Are neutral-neutral reactions effective for the carbon-chain growth of cyanopolyynes and polyacetylenes in interstellar space?

Ab initio molecular quantum-mechanical methods have been applied to explore the possibility of neutral-neutral reactions leading to the formation of cyanopolyynes and polyacetylenes in interstellar cloud. Potential energy surfaces for the reactions between the CN radical and polyacetylenes indicate that all reactions, C2nH2 + CN (n = 1-4), which form HC2n + 1N molecules, are exothermic and have no energy barriers. We have also examined the possibility of the various product channels from the reactions C2nH2 + CN based on the thermochemical relationships. The theoretical results show that none of the product channels is exothermic except the case which produces HC2n + 1N, with the carbon chain being longer than the reactant polyacetylene. Similar results are obtained for the regioselectivity of the reactions, C2nH2 + C2H (n = 1-4), to produce hydrocarbons. The only possible products from the latter reactions are shown to be HC2n + 2H + H under the conditions of interstellar space. These results clearly demonstrate the importance of the neutral-neutral reactions on the carbon-chain growth of cyanopolyynes and polyacetylenes.