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O. Asvany, Y.T. Lee, R.I. Kaiser (Institute of Atomic and Molecular Sciences)
In Saturn's moon Titan, the C2H(2\Sigma+) radical is formed via photodissociation of acetylene, C2H2, by the solar ultraviolet radiation. Since C2H is isoelectronic to the cyano radical, CN(2\Sigma+), the reactions of C2H with unsaturated hydrocarbons such as acetylene, C2H2, and methylacetylene, CH3CCCH, are strongly expected as the key reactions to form (substituted) diacetylenes and their allene isomers in Titans atmosphere:
(1) C2H + C2H2 -> HCCCCH + H
(2) C2H + CH3CCH -> CH3CCCCHC2H + H / H2CCCHCCH + H
Despite the crucial importance of these C2H reactions in the understanding of Titan's atmospheric chemistry, only kinetic studies monitoring the decay of the C2H radical have been performed. These investigation showed the reactions are very fast and proceed with almost unit collision efficiency close to gas kinetics; however, reaction products have never been identified. Here, we report on the very first systematic investigation of reactions (1) and (2) employing the crossed molecular beam technique with mass spectrometric detection. To facilitate the product identification we performed reactions with the deuterated radical C2D instead of C2H.
Our results show that both reactions proceed through the formation of a long lived complex following the addition of C2D radical to the \pi system of the unsaturated hydrocarbon. Each complex decomposes to form the closed shell hydrocarbon molecule and a H atom. No D atom emission was observed indicating that the C2D group is conserved in the reaction. This solid identification of the C2D versus H atom exchange represents the first evidence that diacetylene HCCCCH can be formed via reaction (1) in Titan's atmosphere. Further, results of reaction (2) clearly indicate the formation of two distinct isomers, i.e. methyldiacetylene, CH3CCCCH, and its allenic isomer H2CCCHCCH. This exchange channel opens a versatile route to form highly unsaturated hydrocarbons in Titan's atmosphere. Most important, our studies supply a well defined data base on reaction products and shall guide chemical investigation of the NASA-ESA Cassini-Huygens mission to identify these molecules in Titan and future modeling attempts.
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The author(s) of this abstract have provided an email address for comments about the abstract: kaiser@po.iams.sinica.edu.tw