Journal of physical chemistry vol:99 issue:11 pages:3583-3591
Recently, the minor reaction channel HCCO + O --> CH(a(4) Sigma(-) and/or X(2)II) + CO2 (r2b) was identified and quantified at 290 K; in an accompanying ab initio study, a CH(a(4) Sigma(-))-forming and a CH(X(2)II)-producing pathway were characterized. presently, the CO2 plus CH yield k(2b)/k(2) was determined at 405, 500, 650, and 960 K from the CO2 produced in low-pressure C2H2/O systems, measured by the discharge-flow/molecular beam sampling mass spectrometry technique. The only critical parameters are the rate constant k(1a) of the primary reaction channel C2H2 + O --> HCCO + H, which is well established, and the ratio k(3)/k(2) of the rate constants of the reactions HCCO + H (r3) and HCCO + O (r2), which was determined here simultaneously. In the temperature range of interest, the k(3)/k(2) ratio was found to be nearly constant, with an average value of 1.45 +/- 0.30 (2 sigma error). In the determination of the CO2 plus CH yield k(2b)/k(2) from the observed CO2, contributions of additional CO2 sources were corrected for by kinetic modeling. In this way, k(2b)/k(2) was found to be 0.079 +/- 0.033 at 405 K, 0.110 +/- 0.049 at 500 K, 0.132 +/- 0.059 at 650 K, and 0.149 +/- 0.070 at 960 K (overall 95% confidence range). Using the known k(2), the values of k(2b) at the four temperatures were derived; they can be fitted by k(2b) = (4.9 +/- 2.6) x 10(-11) exp[-(560 +/- 300)/(T/K)] cm(3) molecule(-1) s(-1) The increase of k(2b) with temperature is in agreement with the earlier prediction based on an ab initio characterization of the HCCO + O reaction surfaces. Finally, it is argued that the reaction channel r2b is an important CH-source in hydrocarbon flames.