Arrays with Local Centers of Symmetry in Space Groups Pca2₁ and Pna2₁

Of the several hundred structures in the Cambridge Structural Database [version 4.6 (1992), Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, England] having space groups Pca 2 1 or Pna 2 1 and more than one molecule in the asymmetric unit ( Z > 4), approximately three-quarters contain local centers of symmetry. These local centers, which are not crystallographic centers, occur predominantly near x = 18, y = 14 in Pca 2 1 or near x = 18, y = 0 in Pna 2 1 ; this also holds for the limited number of examples we have examined of pseudo-centrosymmetric molecules with Z = 4. Local centers at these points create unusual correlations between corresponding atoms in the two molecules.