A Unified View of Halogen Bonding, Hydrogen Bonding and Other σ-Hole Interactions

Abstract

The term “σ-hole” refers to a region of diminished electronic density along the extension of a covalent single bond to a hydrogen or an atom of Groups IV—VII. This region often has a positive electrostatic potential through which the atom can interact attractively with a negative site (such as a lone pair of a Lewis base, π electrons or an anion) to form a noncovalent complex. Hydrogen bonding and halogen bonding are the most prominent examples of such σ-hole interactions, although they have long been known experimentally for Groups IV—VI as well (but without the σ-hole label). σ-Holes result from the anisotropic charge distributions of covalently-bonded atoms. It follows from the Hellmann-Feynman theorem that σ-hole interactions can be understood and described as Coulombic, which includes polarization and dispersion. In the context of noncovalent interactions, charge transfer is simply a mathematical formulation of polarization.