Hydrogen vs Halogen-Bonded R22(8) Rings in Organic Crystal Structures

Hydantoins and halohydantoins are five-membered heterocyclic rings where the N–X (X = H, Cl, Br, I) units exhibit similar geometries, such that they should form analogous hydrogen or halogen bond geometries. However, although an R22(8) hydrogen-bonded ring interacting through the amide moieties is the most common supramolecular synthon for hydantoins published in the Cambridge Structural Database (CSD), only one published halohydantoin crystal structure exhibits a similar halogen-bonded motif. Analysis of the structures in the CSD and density functional theory calculations show that the origin of this difference is not the relative strengths of hydrogen vs halogen bonding, as the latter are typically stronger, or the greater directionality of halogen bonding, but rather the presence of close, repulsive X···X contacts in the halogen-bonded rings.