Theoretical study on reaction kinetics of the absent HO2 concerted addition reaction on ester group of methyl esters: Case studies of methyl formate and methyl propanoate

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In this work, a comprehensive study is performed to specify the contributions of H-abstraction, addition and addition-dissociation reactions of methyl esters + HO2, particularly to the low temperature oxidation. It is imperative to investigate these reactions in detail despite the generally higher bond strength of C=O compared to C=C. The two representative methyl esters, methyl formate (MF, HC(=O)OCH3) and methyl propanoate (MP, CH3CH2C(=O)OCH3), are chosen to theoretically explore the reaction kinetics of the C=O of ester group and HO2. The reaction pathways and potential energy profiles including H-abstraction, addition and addition-dissociation of MF+HO2 and MP+HO2 are investigated at the CCSD(T)/cc-pVxZ(x = T, Q)//M062X/6–311+G(2df,2p) and DLPNO-CCSD(T)/cc-pVxZ(x = T, Q)//M062X/6–311+G(2df,2p) level of theory, respectively. The rate constants of MF/MP + HO2 are determined by using the RRKM/master equation coupled with the 1-D hindered rotor approximation and Eckart tunneling correction. This work delves into the temperature and pressure-dependence of these reaction pathways, as well as the competition relationship among of them. The results indicate that for small ester (MF), the H-abstraction is more competitive across all reaction pathways, but under low temperature (≤ 700 K) and high pressure (≥ 10 atm) conditions, the collisional stabilization of peroxy-alcohol radicals (MF+HO2 = HO-ROO) becomes predominant. For larger ester (MP), this is not necessarily the case as such reactions are unimportant for combustion conditions (rates only appear at 400 K, 100 atm). Furthermore, to assess the impact of these reactions on kinetic modeling, the new rate constants of MF/MP + HO2 are incorporated into the kinetic models of MF and MP, respectively. The updated kinetic models reveal the significant role of H-abstraction reactions in ignition delay time and fuel consumption and the formation of peroxy-alcohol radicals is favorable for decomposition pathways, notably affecting fuel consumption at low temperatures. Consequently, this study highlights the important role of the HO2 addition to the C=O of ester group in the low temperature oxidation mechanism of methyl esters, which has been overlooked in previous studies.