Molecular Interactions in Crystal Structures with Z′ > 1

We describe an algorithm for enumerating and classifying molecular interaction motifs in crystal structures taken from the Cambridge Structural Database. It was used to determine which motifs have the highest propensity to occur in structures with Z' > 1 and to show that these motifs also have a strong tendency to occur between symmetry-independent rather than symmetry-related molecules in such structures. In noncentrosymmetric structures, they are predominately stacking interactions and cyclic motifs containing strong or weak hydrogen bonds. In centrosymmetric structures, OH center dot center dot center dot O hydrogen bonds and C-H center dot center dot center dot pi interactions predominate. The proportions of centrosymmetric and noncentrosymmetric structures with Z' = 2 that might be explained by the presence of one or more of these motifs were estimated. Motifs with a strong tendency to form across inversion centers in structures with Z' = 1 were also identified ("inversion-favoring motifs"). They are similar to those preferentially occurring between symmetry-independent molecules in noncentrosymmetric structures with Z' = 2 but dissimilar to those in centrosymmetric structures. However, a significant number of centrosymmetric structures with Z' = 2 have inversion-favoring motifs both across crystallographic inversion centers and between symmetry-independent molecules. Several other results were obtained: for example, crystallization with Z' > 1 is more likely for molecules with 1-3 hydrogen-bonding atoms.