MOLECULAR-DYNAMICS SIMULATION OF SOLID BIPHENYL

Constrained molecular dynamics simulation of solid biphenyl is reported. The calculations were carried out for 432 molecules, i.e., 216 (6 x 12 x 3) unit cells and also for 108 molecules i.e. 54 (3 x 6 x 3) unit cells of the monoclinic crystal. The molecules were handled as rigid bodies except for the possibility of the torsional twist of the phenyl rings around their long molecular axes. Williams-type atom-atom pair-potentials were used to describe the intermolecular interactions. A simple double-well torsional potential governed the intramolecular motions. Starting from the experimentally determined room temperature arrangement, we studied the structure as a function of temperature. Our simulations indicate that the second order phase-transition at approximately 40 K known from experiments, is associated with the ordering of the torsion angles in neighbouring molecules, but the resulting structure is qualitatively different from what was assumed from X-ray diffraction measurements. Rather than all the molecules having the same average torsion angle of approximately 11-degrees, as has hitherto been assumed, it has been found that the system can substantially reduce its free energy if some molecules have a low torsion angle (0-2-degrees) and others a higher (approximately 22-degrees) one. This splitting into two basically different conformations is rather insensitive to the simulation parameters but the proportions of molecules falling in the two regimes is sensitive to the scaling of the potentials. The structure accommodates these different torsion angles by tending to form an alternation of high and low torsion angle in successive sites along the b direction.