Crossed molecular beams study on the formation of vinylacetylene in Titan's atmosphere

The reaction of ground-state ethynyl radicals, C(2)H(X(2)Sigma(+)), with d(4)-ethylene, C(2)D(4)(X(1)A(g)), was investigated at a collision energy of 20.6 +/- 0.4 kJ mol(-1) utilizing the crossed-beams technique. Combined with electronic structure calculations, our results elucidate that this reaction follows indirect reaction dynamics via a doublet radical complex. The reaction is initiated by a barrierless addition of the ethynyl radical to a carbon atom of the d(4)-ethylene molecule to form a C(4)HD(4) intermediate. The latter is long-lived compared with its rotational period and decomposes via a tight exit transition state to form the d(3)-vinylacetylene product (HCCC(2)D(3)) plus a deuterium atom while conserving the ethynyl group; the center-of-mass angular distribution suggests that the deuterium atom leaves almost perpendicularly to the rotational plane of the fragmenting C(4)HD(4) complex. The overall reaction is found to be exoergic by 94 +/- 20 kJ mol(-1); this value agrees nicely with a computational data of 103 +/- 5 kJ mol(-1). This study indicates that the analogous vinylacetylene molecule (HCCC(2)H(3)) can be synthesized in a low-temperature environment such as Titan's atmosphere via the neutral-neutral reaction of ethynyl radicals with ubiquitous ethylene. The similarity between this reaction and that of the isoelectronic cyano radical, CN(X(2)Sigma(+)), with ethylene-yielding vinylcyanide (C(2)H(3)CN) is also discussed.