Reactions of β-hydroxypropyl radicals with O2 on the HOC3H6OO• potential energy surfaces: A theoretical study

Abstract

Although addition reactions of β-hydroxypropyl radicals with O2 are important steps in the reaction pathways of propylene oxidation at low temperature, there is a lack of available accurate rate constants for these reactions in the literature. To obtain complete reaction channels and relatively accurate rate constants over a wide range of temperatures and pressures, a systematic method, which combines the high level ab initio calculation, the conventional transition state theory (TST), the variational transition state theory (VTST) and Rice-Ramsberger-Kassel-Marcus/Master-Equation theory (RRKM/ME), was used to investigate these reactions. The CCSD(T)/cc-pVTZ method was applied to calculate potential energy surfaces. High-pressure limit rate constants of barrierless reactions and those reactions with tight transition states were obtained using the VTST and the TST, respectively. RRKM/ME theory was used to compute rate constants in the fall-off range for pressure-dependent reactions. Take CC•COH as an example, the peroxy adduct of CC•COH with O2 can further react to form more stable products, and subsequent reactions include H-shift reactions, HO2 elimination reactions, OH elimination reactions and Waddington reaction. In these reactions, Waddington reaction is one of the most competitive advantage paths, as it has the lowest barrier. This conclusion is similar to that of those reactions about O2 with products from OH addition to ethylene or isobutene. Rate constants of relevant reactions were fitted to the form of a modified Arrhenius expression, and were implemented into the existing mechanism to simulate concentration profiles of main species of propylene oxidation in a jet-stirred reactor. The results show that predicted results of the revised mechanism are closer to the experimental data than those of the original mechanism. Our calculated rate constants over a wide range of temperatures and pressures for these reactions are transferable and can be applied to a wider range of oxidation conditions.