Halogen bonds recently experienced an increased interest in the crystallographic society due to their high directionality and tunable properties. These properties are a result of the aspherical distribution of the electron density at the halogen atom; the density is shifted perpendicular to the bond forming a ‘sigma hole’. This effect can be investigated with charge density studies based on high resolution diffraction data. Up to now, the focus in those studies was mainly on the halogen atom and its surroundings, whereas the influence of the environment of the halogen bond acceptor has not been investigated in detail yet.

We investigated five crystal structures containing a halogen bond between iodine in a pentafluriodobenzene molecule and nitrogen in four different pyridine derivatives and triethylamine. After a multipole refinement of the diffraction data, the electron density distribution was analyzed according to Bader's Atoms in Molecules theory.

All structures show a polarization of the electron density in the nitrogen molecule towards the halogen direction, but the electron density distribution of the iodine molecule varies widely between the structures. The crystal structures containing pyridine and paraphenyl pyridine express the most pronounced formation of a sigma hole at the iodine atom. The origin of the differences in polarization must be further examined to determine whether it is due to additional interactions of the pentafluoriodobenze with other molecules in the crystal structure or the different structures of the acceptor molecule.