Halogen bond

A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity.

Comparison between hydrogen and halogen bonding:

In both cases, D (donor) is the atom, group, or molecule that is electron rich and donates them to the electron poor species (H or X). H is the hydrogen atom involved in hydrogen bonding (HB), and X is the halogen atom involved in XB. A (acceptor) is the electron poor species withdrawing the electron density from H or X, accordingly. H-A and X-A, when both atoms are considered together, are called hydrogen/halogen bond donors, accordingly, and D is HB/XB acceptor. A difference between HB and XB is since halogen atoms are Lewis bases, a halogen atom can both donate and accept in a halogen bond.

A parallel relationship can easily be drawn between halogen bonding and hydrogen bonding (HB). In both types of bonding, an electron donor/electron acceptor relationship exists. The difference between the two is what species can act as the electron donor/electron acceptor. In hydrogen bonding, a hydrogen atom acts as the electron acceptor and forms a non-covalent interaction by accepting electron density from an electron rich site (electron donor). In halogen bonding, a halogen atom is the electron acceptor. Simultaneously, the normal covalent bond between H or X and A weakens, so the electron density on H or X appears to be reduced. Electron density transfers results in a penetration of the van der Waals volumes.

Halogens participating in halogen bonding include: iodine (I), bromine (Br), chlorine (Cl), and sometimes fluorine (F). All four halogens are capable of acting as XB donors (as proven through theoretical and experimental data) and follow the general trend: F < Cl < Br < I, with iodine normally forming the strongest interactions.

Dihalogens (I₂, Br₂, etc.) tend to form strong halogen bonds. The strength and effectiveness of chlorine and fluorine in XB formation depend on the nature of the XB donor. If the halogen is bonded to an electronegative (electron withdrawing) moiety, it is more likely to form stronger halogen bonds.

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