Conformational Interconverion of Stilbene-type of Compounds in Crystals.

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Although it has been known that molecules are in motion in crystals, most of them are samll amplitude thermal vibrations around equilibrium structures or rotational vibrations of whole molecules. Large amplitude intramolecular motions or conformational interconversions are rare. This study reveals that conformational interconversions by which molecular shapes change greatly take place in the crystals of certain compounds. For the crystals of stilbene-type compounds, an orientational disorder in which two molecules are approximately related to one another by 180ﾟrotation around their longest axes. Although the disorder has been studied repeatedly, it has remained unclear whether the disorder is dynamic or static. This study revealed on the basis of variable temperature X-ray crystallographic analyses that the disorder is dynamic. Thus, the disorder disappears with lowering the temperature. The disorder is accounted for by conformational interconversion during which a pedal motion occurs. The torsional vibration around the N-Ph bonds occurs so that two benzene rings remain parallel to each other and the orientation of the benzene rings to the crystalline lattice do not change. When the amplitude of the torsional vibration becomes much larger, a molecule with one of two orientations changes into the the conformer with the other conformation through the conformation in which the azo unit (C-N=N-C) is perpendicular to the benzene rings. The populations of the two conformers show a temperature dependence if two conformers have different stabilities in crystals. This phenomenon is of interest in relation to solid state properties and reactivities in organic crystals.