Halogen bonding is a non-covalent interaction featuring halogen atoms as electron acceptors. It can be compared to hydrogen bonding. The halogen atom X is attached to an electron-withdrawing group A and accepts electron density from another molecule D, forming a complex A—X- – -D.

In contrast to hydrogen bonds, halogen bonds are directional, usually showing angles of 180°. This is caused by the anisotropic charge distribution at the halogen atom (see left in the picture above). The lower electron density in the direction of the A—X bond, called a σ-hole, is responsible for the halogen’s ability to act as an electron acceptor. Fluorine is different from the other halogens in that it does not typically feature a σ-hole. It can, however, form a non-covalent interaction with electron donors if the acceptor group is sufficiently electron-withdrawing.

To clarify whether these interactions can be categorized as “traditional” halogen bonds, Kiamars Eskandari and Mina Lesani, Isfahan University of Technology, Iran, have performed calculations on complexes of fluorine-containing molecules, such as F₂, FCN, and C₂F₂, with ammonia as the electron donor. They compared these complexes to analogous chlorine and bromine compounds.

The study shows that, unlike other halogens, fluorine has a spherical charge distribution in A—F complexes (see right in the picture above), and the complexes have F- – -N bond distances greater than the sum of van der Waals-radii. Using QTAIM calculations (Quantum Theory of Atoms in Molecules), the researchers show that the majority of fluorine-nitrogen interactions are electronically quite distinct from traditional halogen bonds. The team proposes categorizing the interactions as distinct “fluorine bonds” rather than “halogen bonds” due to these differences.

• Does Fluorine Participate in Halogen Bonding?,
  Kiamars Eskandari, Mina Lesani,
  Chem. Eur. J. 2015.
  DOI: 10.1002/chem.201405054