Infrared frequency-modulation probing of product formation in alkyl plus O-2 reactions: I. The reaction of C2H5 with O-2 between 295 and 698 K

The production of HO2 in the reaction of ethyl radicals with molecular oxyg en has been investigated using laser photolysis/cw infrared frequency modul ation spectroscopy. The ethyl radicals are formed by reaction of photolytic ally produced Cl atoms with ethane, initiated via pulsed laser photolysis o f Cl-2 and the progress of the reaction is monitored by frequency-modulatio n spectroscopy of the HO2 product. The yield of HO2 in the reaction is meas ured by comparison with the Cl-2/CH3OH/O-2 system, which quantitatively con verts Cl atoms to HO2. At low temperatures stabilization to C2H5O2 dominate s, but at elevated temperatures (> 575 K) dissociation of the ethylperoxy r adical begins to contribute. Biexponential time behavior of the HO2 product ion allows separation of prompt, "direct" HO2 formation from HO2 produced a fter thermal redissociation of an Initial ethylperoxy adduct, The prompt HO 2 yield exhibits a smooth increase with increasing temperature, but the tot al HO2 yield, which includes contributions from the redissociation of ethyl peroxy radicals, rises sharply from similar to 10% to 100% between 575 and 675 K. Because of the separation of time scales in the HO2 production, this rapid rise can unambiguously be assigned to ethylperoxy dissociation. No O H was observed in the reaction, and an upper limit of 6% can be placed on d irect OH formation from the C2H5 + O-2 reaction at 700 K. The time behavior of the HO2 production is at variance with the predictions of Wagner et al. 's RRKM-based parameterization of this reaction (J. Phys. Chem. 1990, 94, 1 853). However, a simple ad hoc correction to that model, which takes into a ccount a recent reinterpretation of the equilibrium constant for C2H5 + O-2 Ct C2H5O2, predicts yields and time constants consistent with the present measurements. The reaction mechanism is further discussed in terms of recen t quantum chemical and master equation studies of this system, which show t hat the present results are well described by a coupled mechanism with HO2 + C2H4 formed by direct elimination from the C2H5O2 adduct.