Reactions of ethynyl radicals as a source of C4 and C5 hydrocarbons in Titan's atmosphere

Crossed molecular beam experiments augmented by electronic structure computations of neutral-neutral reactions of the ethynyl radical (C2H, X(2)Sigma(+)) with the unsaturated hydrocarbons acetylene (C2H2), methylacetylene (CH3CCH), and allene (H2CCCH2) are reviewed briefly. All reactions are characterized by a C2H versus H atom exchange and in the case of the C2H/C2H2 system by an additional molecular hydrogen (H-2) elimination pathway. The attack of the ethynyl radical onto the pi-electron density of the unsaturated hydrocarbons has no entrance barrier and initializes each reaction. Consecutive hydrogen atom migrations may precede the exit channels. Diacetylene (HCCCCH), the butadiynyl radical (HCCCC), methyldiacetylene (CH3CCCCH), ethynylallene (H2CCH(C2H)), and penta-4-diyne (HCC(CH2)C2H) were identified as products of which only diacetylene has yet been observed in Titan's atmosphere. Our results, however, strongly suggest the presence of all these species on Titan, and the Cassini-Huygens mission is likely to detect these upon arrival in the Saturnian system in 2004. (C) 2002 Published by Elsevier Science Ltd.