Ultraviolet absorption spectrum and self-reaction of cyclopentylperoxy radicals

Abstract

The kinetics and mechanism of the self-reaction of cyclopentylperoxy radicals: 2 c-C₅H₉O₂→ 2 c-C₅H₉O + O₂(1a), → c-C₅H₉OH + c-C₅H₈O + O₂(1b), have been studied using both time-resolved and end-product-analysis techniques. Determination of the product yields from the photolysis of Cl₂–c-C₅H₁₀–O₂–N₂ mixtures using FTIR spectroscopy demonstrates that ring-opening of the cyclopentoxy radical formed in channel (1a): c-C₅H₉O + M → CH₂(CH₂)₃CHO + M (3) dominates over reaction with oxygen: c-C₅H₉O + O₂→ c-C₅H₈O + HO₂(2), under atmospheric conditions. Flash photolysis-UV absorption experiments were used to obtain the UV spectrum of the cyclopentylperoxy radical and the kinetics of reaction (1). The spectrum of c-C₅H₉O₂ is similar to those of other alkylperoxy radicals, with a maximum cross-section of (5.22 ± 0.20)× 10^–18 cm² molecule^–1 at 250 nm, measured relative to a value of 4.55 × 10^–18 cm² molecule^–1 for CH₃O₂ at 240 nm. The observed second-order rate constant, k_obs(–d[c-C₅H₉O₂]/dt= 2k_obs[c-C₅H₉O₂]²), for removal of cyclopentylperoxy radicals was dependent on the oxygen partial pressure. Experiments as a function of temperature from 243 to 373 K gave limiting minimum and maximum values of k_obs at low (<1 Torr) and high (>50 Torr) oxygen partial pressures, respectively: k_min/cm³ molecule^–1 s^–1=(1.3 ± 0.4)× 10^–14 exp[(188 ± 83)K/T] and k_max/cm³ molecule^–1 s^–1=(2.9 ± 0.8)× 10^–13 exp[–(555 ± 77)K/T]. At low oxygen partial pressures, the only effective removal channel for cyclopentylperoxy radicals is the molecular channel (1b) and k_min can be equated to k_1b. Simulations suggest that k_max represents an upper limit on k₁ and is at most 25% greater. In light of the present results on the cyclopentylperoxy radical, further experiments were performed on the cyclohexylperoxy radical self-reaction: 2 c-C₆H₁₁O₂→ 2 c-C₆H₁₁O + O₂(16a), → c-C₆H₁₁OH + c-C₆H₁₀O + O₂(16b) at low oxygen partial pressures, giving k_16b/cm³ molecule^–1 s^–1=(1.3 ± 0.3)× 10^–14 exp[(185 ± 15) K/T] and an estimated k₁₆/cm³ molecule^–1 s^–1= 7.7 × 10^–14 exp(–184 K/T). The above errors are 1σ and represent experimental uncertainty only.