Intermolecular Nonbonded Contact Distances in Organic Crystal Structures: Comparison with Distances Expected from van der Waals Radii

The conclusions derived from systematic analyses of intermolecular contact distances in organic crystals may be affected by the values assumed for van der Waals radii. The most widely used tabulations of van der Waals radii date back 30 years or more. Moreover, many of the tabulated values were chosen to reproduce volumes, not contact distances in crystals. Literally millions of nonbonded contact distances have been characterized by crystallography since the tabulations were compiled. A study has therefore been performed to establish the degree of consistency between these accumulated crystallographic data and the van der Waals radii of the common nonmetallic elements, as compiled by Pauling and Bondi. For halogens and sulfur, the results show a remarkable agreement between observed contact distances and the Bondi radii. Agreement is slightly less good for carbon, nitrogen, and oxygen, but discrepancies are still only about 0.05 Å. However, there is a significant difference for hydrogen, where the Bondi value of 1.2 Å is probably too high by about 0.1 Å.