Laboratory investigation on the formation of unsaturated nitriles in Titan's atmosphere

Crossed molecular beam experiments of ground state cyano radicals, CN(X(2)Sigma(+)), with hydrocarbons acetylene (C2H2), ethylene (C2H4), methylacetylene (CH3CCH), allene (H2CCCH2), dimethylacetylene (CH3CCCH3), and benzene (C6H6) were performed to investigate the formation of unsaturated nitriles in Titan's atmosphere. These radical-neutral reactions have no entrance barrier, depict an exit barrier well below the energy of the reactant molecules, and are all exothermic. The CN radical attacks the pi electron density at the olefine, alkyne, and aromatic molecules; the formation of an initial addition complex is a common pathway on the involved potential energy surfaces for all reactions. A subsequent carbon-hydrogen bond rupture yields the unsaturated nitriles HCCCN, C2H3CN, CH3CCCN, H2CCCH(CN), H2CCCH2CN and C6H5CN as detected in our experiments. The explicit identification of this CN vs H atom exchange pathway under single collision, makes this reaction-class a compelling candidate to synthesize unsaturated nitriles in Titan's atmosphere. This versatile concept makes it even possible to predict the formation of nitriles once the corresponding unsaturated hydrocarbons are identified in Titan. Here, the C2H2 as well as cyanoacetylene, HCCCN, have been already identified unambiguously in Titan's troposphere. Those nitriles as sampled in our crossed beam experiments resemble an ideal challenge to be detected in the framework of the NASA-ESA Cassini-Huygens mission to Titan. (C) 2000 Elsevier Science Ltd. All rights reserved.