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 c yanopolyynes and polyacetylenes in interstellar cloud. Potential energy sur faces for the reactions between the CN radical and polyacetylenes indicate that all reactions, C2nH2 + CN (n = 1-4), which form HC2n + 1N molecules, a re exothermic and have no energy barriers. We have also examined the possib ility of the various product channels from the reactions C2nH2 + CN based o n 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. Si milar 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.