Theory, measurements, and modeling of OH and HO₂ formation in the reaction of cyclohexyl radicals with O₂

The production of OH and HO₂ in Cl-initiated oxidation of cyclohexane has been measured using pulsed-laser photolytic initiation and continuous-laser absorption detection. The experimental data are modeled by master equation calculations that employ new G2(MP2)-like ab initio characterizations of important stationary points on the cyclo-C₆H₁₁O₂ surface. These ab initio calculations are a substantial expansion on previously published characterizations, including explicit consideration of conformational changes (chair–boat, axial–equatorial) and torsional potentials. The rate constants for the decomposition and ring-opening of cyclohexyl radical are also computed with ab initio based transition state theory calculations. Comparison of kinetic simulations based on the master equation results with the present experimental data and with literature determinations of branching fractions suggests adjustment of several transition state energies below their ab initio values. Simulations with the adjusted values agree well with the body of experimental data. The results once again emphasize the importance of both direct and indirect components of the kinetics for the production of both HO₂ and OH in radical + O₂ reactions.