Symmetry, pseudosymmetry and hypersymmetry of organic crystals

Abstract

Topological aspects of the structure of molecular crystals and the distribution of organic crystals among structural classes (SCs) are considered. As the space group alone does not characterise a molecular arrangement sufficiently, the list of orbits occupied by molecules should be specified to give the SC. The method of symmetry of potential functions (MSPF) is proposed to explain molecular arrangements which occur in organic crystals and the distribution of these crystals among SCs. Some specific effects typical for molecular crystals are also considered, including the existence of pseudosymmetric subsystems of different types (island agglomerates, chains, layers, three-dimensional subsystems). Sometimes molecules retain their intrinsic symmetry, partially or fully, in groups of pseudosymmetry of such subsystems, while it is lost in the true space group. In polysystem crystals, where molecules occupy two or more orbits, one can usually (but not always) observe the phenomenon of hypersymmetry, i.e. the transformation of symmetrically independent molecules into each other by means of non-trivial (hypersymmetrical) operations. Genuine hypersymmetry cannot be reduced to pseudosymmetry (or local symmetry).