An Overview of Strengths and Directionalities of Noncovalent Interactions: σ-Holes and π-Holes

Abstract

Quantum mechanics, through the Hellmann–Feynman theorem and the Schrödinger equation, show that noncovalent interactions are classically Coulombic in nature, which includes polarization as well as electrostatics. In the great majority of these interactions, the positive electrostatic potentials result from regions of low electronic density. These regions are of two types, designated as σ-holes and π-holes. They differ in directionality; in general, σ-holes are along the extensions of covalent bonds to atoms (or occasionally between such extensions), while π-holes are perpendicular to planar portions of molecules. The magnitudes and locations of the most positive electrostatic potentials associated with σ-holes and π-holes are often approximate guides to the strengths and directions of interactions with negative sites but should be used cautiously for this purpose since polarization is not being taken into account. Since these maximum positive potentials may not be in the immediate proximities of atoms, interatomic close contacts are not always reliable indicators of noncovalent interactions. This is demonstrated for some heterocyclic rings and cyclic polyketones. We briefly mention some problems associated with using Periodic Table Groups to label interactions resulting from σ-holes and π-holes; for example, the labels do not distinguish between these two possibilities with differing directionalities.

Symmetrical Noncovalent Interactions Br···Br Observed in Crystal Structure of Exotic Primary Peroxide

“…Although both types of Hal···Hal contacts have similar abundance, today halogen bond (type II) has drawn significantly higher attention and similarly to hydrogen bonds, metallophilic contacts, Although both types of Hal···Hal contacts have similar abundance, today halogen bond (type II) has drawn significantly higher attention and similarly to hydrogen bonds, metallophilic contacts, and stacking interactions, halogen bonding is now widely used in crystal growth and design and in and stacking interactions, halogen bonding is now widely used in crystal growth and design and in supramolecular engineering [1,[3][4][5]. The recently published relevant reviews are devoted to theoretical approaches to Hal···Hal contacts [6,7] and also application of Hal···Hal contacts in supramolecular engineering [8][9][10][11][12], catalytic reactions [13], organometallic and coordination chemistry [14][15][16][17][18], polymer science [19], medical chemistry and drug discovery [20][21][22][23], and to the involvement of Hal···Hal contacts in human function [24].…”

Characterization of the dimeric interactions of dimeric and tetrameric conformations of the PvNV protrusion-domain using a mixed DFT/QTAIM approach

“…To gain insight into the dimer-dimer interfaces of PvNV P-domains, one must characterize accurately the nature of the non-covalent intermolecular interactions involved in these interfaces using applicable computational methods. These dimeric interactions comprise hydrogen-bonding (H-bonding), electrostatic, and van der Waals interactions [9,10] that all of them fall under the umbrella of Coulombic interactions [11–13]. Among them, hydrogen bonds (H-bonds) play the most important role in maintaining the dimeric PvNVPd interfaces.…”

4,4′-Dipyridyl Dioxide·SbF3 Cocrystal: Pnictogen Bond Prevails over Halogen and Hydrogen Bonds in Driving Self-Assembly

“…17,18 σ-Hole interactions present several common features, independent from the group of the periodic table to which the electrophilic atom belongs. 8,9,19 In general, interaction strength and positive electrostatic potential at the σ-hole run parallel. An atom forming n covalent bonds can have on its surface up to n positive σ-holes which can be involved in up to n net attractive interactions with electron rich sites in surrounding molecular entities.…”