An overview of σ-hole bonding, an important and widely-occurring noncovalent interaction

Abstract

σ-Hole bonding is a highly directional noncovalent interaction between a positive region on a covalently-bonded Group IV-VII atom and a negative site on another molecule, e.g., a lone pair of a Lewis base. The positive region reflects the electron deficiency in the outer lobe of the p-type orbital involved in the covalent bond and is along the extension of this bond. There is now considerable experimental and computational evidence for σ-hole bonding. Within a given group, and for a particular Lewis base, the strength of the interaction increases with the polarizability of the atom and the electron-withdrawing power of the remainder of the molecule. For Groups IV-VI, there can be more than one σ-hole on the atom. For a series of Group IV-VII molecules, we give computed values of the positive surface electrostatic potentials associated with the σ-holes, and we also present some calculated interaction energies with different Lewis bases. σ-Hole bonding is competitive with hydrogen bonding, but the two can also accompany each other. The positive σ-holes usually exist in conjunction with negative regions on the remainders of the atoms' surfaces, so that interactions with electrophiles as well as nucleophiles are possible. It is, therefore, not valid to assign single global atomic charges in these instances. Examples of σ-hole bonding in molecular biology and crystal engineering are discussed. © 2010 Springer Science+Business Media B.V.