Hydrogen bond types which do not fit accepted definitions.

There are various interactions that either partially fit or do not fit the currently accepted definitions of the hydrogen bond. However, they possess characteristics of this interaction. It seems that it is partly connected to the fact that these definitions are not precise.

Theoretical Insights into Bifurcated Intramolecular Dihydrogen Bonds.

Two-ring intramolecular π-electron delocalization assisted dihydrogen bonds existing in (1Z,4Z)-1,4-dipentene-3-bora-1,5-diol and its symmetrically substituted derivatives have been analysed here since the MP2/6-311++G(d,p) calculations on these systems were performed. The influence of the coexistence of two intramolecular dihydrogen bonded rings in these molecular structures on properties of intramolecular dihydrogen bonds as well as on the π-electron delocalization within these rings was investigated. The comparison with corresponding structures of typical two-ring, so-called resonance-assisted, RAHB, systems was performed.

Double Centrosymmetric Si···π Tetrel Bonds as New Synthons─Evidence from Crystal Structures and DFT Calculations.

The crystal structure of bis((μ-ethynylsilyloxo)-dichloro-aluminum), BEDCA, and a few related structures are characterized by the occurrence of tetrel bonds that link molecules. Particularly, centosymmetric dimers in such structures occur that are connected by two equivalent Si···π tetrel bonds. The dimer of BEDCA and dimers of other model species that similarly are linked by two equivalent Si···π tetrel bonds are analyzed theoretically.

Ga···C Triel Bonds-Why They Are Not Strong Enough to Change Trigonal Configuration into Tetrahedral One: DFT Calculations on Dimers That Occur in Crystal Structures.

Structures characterized by the trigonal coordination of the gallium center that interacts with electron rich carbon sites are described. These interactions may be classified as Ga···C triel bonds. Their properties are analyzed in this study since these interactions may be important in numerous chemical processes including catalytical activities; additionally, geometrical parameters of corresponding species are described.

Halogen bonds with carbenes acting as Lewis base units: complexes of imidazol-2-ylidene: theoretical analysis and experimental evidence.

ωB97XD/aug-cc-pVDZ and ωB97XD/aug-cc-pVTZ calculations were performed on complexes of imidazol-2-ylidene that are linked by halogen bonds. This singlet carbene acts as the Lewis base through a lone electron pair located at the carbon centre. The XCCH, XCN and X units were chosen here as those that interact through the X Lewis acid halogen centre (X = Cl, Br and I); if X = F the complexes are linked by interactions which are not classified as halogen bonds.

Sandwich, Triple-Decker and Other Sandwich-like Complexes of Cyclopentadienyl Anions with Lithium or Sodium Cations.

Density functional theory, DFT, calculations were carried out on complexes containing cyclopentadienyl anions and lithium or sodium cations; half-sandwich, sandwich and sandwich-like complexes (among them triple-decker ones) are analyzed. Searches performed through the Cambridge Structural Database revealed that crystal structures containing these motifs exist, mostly structures with sodium cations. The DFT calculations performed here include geometry optimization and frequency calculations of the complexes at the ωB97XD/aug-cc-pVTZ level, followed by the partitioning of the energy of interaction via the Energy Decomposition Analysis scheme, EDA, at the BP86-D3/TZ2P level.

Enhancing Effects of the Cyano Group on the C-X∙∙∙N Hydrogen or Halogen Bond in Complexes of X-Cyanomethanes with Trimethyl Amine: CH(CN)X∙∙∙NMe, (n = 0-3; X = H, Cl, Br, I).

In this paper, density functional theory and wave function theory calculations are carried out to investigate the strength and nature of the intermolecular C-X∙∙∙N bond interaction as a function of the number of cyano groups, CN, in the X-bond donor while maintaining the X-bond acceptor as fixed. Specifically, complexes of X-cyanomethanes with trimethyl amine CH(CN)X∙∙∙NMe (n = 0-3; X = H, Cl, Br, I) are used as model systems. Geometrical parameters and vibrational C-X-stretching frequencies as well as interaction energies are used as relevant indicators to gauge hydrogen or halogen bond strength in the complexes.

Proton and Lithium Cations Linked to π-Electron and σ-Electron Systems: Are Such Interactions beyond or within the Definition of Hydrogen/Lithium Bond?

MP2/aug-cc-pVTZ calculations were performed on systems containing a proton or a lithium cation located between two π-electron systems or between π-electron and σ-electron units. The proton or the lithium cation attached to the acetylene or its derivative may be treated as the Lewis acid unit while the remaining part of the complex, the π-electron species or the dihydrogen, act as the Lewis base through their π-electrons or σ-electrons, respectively. The complexes analysed here are linked by the π⋅⋅⋅H /Li ⋅⋅⋅π and π⋅⋅⋅H /Li ⋅⋅⋅σ interactions.

