The σ-holeσ-hole stacking interaction: An unrecognized type of noncovalent interaction

The sigma -hole sigma -hole stacking interaction, an unrecognized type of noncovalent interaction, has been found to be present in large quantities in the Cambridge Structural Database. In the sigma -hole sigma -hole stacking interaction, each of the two interacting sigma -holes has the dual electron donor/electron acceptor character; when one sigma -hole acts as an electron donor, the other sigma -hole acts as an electron acceptor, and vice versa. The sigma -hole sigma -hole stacking interaction is clearly different from the sigma -hole bond in which the charge transfer occurs mainly from the electron donor to the sigma -hole. Energy component analysis shows that the sigma -hole sigma -hole stacking interaction is dominated by the dispersion energy, which is similar to the nature of the aromatic stacking interaction between unsaturated molecules or the sigma sigma stacking interaction between saturated molecules.