High pressure phase of biphenyl at room temperature: A Monte Carlo study

A new pressure-induced solid phase of biphenyl is reported at room temperature. Isothermal-isobaric ensemble variable shape simulation cell Monte Carlo calculations are reported on biphenyl at 300 K as a function of pressure between 0 and 4 GPa. The potential proposed by Williams for intermolecular and Benkert-Heine-Simmons (BHS) for intramolecular interactions have been employed. Different properties indicating changes in the crystal structure, molecular structure, distributions of inter- and intramolecular energy are reported as a function of pressure. With increase in pressure beyond 0.8 GPa, the dihedral angle distribution undergoes a change from a bimodal to an unimodal distribution. The changes in IR and Raman spectra across the transition computed from ab initio calculations are in agreement with the experimental measurements. It is shown that at pressures larger than 0.8 GPa, competition between intermolecular interactions with intramolecular terms viz., conjugation energy and the ortho-ortho repulsion favors a planar biphenyl due to better packing and consequently a predominant intermolecular term. The exact value of the transition pressure will depend on the accuracy of the inter- and intramolecular potentials employed here.