Prevalence and Significance of Approximate Symmetry in Organic Pc Structures

A survey has been made of the well determined (R≤0.050) organic crystal structures that have only a single glide plane (e. g., are in space group Pc) and that have more than one crystallographically independent formula unit (Z′>1); the goal was to discover what fraction have additional approximate symmetry. Of the 377 unique structures examined 8 % should almost certainly been refined in a higher-symmetry unit cell; 86 % of the remaining 347 have approximate symmetry that is periodic in at least one dimension. While these percentages are similar to those found for P1 structures (C. P. Brock, Acta Crystallogr., Sect. B 2022 , 78, 576–588), the types of approximate symmetry differ because 89 % of the P1, Z′>1 structures were composed of enantiopure material. The 347 reliable Pc structures include 118 that are slightly distorted or mimicked Cc and P2₁/c structures, 15 of which were reported as having been determined at room temperature. The distortions in another 72 are so large that the approximate symmetry must be seen as periodic in two dimensions only. These results suggest that symmetry lowering may accompany transformation of a crystal nucleus to a macroscopic crystal.