CH3CO + O2 + M (M = He, N2) Reaction Rate Coefficient Measurements and...

Papadimitriou, V. C., E. S. Karafas, T. Gierczak, and J. Burkholder (2015), CH3CO + O2 + M (M = He, N2) Reaction Rate Coefficient Measurements and Implications for the OH Radical Product Yield, J. Phys. Chem. A, 119, 7481-7497, doi:10.1021/acs.jpca.5b00762.

The gas-phase CH3CO + O2 reaction is known to proceed via a chemical activation mechanism leading to the formation of OH and CH3C(O)OO radicals via bimolecular and termolecular reactive channels, respectively. In this work, rate coefficients, k, for the CH3CO + O2 reaction were measured over a range of temperature (241−373 K) and pressure (0.009−600 Torr) with He and N2 as the bath gas and used to characterize the bi- and ter-molecular reaction channels. Three independent experimental methods (pulsed laser photolysis−laser-induced fluorescence (PLP−LIF), pulsed laser photolysis−cavity ring-down spectroscopy (PLP− CRDS), and a very low-pressure reactor (VLPR)) were used to characterize k(T,M). PLP−LIF was the primary method used to measure k(T,M) in the high-pressure regime under pseudo-first-order conditions. CH3CO was produced by PLP, and LIF was used to monitor the OH radical bimolecular channel reaction product. CRDS, a complementary high-pressure method, measured k(295 K,M) over the pressure range 25−600 Torr (He) by monitoring the temporal CH3CO radical absorption following its production via PLP in the presence of excess O2. The VLPR technique was used in a relative rate mode to measure k(296 K,M) in the low-pressure regime (9−32 mTorr) with CH3CO + Cl2 used as the reference reaction. A kinetic mechanism analysis of the combined kinetic data set yielded a zero pressure limit rate coefficient, kint(T), of (6.4 ± 4) × 10−14 exp((820 ± 150)/T) cm3 molecule−1 s−1 (with kint(296 K) measured to be (9.94 ± 1.3) × 10−13 cm3 molecule−1 s−1), k0(T) = (7.39 ± 0.3) × 10−30 (T/300)−2.2±0.3 cm6 molecule−2 s−1, and k∞(T) = (4.88 ± 0.05) × 10−12 (T/300)−0.85±0.07 cm3 molecule−1 s−1 with Fc = 0.8 and M = N2. A He/N2 collision efficiency ratio of 0.60 ± 0.05 was determined. The phenomenological kinetic results were used to define the pressure and temperature dependence of the OH radical yield in the CH3CO + O2 reaction. The present results are compared with results from previous studies and the discrepancies are discussed.

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Upper Atmosphere Research Program (UARP)