Infrared Frequency-Modulation Probing of Product Formation in Alkyl + O₂ Reactions:  I. The Reaction of C₂H₅ with O₂ between 295 and 698 K

The production of HO 2 in the reaction of ethyl radicals with molecular oxygen has been investigated using laser photolysis/cw infrared frequency modulation spectroscopy. The ethyl radicals are formed by reaction of photolytically produced Cl atoms with ethane, initiated via pulsed laser photolysis of Cl 2, and the progress of the reaction is monitored by frequency-modulation spectroscopy of the HO 2 product. The yield of HO 2 in the reaction is measured by comparison with the Cl 2 /CH 3 OH/O 2 system, which quantitatively converts Cl atoms to HO 2 . At low temperatures stabilization to C 2 H 5 O 2 dominates, but at elevated temperatures (> 575 K) dissociation of the ethylperoxy radical begins to contribute. Biexponential time behavior of the HO 2 production allows separation of prompt, “direct” HO 2 formation from HO 2 produced after thermal redissociation of an initial ethylperoxy adduct. The prompt HO 2 yield exhibits a smooth increase with increasing temperature, but the total HO 2 yield, which includes contributions from the redissociation of ethylperoxy radicals, rises sharply from ∼10% to 100% between 575 and 675 K. Because of the separation of time scales in the HO 2 production, this rapid rise can unambiguously be assigned to ethylperoxy dissociation. No OH was observed in the reaction, and an upper limit of 6% can be placed on direct OH formation from the C 2 H 5 + O 2 reaction at 700 K. The time behavior of the HO 2 production is at variance with the predictions of Wagner et al.'s RRKM-based parameterization of this reaction ( J. Phys. Chem. 1990, 94, 1853). However, a simple ad hoc correction to that model, which takes into account a recent reinterpretation of the equilibrium constant for C 2 H 5 + O 2 ↔ C 2 H 5 O 2, predicts yields and time constants consistent with the present measurements. The reaction mechanism is further discussed in terms of recent quantum chemical and master equation studies of this system, which show that the present results are well described by a coupled mechanism with HO 2 + C 2 H 4 formed by direct elimination from the C 2 H 5 O 2 adduct.