Sławomir J. Grabowski

Hydrogen and halogen bonds are ruled by the same mechanisms

Hydrogen and halogen bonds are compared on the basis of ab initio calculations performed for complexes linked through these interactions. The Quantum Theory of Atoms in Molecules (QTAIM) and the Natural Bond Orbitals (NBO) method are applied for a deeper understanding of the nature of interactions. Both interactions are ruled by the same effects of hyperconjugation and rehybridization. In general for both kinds of interactions the same processes of the electron charge redistribution being the result of complexation are observed. As a consequence similar characteristics are also observed for the hydrogen and halogen bonds for example the increase of the positive charge of the atom being in contact with the Lewis base (hydrogen and chlorine or bromine for complexes analyzed here) and the decrease of its volume as a result of the complex formation. The halogen bond is enhanced by the charge assistance, similarly to the hydrogen bond.

σ-Hole Bond Versus Hydrogen Bond: From Tetravalent to Pentavalent N, P, and As Atoms

Ab initio calculations were performed on complexes of ZH4 (+) (Z=N, P, As) and their fluoro derivatives, ZFH3 (+) and ZF4 (+) , with a HCN (or LiCN) molecule acting as the Lewis base through the nitrogen electronegative center. It was found that the complexes are linked by the Zuf8ffH⋅⋅⋅N hydrogen bond or another type of noncovalent interaction in which the tetravalent heavy atom of the cation acts as the Lewis acid center, that is, when the Z⋅⋅⋅N link exists, which may be classified as the σ-hole bond. The formation of the latter interaction is usually preferable to the formation of the corresponding hydrogen bond. The Z⋅⋅⋅N interaction may be also considered as the preliminary stage of the SN 2 reaction. This is supported by the observation that for a short Z⋅⋅⋅N contact, the corresponding complex geometry coincides with the trigonal-bipyramidal geometry typical for the transition state of the SN 2 reaction. The Z⋅⋅⋅N interaction for some of complexes analyzed here possesses characteristics typical for covalent bonds. Numerous interrelations between geometrical, topological and energetic parameters are discussed. The natural bond orbital method as well as the Quantum Theory of Atoms in Molecules is applied to characterize interactions in the analyzed complexes. The experimental evidences of the existence of these interactions, based on the Cambridge Structure Database search, are also presented. In addition, it is justified that mechanisms of the formation of the Z⋅⋅⋅N interactions are similar to the processes occurring for the other noncovalent links. The formation of Z⋅⋅⋅N interaction as well as of other interactions may be explained with the use of the σ-hole concept.

Cooperativity of hydrogen and halogen bond interactions

The cooperativity effects in the Cl−···HCCH···HF, Cl−···ClCCH···HF and F−···ClCCH···HF complexes are analyzed here. The results show that the formation of the hydrogen and halogen bonds is ruled by the same mechanisms and that the cooperativity enhances these interactions. The MP2(full)/6-311xa0++G(d,p) calculations were performed for the above triads and the corresponding sub-units; dyads linked by the hydrogen or halogen bonds and monomers. The NEDA scheme of the decomposition of the interaction energy was applied here. It was found that for the halogen bonded systems, the most important is the polarization term of the energy of interaction while for the hydrogen bonds the charge transfer interaction energy and next the electrostatic contribution. The interaction between orbitals is also analyzed here in terms of the Natural Bond Orbitals method.

Non-covalent interactions – QTAIM and NBO analysis

AbstractMP2(full)/6-311++G(3df,3pd) calculations were carried out on complexes linked through various non-covalent Lewis acid – Lewis base interactions. These are: hydrogen bond, dihydrogen bond, hydride bond and halogen bond. The quantum theory of ´atoms in molecules´ (QTAIM) as well as the natural bond orbitals (NBO) method were applied to analyze properties of these interactions. It was found that for the A-H…B hydrogen bond as well as for the A-X…B halogen bond (X designates halogen) the complex formation leads to the increase of s-character in the A-atom hybrid orbital aimed toward the H or X atom. In opposite, for the A…H-B hydride bond, where the H-atom possesses negative charge, the decrease of s-character in the B-atom orbital is observed. All these changes connected with the redistribution of the electron charge being the effect of the complex formation are in line with Bent´s rule. The numerous correlations between energetic, geometrical, NBO and QTAIM parameters were also found.n FigureQTAIM atomic radii for NH4+…HMgH and Na+…HBeH

Are Various σ-Hole Bonds Steered by the Same Mechanisms?

Representative Lewis acid-Lewis base complexes linked by tetrel, pnicogen, chalcogen, and halogen bonds have been studied within the quantum theory of atoms in molecules (QTAIM) approach and the hybrid variation-perturbation theory (HVPT) to analyze possible relationships between these σ-hole dimers. Results obtained at the MP2/aug-cc-pVTZ level indicate numerous correlations similar to hydrogen-bonded systems.

