Rotational disorder in crystalline pyrene as a model for layered aromatic structures

Abstract

The lattice energy surface of crystalline pyrene, as a model of layered aromatic structures, is analyzed numerically as a function of molecular reorientation. Cooperative effects are simulated by concerted rotation of neighboring molecules and the preservation of translational symmetry. The potential energy surface calculated by intermolecular atom–atom potentials corresponds to those of more rigid binary units like metallocenes and π-molecular compounds of the charge-transfer type. Common features in physical properties and low-temperature phase transitions with crystal fracture, observed for all these compounds, are readily accounted for in terms of domain disorder, calculated for the room-temperature structures. Because of the high molecular symmetry, rotationally disordered arrangements 60° apart are difficult to distinguish crystallographically and the predicted order–disorder transition for pyrene has only been characterized spectroscopically.