Hydrogen and Lithium Bonds-Lewis Acid Units Possessing Multi-Center Covalent Bonds.

MP2/aug-cc-pVTZ calculations were carried out on complexes wherein the proton or the lithium cation is located between π-electron systems, or between π-electron and σ-electron units. The acetylene or its fluorine and lithium derivatives act as the Lewis base π-electron species similarly to molecular hydrogen, which acts as the electron donor via its σ-electrons. These complexes may be classified as linked by π-H∙∙∙π/σ hydrogen bonds and π-Li∙∙∙π/σ lithium bonds.

A-X⋯σ Interactions-Halogen Bonds with σ-Electrons as the Lewis Base Centre.

CCSD(T)/aug-cc-pVTZ//ωB97XD/aug-cc-pVTZ calculations were performed for halogen-bonded complexes. Here, the molecular hydrogen, cyclopropane, cyclobutane and cyclopentane act as Lewis base units that interact through the electrons of the H-H or C-C σ-bond. The FCCH, ClCCH, BrCCH and ICCH species, as well as the F, Cl, Br and I molecular halogens, act as Lewis acid units in these complexes, interacting through the σ-hole localised at the halogen centre.

Classification of So-Called Non-Covalent Interactions Based on VSEPR Model.

The variety of interactions have been analyzed in numerous studies. They are often compared with the hydrogen bond that is crucial in numerous chemical and biological processes. One can mention such interactions as the halogen bond, pnicogen bond, and others that may be classified as σ-hole bonds.

Coexistence of Intra- and Intermolecular Hydrogen Bonds: Salicylic Acid and Salicylamide and Their Thiol Counterparts.

The ωB97-XD/6-311++G(d,p) calculations were carried out on dimers and monomers of salicylic acid and salicylamide as well as on their thiol counterparts; different conformations of these species were considered. The searches through the Cambridge Structural Database were performed to find related structures; thus the analysis of results of these searches is presented. Various approaches were applied to analyze inter- and intramolecular hydrogen bonds occurring in the above-mentioned species: natural bond orbital (NBO) method, symmetry-adapted perturbation theory (SAPT) approach, the quantum theory of atoms in molecules (QTAIM), and the electron localization function (ELF) method.

Hydrogen Bond and Other Lewis Acid-Lewis Base Interactions as Preliminary Stages of Chemical Reactions.

Various Lewis acid-Lewis base interactions are discussed as initiating chemical reactions and processes. For example, the hydrogen bond is often a preliminary stage of the proton transfer process or the tetrel and pnicogen bonds lead sometimes to the S2 reactions. There are numerous characteristics of interactions being first stages of reactions; one can observe a meaningful electron charge transfer from the Lewis base unit to the Lewis acid; such interactions possess at least partly covalent character, one can mention other features.

Molecular Hydrogen as a Lewis Base in Hydrogen Bonds and Other Interactions.

The second-order Møller-Plesset perturbation theory calculations with the aug-cc-pVTZ basis set were performed for complexes of molecular hydrogen. These complexes are connected by various types of interactions, the hydrogen bonds and halogen bonds are most often represented in the sample of species analysed; most interactions can be classified as σ-hole and π-hole bonds. Different theoretical approaches were applied to describe these interactions: Quantum Theory of 'Atoms in Molecules', Natural Bond Orbital method, or the decomposition of the energy of interaction.

The Nature of Triel Bonds, a Case of B and Al Centres Bonded with Electron Rich Sites.

The second-order Møller-Plesset perturbation theory calculations with the aug-cc-pVTZ basis set were performed on complexes of triel species: BCl, BH, AlCl, and AlH acting as Lewis acids through the B or Al centre with Lewis base units: NCH, N, NH, and Cl anion. These complexes are linked by triel bonds: B/Al⋅⋅⋅N or B/Al⋅⋅⋅Cl. The Quantum Theory of ´Atoms in Molecules´ approach, Natural Bond Orbital method, and the decomposition of energy of interaction were applied to characterise the latter links.

S-H…O and O-H…O Hydrogen Bonds-Comparison of Dimers of Thiocarboxylic and Carboxylic Acids.

ωB97-XD/aug-cc-pVTZ calculations were performed on dimers of selected thiocarboxylic acids and on analogous carboxylic acids. The sample of calculated thiocarboxylic acids is an extension of the Cambridge Structural Database search that contains only a few such structures. The Natural Bond Orbital (NBO) method, Symmetry-Adapted Perturbation Theory (SAPT) approach, Non-Covalent Interaction (NCI) method and Quantum Theory of Atoms in Molecules (QTAIM) were applied additionally to analyse interactions in dimers of thiocarboxylic and carboxylic acids.

