Theoretical calculation of low-temperature oxidation of heptyl radicals and O2

Abstract

n-Heptane, as an alternative gasoline fuel, was used for research on low-temperature oxidation in this study, where the reactions of alkyl radicals (C7H15) and O2 play an important role. The related chemical reaction kinetics on four C7H15 radicals and O2 were investigated. The potential energy surfaces (PESs) of C7H15O2 were obtained by the quantum chemical calculation method of CBS-QB3. The pressure- and temperature- dependence of the rate constants was also calculated by solving master equations based on Rice–Ramsperger–Kassel–Marcus theory. In this work, all possible reaction pathways in the oxidation process were considered and calculated. Formation, concerted elimination, and intramolecular H atom transfer reactions of initial product RO2 are very critical to the low-temperature combustion. For four C7H15 radicals, the energy barriers that form RO2 from the HR + O2 association are typically zero, and barriers to subsequent reactions are different by up to 30 kcal/mol. It is most advantageous to form QOOH from RO2 through a six- or seven-member ring transition state. In terms of kinetic aspect, compared with the reaction of O2 plus secondary radicals (HR2, HR3, HR4), the reaction of adding O2 to primary radical (HR1) is less competitive at low temperatures. Moreover, the kinetic data obtained in this study will play an important role in the kinetic model of low-temperature combustion for n-heptane.