Reactions of ethynyl radicals as a source of C 4 and C 5 hydrocarbons in Titan's atmosphere

Crossed molecular beam experiments augmented by electronic structure computations of neutral-neutral reactions of the ethynyl radical (C ₂H, X ²Σ⁺) with the unsaturated hydrocarbons acetylene (C ₂H ₂), methylacetylene (CH ₃CCH), and allene (H ₂CCCH ₂) are reviewed briefly. All reactions are characterized by a C ₂H versus H atom exchange and in the case of the C ₂H/C ₂H ₂ system by an additional molecular hydrogen (H ₂) elimination pathway. The attack of the ethynyl radical onto the π-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 (CH ₃CCCCH), ethynylallene (H ₂CCH(C ₂H)), and penta-4-diyne (HCC(CH ₂)C ₂H) 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.