Tetravalent Oxygen and Sulphur Centres Mediated by Carborane Superacid: Theoretical Analysis.

The tetravalent oxygen or sulphur centres, especially in H O and H S dications, were analysed experimentally and theoretically in various studies. Herein, we discuss stabilities of such centres in related H(CH ) O and H(CH ) S dications mediated by carborane superacid. The ωB97X-D/6-311++G(d,p) calculations were performed for a gas phase and for different solvents characterized by a wide range of dielectric constants for complexes of these dications with the conjugated base of H(CHB F ) carborane superacid, CHB F , which indicate that these complexes are linked by hydrogen bonds.

A-H…σ Hydrogen Bonds: Dihydrogen and Cycloalkanes as Proton Acceptors.

ωB97XD/aug-cc-pVTZ calculations were performed for complexes of dihydrogen, cyclopropane, cyclobutane and cyclopentane, with simple proton donating species such as hydrogen fluoride, hydrogen chloride, water, hydrogen cyanide and acetylene. Numerous dependencies between geometrical, energetic and topological parameters of complexes considered were found, since various theoretical approaches were applied: Quantum Theory of 'Atoms in Molecules' (QTAIM), Natural Bond Orbital (NBO) method and energy decomposition analysis (EDA). It was confirmed that complexes of dihydrogen and cyclopropane are linked through the A-H…σ interactions that may be classified as hydrogen bonds.

Thieno[3,4-c]pyrrole-4,6-dione Oligothiophenes Have Two Crossed Paths for Electron Delocalization.

A new series of electron-deficient oligothiophenes, thieno[3,4-c]pyrrole-4,6-dione oligothiophenes (OTPD ), from the monomer to hexamer, is reported. The optical and structural properties in the neutral states have been analyzed by absorption and emission spectroscopy together with vibrational Raman spectroscopy. In their reduced forms, these molecules could stabilize both anions and dianions in similar ways.

Tetrel Bonds with π-Electrons Acting as Lewis Bases-Theoretical Results and Experimental Evidences.

MP2/aug-cc-pVTZ calculations were carried out for the ZFH₃-B complexes (Z = C, Si, Ge, Sn and Pb; B = C₂H₂, C₂H₄, C₆H₆ and C₅H₅⁻; relativistic effects were taken into account for Ge, Sn and Pb elements). These calculations are supported by other approaches; the decomposition of the energy of interaction, Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) method. The results show that tetrel bonds with π-electrons as Lewis bases are classified as Z···C links between single centers (C is an atom of the π-electron system) or as Z···π interactions where F‒Z bond is directed to the mid-point (or nearly so) of the CC bond of the Lewis base.

Coordination of Be and Mg Centres by HCN Ligands - Be…N and Mg…N Interactions.

ωB97XD/aug-cc-pVTZ calculations were performed for clusters of Z cations (Z=Be and Mg) and HCN molecules (up to six molecules). The clusters of Be(CH ) and Mg(CH ) with HCN species were also calculated to analyse the influence of the Be/Mg-C formally covalent bonds on interactions of Be or Mg centre with ligands. The beryllium and magnesium centres possess different areas of a positive electrostatic potential that depend on a number of HCN ligands in the cluster considered.

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

MP2/aug-cc-pVTZ calculations were performed for complexes linked by hydrogen bonds. Three types of proton donating species were taken into account: HO, CClH, and HO. These calculations are supported by the natural bond orbital (NBO) method and the quantum theory of atoms in molecules (QTAIM) approach.

New Type of Halogen Bond: Multivalent Halogen Interacting with π- and σ-Electrons.

MP2/aug-cc-pVTZ calculations were performed for complexes of BrF₃ and BrF₅ acting as Lewis acids through the bromine centre, with species playing a role of Lewis base: dihydrogen, acetylene, ethylene, and benzene. The molecular hydrogen donates electrons by its σ-bond, while in remaining moieties-in complexes of hydrocarbons; such an electron transfer follows from π-electrons. The complexes are linked by a kind of the halogen bond that is analyzed for the first time in this study, i.

Hydrogen bonds, and σ-hole and π-hole bonds - mechanisms protecting doublet and octet electron structures.

The hydrogen bond interaction and σ-hole and π-hole bonds are steered by the same mechanisms. There is electron charge transfer from the Lewis base to the Lewis acid unit, and further, for various interactions the same mechanisms try to protect the former electronic structure of the Lewis acid centre. The increase of the polarization of bonds to this centre seems to be the common effect.