Distance criteria for crystal packing analysis of supramolecular motifs

Supramolecular phenomena are determined by energies, but a different property, geometry, is commonly measured, and abundant data are available. The sum of van der Waals radii, the conventional distance criterion in assessing geometrical data, needs to be related to the relevant intermolecular potential, with cognisance of the chemistry of the molecular surface. The relationship between distance distributions in crystals and the distance scale of the intermolecular potential is evaluated, and the relevant concepts (and mis-concepts) are elaborated. The position of the minimum in the intermolecular potential, that is the most stabilising distance, is ca. 0.4 Å larger than the van der Waals distance. In crystals a small number of destabilising contacts less than the van der Waals distance can be enforced by a much larger number of longer stabilising distances. Crystal packing analyses with cut-offs at the van der Waals distance are likely to miss key supramolecular features.