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

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 $75.7kcal∕mol$ 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, $70.2kcal∕mol$ below the reactants. This adduct loses a hydrogen atom from the nonterminal position to give the HCCCCCCH (triacetylene) product exothermic by $29.7kcal∕mol$ via an exit barrier of $5.3kcal∕mol$⁠. 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 $10−10cm3molecule−1s−1$ for $T=298–63K$⁠, and to show a negative temperature dependence at low $T$⁠. A general mechanism for the growth of polyyne chains involving $C2H+H(CC)nH→H(CC)n+1H+H$ 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.