Rate constant and reaction channels for the reaction of atomic nitrogen with the ethyl radical

Abstract

The absolute rate constant and primary reaction products have been determined at T=298 K for the atom–radical reaction N(4S)+C2H5 in a discharge flow system with collision-free sampling to a mass spectrometer. The rate constant measurements employed low energy electron impact ionization while the product study used dispersed synchrotron radiation as the photoionization source. The rate constant was determined under pseudo-first-order conditions by monitoring the decay of C2H5 or C2D5 as a function of time in the presence of excess N atoms. The result is k=(1.1±0.3)×10−10 cm3 molecule−1 s−1. For the reaction product experiments using photoionization mass spectrometry, products observed at 114 nm (10.9 eV) were CD3, D2CN and C2D4 for the N+C2D5 reaction. The product identification is based on the unambiguous combination of product m/z values, the shift of the m/z peaks observed for the N+C2D5 reaction products with respect to the N+C2H5 reaction products and the photoionization threshold measured for the major products. The observed products are consistent with the occurrence of the reaction channels D2CN+CD3(2a) and C2D4+ND(2c). Formation of C2D4 product via channel (2c) accounts for approximately 65% of the C2D5 reacted. Most, if not all, of the remaining 35% is probably accounted for by channel (2a). These rate constant and product results are compared with those for the N+CH3 reaction as well as other atom+C2H5 reactions. The role of the N+C2H5 reaction in the formation of HCN in the atmospheres of Titan and Neptune is briefly considered. In addition, the appearance energy for the formation of C2D+3 from C2D5 was determined from photoionization threshold measurements, AE0(C2D+3,C2D5)=239.5 kcal mol−1. From this, values are derived for the zero Kelvin heats of formation of C2D+3 (266 kcal mol−1) and C2D3 (71.6 kcal mol−1).