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

Crossed molecular beam experiments of ground state cyano radicals, CN(X(2)S igma(+)), with hydrocarbons acetylene (C2H2), ethylene (C2H4), methylacetyl ene (CH3CCH), allene (H2CCCH2), dimethylacetylene (CH3CCCH3), and benzene ( C6H6) were performed to investigate the formation of unsaturated nitriles i n Titan's atmosphere. These radical-neutral reactions have no entrance barr ier, 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 a ddition complex is a common pathway on the involved potential energy surfac es for all reactions. A subsequent carbon-hydrogen bond rupture yields the unsaturated nitriles HCCCN, C2H3CN, CH3CCCN, H2CCCH(CN), H2CCCH2CN and C6H5 CN as detected in our experiments. The explicit identification of this CN v s H atom exchange pathway under single collision, makes this reaction-class a compelling candidate to synthesize unsaturated nitriles in Titan's atmos phere. This versatile concept makes it even possible to predict the formati on of nitriles once the corresponding unsaturated hydrocarbons are identifi ed in Titan. Here, the C2H2 as well as cyanoacetylene, HCCCN, have been alr eady identified unambiguously in Titan's troposphere. Those nitriles as sam pled in our crossed beam experiments resemble an ideal challenge to be dete cted in the framework of the NASA-ESA Cassini-Huygens mission to Titan. (C) 2000 Elsevier Science Ltd. All rights reserved.