Definition of the Halogen Bond (IUPAC Recommendations 2013): A Revisit

This Article revisits the “Definition of the Halogen Bond (IUPAC Recommendations 2013)” [Desiraju, G. R. Pure Appl. Chem. 2013, 85 (8), 1711–1713], recommendations that fail to include the fundamental, underlying concept of (electrophilic) σ- and p-/π-hole theory and orbital-based charge transfer interactions that accompany halogen bond formation. An electrophilic σ-hole, or p-/π-hole, is an electron-density-deficient region of positive polarity (and positive potential) on the electrostatic surface on the side of halogen along, or orthogonal to, a covalently bonded halogen in a molecular entity that leads to the development of a noncovalent interaction─a halogen bond─when in close proximity to an electron-density-rich nucleophilic region on the same or another identical or different molecular entity, with which it interacts. This Article re-examines the characteristic features of the halogen bond and lists a wide variety of donors and acceptors that participate in halogen bonding. We add caveats that are essential for identifying halogen bonding in chemical systems, necessary for the appropriate use of the terminologies involved. Illustrative examples of chemical systems that feature inter- and intramolecular halogen bonds and other noncovalent interactions in the crystalline phase are given, together with a case study of some dimer systems using first-principles calculations. We also point out that the π-hole/belt (or p-hole/belt) that may develop on the surface of a halogen derivative in halogenated molecules may be prone to forming a π-hole/belt (or p-hole/belt) halogen bond when in close proximity to nucleophiles on another similar or different molecular entity.