Infrared Frequency-Modulation Probing of Product Formation in Alkyl + O2 Reactions: III. The Reaction of Cyclopentyl Radical (c-C5H9) with O2 between 296 and 723 K

The production of HO 2 from the reaction of c -C 5 H 9 + O 2 has been investigated as a function of temperature (296−723 K) by using laser photolysis/CW infrared frequency modulation spectroscopy. The HO 2 yield is derived by comparison with the Cl 2 /CH 3 OH/O 2 system and is corrected to account for HO 2 signal loss due to competing reactions involving HO 2 radical and the adduct c -C 5 H 9 O 2 . The time behavior of the HO 2 signal following cyclopentyl radical formation displays two separate components. The first component is a prompt production of HO 2, which increases with temperature and is the only component observed between 296 and 500 K. The yield from the prompt production rises from less than 1% at 296 K to ∼23% at 693 K. At temperatures above 500 K a second slower rise in the HO 2 signal is also observed. The production of HO 2 on a slower time scale is attributable to cyclopentylperoxy radical decomposition. The total HO 2 yield, including the contribution from the slower rise, increases dramatically with temperature from ∼2% at 500 K to ∼100% at 683 K. From 683 to 723 K the total HO 2 yield remains constant. The second slower rise accounts for a majority of the product formation at these higher temperatures. The biexponential time behavior of the HO 2 production from c -C 5 H 9 + O 2 is similar to that previously observed in studies of C 2 H 5 + O 2 and C 3 H 7 + O 2 reactions. The rate of formation for delayed HO 2 production from c -C 5 H 9 + O 2 is larger than the rate of formation from either C 2 H 5 + O 2 or C 3 H 7 + O 2 at each temperature. However, apparent activation energies, obtained by an Arrhenius plot of the rates of formation for delayed HO 2 formation, are very similar for the three systems (C 2 H 5 + O 2, C 3 H 7 + O 2, and c -C 5 H 9 + O 2 ). The results suggest a similar coupled mechanism for HO 2 production in the C 2 H 5 + O 2, C 3 H 7 + O 2, and c -C 5 H 9 + O 2 reactions, with concerted elimination of HO 2 from the RO 2 radical responsible for HO 2 + alkene production.