EXPERIMENTAL INVESTIGATION OF THE REACTION BETWEEN NITRIC-OXIDE AND KETENYL RADICALS (HCCO+NO) - RATE COEFFICIENT AT T=290-670-K AND PRODUCT DISTRIBUTION AT 700-K

The HCCO + NO reaction (r5) was investigated in C2H2/O/NO systems at a pressure of 2 Torr (He bath gas) using discharge flow-molecular beam mass spectrometry techniques (DF-MBMS). The first rate coefficient data at temperatures > 300 K are presented. The coefficient was measured relative to the known k(HCCO+O) from the change of steady-state HCCO signals upon adding increasing amounts of NO. Thus, in the temperature range from 290 to 670 K, the k(HCCO+NO) coefficient was found to exhibit a slight but significant temperature dependence: k(T) = (1.0 +/- 0.3) x 10-(10) e(-(350+/-150)/(T/K)) cm(3) molecule(-1) s(-1) (T = 290-670 K). The product distribution was determined at 700 K. The experiments relied on the fact that all reaction pathways yield either CO or CO2. Ketenyl radicals, generated by quantitative reaction of a known amount of O atoms with C2H2 in high excess, were reacted with a large excess of NO, ensuring quantitative conversion into the products CO2 and CO, The product distribution was essentially deduced from the ratios [CO2]formed/[O]input and [CO]formed/ [O]input. Formation of CO together with CH2(B-3(1)) in a minor (25 +/- 15%) channel of the C2H2 + O reaction was taken into account. Small corrections for secondary reactions such as HCCO + O --> H + 2 CO were made by kinetic modeling, Thus, the following yields were obtained: for HCCO + NO --> (CHNO) + CO, 77 +/- 9%; for HCCO + NO --> (CHN) + CO2, 23 +/- 9%. Strong product signals were also observed at m/e = 43 and m/e = 27, confirming that CHNO isomers and CHN isomers are formed along with CO and CO2, respectively. Theoretical predictions regarding the CO/CO2 yield ratio, presented in a companion paper in this issue, can be reconciled with the experimental product distribution only when an as yet unidentified entrance pathway to the formyl isocyanate intermediate is assumed to exist and to be thermally accessible.