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

Intermolecular bonding attraction at π-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 Å 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) nD-π*AX interactions that are fully analogous to the nD-σ*AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi–Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that “deletion” of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi–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.

Magnesium–halobenzene bonding: mapping the halogen sigma-hole with a Lewis-acidic complex

Complexes of a highly Lewis acidic Mg cation and the full series of Ph–X (X = F, Cl, Br, I) have been structurally characterized. The Mg⋯X–Ph angle decreases with halogen size on account of the growing halogen σ-hole.

Simulating Chalcogen Bonding Using Molecular Mechanics: A Pseudoatom Approach to Model Ebselen.

The organoselenium compound ebselen has recently been investigated as a treatment for COVID-19, however<br>efforts to model ebselen in silico have been hampered by the lack of a efficient and accurate method to assess<br>its binding to biological macromolecules. We present here a Generalized Amber Force Field modification which<br>incorporates classical parameters for the selenium atom in ebselen, as well as a positively charged pseudoatom to<br>simulate the sigma?-hole, a quantum mechanical phenomenon that dominates the chemistry of ebselen. Our approach<br>is justified using an energy decomposition analysis of a number DFT optimised structures, which shows that the<br>?sigma-hole interaction is primarily electrostatic in origin. Finally, our model is verified by conducting MD simulations<br>on a number of simple complexes, as well the clinically relevant SOD1, which is known to bind to ebselen.

Simulating Chalcogen Bonding Using Molecular Mechanics: A Pseudoatom Approach to Model Ebselen.

The organoselenium compound ebselen has recently been investigated as a treatment for COVID-19, however<br>efforts to model ebselen in silico have been hampered by the lack of a efficient and accurate method to assess<br>its binding to biological macromolecules. We present here a Generalized Amber Force Field modification which<br>incorporates classical parameters for the selenium atom in ebselen, as well as a positively charged pseudoatom to<br>simulate the sigma?-hole, a quantum mechanical phenomenon that dominates the chemistry of ebselen. Our approach<br>is justified using an energy decomposition analysis of a number DFT optimised structures, which shows that the<br>?sigma-hole interaction is primarily electrostatic in origin. Finally, our model is verified by conducting MD simulations<br>on a number of simple complexes, as well the clinically relevant SOD1, which is known to bind to ebselen.

Theoretical Description of R–X⋯NH3 Halogen Bond Complexes: Effect of the R Group on the Complex Stability and Sigma-Hole Electron Depletion

In the present work, a number of R–X⋯NH3 (X = Cl, Br, and I) halogen bonded systems were theoretical studied by means of DFT calculations performed at the ωB97XD/6-31+G(d,p) level of theory in order to get insights on the effect of the electron-donating or electron-withdrawing character of the different R substituent groups (R = halogen, methyl, partially fluorinated methyl, perfluoro-methyl, ethyl, vinyl, and acetyl) on the stability of the halogen bond. The results indicate that the relative stability of the halogen bond follows the Cl < Br < I trend considering the same R substituent whereas the more electron-withdrawing character of the R substituent the more stable the halogen bond. Refinement of the latter results, performed at the MP2/6-31+G(d,p) level showed that the DFT and the MP2 binding energies correlate remarkably well, suggesting that the Grimme’s type dispersion-corrected functional produces reasonable structural and energetic features of halogen bond systems. DFT results were also observed to agree with more refined calculations performed at the CCSD(T) level. In a further stage, a more thorough analysis of the R–Br⋯NH3 complexes was performed by means of a novel electron localization/delocalization tool, defined in terms of an Information Theory, IT, based quantity obtained from the conditional pair density. For the latter, our in-house developed C++/CUDA program, called KLD (acronym of Kullback–Leibler divergence), was employed. KLD results mapped onto the one-electron density plotted at a 0.04 a.u. isovalue, showed that (i) as expected, the localized electron depletion of the Br sigma-hole is largely affected by the electron-withdrawing character of the R substituent group and (ii) the R–X bond is significantly polarized due to the presence of the NH3 molecule in the complexes. The afore-mentioned constitutes a clear indication of the dominant character of electrostatics on the stabilization of halogen bonds in agreement with a number of studies reported in the main literature. Finally, the cooperative effects on the [Br—CN]n system (n = 1–8) was evaluated at the MP2/6-31+G(d,p) level, where it was observed that an increase of about ~14.2% on the complex stability is obtained when going from n = 2 to n = 8. The latter results were corroborated by the analysis of the changes on the Fermi-hole localization pattern on the halogen bond zones, which suggests an also important contribution of the electron correlation in the stabilization of these systems.

Computational insight into the halogen bonded self-assembly of hexa-coordinated metalloporphyrins

A direct influence of porphyrin's ring current on the sigma-hole potential of halogen atoms at the axial position of metalloporphyrins during halogen bonded self-assembly is determined in this study.

Biological control of S-nitrosothiol reactivity: potential role of sigma-hole interactions

S-Nitrosothiols, ubiquitous biological derivatives of nitric oxide, can engage in σ-hole/bonding with Lewis bases, which, in combination with hydrogen bonding with Lewis acids, could be the basis of enzymatic control of S-nitrosothiol reactions.