Formation of the 1 , 3 , 5 - Hexatriynyl Radical ( C 6 H ( X 2 Π ) ) via the Crossed Beams Reaction of

Crossed molecular beams experiments were conducted to investigate the chemical dynamics of the reaction of dicarbon molecules, C2(X Σg /aΠu), with diacetylene, C4H2(X Σg ) at two collision energies of 12.1 and 32.8 kJmol. The dynamics were found to be indirect, involved C6H2 intermediates, and were dictated by an initial addition of the dicarbon molecule to the carbon-carbon triple bond of diacetylene. The initial collision complexes could isomerize. On the singlet surface, the resulting linear triacetylene molecule (C6H2(X Σg )) decomposed without an exit barrier to form the linear 1,3,5-hexatriynyl radical (C6H(X Π)). On the triplet surface, the dynamics suggested at least a tight exit transition state involved in the fragmentation of a triplet C6H2 intermediate to yield the 1,3,5-hexatriynyl radical (C6H(X Π)) plus atomic hydrogen. On the basis of the experimental data, we recommend an experimentally determined enthalpy of formation of the 1,3,5hexatriynyl radical of 1014 ( 27 kJmol at 0 K. Our experimental results and the derived reaction mechanisms gain full support from electronic structure calculations on the singlet and triplet C6H2 potential-energy surfaces. The identification of the 1,3,5-hexatriynyl radical under single collision conditions implies that the neutral-neutral reaction of dicarbon with diacetylene can lead to the formation of 1,3,5-hexatriynyl radicals in the interstellar medium and possibly in the hydrocarbon-rich atmospheres of planets and their moons such as Saturn’s satellite Titan.