Non-conventional bonding between organic molecules. The 'halogen bond' in crystalline systems

The intermolecular interaction in organic crystals, when close contact between a halogen atom and an oxygen or nitrogen atom is present, is investigated by surveys of existing crystal structure determinations in the Cambridge Structural Database and by theoretical methods. Short halogen-oxygen and-nitrogen contacts are restricted to systems with peculiar electronic and steric properties. Energy well depths for sample systems range from almost nil to about 20 kJ mol(-1), considerably less than for hydrogen bonding, with which halogen bonding can hardly compete. The width of the energy wells suggests that some short contacts may correspond to just permissive (i.e. energetically neutral) approach, or even to compressed bonding. The strongest bond is attainable only by aromatic iodine, highly activated by electron-attracting substituents, in molecular complexes with strong and sterically unhindered Lewis bases; only in such special cases is the halogen bond the most relevant cohesive factor in the crystal structure. In the PIXEL energy dissection scheme, the largest contribution to halogen bonding comes from Coulombic plus first-order polarization terms. Dispersive interactions between parallel aromatic systems are often more stabilizing and should not be neglected in assessing the tendency of halogen compounds to form linear aggregates in polar crystal structures.