title: A crossed molecular beams and ab initio study on the formation of C6H3 radicals. an interface between resonantly stabilized and aromatic radicals.
abstract: The crossed molecular beams reaction of dicarbon molecules, C(2)(X(1)Σ(g)(+)/a(3)Π(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 C(6)H(4) 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 C(6)H(4) 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 (C(6)H(3); H(2)CCCCCCH; C(2v)). 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 C(6)H(x) (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 C(6)H(3) system (aromatic 1,2,3-tridehydrobenzene versus the resonantly stabilized free radical 1-hexene-3,4-diynyl-2). Therefore, the C(6)H(3) system can be seen as a "transition" stage among the C(6)H(x) (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).
authors.affiliation: Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA.
authors.forename: R I
authors.initials: RI
authors.lastname: Kaiser
authors.forename: M
authors.initials: M
authors.lastname: Goswami
authors.forename: P
authors.initials: P
authors.lastname: Maksyutenko
authors.forename: F
authors.initials: F
authors.lastname: Zhang
authors.forename: Y S
authors.initials: YS
authors.lastname: Kim
authors.forename: Alexander
authors.initials: A
authors.lastname: Landera
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Mebel