Infrared frequency-modulation probing of product formation in alkyl+O-2 reactions: III. The reaction of cyclopentyl radical (c-C5H9) with O-2 between296 and 723 K

The production of HO2 from the reaction of c-C5H9 + O-2 has been investigat ed as a function of temperature (296-723 K) by using laser photolysis/CW in frared frequency modulation spectroscopy. The HO2 yield is derived by compa rison with the Cl-2/CH3OH/O-2 system and is corrected to account for HO2 si gnal loss due to competing reactions involving HO2 radical and the adduct c -C5H9O2 The time behavior of the HO2 signal following cyclopentyl radical f ormation displays two separate components. The first component is a prompt production of HO2, which increases with temperature and is the only compone nt observed between 296 and 500 K. The yield from the prompt production ris es from less than 1% at 296 K to similar to 23% at 693 K. At temperatures a bove 500 K a second slower rise in the HO2 signal is also observed. The pro duction of HO2 on a slower time scale is attributable to cyclopentylperoxy radical decomposition. The total HO2 yield, including the contribution from the slower rise, increases dramatically with temperature from similar to2% at 500 K to similar to 100% at 683 K. From 683 to 723 K the total HO2 yiel d remains constant. The second slower rise accounts for a majority of the p roduct formation at these higher temperatures. The biexponential time behav ior of the HO2 production from c-C5H9 + O-2 is similar to that previously o bserved in studies of C2H5 + O-2 and C3H7 + O-2 reactions. The-rate of form ation for delayed HO2 production from c-G(5)H(9) + O-2 is larger than the r ate of formation from either C2H5 + O-2, or C3H7 + O-2, at each temperature . However, apparent activation energies, obtained by an Arrhenius plot of t he rates of formation for delayed HO2 formation, are very similar for the t hree systems (C2K5 + O-2, C3H7 + O-2, and c-C5H9 + O-2) The results suggest a similar coupled mechanism for HO2 production in the C2H5 + O-2, C3H7 + O -2, and c-C5H9 + O-2 reactions, with concerted elimination of HO2 from the RO2 radical responsible for HO2 alkene production.