CH3CO + O2 + M (M = He, N2) Reaction Rate Coefficient Measurements and Implications for the OH Radical Product Yield

The gas-phase CH3CO + O-2 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 + O-2 reaction were measured over a range of temperature (241-373 K) and pressure (0.009-600 Torr) with He and N-2 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 O-2. 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 + Cl-2 used as the reference reaction. A kinetic mechanism analysis of the combined kinetic data set yielded a zero pressure limit rate coefficient, k(int)(T), of (6.4 +/- 4) x 10(-14) exp((820 +/- 150)/T) cm(3) molecule(-1) s(-1) (with k(int)(296 K) measured to be (9.94 +/- 1.3) x 10(-13) cm(3) molecule(-1) s(-1)), k(0)(T) = (7.39 +/- 0.3) x 10(-30) (T/300)(-2.2 +/- 0.3) cm(6) molecule(-2) s(-1), and k(infinity)(T) = (4.88 +/- 0.05) x 10(-12) (T/300)(-0.85 +/- 0.07) cm(3) molecule(-1) s(-1) with F-c = 0.8 and M = N-2. A He/N-2 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 + O-2 reaction. The present results are compared with results from previous studies and the discrepancies are discussed.