Unraveling the low-temperature oxidation mechanism between methyl crotonate radicals and O2

In this paper, chemical reaction kinetics at low temperatures on three different methyl crotonate (MC, C5H8O2) radicals with O2 were conducted via quantum chemical methods. The potential energy surfaces (PESs) for these reactions were investigated by M062x/6-311++G(d,p) and CBS-QB3 methods. The related rate coefficients also have been solved by master equations based on Rice-Ramsperger-Kassel-Marcus theory, predicting the competitive relationships over 300 to 1500 K and 0.001 to 100 atm. The calculated results indicated that the rate constants of O2 addition reaction at ester methylic site were higher than those at allylic sites, which showed that the conjugation effect caused by the C=C double bond has a crucial effect on the reaction process. Formation of initial adducts and intramolecular H-transfer reactions play a great role in the low temperature oxidation of MC. Furthermore, the mechanism of O2 addition to MC radicals was verified by the previous combustion model. The updated model did a good job to replicate the previous experimental results. This work not only provides the necessary rate constants for the reaction mechanism of MC combustion but also serves as a solid starting point for the further understanding of combustion kinetics of large molecule unsaturated biodiesels.