The multichannel i-propyl + O-2 reaction system: A model of secondary alkyl radical oxidation

The i-propyl + O-2 reaction mechanism has been investigated by definitive quantum chemical methods to establish this system as a benchmark for the combustion of secondary alkyl radicals. Focal point analyses extrapolating to the ab initio limit were performed based on explicit computations with electron correlation treatments through coupled cluster single, double, triple, and quadruple excitations and basis sets up to cc-pV5Z. The rigorous coupled cluster single, double, and triple excitations/cc-pVTZ level of theory was used to fully optimize all reaction species and transition states, thus, removing some substantial flaws in reference geometries existing in the literature. The vital i-propylperoxy radical (MIN1) and its concerted elimination transition state (TS1) were found 34.8 and 4.4 kcal mol(-1) below the reactants, respectively. Two beta-hydrogen transfer transition states (TS2, TS2') lie above the reactants by (1.4, 2.5) kcal mol(-1) and display large Born-Oppenheimer diagonal corrections indicative of nearby surface crossings. An alpha-hydrogen transfer transition state (TS5) is discovered 5.7 kcal mol-1 above the reactants that bifurcates into equivalent alpha-peroxy radical hanging wells (MIN3) prior to a highly exothermic dissociation into acetone + OH. The reverse TS5 -> MIN1 intrinsic reaction path also displays fascinating features, including another bifurcation and a conical intersection of potential energy surfaces. An exhaustive conformational search of two hydroperoxypropyl (QOOH) intermediates (MIN2 and MIN3) of the i-propyl + O-2 system located nine rotamers within 0.9 kcal mol(-1) of the corresponding lowest-energy minima. Published under an exclusive license by AIP Publishing.

Automated summary

The reaction mechanism of i-propyl + O-2 was studied using quantum chemical methods, establishing it as a benchmark for secondary alkyl radical combustion. Focal point analyses were performed, extrapolating to the ab initio limit. The optimization of all reaction species and transition states was carried out, removing flaws in literature reference geometries. Important species and transition states were identified, and exhaustive conformational searches were conducted.