Mechanisms and rates of the reactions C2H5+O and 1-C3H7+O

The mechanisms and rates of the reactions of the primary alkyl radicals ethyl and I-propyl with oxygen atoms at room temperature and low pressure (around 5 mbar) have been studied using two independent experimental arrangements. The reactants were generated by UV-laser flash photolysis with different precursors (C2H5COC2H5, C2H6 + CFCl3, C2H5I, C3H7COC3H7, SO2). Stable species concentrations were measured quantitatively by Fourier transform IR and OH radical concentrations of the ground (V = 0) and first vibrational (v = 1) state by time-resolved laser-induced fluorescence. For both reaction 1 and reaction 2, the mechanism is explained in terms of the formation and subsequent decomposition of a chemically activated alkoxy radical and a competing abstraction channel leading directly to OH and the alkene: C2H5 + O --> C2H5O (reaction 1a)/C2H5O --> HCHO + CH3 (reaction 1a(1))/CH3CHO + H (reaction 1a(2))//C2H5 + O --> C2H4 + OH (reaction 1b). The absolute branching ratio was determined preferentially using diethyl ketone as the C2H5 radical source leading to (1a(1))/(1a(2))/(1b), 32/44/24. Relative branching ratios for the C2H5 radical sources C2H6 + Cl and C2H5I were derived as (1a(1))/(1a(2)) = 1/1.5 and 1/1.55, respectively. The overall rate coefficient of the reaction C2H5 + O was measured as k(1) = (1.04 +/- 0.1) X 10(14) cm(3) mol(-1) s(-1) and in addition k(C2H5 + OH) = (7.0 +/- 1) X 10(13) cm(3) mol(-1) s(-1). The mechanism and the rate of reaction 2 were found as 1-C3H7 + O --> 1-C3H7O (reaction 2a)/I-C3H7O --> HCHO + C2H5 (reaction 2a(1))/C2H5CHO + H (reaction 2a(2))//1-C3H7 + O --> C3H6 + OH (reaction 2b) (branching ratio (2a(1))/(2a(2))/(2b), 44/ 32/ 24) and k(2) = (8.2 +/- 1) X 10(13) cm(3) mol(-1) s(-1). The results are discussed in terms of statistical rate theory.