A Crossed Molecular Beams and Ab Initio Study on the Formation of C6H3 Radicals. An Interface between Resonantly Stabilized and Aromatic Radicals

The crossed molecular beams reaction of dicarbon molecules, C-2(Chi(1)Sigma(g)+/a(3)Pi(u)) with vinylacetylene was studied under single collision conditions at a collision energy of 31.0 kJ mol(-1) and combined with electronic structure calculations on the singlet and triplet C6H4 potential energy surfaces. The investigations indicate that both reactions on the triplet and singlet surfaces are dictated by a barrierless addition of the dicarbon unit to the vinylacetylene molecule and hence indirect scattering dynamics via long-lived C6H4 complexes. On the singlet surface, ethynylbutatriene and vinyldiacetylene were found to decompose via atomic hydrogen loss involving loose exit transition states to form exclusively the resonantly stabilized 1-hexene-3,4-diynyl-2 radical (C6H3; H2CCCCCCH; C-2u). On the triplet surface, ethynylbutatriene emitted a hydrogen atom through a tight exit transition state located about 20 kJ mol(-1) above the separated stabilized 1-hexene-3,4-diynyl-2 radical plus atomic hydrogen product; to a minor amount (<5%) theory predicts that the aromatic 1,2,3-tridehydrobenzene molecule is formed. Compared to previous crossed beams and theoretical investigations on the formation of aromatic C6Hx (x = 6, 5, 4) molecules benzene, phenyl, and o-benzyne, the decreasing energy difference from benzene via phenyl and o-benzyne between the aromatic and acyclic reaction products, i.e., 253, 218, and 58 kJ mol(-1), is narrowed down to only kJ mol(-1) for the C6H3 system (aromatic 1,2,3-tridehydrobenzene versus the resonantly stabilized free radical 1-hexene-3,4-diynyl-2). Therefore, the C6H3 system can be seen as a transition stage among the C6Hx (x = 6-1) systems, in which the energy gap between the aromatic isomer (x = 6, 5, 4) is reduced compared to the acyclic isomer as the carbon-to-hydrogen ratio increases and the acyclic isomer becomes more stable (x = 1, 2).