Zur Konformation des Bicyclo[2.2.2]octan-Systems

Crystals of bicyclo 2.2.2] octane-1, 4-dicarboxylic acid are monoclinic, a = 6.01 Å, b = 16.73 Å, c = 10.29 Å, β = 115.08°, space group P2₁/c, with 4 molecules in the unit cell. The structure was solved with the help of direct methods and refined by full-matrix least-squares analysis of the three-dimensional intensity data. Within experimental error the bicyclo 2.2.2]-octane (BCO) skeleton has apparent D_3h-symmetry, corresponding to the totally eclipsed conformation. Analysis of the thermal ellipsoids in terms of the translational and rotational motion of the BCO skeleton leads to an r.m.s. amplitude of 5.9 ± 0.2° for rotation about the threefold axis. On the assumption that the bond lengths remain effectively constant during a torsional vibration of BCO, the potential energy surface has been calculated for a range of semi-empirical potential functions. These calculations show that the energy minimum may be slightly displaced from D_3h symmetry, but if so the barrier between the two such equivalent minima is only about 0.1 kcal mole^?1. The energy eigenvalues and eigenfunctions for a typical variation of potential energy vs torsion angle have been calculated. From the form of the eigenfunction of the ground vibrational state we conclude that BCO has effective D_3h symmetry at all temperatures as far as diffraction methods are concerned.