Theoretical study of the C6H3 potential energy surface and rate constants and product branching ratios of the C2H(2Σ+)+C4H2(1Σg+) and C4H(2Σ+)+C2H2(1Σg+) reactions

Ab initio CCSD(T)/cc-pVTZ//B3LYP/6-311G(**) and CCSD(T)/complete basis set (CBS) calculations of stationary points on the C6H3 potential energy surface have been performed to investigate the reaction mechanism of C2H with diacetylene and C4H with acetylene. Totally, 25 different C6H3 isomers and 40 transition states are located and all possible bimolecular decomposition products are also characterized. 1,2,3- and 1,2,4-tridehydrobenzene and H2CCCCCCH isomers are found to be the most stable thermodynamically residing 77.2, 75.1, and 75.7 kcal/mol lower in energy than C2H+C4H2, respectively, at the CCSD(T)/CBS level of theory. The results show that the most favorable C2H+C4H2 entrance channel is C2H addition to a terminal carbon of C4H2 producing HCCCHCCCH, 70.2 kcal/mol below the reactants. This adduct loses a hydrogen atom from the nonterminal position to give the HCCCCCCH (triacetylene) product exothermic by 29.7 kcal/mol via an exit barrier of 5.3 kcal/mol. Based on Rice-Ramsperger-Kassel-Marcus calculations under single-collision conditions, triacetylene+H are concluded to be the only reaction products, with more than 98% of them formed directly from HCCCHCCCH. The C2H+C4H2 reaction rate constants calculated by employing canonical variational transition state theory are found to be similar to those for the related C2H+C2H2 reaction in the order of magnitude of 10(-10) cm(3) molecule(-1) s(-1) for T=298-63 K, and to show a negative temperature dependence at low T. A general mechanism for the growth of polyyne chains involving C2H+H(C C)(n)H -> H(C C)(n+1)H+H reactions has been suggested based on a comparison of the reactions of ethynyl radical with acetylene and diacetylene. The C4H+C2H2 reaction is also predicted to readily produce triacetylene+H via barrierless C4H addition to acetylene, followed by H elimination. (c) 2008 American Institute of Physics.