Trishomocyclopropenylium Cations. Structure, Stability, Magnetic Properties, and Rearrangement Possibilities

Potentially trishomoaromatic cations possessing the 6-X-bicyclo[3.1.0]hex-3-yl (X = CH(2), BH, NH, O) or bicyclo[3.2.0]hept-3-yl unit have been investigated at the Hartree-Fock, second-order, third-order, and fourth-order (single, double, quadruple excitations) Møller-Plesset perturbation level employing the 6-31G(d) basis set. IGLO/6-31G(d) chemical shift calculations have been carried out at optimized geometries. Through-space interactions between the symmetric Walsh orbital of the three-membered ring and ppi(C3) orbital have been analyzed as a function of orbital energies and orbital overlap. The best indicators for trishomoaromaticity are NMR chemical shifts and magnetic susceptibility. There is a simple relationship between the conformation of the trishomocyclopropenylium cation, its charge distribution, and delta(13)C3, which can be used to determine the conformation or the C1C3 interaction distance from NMR measurements. Trishomocyclopropylium cations investigated can rearrange to an envelope form of higher energy where the height of the inversion barrier and the chair-envelope energy difference are a measure for the homoaromatic stabilization energy. The bicyclo[3.1.0]hex-3-yl cation in its envelope form can rearrange with a barrier of just 1 kcal/mol to the bicyclo[3.1.0]hept-2-yl cation. In the case of the bicyclo[3.2.0]hept-3-yl cation, there exists just the envelope form, which can rearrange to a ethano-bridged center-protonated spirocyclopentyl cation. The later cation should be an interesting target of chemical synthesis since it contains a pentacoordinated carbon atom and possesses unusual properties.