MOLECULAR-DYNAMICS SIMULATION STUDY OF SOLID POLYPHENYLS - STRUCTURES DETERMINED BY THE INTERPLAY BETWEEN INTRAMOLECULAR AND INTERMOLECULAR FORCES

As a continuation of our previous work we have performed constrained molecular dynamics simulations on models of solid biphenyl and para-terphenyl in order to study their conformation instability. We determined a 'critical' torsion potential range with respect to widely used Williams type intermolecular interactions and the experimentally determined densities. Torsion potentials weaker than this critical one will not produce phase transitions. The critical torsion potential was found to be similar for both molecules in the solid state. The fine details of the structures for both compounds vary with simulation parameters but the major characteristics of the arrangements are rather insensitive in this respect. We compared our findings with experimental results. For biphenyl -as we reported in our previous paper-our results are qualitatively different from the structure assumed from X-ray diffraction measurements. Rather than all the molecules having the same torsion angle on average, alternate molecules in the b direction have a very small (0-2-degrees) or a much larger (approximately 22-degrees) torsion angle. For p-terphenyl our results are qualitatively in agreement with what was found in X-ray measurements, with each of four molecules in the low temperature unit cell having different torsion angles. However, the same trends as for biphenyl were observed with at least one of the sites containing molecules with a very small torsion angle.