Halogen bonding, XB, is the product of a non-covalent interaction between a halogen X and a negative site B (e.g., Lewis base). The halogen, X, is usually part of an R-X molecule where R can be another halogen, an organic or an inorganic electron-donating-group. Halogen bonding (XB) is in some ways analogous to hydrogen bonding (HB). In the latter, a hydrogen atom is shared between an atom, group or molecule that “donates” and another that “accepts” it.[1-3] In halogen bonding, it is a halogen atom X that is shared between a donor R and an acceptor Y. Thus the two forms of interaction can be illustrated by:
HB : R_H…Y
XB : R_X…Y
Because of their high electronegativity; halogen atoms in halo-organics are classically considered as sites of high
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A typical halogen bond is denoted by the three dots in R–X…Y. R–X is the halogen bond donor, X is any halogen atom with an electrophilic (electron-poor) region, and R is a group covalently bound to X. In some cases, X may be covalently bound to more than one group. It may also form more than one halogen bond. Y is the halogen bond acceptor and is typically a molecular entity possessing at least one nucleophilic (electronrich) region.[2,4, 48]
1.3 List of Features (as proposed by the IUPAC)*
Existence of a halogen bond may be evident by experimental or theoretical, or even both. Some features that are useful as signs for the halogen bond are mentioned below. The more the number of features met, the more consistent the description of an interaction as a halogen bond is. [4]
In a typical halogen-bonded complex R–X…Y:
• The interatomic distance between X and the appropriate nucleophilic atom of Y tends to be less than the sum of the van der Waals radii.
• The length of the R–X covalent bond usually increases relative to the unbonded
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• The binding energies of the peaks associated with X with the X-ray photoelectron spectrum (XPS) of the complex shift to lower energies relative to unbonded X.
1.4 Significance of the Halogen Bond
Halogen bonding importance in biological structures and functioning has been surveyed. It occurs, for instance, in various forms including, yet not confined, to ligand binding, recognition, and molecular folding. This opened the door to a new era of application of halogen bonding in biological advancements both on the level of further understanding of biological processes and the implication of new pharmacological designs. [46, 50-54]
Another vital and fascinating application of halogen bonding is in crystal engineering. This encompasses framing cocrystals with detailed preferred characteristics, such as the spatial orientations and/or component separation. Such implications open the door to a limitless future possibility in the material designing and development. [1,3, 53-57]
2. General Aspects and Nature of the Halogen