The production of HO2 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 Cl2, and the progress of the reaction is monitored by frequency-modulation spectroscopy of the HO2 product. The yield of HO2 in the reaction is measured by comparison with the Cl2/CH3OH/O2 system, which quantitatively converts Cl atoms to HO2. At low temperatures stabilization to C2H5O2 dominates, but at elevated temperatures (> 575 K) dissociation of the ethylperoxy radical begins to contribute. Biexponential time behavior of the HO2 production allows separation of prompt, “direct” HO2 formation from HO2 produced after thermal redissociation of an initial ethylperoxy adduct. The prompt HO2 yield exhibits a smooth increase with increasing temperature, but the total HO2 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 HO2 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 C2H5 + O2 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, 1853). However, a simple ad hoc correction to that model, which takes into account a recent reinterpretation of the equilibrium constant for C2H5 + O2 ↔ C2H5O2, 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 HO2 + C2H4 formed by direct elimination from the C2H5O2 adduct.

The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO 2 Ϫ ϩប 351.1 nm →HO 2 ϩe Ϫ EA͓HO 2 ,X 2 AЉ͔ ϭ1.089Ϯ0.006 eV, ͑CH 3 ͒ 3 COO Ϫ ϩប 351.1 nm →͑CH 3 ͒ 3 COOϩe Ϫ EA͓͑CH 3 ͒ 3 COO,X 2 AЉ]ϭ1.196 Ϯ0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be ⌬E(X 2 AЉ-Ã 2 AЈ)͓HO 2 ͔ϭ0.871Ϯ0.007 eV and ⌬E(X 2 AЉ-2 AЈ)͓͑CH 3 ͒ 3 COO͔ϭ0.967Ϯ0.011 eV. The gas phase acidity of ͑CH 3 ͒ 3 COOH was measured in a tandem flowing afterglow-selected ion flow tube ͑FA-SIFT͒ to be ⌬ acid G 298 ϭ363.2Ϯ2.0 kcal mol Ϫ1 and we find ⌬ acid H 298 ͓͑CH 3 ͒ 3 COO-H͔ϭ370.9Ϯ2.0 kcal mol Ϫ1. Use of ⌬ acid H 298 ͓͑CH 3 ͒ 3 COO-H͔ and EA͓͑CH 3 ͒ 3 COO͔ leads to the bond energies DH 298 ͓͑CH 3 ͒ 3 COO-H͔ϭ85Ϯ2 kcal mol Ϫ1 and D 0 ͓͑CH 3 ͒ 3 COO-H͔ϭ83Ϯ2 kcal mol Ϫ1. The thermochemistry of the alkylperoxyl radicals, RO 2 , is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed ͓RO 2 ͔X 2 AЉ→͓RO 2 ͔*Ã 2 AЈ→aldehydes/ketonesϩHO(2 ⌸), ͓RO 2 ͔X 2 AЉ→͓RO 2 ͔*Ã 2 AЈ →alkenesϩHO 2 (X 2 AЉ).

The production of HO2 from the reaction of C3H7 and O2 has been investigated as a function of temperature (296−683 K) using laser photolysis/CW infrared frequency-modulation spectroscopy. The HO2 yield is derived by comparison with the Cl2/CH3OH/O2 system and is corrected to account for HO2 signal loss due to competing reactions involving HO2 radical and the adduct C3H7O2. The time behavior of the HO2 signal following propyl radical formation was observed to have two separate components. The first component is a prompt production of HO2, which increases with temperature and is the only HO2 production observed between 296 and 550 K. This prompt yield increases from less than 1% at 296 K to ∼16% at 683 K. At temperatures above 550 K, a second, slower rise in the HO2 signal is also observed. The production of HO2 on a slower time scale is attributable to propylperoxy radical decomposition. The total HO2 yield, including the contribution from the slower rise, increases rapidly with temperature from 5% at 500 K to 100% at 683 K. The second slower rise accounts for nearly all of the product formation at these higher temperatures. The biexponential time behavior of the HO2 production from C3H7 + O2 is similar to that previously observed in studies of the C2H5 + O2 reaction. The temperature dependence of the prompt yield for the two reactions is very similar, with the C3H7 + O2 reaction having a slightly lower yield at each temperature. The temperature dependence of the total HO2 yield is also very similar for the two reactions, with the sharp increase in the total HO2 yield at high temperatures occurring in very similar temperature ranges. The phenomenological rate constant for delayed HO2 production from C3H7 + O2 is slightly larger than that for C2H5 + O2 at each temperature. Apparent activation energies, obtained from an Arrhenius plot of the inverse of the time constants for delayed HO2 production, are similar for the two systems, being 24.6 and 26.0 kcal mol-1 for C2H5 + O2 and C3H7 + O2, respectively. These results suggest similar coupled mechanisms for HO2 production in the C2H5 + O2 and C3H7 + O2 reactions, with similar concerted HO2 elimination pathways from the RO2 species.

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group in this state of PhN. This delocalization is favorable energetically because in 2 the non bonding and electrons have opposite spins, so that the motions of these two electrons are not correlated by the Pauli exclusion principle. Hence, in the ' state of HN these two nonbonding electrons have a large Coulombic repulsion energy.14 However, in PhN, delocalization of the electron into the phenyl group allows these two electrons to occupy different regions of space, thus minimizing their Coulombic repulsion energy.14•15 In carbenes, too, an adjacent ir bond provides selective stabilization for the open-shell singlet state (1A").16,17Despite the selective stabilization of !A2 in PhN, we still compute it to lie about 18 kcal/mol above the 3A2 ground state. As shown in Table I, neither this calculated energy difference nor that between the ' and 32" states of NH shows much sensitivity to the amount of electron correlation provided.As is the case in calculations on methylene,18 the results of our calculations and previous12 calculations on HN suggest that very large basis sets appear to be necessary to correlate the two electrons lations were performed. We also thank the San Diego Supercomputer Center for a generous allocation of computer time, Professor Matthew S. Platz for conversations that stimulated this computational study, Professor G. Barney Ellison for agreeing to simultaneous publication, and Professor Richard N. McDonald for communicating his results to us in advance of publication.