Triel bonds-complexes of boron and aluminum trihalides and trihydrides with benzene

Triel bond is an interaction of an atom of the 13th Group of periodic system that acts as a Lewis acid with an electron rich species; it is analyzed here in complexes of benzene with boron and aluminum trihalides and trihydrides. MP2/aug-cc-pVTZ calculations were performed for these complexes and the interactions were analyzed with the use of Quantum Theory of “Atoms in Molecules.” It was found that benzene acts as the Lewis base not through a π-electron system but through one of carbon centers that is characterized by the most negative charge if compared with other carbon atoms of benzene. Thus, the B…C and Al…C bond paths are analyzed which correspond to preferable interactions; for some of complexes, the additional halogen (X)–carbon, X…C, intermolecular bond paths exist.

Comparison of Hydrogen and Gold Bonding in [XHX](-) , [XAuX](-) , and Isoelectronic [NgHNg](+) , [NgAuNg](+) (X=Halogen, Ng=Noble Gas).

Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX](-) and [XAuX](-) decrease from [FHF](-) to [IHI](-) and from [FAuF](-) to [IAuI](-) . The BDEs of the noble gas compounds [NgHNg](+) and [NgAuNg](+) become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI](-) , in which case the gold atom has a small negative charge of -0.03u2005e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in [XHX](-) . The covalent character of the bonding in the gold systems [NgAuNg](+) and [XAuX](-) is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X(-) →H(+) ←X(-) or Ng→H(+) ←Ng while the heavier homologues exhibit electron sharing through two-electron, three-center bonds. Dative bonds X(-) →Au(+) ←X(-) and Ng→Au(+) ←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron-shared bonds.

Clusters of Ammonium Cation—Hydrogen Bond versus σ‐Hole Bond

MP2/6-311++G(d,p) calculations were performed on the NH4 (+) ⋅⋅⋅(HCN)n and NH4 (+) ⋅⋅⋅(N2 )n clusters (n=1-8), and interactions within them were analyzed. It was found that for molecules of N2 and HCN, the N centers play the role of the Lewis bases, whereas the ammonium cation acts as the Lewis acid, as it is characterized by sites of positive electrostatic potential, that is, H atoms and the sites located at the N atom in the extension of the Huf8ffN bonds. Hence, the coordination number for the ammonium cation is eight, and two types of interactions of this cation with the Lewis base centers are possible: Nuf8ffH⋅⋅⋅N hydrogen bonds and Huf8ffN⋅⋅⋅N interactions that are classified as σ-hole bonds. Redistribution of the electronic charge resulting from complexation of the ammonium cation was analyzed. On the one hand, the interactions are similar, as they lead to electronic charge transfer from the Lewis base (HCN or N2 in this study) to NH4 (+) . On the other hand, the hydrogen bond results in the accumulation of electronic charge on the N atom of the NH4 (+) ion, whereas the σ-hole bond results in the depletion of the electronic charge on this atom. Quantum theory of atoms in molecules and the natural bond orbital method were applied to deepen the understanding of the nature of the interactions analyzed. Density functional theory/natural energy decomposition analysis was used to analyze the interactions of the ammonium ion with various types of Lewis bases. Different correlations between the geometrical, energetic, and topological parameters were found and discussed.

Two faces of triel bonds in boron trihalide complexes

The N⋅⋅⋅B triel bonds in complexes of boron trihalides, BX3 (Xu2009=u2009F, Cl, Br, and I), with species acting as Lewis bases through the nitrogen center, NH3, N2, and HCN, are analyzed theoretically (MP2/aug‐cc‐pVTZ calculations). It is confirmed that stronger Lewis acid properties of the boron center are observed for the BCl3 moiety than for the BF3 one in complexes with the strong Lewis base (NH3); while the opposite order is observed for complexes with the weak Lewis base (N2). The BX3uf8ffNCH complexes (for Xu2009=u2009Cl, Br, and I) are characterized by two tautomeric forms and by two corresponding N⋅⋅⋅B distances, the shorter one possesses characteristics of the covalent bond. In a case of the BF3uf8ffNCH complex one energetic minimum is observed. Ab initio calculations are supported by an analysis of molecular electrostatic potentials (EPs) and electron density distributions. The quantum theory of ‘atoms in molecules’ and the decomposition of the energy of interaction are applied. The aforementioned acidity orders as well as the existence of two tautomers for some of complexes result partly from the electrostatic interactions balance; the EP distribution is different for the BF3 species than for the other BX3 species where Xu2009=u2009Cl, Br, and I.

Hydrogen bonds and other interactions as a response to protect doublet/octet electron structure

AbstractMP2/aug-cc-pVTZ calculations were performed for complexes linked by hydrogen bonds. Three types of proton donating species were taken into account: H2O, CCl3H, and H3O+. These calculations are supported by the natural bond orbital (NBO) method and the quantum theory of atoms in molecules (QTAIM) approach. Numerous correlations between parameters of H-bonded systems were found. The most important are those which show the response of the system on the H-bond formation; for example, the increase of polarization of the A-H bond correlates with the strength of the hydrogen bond. Similar relationships were found for the σ-hole bonds while the π-hole bonds do not follow the trends known for the hydrogen bonds.n Graphical abstractHydrogen bonds and other interactions as a response to protect doublet/octet electron structureᅟ