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A chemical kinetic submechanism proposed here is an essential prerequisite to model autoignition of methyl esters at low temperature region (less than ∼900 K), where reactions of methyl-ester peroxy radicals (ROO∙) and hydroperoxy methyl-ester radicals (Q∙OOH) are crucial and relatively unexplored. The potential energy surfaces of the methyl butanoate peroxy radicals + O2 systems are computed by the G3MP2B3 composite approach. 114 pathways are identified leading to the formation of key radicals in the ignition kernel such as OH and HO2. Particular attention is focused on: (1) intramolecular H-migration of ROO∙, (2) unimolecular dissociations of ROO∙ and Q∙OOH and (3) reactions of ROO∙ + HO2. Using the canonical transition state theory, the high-pressure limit rate constants for reactions in the kinetic submechanism are calculated. Standard enthalpy of formation, entropy and heat capacities are evaluated for intermediates and products formed during combustion by means of the standard statistical mechanics formulae. The agreement and disagreement between our calculated kinetic parameters and previous estimates offer further insight into the uncertainty associated with theoretical estimation. We perform the branching ratio analysis for the competing channels between the reverse dissociation of ROO∙ (ROO∙ → R∙ + O2) and explored unimolecular reactions decomposing ROO∙. Additionally, we also quantify the similarity and dissimilarity between the rate constants determined here and those previously calculated for normal alkanes. Finally, the effect of transition state size on the rate constants for the isomerizations of methyl-ester peroxy radicals is systematically analyzed. [ABSTRACT FROM AUTHOR]

In an attempt to construct detailed kinetic mechanisms for biodiesel fuels on the fly, high-pressure rate rules for the concerted HO 2 elimination reaction class were derived using a comprehensive training reaction set of more than 60 reactions involving different alkyl methyl/ethyl ester peroxy radicals (RCOOR')-OO • . Using the composite electronic structure method CBS-QB3 in combination with classical statistical mechanics and the transition state theory (TST) rate model, high-pressure rate constants for the reactions in the training set as well as thermodynamic properties for the species involved were calculated. The corrections from Eckart tunneling and hindered internal rotation (HIR) treatments were also included in the calculations. The results reveal that the ester group (-COO-) selectively promotes the reaction when compared with the traditional hydrocarbon fuels; thus it is recommended that the seven derived rate rules for the title reaction class (including the thermodynamic data of the species involved in the NASA format) should be used for construction of detailed kinetic mechanisms for real biodiesel molecules.