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

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journal contribution
posted on 22.09.2011, 00:00 authored by R. I. Kaiser, M. Goswami, P. Maksyutenko, F. Zhang, Y. S. Kim, Alexander Landera, Alexander M. Mebel
The crossed molecular beams reaction of dicarbon molecules, C2(X1Σg+/a3Πu) 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; C2v). 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 ∼7 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).