Origins of the bond distance anomalies observed for the X-ray structures of a series of organic compounds are discussed in terms of molecular motion and reactions in crystals. The unusual shortening of the central bond in stilbene-type compounds has been ascribed to an artifact originated from molecular motion in crystals. The anomalous elongation of the central C - C bond in bi (anthracene-9, 10-dimethylene) photoisomer has been ascribed to an artifact originated from a cycloreversion reaction in crystals.
The application of the theory of superspace groups to the incommensurate composite crystal is reviewed. An introduction is given of the methods for assignment of the diffraction pattern, description of the symmetry, and determination and description of the crystal structure especially for a composite crystal with a four-dimensional symmetry. Some discussions are given about interatomic distances and the relation between the three-dimensional and four-dimensional lattices.
Self-organization has been found out in the crystals of the metal complexes (Cu (II), Pd (II) ) with arginine (Arg) or glutamate (Glu) . The [Cu (L-Arg) 2] -X and [Cu (D-Arg) 2] -X (X=isophthalate, pyridine-2, 6-dicarboxylate, pyridine-3, 5-dicarboxylate) revealed infinite right- and left-handed double-helical structures, respectively. The [Cu (Arg) 2] -X (X=SO42-) afforded a single-helical structure, and [Cu (Arg) 2] -X (X=terephthalic acid, pyromellitic acid) gave a layer structure. It indicates that self-organizations in these systems are controlled by dianions. The reaction of [Pd (Arg) 2] 2+complex with isophthalate has also been examined in DMSO-d6 by 1H- and 13C-NMR spectroscopies, in which the molecular process of self assembly in the [Cu (L-Arg) 2] -isophthalate system was successfully followed. Furthermore, the [Pd (L-/D-Glu) (H2O) ] (H2O) complexes formed infinite left- and right-handed single-helical structures, respectively, which exhibited not only self-organization but also self-recombination.