Herein, two unconventional type III halogen···halogen interactions, namely, σ-hole···σ-hole and di-σ-hole interactions, were reported in a series of halogenated complexes. In type III, the A-halogen···halogen angles are typically equal to 180°, and the occurrence of σ-hole on halogen atoms is mandatory. Using diverse quantum mechanical calculations, it was demonstrated that the occurrence of such interactions with binding energies varied from −0.35 to −1.30 kcal/mol. Symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) revealed that type III interactions are dominated by dispersion forces, while electrostatic forces are unfavorable. Cambridge Structure Database (CSD) survey unveiled the experimental evidence for the manifestation of σ-hole···σ-hole interactions in crystal structures. This work might be deemed as a foundation for a vast number of forthcoming crystal engineering and materials science studies.

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Recently, noncovalent interactions in complexes and crystals have attracted considerable interest. The current study was thus designed to gain a better understanding of three seminal types of noncovalent interactions, namely: hydrogen, halogen and tetrel interactions with p-systems. This study was performed on three models of Lewis acids: X 3 -C-H, F 3 -C-X and F-T-F 3 (where X ¼ F, Cl, Br and I; andT ¼ C, Si, Ge and Sn) and three p-systems as Lewis bases: benzene (BZN), 1,3,5-trifluorobenzene (TFB) and hexafluorobenzene (HFB). Quantum mechanical calculations, including geometrical optimization, molecular electrostatic potential (MEP), maximum positive electrostatic potential (V s,max ), Point-of-Charge (PoC), potential energy surface (PES), quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) calculations, were carried out at the MP2/aug cc-pVDZ level of theory.The binding energies were additionally benchmarked at the CCSD(T)/CBS level. The results showed that:(i) the binding energies of the X 3 -C-H/p-system complexes were unexpectedly inversely correlated with the V s,max values on the hydrogen atom but directly correlated with the X atomic sizes; (ii) the binding energies for the F 3 -C-X/p-system and F-T-F 3 /p-system complexes were correlated with the s-hole magnitudes of the X and T atoms, respectively; and (iii) for the F 3 -C-F/p-system complexes, the binding energy was as strong as the p-system was electron-deficient, indicating the dominating nucleophilic character of the fluorine atom. NCI analysis showed that the unexpected trend of X 3 -C-H/p-system binding energies could be attributed to additional attractive interactions between the X atoms in the X 3 -C-H molecule and the carbon atoms of the p-system. Furthermore, the I 3 -Sn-H molecule was employed as a case study of hydrogen, halogen and tetrel interactions with p-systems. It was found that hydrogen and halogen interactions of the I 3 -Sn-H molecule correlated with the electron-richness of the p-system. In contrast, tetrel interactions correlated with the electron deficiency of the p-system.

σ-hole and lone-pair (lp) hole interactions of trivalent pnicogen-bearing compounds were comparatively investigated under field-free and external electric field (EEF) conditions.

The point-of-charge (PoC) approach was employed to investigate the characteristics of the tetrel bond from an electrostatic perspective. W-T-XYZ···B nomenclature was suggested where T is a tetrel atom, W is the atom along the σ-hole extension, B is a Lewis base, and X, Y, and Z are three atoms on the same side of the σ-hole. Quantum-mechanical calculations were carried out on F-T-F systems (where T = C, Si, Ge, or Sn) at the MP2/aug-cc-pVTZ level of theory, with PP functions for Ge and Sn atoms. The tetrel bond strength was estimated via the molecular stabilization energy. Tetrel bond strength was found to increase with increasing PoC negativity (i.e., Lewis basicity) and the electronegativity of the W atom. Moreover, the effects of the T···PoC distance, the W-T···PoC angle, and the aqueous medium on the tetrel bond strength were also investigated. Correlations between tetrel bond strength and several atomic and molecular descriptors such as the natural charge on the tetrel atom, E, and the p-orbital contribution to W-T bond hybridization were observed. Contrary to expectations, the tetrel bond strength in F-C-X increased as the electronegativity of X decreased. The σ-node criteria for the studied molecules were also introduced and discussed. The ability of these molecules to simultaneously form more than one tetrel bond was examined via the σ-hole test. In conclusion, the tetrel bond strength was found to be governed by the strengths of (i) the attractive electrostatic interaction of the Lewis base with the σ-hole, (ii) the attractive/repulsive interaction between the Lewis base and the X, Y, and Z atoms, and (iii) the van der Waals interaction between the Lewis base and the X, Y, and Z atoms. Graphical Abstract Characterization of tetrel bond using the Point-of-Charge (PoC) approach.

In the current study, unexplored type IV halogen⋯halogen interaction was thoroughly elucidated, for the first time, and compared to the well-established types I–III interactions by means of the second-order Møller–Plesset (MP2) method. For this aim, the halobenzene⋯halobenzene homodimers (where halogen = Cl, Br, and I) were designed into four different types, parodying the considered interactions. From the energetic perspective, the preference of scouted homodimers was ascribed to type II interactions (i.e., highest binding energy), whereas the lowest binding energies were discerned in type III interactions. Generally, binding energies of the studied interactions were observed to decline with the decrease in the σ-hole size in the order, C6H5I⋯IC6H5 > C6H5Br⋯BrC6H5 > C6H5Cl⋯ClC6H5 homodimers and the reverse was noticed in the case of type IV interactions. Such peculiar observations were relevant to the ample contributions of negative-belt⋯negative-belt interactions within the C6H5Cl⋯ClC6H5 homodimer. Further, type IV torsional trans → cis interconversion of C6H5X⋯XC6H5 homodimers was investigated to quantify the π⋯π contributions into the total binding energies. Evidently, the energetic features illustrated the amelioration of the considered homodimers (i.e., more negative binding energy) along the prolonged scope of torsional trans → cis interconversion. In turn, these findings outlined the efficiency of the cis configuration over the trans analog. Generally, symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) demonstrated the predominance of all the scouted homodimers by the dispersion forces. The obtained results would be beneficial for the omnipresent studies relevant to the applications of halogen bonds in the fields of materials science and crystal engineering.

Chalcogen∙∙∙Chalcogen interactions were investigated within four types of like∙∙∙like and unlike Y=C=Y∙∙∙Y=C=Y complexes (where Y = O, S, and Se). A plethora of quantum mechanical calculations, including molecular electrostatic potential...

The versatility of the X-T-X3 compounds (where T = C, Si, and Ge, and X = F, Cl, and Br) to participate in tetrel- and halogen-bonding interactions was settled out, at the MP2/aug-cc-pVTZ level of theory, within a series of configurations for (X-T-X3)2 homodimers. The electrostatic potential computations ensured the remarkable ability of the investigated X-T-X3 monomers to participate in σ-hole halogen and tetrel interactions. The energetic findings significantly unveil the favorability of the tetrel···tetrel directional configuration with considerable negative binding energies over tetrel···halogen, type III halogen···halogen, and type II halogen···halogen analogs. Quantum theory of atoms in molecules and noncovalent interaction analyses were accomplished to disclose the nature of the tetrel- and halogen-bonding interactions within designed configurations, giving good correlations between the total electron densities and binding energies. Further insight into the binding energy physical meanings was invoked through using symmetry-adapted perturbation theory-based energy decomposition analysis, featuring the dispersion term as the most prominent force beyond the examined interactions. The theoretical results were supported by versatile crystal structures which were characterized by the same type of interactions. Presumably, the obtained findings would be considered as a solid underpinning for future supramolecular chemistry, materials science, and crystal engineering studies, as well as a fundamental linchpin for a better understanding of the biological activities of chemicals.