Reassessing the Role of σ Holes in Noncovalent Interactions: It is Pauli Repulsion that Counts

Abstract

A number of prototypical weak electron donor–electron acceptor complexes are investigated by the Symmetry Adapted Perturbation Theory, some of which belong to novel classes of weak bonds such as halogen and chalcogen bonds. Also included are complexes involving strong Lewis acids such as BeO and AuF. The common view in the literature is to associate these novel bonds with a variety of “holes”, σ, π, δ, or positive areas in their electrostatic potential maps. The presumption is that these positive areas of the electrostatic potential are indicative of the electrostatic nature of these noncovalent bonds. The electrostatic view extends to the explanations of the directionality of approaches between the subsystems forming these bonds. This work demonstrates that one common feature of these electrostatic potential “holes” is the local depletion of electron density of which the best detector is the first-order Pauli repulsion. The minimization of this repulsion determines the bond directionality and its relative angular rigidity. In relatively strong complexes of BeO with rare gases, where BeO shows a clear cavity in electron density—an ultimate “σ hole”—the electrostatic effect does not control the bending potential—the exchange repulsion does. In halogen bonds, the halogen atom is nonspherical, displaying an axial “σ hole” in its electrostatic potential. However, in no examined case, from rare gas acting as an electron donor to a polar donor to an anionic donor, is the electrostatic energy responsible for the directionality of the halogen bond. In fact, it is not even maximized in the direction of the σ hole in N2-ClF and NH3-ClF. Yet, in all the cases, the exchange repulsion is minimized in the direction of the σ hole. The minimized exchange repulsion associated with the subtle and less subtle depletions of the electron density occur on the nodal planes or on the intersections thereof in the highest occupied molecular orbitals of Lewis acids, provided that the systems are closed-shell. The role of nodal planes in covalent and coordinate covalent bonds is well recognized. This work points to their similarly equal importance in certain types of donor–acceptor noncovalent interactions.

Three-body interaction effects in heterolytic hydrogen splitting by frustrated Lewis pairs

“…This term is attributed to the charge transfer interaction (see above, and the discussion in Ref. 20).…”

“…The interaction energy analysis is carried out using SAPT 17,18 in its variant SAPT0 alternatively called SAPT(HF). The interaction energy at this level of theory consists of the electrostatic (es), exchange (exch), induction (ind), exchange-induction (exind), dispersion (disp), and its exchange counterpart (ex-disp): (10) es + E (10) exch + E (20) ind,r + E (20) ex-ind,r + E (20) disp + E (20) ex-disp + d HF (1) where ''r'' in the subscripts refers to the response-inclusive treatment of induction. The numbers in the superscripts in the E (VW) notation correspond to the orders in the intermolecular perturbation operator V and the intramonomer correlation operator W. The last term d HF stands for higher order induction and other residual effects:…”

“…Thus, δ HF is expected to include not only the higher than second-order induction terms but also, in donor–acceptor complexes, the charge transfer term (see the discussion in Ref. 20). To reiterate, SAPT is here applied to the acid–base + H 2 pseudo-dimer (denoted as A–B + H 2 ) as well as to acid + H 2 (A + H 2 ) and base + H 2 (B + H 2 ), all 2-body interactions.…”

Enhancement of Halogen Bond Strength by Intramolecular H-Bonds

“…These and many other attributes of the HB have been thoroughly elucidated over more than a century, and are laid out in countless papers and monographs. − The study of the parallel XB is advancing quickly, leading to the establishment of a number of general rules and principles that guide our understanding of this phenomenon. − It is commonly understood, for example, that this bond strengthens as the X atom grows larger, commonly attributed to an associated rise in its electropositivity and polarizability. The highly compact and electronegative F atom very rarely participates in a XB, and then only under certain limited conditions .…”

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

“…The variety of noncovalent bond which has garnered the earliest and most widespread attention to date, has been the halogen bond (XB). − In its simplest manifestation, the R substituent in an RX molecule draws electron density toward itself along the R–X σ-bond, leaving a deficit along this axis. This density depletion, and the accompanying positive electrostatic potential, is commonly referred to as a σ-hole, which contrasts with the negative potential around the equator of the R–X axis, in the vicinity of the X lone pairs.…”