Reassessing the Role of σ Holes in Noncovalent Interactions: It is Pauli Repulsion that Counts

Elucidating the Role of Electron-Donating Groups in Halogen Bonding

Computational quantum chemical techniques were utilized to systematically examine how electron-donating groups affect the electronic and spectroscopic properties of halogen bond donors and their corresponding complexes. Unlike the majority of studies on halogen bonding, where electron-withdrawing groups are utilized, this work investigates the influence of electron-donating substituents within the halogen bond donors. Statistical analyses were performed on the descriptors of halogen bond donors in a prescribed set of archetype, halo-alkyne, halo-benzene, and halo-ethynyl benzene halogen bond systems. The σ-hole magnitude, binding and interaction energies, and the vibrational X···N local force constant (where X = Cl, Br, I, and At) were found to correlate very well in a monotonic and linear manner with all other properties studied. In addition, enhanced halogen bonds were found when the systems contained electron-donating groups that could form intramolecular hydrogen bonds with the electronegative belt of the halogen atom and adjacent linker features.

Pd and Pt metal atoms as electron donors in σ-hole bonded complexes

Quantum calculations provide a systematic assessment of the ability of Group 10 transition metals M = Pd and Pt to act as an electron donor within the context of pnicogen, chalcogen, and halogen bonds. These M atoms are coordinated in a square planar geometry, attached to two N atoms of a modified phenanthrene unit, as well as two ligand atoms Cl, Br, or I. As the Lewis acid, a series of AFn molecules were chosen, which could form a pnicogen bond (A = P, As, Sb), chalcogen bond (A = S, Se, Te) or halogen bond (A = Cl, Br, I) with M. These noncovalent bonds are fairly strong, varying between 6 and 20 kcal mol-1, with the occupied dz2 orbital of M acting as the origin of charge transferred to the acid. Pt forms somewhat stronger bonds than Pd, and the bond strength rises with the size of the A atom of the acid. Within the context of smaller A atoms, the bond strength rises in the order pnicogen < chalcogen < halogen, but this distinction vanishes for the fifth-row A atoms. The nature of the ligand atoms on M has little bearing on the bond strength. Based on the Harmonic Oscillator Model of Aromaticity (HOMA) index, the ZB, YB and XB bonds were shown to have only a subtle effect on the ring electronic structures.

Enhancement of Halogen Bond Strength by Intramolecular H-Bonds

Quantum calculations study the potential of an intramolecular H-bond between the halogen atom (X) of a halobenzene and a substituent placed ortho to it, to amplify the ability of X to engage in a halogen bond (XB) with a Lewis base. H-bonding substituents NH₂, CH₂CH₂OH, CH₂OH, OH, and COOH were added to halobenzenes (X = Cl, Br, I). The amino group had little effect, but those containing OH increased the CX···N XB energy to a NH₃ nucleophile by about 0.5 kcal/mol; the increment associated with COOH is larger, nearly 2 kcal/mol. These energy increments were approximately doubled if two such H-bonding substituents are present. Combining a pair of ortho COOH groups with an electron-withdrawing NO₂ group in the para position has a particularly large effect, raising the XB energy by about 4 kcal/mol, which can amount to as much as a 4-fold magnification.

Three-body interaction effects in heterolytic hydrogen splitting by frustrated Lewis pairs

The reaction of heterolytic dihydrogen splitting by frustrated Lewis pairs P(R)₃ and B(C₆F₅)₃ (where R = t-butyl and 1-adamantene) is driven by strong three-body contributions which originate from the induction and charge transfer effects. The three-body effect increases dramatically as a function of inter-hydrogen distance. As predicted by the symmetry adapted perturbation theory, the "frustration" of Lewis pairs originates from the dual role of the exchange effects. First, the exchange manifests itself in the first-order Pauli repulsion by keeping the pairs away. Second, and equally important, the second-order exchange-induction almost completely cancels the effects of the second-order induction. This suppression of induction effects eases up upon the interaction of the frustrated pairs with H₂. The activation of induction in this instance constitutes the three-body effect.

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

Open and cyclic chains from two to eight units of ICl and IF are constructed and examined by density functional theory (DFT) calculations. These chains contain either I···I or I···X halogen bonds (XBs) where X refers to Cl or F. The closed rings are more stable than the open chains due to the presence of an additional XB and enhanced cooperativity. This pattern is true even for most trimers where there is sizable geometric distortion in the rings. I···F rings are generally more stable than the corresponding I···I cycles as the I···F bond is stronger than I···I even in the simple dimer. However, I···I rings are comparable in energy to I···Cl. It is possible to construct I···I rings of at least as large as eight units, which are held together exclusively by XBs. On the other hand, the maximum possible size of I···X rings is 6. Red shifts are observed in the I-X stretching frequency bands, which magnify as the chain, both cyclic and open, grows longer. The NMR chemical shielding of the I atoms increases for I···I chains but diminishes when I···Cl bonds are present.