ON THE BARRIER TO MOLECULAR AXIAL ROTATION IN CRYSTALLINE BENZENE

The barrier to axial rotation of a benzene molecule in crystalline ben zene has been computed by ab initio methods. A cluster consisting of a central molecule surrounded by its four nearest neighbours was treate d at the Hartree-Fock level. Beyond this, the lattice was represented by (potential-derived) point charges placed at the nuclear positions. The energy barrier obtained in the best model was 10.1 kJ mol(-1), whi ch compares with 16.8 kJ mol(-1) found by relaxation time solid-state NMR measurements. A point charge-only lattice model gives a somewhat s maller estimate of the barrier height.