This recommendation proposes a definition for the term "halogen bond", which designates a specific subset of the inter-and intramolecular interactions involving a halogen atom in a molecular entity.

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Abstract:The term "hydrogen bond" has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X-HؒؒؒY hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Similar and yet also notably different are the B small middle dot small middle dot small middle dotXY and B small middle dot small middle dot small middle dotHX complexes in the gas phase, where B is a simple Lewis base, XY is a homo- or heterodihalogen molecule, and HX is a hydrogen halide. This is demonstrated, for example, by the structures of oxirane small middle dot small middle dot small middle dotClF and oxirane small middle dot small middle dot small middle dotHCl (see picture). Both bonds are dominated by simple electrostatic interactions, but differ in terms of their propensity for nonlinearity.

There has been an upsurge of interest in the halogen bond during the last decade. This non-covalent interaction is less familiar than the hydrogen bond, but is similar to it in several respects. In this article, we first discuss the nature of the halogen bond in the gas phase, as established by systematic investigations of the rotational spectra of complexes B...XY, where B is a simple Lewis base and XY is a dihalogen molecule. The geometry of a given B...XY is found to be isomorphic with that of the corresponding hydrogen-bonded system B...HX, an observation that leads an interim definition of the halogen bond similar to that recently proposed for the hydrogen bond. Selected novel applications of the halogen bond made in the last decade in various areas of chemistry/materials (namely crystal engineering, liquid crystals, nano-materials, polymer chemistry and inorganic chemistry) are then reviewed. These applications generally involve molecules of the type XR (where R is an electron-withdrawing group) acting as the electron donor, rather than dihalogens XY.

This recommendation proposes a definition for the term “chalcogen bond”; it is recommended the term is used to designate the specific subset of inter- and intramolecular interactions formed by chalcogen atoms wherein the Group 16 element is the electrophilic site.

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Tetrel, pnictogen and chalcogen-bonded complexes: old bonds but new names.