Temperature and pressure dependence of the absolute rate constant for the reactions of NH2 radicals with acetylene and ethylene

Publisher Website
Google Scholar
Abstract
Absolute rate constants for reaction of the NH2 free radical with acetylene and ethylene have been measured as a function of pressure and temperature using the technique of flash photolysis combined with time resolved detection of NH2 via laser induced fluorescence. NH2 radicals were produced by flash photolysis of ammonia highly diluted in argon and the decay of fluorescent NH2 photons was measured by multiscaling techniques. At each temperature employed in the study, the observed pseudo‐first‐order rate constants were independent of flash intensity (i.e., initial [NH2]) and were a linear function of [C2H2] or [C2H4]. The bimolecular rate constant for the reaction NH2+C2H2 (1) increased with an increase in pressure at 373 and 459 K but not at lower temperatures. Results at or near the high pressure limit are well represented by the Arrhenius expression k∞1 =(1.11±0.36)×10−13 exp[−(1852±100)/T] cm3 molecule−1 s−1 over the temperature range 241–459 K. For the reaction NH2+C2H4 (2), a smaller increase in the bimolecular rate constant with an increase in pressure was observed and only at 465 K. Results representing the high pressure limiting rate constant over the temperature interval 250–465 K are given by k∞2 =(3.41±0.12)×10−14 exp[−(1318±23)/T] cm3 molecule−1 s−1. The observed pressure dependence confirms expectations from previous studies that both reactions involve addition of NH2 to the unsaturated molecule followed by collisional stabilization of the adduct radical. The results are compared with two previous determinations using the techniques of flash photolysis–laser absorption and discharge flow–laser induced fluorescence. There is substantial disagreement between these two previous studies; our results for both NH2+C2H2 and NH2+C2H4 are much more consistent with the flash photolysis–laser absorption results. The implications of these results for atmospheric and combustion chemistry are briefly considered.