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.

No abstract

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.