TRISHOMOCYCLOPROPENYLIUM CATIONS - STRUCTURE, STABILITY, MAGNETIC-PROPERTIES, AND REARRANGEMENT POSSIBILITIES

Potentially trishomoaromatic cations possessing the 6-X-bicyclo[3.1.0] hex-3-yl (X = CH2, BH, NH, O) or bicyclo[3.2.0]hept-3-yl unit have bee n investigated at the Hartree-Fock, second-order, third-order, and fou rth-order (single, double, quadruple excitations) Moller-Plesset pertu rbation level employing the 6-31G(d) basis set. IGLO/6-31G(d) chemical shift calculations have been carried out at optimized geometries. Thr ough-space interactions between the symmetric Walsh orbital of the thr ee-membered ring and p pi(C3) orbital have been analyzed as a function of orbital energies and orbital overlap. The best indicators for tris homoaromaticity are NMR chemical shifts and magnetic susceptibility. T here is a simple relationship between the conformation of the trishomo cyclopropenylium cation, its charge distribution, and delta(13)C3, whi ch can be used to determine the conformation or the C1C3 interaction d istance from NMR measurements. Trishomocyclopropylium cations investig ated can rearrange to an envelope form of higher energy where the heig ht of the inversion barrier and the chair-envelope energy difference a re 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 spiroc yclopentyl cation. The later cation should be an interesting target of chemical synthesis since it contains a pentacoordinated carbon atom a nd possesses unusual properties.