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ABSTRACT The interaction energies within noncovalent bonds can be partitioned into electrostatic, induction, and dispersive attractive elements. A set of complexes comprising halogen, chalcogen, pnicogen, and tetrel bonds, are studied by quantum chemical calculations to assess how each of these components can be understood on the basis of properties of the constituent monomers. The variation of the electrostatic term, which accounts for over half of the total attractive energy, can be approximated, but with only modest accuracy, by combination of the maximum and minimum of the electrostatic potential on the two subunits. Induction represents a smaller contribution to the total, but is well connected with the NBO interorbital transfer energy, as opposed to the reciprocal of the HOMO‐LUMO gap which behaves quite differently than IND. Of the various AIM parameters, both the bond critical point density and energy density are closely related to the full interaction energy. 

Abstract Both methyl groups and benzene rings are exceedingly common, and they lie near one another in many chemical situations. DFT calculations are used to gauge the strength of the attractive forces between them, and to better understand the phenomena that underlie this attraction. Methane and benzene are taken as the starting point, and substituents of both electron‐withdrawing and donating types are added to each. The interaction energy varies between 1.4 and 5.0 kcal/mol, depending upon the substituents placed on the two groups. The nature of the binding is analyzed via Atoms in Molecules (AIM), Natural Bond Orbital (NBO), Symmetry‐Adapted Perturbation Theory (SAPT), nuclear magnetic resonance (NMR) chemical shifts, and electron density shift diagrams. While there is a sizable electrostatic component, it is dispersion that dominates these interactions, particularly the weaker ones. As such, these interactions cannot be categorized unambiguously as either H‐bonds or tetrel bonds. 

Abstract A halobenzene molecule contains several sites that are capable of acting in an electron‐donating capacity within a H−bond. One set of such sites comprise the lone electron pairs of the halogen (X) atoms on the periphery of the ring. The π‐electron system above the ring plane can also fulfill this function in many cases. DFT calculations are applied to compare and contrast the propensity of these two site types to engage in such a H−bond within the context of mono, di, tri, tetra, and hexasubstituted halobenzenes. The X atoms chosen for study comprise the full set: F, Cl, Br, and I. It is found that even when the electrostatic potential of the X lone pair is more negative than that above the ring, it is the latter position which is the preferred binding site of HCl in most cases. This preference switches over to the X lone pair only for higher order of substitution, with n=4 or 6. This pattern is explained in large measure by the higher contribution of dispersion when the proton donor is located above the ring. 

Abstract The ability of an anion to serve as electron‐accepting Lewis acid in a noncovalent bond is assessed via DFT calculations. NH₃is taken as the common base, and is paired with a host of ACl_n⁻anions, with central atom A=Ca, Sr, Mg, Te, Sb, Hg, Zn, Ag, Ga, Ti, Sn, I, and B. Each anion reacts through its σ or π‐hole although the electrostatic potential of this hole is quite negative in most cases. Despite the contact between this negative hole and the negative region of the approaching nucleophile, the electrostatic component of the interaction energy of each bond is highly favorable, and accounts for more than half of the total attractive energy. The double negative charge of dianions precludes a stable complex with NH₃. 

Abstract The interaction between two square palladium (II) dianions PdX₄²⁻(X=Cl, Br) is evaluated by crystal study and analyzed by quantum chemical means. The arrangement within the crystal between each pair of PdX₄²⁻neighbors is suggestive of a Pd⋅⋅⋅X noncovalent bond, which is verified by a battery of computational protocols. While the potential between these two bare dianions is computed to be highly repulsive, the introduction of even just two counterions makes this interaction attractive, as does the presence of a constellation of point charges. It is concluded that there is indeed a stabilizing Pd⋅⋅⋅X bond, but it is incapable of overcoming the strong coulombic repulsive force between two dianions. While the QTAIM, NBO, and NCI tools can indicate the presence of a noncovalent bond, they are unable to distinguish an attractive from a repulsive interaction. 

