Automated derivation of structural class symbols and extended Z′ descriptors for molecular crystal structures in the Cambridge Structural Database

Abstract

Extended Z′ descriptors that distinguish cases in which molecules occupy crystallographically distinct special equivalent positions (for example, Z′ = ½ + ½) are derived for 173 497 single-component molecular crystal structures in the Cambridge Structural Database. The extended Z′ descriptors are closely related to the structural class (SC) symbols developed by Belsky and Zorky, and the SC symbol is also derived for each CSD entry. SC symbols express more information than Z′ descriptors, listing explicitly the crystallographic site symmetry for all crystallographically distinct molecules: for example, P, Z = 2(, ). The Z′ descriptors are obtained from the SC symbols by taking the reciprocal of the order of each distinct crystallographic site-symmetry group. The automated methodology permits convenient identification and quantification of structures in the CSD that have more than one crystallographically distinct molecule, and it is shown that the proportion of these structures is slowly increasing.