(670d) On the Role of O2 + QOOH In Low-Temperature Ignition of Alkanes

The kinetics of the reaction of molecular oxygen with hydroperoxyalkylperoxy radicals have been studied theoretically.  These reactions – often referred to as second O₂ addition, or O₂ + QOOH reactions – are believed to be responsible for low-temperature chain branching in hydrocarbon oxidation.  The O₂ + propyl system was chosen as a model system.  High-level ab initio calculations of the C₃H₇O₂ and C₃H₇O₄ potential energy surfaces are coupled with RRKM master equation methods to compute the temperature- and pressure-dependence of the rate coefficients.  Variable reaction coordinate transition state theory is used to characterize the barrierless transition states for the O­₂ + QOOH addition reactions as well as subsequent C₃H₆O₃ dissociation reactions.  A simple kinetic mechanism is developed to quantify the extent to which the second O₂ addition increases the number of radicals.  Based upon the simulations, chain branching increases with pressure.  For the initial conditions considered here, chain branching is maximized in the temperature range of 700 to 900 K.  Low-temperature chain branching is significant only when the isomerization from a peroxy radical to a hydroperoxyalkyl radical proceeds via a transition state with six or more members.  The formally direct (or well skipping) reaction O₂ + QOOH → OH + OH + oxy-radical and the corresponding sequential reactions O₂ + QOOH → OOQOOH   → OH + ketohydroperoxide → OH + OH + oxy-radical increase the total number of radicals.  An analysis of the influence of 1D, separable models for hindered internal rotation on the phenomenological rate coefficients suggests that the uncertainty in the rate constants due to the hindered rotor model is less than a factor of two.  The results confirm that n-propyl is the smallest alkyl radical to exhibit the low-temperature combustion properties of larger alkyl radicals, but n-butyl is perhaps a truer combustion archetype.