σ-hole bonding between like atoms; a fallacy of atomic charges

Abstract

Covalently bonded atoms, at least in Groups V–VII, may have regions of both positive and negative electrostatic potentials on their surfaces. The positive regions tend to be along the extensions of the bonds to these atoms; the origin of this can be explained in terms of the σ-hole concept. It is thus possible for such an atom in one molecule to interact electrostatically with its counterpart in a second, identical molecule, forming a highly directional noncovalent bond. Several examples are presented and discussed. Such “like-like” interactions could not be understood in terms of atomic charges assigned by any of the usual procedures, which view a bonded atom as being entirely positive or negative. Figure Calculated electrostatic potential on the surface of SCl₂. The sulfur is in the foreground, the chlorines are at the back. Color ranges (kcal mol⁻¹): purple negative, blue between 0 and 8, green between 8 and 15, yellow between 15 and 20, red more positive than 20. Note that the sulfur has regions of both positive (red) and negative (purple) electrostatic potential