The reaction between N(4S) and C2H3: Rate constant and primary reaction channels

Abstract

The rate constant and the product branching ratios have been determined at T=298 K for the reaction between ground state atomic nitrogen (4S) and the vinyl radical (C2H3) at a nominal pressure of 1 Torr He. The kinetic technique employed was discharge-flow coupled to a collision-free sampling mass spectrometer. The rate constant was determined by monitoring the decay of the vinyl radical in the presence of excess [N], yielding a value for k(N+C2H3) of (7.7±2.9)×10−11 cm3 molecule−1 s−1. Three primary reaction channels have been experimentally observed: N+C2H3→C2H2+NH (1a), C2H2N+H (1b), and C2H3N (1c). The lowest energy isomers of the C2H2N radical and the C2H3N adduct molecule are CH2CN and CH3CN, respectively and their identification as products of the reaction is consistent with experimental results. Contributions from the higher energy isomers CH2NC or cyc-C2H2N in channel (1b) and CH3NC or H2C=C=NH in channel (1c) are not consistent with the experimental results and can be ruled out. Contribution from other higher energy isomers such as the radicals HC–CH=N, HC=C=NH and H2C=C=N in channel (1b) and the adduct species vinyl nitrene and 2H-azirine in channel (1c) cannot be ruled out in the absence of knowledge of heats of formation of the radical species and ionization energies for both the radical and adduct species. The following branching ratios were determined at T=298 K: Γ1a=0.16, Γ1c=0.04. No other potential products were detected. It can therefore be inferred that channel (1b) accounts for most if not all of the remaining products, i.e., Γ1b=0.80. The magnitude of the rate constant and the nature of the observed products for N+C2H3 are compared with those for the reactions N+CH3 and N+C2H5. The formation of the adduct molecule is considered in terms of initial formation of vinyl nitrene or 2H-azirine followed by a series of ring openings, ring closings, and an H atom transfer to yield the lowest energy isomer acetonitrile, CH3CN. The possible role of the N+C2H3 reaction in the atmospheric chemistry of Titan, Neptune, and Triton is briefly considered.