Attraction between electrophilic caps: A counterintuitive case of noncovalent interactions

Abstract

Intermolecular attractive interaction between electrophilic sites is a counterintuitive phenomenon, as the electrostatic interaction therein is repulsive and destabilizing. Here, we confirm this phenomenon in four representative complexes, using state-ofthe-art quantum mechanical methods. By employing the blocklocalized wavefunction (BLW) method, which can turn off intermolecular charge transfer interactions, we profoundly demonstrated the significance of charge transfer interactions in these seemingly counterintuitive complexes. Indeed, after being "turned off" the intermolecular charge transfer interaction in, for example, the FNSiÁÁÁBrF complex, the originally attractive intermolecular interaction turns to be repulsive. The energy decomposition approach based on the BLW method (BLW-ED) can partition the overall stability gained on the formation of intermolecular noncovalent interaction into several physically meaningful components. According to the BLW-ED analysis, the electrostatic repulsion in these counterintuitive cases is overwhelmed by the stabilizing polarization, dispersion interaction, and most importantly, the charge transfer interaction, resulting in the eventual counterintuitive overall attraction. The present study suggests that, predicting bonding sites of noncovalent interactions using only the "hole" concept may be not universally sufficient, because other significant stabilizing factors will contribute to the stability and sometimes, play even bigger roles than the electrostatic interaction and consequently govern the complex structures.

Halogen Bonding: A Halogen-Centered Noncovalent Interaction Yet to Be Understood

“…This is in agreement with the views of Bayse [14] that even though halogen bonding interactions are often discussed in terms of an area of positive electrostatic potential on the halogen center along the outer portion of the bond axis, there are several others who have noted a lack of completeness in this model. This is also in agreement with Wang et al [51], who view the intermolecular attractive interaction between electrophilic (σ-hole) sites as a counterintuitive phenomenon because the electrostatic component of the interaction is repulsive and plays a destabilizing role. Using the block-localized wave function (BLW) energy decomposition method [211], they demonstrated that charge transfer seemingly plays the main role, and that the predicted bonding sites related with the "hole" concept are universally insufficient to describe the noncovalent interactions in these counterintuitive complexes.…”

“…Riley et al suggested that the C-Br• • • Cl angles for some halogen bonded complexes with cationic halogen bond donors the secondary interaction that occurs between Cl − and the cation ammonium group (angles greater than~120 • ) enhances exchange repulsion associated with the halogen's oblate shape, which is probably chiefly responsible for halogen bond directionality, as has been found for neutral halogen bonds [203]. Others argued that the contribution due to electrostatics may be significant in some complexes, as significant as dispersion and exchange in the others, but the equilibrium geometry of a complex system that is stabilized by a noncovalent interaction (no matter what type of bond it is) is always driven by all of them [12,45,47,49,51,52,204]. This suggests that the directionality of the interaction is simply a representation of the delicate balance between all component interactions acting on the nuclei of atoms.…”

“…Debate is ongoing to validate the IUPAC recommended characteristics [39,40,50,55] of halogen bonding, the underlying concepts of σ-hole theory [16,49,51], the origin of directionality [175][176][177], and the geometric definition by which halogen bonding (and other σ-hole interactions) may be identified [49,169]. For instance, it was suggested that a minimum of four atoms or group of atoms should be considered to define a hydrogen bond (or a halogen bond) [39,177].…”

“…The covalent character of a halogen bond increases in the way 3c-4e (three-center-four-electron) bonding becomes possible [93]. Wang et al [51] have shown that for the FNSi• • • BrF complex, and similar others, the energy decomposition approach based on the block-localized wave function (BLW) energy method (BLW-ED) can partition the overall stability gained on the formation of intermolecular noncovalent interaction into several physically meaningful components. The electrostatic repulsion in these counterintuitive cases is overwhelmed by the stabilizing polarization, dispersion interaction, and most importantly, the charge transfer interaction, resulting eventually counterintuitively in an overall attraction.…”

Nature of halogen‐centered intermolecular interactions in crystal growth and design: Fluorine‐centered interactions in dimers in crystalline hexafluoropropylene as a prototype

“…). The noncovalent chemistry of analogous interactions has been discussed on a number of occasions, including by Wang et al, but are yet to be fully appreciated for fluorine‐centered interactions. Several criticisms have appeared recently on the reliability of the IUPAC recommendations …”