Ethylene radical cation

The novel four-center, two-electron delocalized a-bishomoaromatic species 182,183,188,190a, and 192 are representatives of a new class of 27i-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations (Fig. 5.11,193). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 194 having a conventional p-type delocalized electron structure (Fig. 5.11). 

Figure 3,5 Energy costs for the 90 degree twist in (a) ethylene (experimental), (b) ethylene radical cation (MP2/6-3IC ), and (c) ethylene radical anion (B3LYP/6-311++C(3df,3pd))A...

V. Hrouda, T. Bally, M. Ingr, and P. Carsky,. Phys. Chem. A, 102, 9297 (1998). The C4Hj Potential Energy Surface. 2. The Reaction of Ethylene Radical Cation with Acetylene and Ensuing Rearrangements. 

The cycloaddition of two ethylenes or the cycloreversion of cyclobutane is one of the textbook examples used in the illustration of the Woodward-Hoffman rules [20] of orbital symmetry. Studies on the cyclobutane radical cation [21,22] showed a low activation energy for the cycloaddition of an ethylene radical cation to ethylene, in remarkable contrast with the high activation energy for the corresponding neutral reaction [23]. The dissociation reaction of cyclobutane radical cation is endothermic. Although there is a cyclobutane ring in the pyrimidine dimer, its electronic structure is likely to be different from cyclobutane itself, because of the presence of the two pyrimidine rings. 

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