Kitajgorodskij's categories

To a close approximation, the relative frequency of the space groups of molecular organic compounds is determined by ease of packing. Kitajgorodskij, with molecular organic structures in mind, divided the space groups of the triclinic, monoclinic and orthorhombic systems into four categories: `closest-packed', `limitingly close-packed', `permissible' and `impossible' [Kitaigorodskii (1955); Kitaigorodskii (1961). Organic Chemical Crystallography, New York: Consultants Bureau]. Empirically, about a dozen of the `impossible' space groups are not rare and several of them (Pc, P2/c, C222<sub>1</sub>, Fdd2 and possibly other orthorhombic groups) can be recategorized as `permissible' on Kitajgorodskij's own criteria. In addition, certain space groups (notably C2/c and Pbca) requiring inherent molecular symmetry for close packing in Kitajgorodskij's categorization may also be placed in the category allowing close packing for molecules in general positions; in such cases both enantiomers may be required.