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.
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