The multichannel n-propyl + O2 reaction surface: Definitive theory on a model hydrocarbon oxidation mechanism

Abstract

The n-propyl + O2 reaction is an important model of chain branching reactions in larger combustion systems. In this work, focal point analyses (FPAs) extrapolating to the ab initio limit were performed on the n-propyl + O2 system based on explicit quantum chemical computations with electron correlation treatments through coupled cluster single, double, triple, and perturbative quadruple excitations [CCSDT(Q)] and basis sets up to cc-pV5Z. All reaction species and transition states were fully optimized at the rigorous CCSD(T)/cc-pVTZ level of theory, revealing some substantial differences in comparison to the density functional theory geometries existing in the literature. A mixed Hessian methodology was implemented and benchmarked that essentially makes the computations of CCSD(T)/cc-pVTZ vibrational frequencies feasible and thus provides critical improvements to zero-point vibrational energies for the n-propyl + O2 system. Two key stationary points, n-propylperoxy radical (MIN1) and its concerted elimination transition state (TS1), were located 32.7 kcal mol−1 and 2.4 kcal mol−1 below the reactants, respectively. Two competitive β-hydrogen transfer transition states (TS2 and TS2′) were found separated by only 0.16 kcal mol−1, a fact unrecognized in the current combustion literature. Incorporating TS2′ in master equation (ME) kinetic models might reduce the large discrepancy of 2.5 kcal mol−1 between FPA and ME barrier heights for TS2. TS2 exhibits an anomalously large diagonal Born-Oppenheimer correction (ΔDBOC = 1.71 kcal mol−1), which is indicative of a nearby surface crossing and possible nonadiabatic reaction dynamics. The first systematic conformational search of three hydroperoxypropyl (QOOH) intermediates was completed, uncovering a total of 32 rotamers lying within 1.6 kcal mol−1 of their respective lowest-energy minima. Our definitive energetics for stationary points on the n-propyl + O2 potential energy surface provide key benchmarks for future studies of hydrocarbon oxidation.