Rate coefficients for cycloalkyl + O reactions and product branching in the decomposition of chemically activated cycloalkoxy radicals: an experimental and theoretical study

Abstract

The kinetics of cycloalkyl + O reactions were studied with respect to their rate coefficients and the product branching ratios from the decomposition of the chemically activated cycloalkoxy radicals. Rate coefficients for the reactions of cyclohexyl (c-C₆H₁₁), cycloheptyl (c-C₇H₁₃) and cyclooctyl (c-C₈H₁₅) radicals with oxygen atoms were determined with an experimental setup consisting of a discharge flow reactor with molecular beam sampling and REMPI/TOF-MS detection. The following rate coefficients were obtained (units: cm³/mol⁻¹ s⁻¹): k(c-C₆H₁₁ + O) = (1.33 ± 0.24) × 10¹⁴(T/298 K)^0.11 (T = 250–600 K), k(c-C₇H₁₃ + O) = (1.85 ± 0.25) × 10¹⁴ (T = 298 K), k(c-C₈H₁₅ + O) = (1.56 ± 0.20) × 10¹⁴(T/298 K)^0.66±0.15 (T = 268–363 K). Stable products were determined by quantitative FTIR spectroscopy. The decomposition of the cycloalkoxy radicals leads besides β-C–H bond fission (yields: 24% for c-C₆H₁₁O, 20–25% for c-C₈H₁₅O) mainly to alkyl radicals by ring-opening via β-C–C bond cleavage. These open-chain alkyl radicals further decompose mainly by β-C–C bond scission. An increase of the total pressure from 4 mbar to 1 bar had no effect on the product distribution for the reaction c-C₆H₁₁ + O, whereas for the reaction c-C₈H₁₅ + O further decomposition of the ring-opening product is significantly suppressed at 1 bar. The experimental results on the channel branching and its pressure dependence were rationalized with the statistical rate theory. A comparison of the experimental and modeling results indicates a significant influence of hindered internal rotations (HIRs) on the reactions of the ring-opening products. The harmonic approximation to describe these modes was shown to be inadequate, while a treatment as one-dimensional HIRs led to a significantly improved agreement between experimental and modeling results. Implications of our findings for the formation of secondary organic aerosol and high-temperature combustion are discussed.