THE REACTION BETWEEN N(S-4) AND C2H3 - RATE-CONSTANT AND PRIMARY REACTION CHANNELS

The rate constant and the product branching ratios have been determine d at T=298 K for the reaction between ground state atomic nitrogen (S- 4) and the vinyl radical (C2H3) at a nominal pressure of 1 Torr He. Th e 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) x 10(-11) cm(3) molec ule(-1) s(-1). Three primary reaction channels have been experimentall y observed: N+C2H3+C2H2+NH (1a), C2H2N+H (1b), and C2H3N (1c). The low est 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. Contribution s from the higher energy isomers CH2NC or cyc-C2H2N in channel (1b) an d CH3NC or H2C=C=NH in channel (1c) are not consistent with the experi mental results and can be ruled out. Contribution from other higher en ergy isomers such as the radicals HC-CH=N, HC=C=NH and H2C=C=N in chan nel (1b) and the adduct species vinyl nitrene and 2H-azirine in channe l (1c) cannot be ruled out in the absence of knowledge of heats of for mation of the radical species and ionization energies for both the rad ical and adduct species. The following branching ratios were determine d at T=298 K: Gamma(1a) = 0.16, Gamma(1c)=0.04. No other potential pro ducts were detected. It can therefore be inferred that channel (1b) ac counts for most if not all of the remaining products, i.e., Gamma(1b) = 0.80. The magnitude of the rate constant and the nature of the obser ved products for N+C2H3 are compared with those for the reactions N+CH 3 and N+C2H5. The formation of the adduct molecule is considered in te rms of initial formation of vinyl nitrene or 2H-azirine followed by a series of ring openings, ring closings, and an H atom transfer to yiel d the lowest energy isomer acetonitrile, CH3CN. The possible role of t he N+C2H3 reaction in the atmospheric chemistry of Titan, Neptune, and Triton is briefly considered. (C) 1996 American Institute of Physics.