Non-fused polyaromatic hydrocarbons: interactions of aromatic and antiaromatic rings through a CC bond

The interaction of the moieties of benzene, cyclobutadiene, cyclopentadinyl anion, and the cyclopentadianide cation upon each other and upon a CC bond connecting pairs of these rings is investigated computationally. The resulting non-fused bicycles include biphenyl, phenylcyclobutadiene, phenylcyclopentadienylium, phenylcyclopentadienide, pentafulvalene, cyclobutadienyl–cyclopentadienylium, cyclobutadienyl–cyclopentadienide, and bicyclobutadiene. The relative stability and aromaticity are assessed from hydrogenation energies, aromatic stabilization energies, ring separation energies, nucleus-independent chemical-shift, harmonic oscillator model of aromaticity, and natural bond orbital analysis. Calculations are performed with density functional theory (B3LYP) and Møller–Plesset perturbation theory of second order (MP2). Enthalpy quantities are also determined by G3. When both rings are aromatic in character, the bridging bond is mostly σ in character. When one or both of the rings is antiaromatic, the bridging bond has significant π character. Systems with contrasting aromaticities have CC bridging bonds of lengths between CC single bond lengths and CC double bond lengths and where the systems were charged, the charge is evenly distributed between the rings.