article · Crystal Growth & Design
This work re-examines the 2013 IUPAC definition of the halogen bond, arguing that the earlier guidance omitted the essential role of electrophilic sigma-hole and p- or pi-hole theory alongside orbital-based charge transfer interactions. A halogen bond forms when an electron-deficient region of positive polarity on a bonded halogen atom interacts with an electron-rich nucleophilic site on the same or another molecular entity. The study reviews the defining characteristics of these interactions, cataloguing a wide range of participating donor and acceptor entities. It introduces essential caveats to guide the correct identification of halogen bonds and the accurate use of related chemical terminology. Supported by first-principles calculations of selected molecular dimers, the analysis highlights crystalline systems displaying both intermolecular and intramolecular interactions, while demonstrating how halogen surface features like pi-hole belts can establish distinct noncovalent bonds with neighbouring nucleophiles.
Understanding how molecules interact is fundamental to controlling chemical structure and crystalline behaviour. By clarifying the physical mechanisms that define halogen bonding, this work provides a more precise and rigorous framework for researchers studying molecular design. A standardised, comprehensive definition helps prevent misclassification of noncovalent interactions across crystallography and molecular chemistry.
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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.
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DOI: 10.1021/acs.cgd.4c00228
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