UV ABSORPTION-SPECTRUM AND SELF-REACTION OF CYCLOHEXYLPEROXY RADICALS

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) x 10(18) cm2 molecule-1 at 250 nm, measured relative to a value of 4.55 x 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) x 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 x 10(-14) cm3 molecule-1 s-1 to be assigned to k1 at 298 K. The above errors are 1-sigma 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.