π⋅⋅⋅H+ ⋅⋅⋅π Hydrogen Bonds and Their Lithium and Gold Analogues: MP2 and CASPT2 Calculations

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. The typical 3c-4e (three centres - four electrons) A-H⋯B hydrogen bond is characterized by the single-atom A and B centres that are highly electronegative. On the other hand, non-typical interactions that do not fit the hydrogen bond definitions well are characterised by uncommon proton donors and/or proton acceptors. The cases of multi-centre proton acceptors, π-electron or σ-electron systems are well known - such interactions are designated as A-H⋯π and A-H⋯σ hydrogen bonds, respectively. However, the cases of interactions with the multi-centre proton donors and proton acceptors do not fit the majority of definitions of hydrogen bond. The π⋯H+⋯π system in the proton-bound homodimer of acetylene is an example. This system can be classified as a hydrogen bond according to the two-sites hydrogen bond, 2sHB, definition. There are various types of interactions discussed in this review; among them, those that are undoubtedly unclassified as hydrogen bonds, i.e., hydride bonds, and charge inverted hydrogen bonds, CIHBs. Special emphasis is also put here on the proton sponges and other systems such as the [FHF]- anion or [NgHNg]+ cation (Ng is the noble gas centre).

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. The results of calculations show that such rings' coexistence leads to the weakening of dihydrogen bonds, similarly as for the typical two-ring RAHB systems. The Quantum Theory of ''Atoms in Molecules'' (QTAIM) was also applied here to get more details about the nature of dihydrogen bonds. Correlations between dihydrogen bond strength measures and other energetic, geometrical and topological parameters were also analysed. It was found that characteristics of bond critical points as well as of ring critical points are useful to estimate the strength of intramolecular dihydrogen bonds in two-ring dihydrogen bonded systems discussed here. The Natural Bond Orbital, NBO, approach parameters are also discussed as useful ones to describe properties of dihydrogen bonded systems.

Heavy pnicogen atoms as electron donors in sigma-hole bonds

DFT calculations evaluate the strength of σ-hole bonds formed by ZH3 and ZMe3 (Z = N, P, As, Sb) acting as electron donor. Bond types considered include H-bond, halogen, chalcogen, pnicogen, and tetrel bond to perfluorinated Lewis acids FH, FBr, F2Se F3As, F4Ge, respectively, as well as their monofluorinated analogues. All of the Z atoms can engage in bonds of at least moderate strength, varying from 3 to more than 40 kcal mol-1. In most cases, N forms the strongest bonds, but the falloff from P to Sb is quite mild. However, this pattern is not characteristic of all cases, as for example in the halogen bonds, where the heavier Z atoms are comparable to, or even stronger than N. Most of the bonds are strengthened by replacing the three H atoms of ZH3 by methyl groups, better simulating the situation that would be generally encountered. Structural and NMR shielding data ought to facilitate the identification of these bonds within crystals or in solution.

The proton and the lithium cation linked with 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 the proton or the 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. It is discussed whether these interactions are classified as hydrogen and lithium bonds. Therefore, different definitions of the latter interactions are presented. The Electron Localization Function (ELF) and the Natural Bond Orbital (NBO) approaches were applied to analyse the above-mentioned complexes. The unique properties of interactions with the proton and with the lithium cation that occur in complexes analysed here are described.

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. The properties of these interactions are discussed, and particularly the Lewis acid units are analyzed, because multi-center π-H or π-Li covalent bonds may occur in these systems. Various theoretical approaches were applied here to analyze the above-mentioned interactions—the Quantum Theory of Atoms in Molecules (QTAIM), the Symmetry-Adapted Perturbation Theory (SAPT) and the Non-Covalent Interaction (NCI) method.

Regium-π bonds: An Unexplored Link between Noble Metal Nanoparticles and Aromatic Surfaces

The ability of metal clusters involving elements from group 11 (Ag, Cu, Au) to favorably interact with π systems of different size and electronic nature was evaluated at the PBE0-D3/def2-TZVPP//PBE0-D3/def2-TZVP level of theory. The M₉ clusters (M=Cu, Ag, Au) were used as σ-hole and σ-lump donors, and benzene, trifluorobenzene, and hexafluorobenzene as aromatic rings. In addition, the study was expanded to the analysis of extended π systems by using naphthalene and anthracene as well as their corresponding perfluorinated derivatives. Furthermore, Bader's theory of Atoms in Molecules as well as natural bonding orbital and spin-density calculations were used to further investigate and characterize the regium-π and σ-lump complexes described herein. Apparently, regium-π bonds have not previously been described in the literature and may be of great importance in the understanding of organocatalytic processes involving aromatic substrates as well as in the design of new materials based on this novel subclass of σ-hole bonding.

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

For various interactions electron charge shifts try to protect the former doublet or octet electronic structure of the Lewis acid centre.