Challenging the electrostatic σ‐hole picture of halogen bonding using minimal models and the interacting quantum atoms approach

Abstract

Among the different noncovalent interactions, halogen bonds have captured wide attention in the last years. Their stability has been rationalized in electrostatic terms by appealing to the σ-hole concept, a charge-depleted region that is able to interact favorably with electron rich moieties. This interpretation has been questioned, and in this work a set of anionic halogen model systems are used to shed some light on this issue. We use the interacting quantum atoms method, which provides an orbital invariant energy decomposition in which pure electrostatic terms are well isolated, and we complement our insights with the analysis of electrostatic potentials (ESPs) as well as with traditional descriptors of charge accumulation like the Laplacian of the electron density. The total electrostatic interaction between the interacting species is surprisingly destabilizing in many of the systems examined, demonstrating that although σ-holes might be qualitatively helpful, much care has to be taken in ascribing the stability of these systems to electrostatics. It is clearly shown that electron delocalization is essential to understand the stability of the complexes. The evolution of atomic charges as the aggregates forms reveals a charge transfer picture in which the central, σ-hole bearing halogen acts as a mere spectator. These systems may then be not far from engaging in a classical 3c-4e interaction. Since the presence of a σ-hole as characterized by the ESP mapped on a suitable molecular envelope isosurface does not guarantee attractive electrostatic interactions, we encourage to employ a wider perspective that takes into account the full charge distribution.

Are Heavy Pnictogen‐π Interactions Really “π Interactions”?

“…Among the established concepts on noncovalent intermolecular interactions, the σ‐hole model and its implications have been controversially discussed in recent years, especially in the context of halogen bonding [42–44] . While all studies agree on the fact that a depletion of electron density opposite of a sigma bond leads to a fairly strong interaction that is strongly directional, [45,46] different analysis tools indicate that it could either be regarded as electrostatic or as charge‐transfer including orbital interactions [47] . For the interaction of heavy pnictogen atoms and the π‐system, as discussed in ref.…”

“…[ 42 , 43 , 44 ] While all studies agree on the fact that a depletion of electron density opposite of a sigma bond leads to a fairly strong interaction that is strongly directional,[ 45 , 46 ] different analysis tools indicate that it could either be regarded as electrostatic or as charge‐transfer including orbital interactions. [47] For the interaction of heavy pnictogen atoms and the π‐system, as discussed in ref. [10] , a thorough analysis of the angle‐dependance of the interaction energies, however, showed that the potential energy surfaces appear very flat.…”

Role of Charge Transfer in Halogen Bonding

“…Brinck and Borrfors studied a series of halogen bonded complexes (R–Br···Br – and R′–CC–Br···Br – where R is a substituted methyl group) and showed that the halogen-bonding strength correlates linearly with the interaction energy from a point-charge model by replacing Br – with a point charge. Subsequently, they concluded that “the halogen bond can be described accurately by electrostatics and polarization without any need to consider charge transfer.” But there have been negative correlations between maximum electrostatic potentials and interaction energies even when polarization is considered in the literature. , In fact, there have been profound theoretical and experimental studies, including ours, confirming the crucial or even dominating role of charge transfer interaction in halogen bonding. − For instance, Wolter and Bickelhaupt compared the halogen bonding in D–X···A – and hydrogen bonding in D–H···A – (D, X, and A are halogen elements) and found that apart from the classical electrostatic attraction, there is substantial HOMO–LUMO interactions between the lone pair of A and the σ* orbital of the D–X or D–H bond . In other words, there is considerable charge transfer interactions from A – to D–X/D–H in both types of bonding.…”

Giant Supramolecular Synthons via Cyclic Halogen···Halogen Contacts in Substituted o-Xylenes

“…In an octameric synthon, the Although the electrostatic model continues to be useful in rationalizing the halogen bonding, 111 our results support the need to consider the weak nonelectrostatic interactions for this purpose. 42,112…”

“…Albeit the presence of σ-holes, the Br···Br interactions present in the synthons have destabilizing or nearly zero Coulombic components according to the IQA analysis. Although the electrostatic model continues to be useful in rationalizing the halogen bonding, our results support the need to consider the weak nonelectrostatic interactions for this purpose. , …”