Direct measurement of •OH and HO2• formation in •R + O2 reactions of cyclohexane and tetrahydropyran

Formation of the key general radical chain carriers, (OH)-O-center dot and HO2 center dot, during pulsed-photolytic Cl-center dot-initiated oxidation of tetrahydropyran and cyclohexane are measured with time-resolved infrared absorption in a temperature-controlled Herriott multipass cell in the temperature range of 500-750 K at 20 Torr. The experiments show two distinct timescales for HO2 center dot and (OH)-O-center dot formation in the oxidation of both fuels. Analysis of the timescales reveals striking differences in behavior between the two fuels. In both cyclohexane and tetrahydropyran oxidation, a faster timescale is strongly related to the ``well-skipping'' (R-center dot + O-2 -> alkene + HO2 center dot or cyclic ether + (OH)-O-center dot) mechanism and is expected to have, at most, a weak temperature dependence. Indeed, the fast HO2 center dot formation timescale is nearly temperature independent both for cyclohexyl + O-2 and for tetrahydropyranyl + O-2 below 700 K. A slower HO2 center dot formation timescale in cyclohexane oxidation is shown to be linked to the sequential R-center dot + O-2 -> ROO center dot -> alkene + HO2 center dot pathway, and displays a strong temperature dependence mainly from the final step ( with energy barrier similar to 32.5 kcal mol(-1)). In contrast, the slower HO2 center dot formation timescale in tetrahydropyran oxidation is surprisingly temperature insensitive across all measured temperatures. Although the (OH)-O-center dot formation timescales in tetrahydropyran oxidation show a temperature dependence similar to the cyclohexane oxidation, the temperature dependence of (OH)-O-center dot yield is opposite in both cases. This significant difference of HO2 center dot formation kinetics and (OH)-O-center dot formation yield for the tetrahydropyran oxidation can arise from contributions related to ring-opening pathways in the tetrahydropyranyl + O-2 system that compete with the typical R-center dot + O-2 reaction scheme. This comparison of two similar fuels demonstrates the consequences of differing chemical mechanisms on (OH)-O-center dot and HO2 center dot formation and shows that they can be highlighted by analysis of the eigenvalues of a system of simplified kinetic equations for the alkylperoxy-centered R-center dot + O-2 reaction pathways. We suggest that such analysis can be more generally applied to complex or poorly known oxidation systems.