Abstract The propensity of the π‐electron system lying above a polycyclic aromatic system to engage in a halogen bond is examined by DFT calculations. Prototype Lewis acid CF₃I is placed above the planes of benzene, naphthalene, anthracene, phenanthrene, naphthacene, chrysene, triphenyl, pyrene, and coronene. The I atom positions itself some 3.3–3.4 Å above the polycyclic plane, and the associated interaction energy is about 4 kcal/mol. This quantity is a little smaller for benzene, but is roughly equal for the larger polycyclics. The energy only oscillates a little as the Lewis acid slides across the face of the polycyclic, preferring regions of higher π‐electron density over minima of the electrostatic potential. The binding is dominated by dispersion which contributes half of the total interaction energy. 

Abstract The triel bond (TrB) formed between Be(CH₃)₂/Mg(CH₃)₂and TrX₃(Tr=B, Al, and Ga; X=H, F, Cl, Br, and I) is investigated via the MP2/aug‐cc‐pVTZ(PP) quantum chemical protocol. The C atoms of the methyl groups in M(CH₃)₂are characterized by a negative electrostatic potential and act as an electron donor in a triel bond with the π‐hole above the Tr atom of planar TrX₃. The interaction energy spans a wide range between −2 and −69 kcal/mol. Mg(CH₃)₂forms a stronger TrB than does Be(CH₃)₂, which comports with the more negative electrostatic potential on its methyl groups. Some of the complexes involving Mg display a high degree of transfer of the methyl group from Mg to Tr, which is accompanied by an inversion of the bridging methyl and a sizable pyramidalization of the TrX₃unit. The geometries of these complexes have the properties of the long sought pentacoordinate C which has eluded identification and characterization in the past. 

Abstract The starting point for this work was a set of crystal structures containing the motif of interaction between methyl groups in homodimers. Two structures were selected for which QTAIM, NCI and NBO analyses suggested an attractive interaction. However, the calculated interaction energy was negative for only one of these systems. The ability of methyl groups to interact with one another is then examined by DFT calculations. A series of (CH₃PnHCH₃)₂homodimers were allowed to interact with each other for a range of Pn atoms N, P, As, and Sb. Interaction energies of these C⋅⋅⋅C tetrel‐bonded species were below 1 kcal/mol, but could be raised to nearly 3 kcal/mol if the C atom was changed to a heavier tetrel. A strengthening of the C⋅⋅⋅C intermethyl bonds can also be achieved by introducing an asymmetry via an electron‐withdrawing substituent on one unit and a donor on the other. The attractions between the methyl and related groups occur in spite of a coulombic repulsion between σ‐holes on the two groups. NBO, AIM, and NCI tools must be interpreted with caution as they can falsely suggest bonding when the potentials are repulsive. 

Abstract As a flat trigonal species, the CR₃⁺carbenium ion contains a pair of deep π‐holes above and below its molecular plane. In the case of CH₃⁺a first base will form a covalent bond with the central C, making the combined species tetrahedral. Approach of a second base to the opposite side results in a longer but rather strong noncovalent tetrel bond (TB). While CMe₃⁺can also form a similar asymmetric complex with a pair of bases, it also has the capacity to form a pair of nearly equivalent TBs, such that the resulting symmetric trigonal bipyramid configuration is only slightly higher in energy. When the three substituents on the central C are phenyl rings, the symmetric configuration with two TBs predominates. These tetrel bonds are quite strong, reaching up to 20 kcal/mol. Adding OPH₂or OCH substituents to the phenyl rings permits the formation of intramolecular C⋅⋅O TBs to the central C, very similar in many respects to the case where these TBs are intermolecular. 

Abstract Adducts between OsO₄and Lewis bases exert a role in important oxidation processes such as epoxidation and dihydroxylation. It has been shown that the attractive interaction driving the formation of these adducts is a σ‐hole bond involving the metal as the electrophilic species; the term Osme Bond (OmB) was proposed for designating it. Here some new adducts between OsO₄and various bases have been characterized through single crystal x‐ray diffraction (XRD) and computational studies (density functional theory, DFT), confirming the existence of a robust correlation between σ‐hole interaction energy and deformation of the tetrahedral geometry of OsO₄. Also, some adducts formed by RuO₄with nucleophiles were investigated computationally.