On the barrier to molecular axial rotation in crystalline benzene

The barrier to axial rotation of a benzene molecule in crystalline benzene has been computed by ab initio methods. A cluster consisting of a central molecule surrounded by its four nearest neighbours was treated 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, which compares with 16.8 kJ mol−1 found by relaxation time solid-state NMR measurements. A point charge-only lattice model gives a somewhat smaller estimate of the barrier height.