Why do crystal structures waste molecular inversion symmetry?

Abstract

Single-component molecular crystal structures containing centrosymmetric molecules with non-centrosymmetric crystallographic site symmetry are retrieved from the Cambridge Structural Database. The distribution amongst space group types is comparable to that for molecular crystals in general, except that there is a significant decrease in the proportion of triclinic structures. The structures are analysed in terms of the symmetrical relationships between them and some specific chemical circumstances that lead to wasted molecular inversion symmetry are identified: (1) planar centrosymmetric molecules form locally centrosymmetric pairs through face-to-face (π–π) interactions and these pairs pack through edge-to-face interactions; (2) molecules form layers that are locally centrosymmetric but the symmetry within the layers is not compatible with the symmetry of the interlayer interactions. Although the non-centrosymmetric environments of isolated centrosymmetric molecules seem to be unusual, the symmetries of the identified structural fragments and the interactions between them are quite normal.