UV absorption spectrum and self-reaction of cyclohexylperoxy radicals

Abstract

The kinetics and mechanism of the self-reaction of cyclohexylperoxy radicals: 2c-C₆H₁₁O₂→ 2c-C₆H₁₁O + O₂(1a), → c-C₆H₁₀O + c-C₆H₁₁OH + O₂(1b) have been studied using both time-resolved and end-product analysis techniques. Determination of the product yields from the photooxidation of Cl₂–c-C₆H₁₂–O₂–N₂ mixtures using FTIR spectrometry demonstrates that the branching ratio for the radical-producing channel (1a) is 0.29 ± 0.02 at 295 K. Furthermore, the dependence of the product yields on oxygen partial pressure shows that ring-opening of the cyclohexyloxy radical formed in channel (1a): c-C₆H₁₁O + M → CH₂(CH₂)₄CHO + M (4) competes with the reaction with oxygen: c-C₆H₁₁O + O₂→ c-C₆H₁₀O + HO₂(2) under atmospheric conditions. Flash photolysis–UV absorption experiments were used to obtain the UV spectrum of the cyclohexylperoxy radical and the kinetics of reaction (1). The spectrum of c-C₆H₁₁O₂ is similar to those of other alkylperoxy radicals, with a maximum cross-section of (4.95 ± 0.51)× 10¹⁸ cm² molecule^–1 at 250 nm, measured relative to a value of 4.55 × 10^–18 cm² molecule^–1 for CH₃O₂ at 240 nm. Reaction (1) is slow compared to the self-reactions of primary alkylperoxy radicals, but is significantly faster than that of isopropylperoxy radicals at room temperature. Experiments as a function of temperature from 253 to 373 K give: k_obs(2.0 ± 0.4)× 10^–13 exp[–(487 ± 64)K/T] cm³ molecule^–1 s^–1 for reaction (1). The room-temperature branching ratio measurement enables a value of 2.84 × 10^–14 cm³ molecule^–1 s^–1 to be assigned to k₁ at 298 K. The above errors are 1σ and represent experimental uncertainty only; assuming a 10% uncertainty in the CH₃O₂ calibration cross-section, absolute uncertainties in the values of the cyclohexylperoxy cross-sections and k_obs are 16% and 17%, respectively.