An atomistic simulation investigation of the inter-ring torsion in crystalline biphenyl

An atomistic simulation of solid-phase biphenyl describes the pi character of the interphenyl bond by a torsional potential which predicts the observed torsional angle of the molecule in the vapour phase. In the crystal lattice the unit cell is defined by a doubling of the b axis vector of the early diffraction studies to allow for an alternation of torsional angles. The results predict a non-planar molecular conformation, with a dihedral angle of 15 degrees between the rings and a torsional barrier of 1.65 kJ mel(-1). Comparison with calculations conducted on the simple cell (in which no doubling of the b axis has been imposed) shows the stabilising effect of the alternation of the torsional angles. Thermal averaging would be expected to result in rapid interconversion of the two degenerate conformers at room temperature. The sensitivity found for the lattice conformation to small changes in the volume of the unit cell is discussed.