Disorder in crystals of the perylene–pyromellitic dianhydride complex

Abstract

Computer simulation of concerted molecular reorientation in terms of lattice potential energy, has been used to rationalize disorder in the room-temperature structure of the perylene–pyromellitic dianhydride molecular complex. A trial structure, based on the simulated disorder of rigid moieties, is not inconsistent with single-crystal x-ray diffractometer data, whereas unconstrained refinement allowing for anisotropic thermal motion converges to a chemically dubious configuration. The disordered trial structure was obtained by simulating gear-like, in-plane concerted rotation of planar molecules as a function of van der Waals lattice energy. Subminima in the potential energy surface were interpreted as possible contributing forms in the disordered arrangement. The total lattice-energy manifold was not explored and positional disorder was not taken into account. The trial structure therefore reflects the main features of the disorder (R=0.14), but is still inadequate to account for all disorder effects measured crystallographically. Vibration analysis as a function of temperature is shown to be an unreliable probe to distinguish between dynamic and static disorder effects.