[1] These interactions are usually rationalized primarily via dispersion, electrostatics, and electron delocalization (similar to Lewis-acid/base coordination) and are characterized by a strong directional preference that allows control over supramolecular chemistry.
[4] Alternatively, sigma hole pair interactions can be conceptualized in terms of the mixing of molecular orbitals.
[1] Several atoms, including those which are relatively electronegative (such as Chlorine,[5] Oxygen,[6] and even Fluorine[5]) can act as positive sites in sigma hole pair interactions.
Counterintuitively, this can occur even when the atom acting as the positive site has an overall negative partial charge.
The solution to this apparent contradiction lies in the anisotropy in the electron cloud introduced by the presence of the sigma bond.
Theoretical studies have shown that the interaction is most stabilizing when the negative site is colinear with the bond that gives rise to the sigma hole.
[1] The table below shows the computed strength (in kcal/mol) of three selected sigma hole interactions at a variety of angles.
[7] At any angle, it can be observed that the interaction is stronger when the Bromine atom hosting the sigma hole is bound to a strongly electron withdrawing cyano group than when this atom is bound to a trifluoromethyl group, which is only moderately electron withdrawing.
[1] The sigma hole formalism has been applied to a wide range of interactions involving electrostatic and dispersive attraction between positively and negatively charged sites.