Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics

Intermolecular bonding attraction at pi-bonded centers is often described as electrostatically driven and given quasi-classical rationalization in terms of a pi hole depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO-, CN-) with simple atomic anions (H-, F-) or with one another. Such anti-electrostatic anion-anion attractions are shown to lead to robust metastable binding wells (ranging up to 20-30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 & ANGS; width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) n(D)-pi*(AX) interactions that are fully analogous to the n(D)-sigma*(AH) interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-sigma* (sigma hole) and n-pi* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Burgi-Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that deletion of such pi*-based donor-acceptor interaction obliterates the characteristic Burgi-Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency ( charge transfer ) rather than envisioned Coulombic properties of unperturbed monomers.

Automated summary

This study demonstrates that intermolecular bonding attraction at pi-bonded centers can occur between closed-shell ions of like charge, forming locally stable complexes. This attraction violates classical electrostatic expectations and is associated with 3-center/4-electron interactions. The study also shows that short-range covalency (charge transfer) is the cause of this attraction, rather than the Coulombic properties of unperturbed monomers.