σ-Holes, π-holes and electrostatically-driven interactions

A positive pi-hole is a region of positive electrostatic potential that is perpendicular to a portion of a molecular framework. It is the counterpart of a sigma-hole, which is along the extension of a covalent bond to an atom. Both sigma-holes and pi-holes become more positive (a) in going from the lighter to the heavier atoms in a given Group of the periodic table, and (b) as the remainder of the molecule is more electron-withdrawing. Positive sigma- and pi-holes can interact in a highly directional manner with negative sites, e.g., the lone pairs of Lewis bases. In this work, the complexes of 13 pi-hole-containing molecules with the nitrogen lone pairs of HCN and NH3 have been characterized computationally using the MP2, M06-2X and B3PW91 procedures. While the electrostatic interaction is a major driving force in pi-hole bonding, a gradation is found from weakly noncovalent to considerably stronger with possible indications of some degree of coordinate covalency.