STRUCTURE OF PERCUBYLCUBANE - RELATIONSHIP TO SUPERCUBANE, S-6-SYMMETRY AND O-SYMMETRY AND CHIRAL ASPECTS

We report the computed equilibrium geometry of percubylcubane which se rves as a model for supercubane. Supercubane is a very dense form of c arbon thought to be composed of cubane subunits. Results for cubane, c ubylcubane and percubylcubane are presented. Calculations were carried out for cubane and cubylcubane with Hartree-Fock and MP2 wavefunction s using the STO-3G and 6-31G basis sets, respectively. A calculation was carried out for percubylcubane with a Hartree-Fock wavefunction an d the STO-3G basis set. A comparison of the calculated geometries of c ubane and cubylcubane with the corresponding experimental data yields correction factors for the equilibrium geometry computed for percubylc ubane. Percubylcubane was determined to have a structure with 0 symmet ry. The dihedral angles for the intercube bonds are rotated approximat ely 15 degrees from the previously postulated Burdett and Lee structur e. The 15 degrees rotation occurs primarily for steric reasons. This r otation may also explain why supercubane is more dense than would be e xpected from the Burdett and Lee structure. Using our structure for pe rcubylcubane as a non-primitive unit cell, we propose an alternative s tructure for supercubane. The primitive unit cell of our proposed stru cture is of Sg symmetry and the crystal group is <R(3)over bar> (C-3i( 2), No. 148).