A crossed beam and ab initio study of the C2(X1Σ+g/a3Πu)+C2H2(X1Σ+g) reactions

The reaction dynamics of C-2 in the ground ((1)Sigma(g)(+)) and first electronically excited ((3)Pi(u)) states with the simplest alkyne, acetylene C2H2(X(1)Sigma(g)(+)), were investigated in a crossed molecular beam setup at a nominal collision energy of 24.1 kJ mol(-1). The experimental data expose the existence of a C-2/H exchange pathway to form C4H + H. The experimental results were combined with electronic structure calculations on the singlet and triplet C4H2 surfaces. The reaction of C-2(X(1)Sigma(g)(+)) was found to proceed via indirect scattering dynamics through addition of C-2 to the carbon-carbon triple bond. On the triplet surface, C-2(a(3)Pi(u)) adds also to the acetylenic triple bond. Dominated by large impact parameters, this process favors the formation of two short lived chain-like triplet C4H2 isomers. For both reactions, the identification of the butadiynyl radical as a relevant product presents an important step to refine the networks of reactions for the modeling of polycyclic, aromatic hydrocarbons formation together with that of their hydrogen deficient precursors in the interstellar medium, in carbon-rich circumstellar envelopes and pre-planetary nebula, and in combustion flames. (C) 2003 Elsevier B.V. All rights reserved.