Temperature dependence of the rate constant and product distribution of the reaction of CH3 radicals with O(3P) atoms

The kinetics and product distribution for the reaction of methyl radicals, CH3, with ground-state, O(P-3) oxygen atoms, have been investigated at temperatures up to 925 K and at constant bath gas (He) concentrations of about 3.2 x 10(16) cm(-3). With a photoionization/time-of-flight mass spectrometer (TOFMS) as an analytical tool, precursor species, reactants, and products were observed simultaneously. The radicals were produced by an excimer laser pulse (lambda = 193 nm), in the cophotolysis of acetone, CH3C(O)CH3 and sulfur dioxide, SO2. In addition to the dominant product, formaldehyde (CH2O), carbon monoxide (CO) was detected as the only other main product. The yields for both products were found to be independent of temperature with values of Phi(CH3+O)(CH2O) = 0.84 +/- 0.12 (2sigma) and Phi(CH3+O)(CO) = 0.15 +/- 0.06 (2sigma). However, the overall rate constant for this reaction shows a slight increase with temperature with k(CH3+O) = (2.4 +/- 0.3) x 10(-10) exp(-(202 +/- 60) K/T) cm(3) molecule(-1) s(-1) between T = 354 and 925 K at [He] = 3.2 x 10(16) cm(-3). In an additional experiment the rate constant for the reaction of deuterated methyl radicals, CD3, with oxygen atoms at 308 K in 3 Torr of He was determined to be k(CD3+O) = (1.3 +/- 0.8) x 10(-10) cm(-1) molecule(-1) s(-1). Using tunable diode laser absorption spectroscopy (TDLAS) the product yield of (CO)-O-18 from the reaction CD3 + O-18(P-3) was measured at room temperature in 5 Torr of Ar indicating an H/D-isotope effect lowering the yield to Phi(CD3+O)((CO)-O-18) = 0.12 +/- 0.01 (2sigma).