abstraction Reactions of Esters with HȮ₂ Radicals

Abstract

Herein, a systematic rate coefficients calculation of the hydrogen atom abstraction reactions of esters (methyl ethanoate, methyl propanoate, methyl butanoate, methyl pentanoate, methyl isobutyrate, ethyl ethanoate, propyl ethanoate and iso-propyl ethanoate) with the hydroperoxyl radical (HȮ 2 ) has been performed. The Møller-Plesset (MP2) method and the 6-311G(d,p) basis set have been used in order to optimize and calculate the frequencies of all of the species involved in the title reactions. MP2/6-311G(d,p) was used to determine the hindrance potentials for the reactants and transition states. The connection between each transition state and the corresponding local minima was validated by intrinsic reaction coordinate calculations. Electronic energies for all of the species are reported, in kcal mol −1 , using the CCSD(T)/cc-pVTZ level of theory with the corresponding zero-point energy corrections. The CCSD(T)/CBS (extrapolated from CCSD(T)/cc-pVXZ, where X = D, T, Q) was calculated for the reactions of methyl ethanoate + HȮ 2 radicals as a benchmark in the electronic energy calculations. High-pressure limit rate coefficients were determined with the use of the conventional transition state theory with corrections for the asymmetric Eckart tunneling for all of the reaction channels in this study from 500-2000 K. In both reactants and transition states, the 1-D hindered rotor approximation has been used in order to calculate the low frequency torsional modes. The calculated individual, average and total rate coefficients are reported for all of the reaction channels in every reaction system in this work. A branching ratio analysis for every reaction site has also been investigated for all of the esters studied in this work.

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

“…By contrast, the few existing theoretical and experimental studies of MP decomposition [21] and H-abstraction by OH [17,22] and HO 2 [23] exhibit large discrepancies between theory [17] and experiments [22] for H-abstraction rates by OH and in species profiles of MP pyrolysis [21] .…”

An experimental and modeling study of diethyl carbonate oxidation

“…These rate constants were taken from theoretical calculations by Mendes et al for ethyl esters with H [25] and H 2 radicals [26]. However, preliminary kinetic modeling simulations showed that the calculated rate constants were too slow in predicting experimental ignition delay times, so that these rates were increased by a factor of 2.5, which is the estimated uncertainty limit stated by Mendes et al [25,26]. For H-atom abstraction reactions from the fuel by other small radicals (e.g., , and ĊH 3 ), the rate constants were based on analogy with alkanes [27] with some modifications noted below.…”

Rate Constant Calculations of H-Atom Abstraction Reactions from Ethers by HȮ2 Radicals

“…This is comparable to our previous work on the ketones 4 and esters. 6 Their relative electronic energies range from −10.0 to −9.3 kcal mol −1 for RC and from 2.7 to 11.9 kcal mol −1 for PC. Figure 4 details the PES for DME + HȮ2 radicals and was obtained using the CCSD(T)/cc-pVTZ method and corresponding extrapolation to the CBS limit, with energies in kcal mol −1 .…”

“…In the same work, the trans reactant conformer of methyl pentanoate has energy Article pubs.acs.org/JPCA barriers of 4.5 and 5.7 kcal mol −1 for the α′−β′ and β′−γ′ hindrance potentials, respectively. 6 Therefore, as with our previous work with ketones 4,5 and esters, 6 we only consider the trans reactant conformers in our calculations. Figure 1 shows a 2D representation of the conformers of a reactant in this study, trans ( Figure 1a) and gauche (Figure 1b).…”