A theoretical study of three and four proton donors on linear HX···BeH2···HX and bifurcate BeH2···2HX trimolecular dihydrogen-bonded complexes with X = CN and NC

Discovering trends in the Lewis acidity of beryllium and magnesium hydrides and fluorides with increasing clusters size

We have reported in the last years the strong effect that Be- and Mg-containing Lewis acids have on the intrinsic properties of typical bases, which become acids upon complexation. In an effort to investigate these changes when the Be and Mg derivatives form clusters of increasing size, we have examined the behavior of the (MX2)n (M = Be, Mg; X = H, F; n = 1, 2, 3) clusters when they interact with ammonia, methanimine, hydrogen cyanide and pyridine, and with their corresponding deprotonated forms. The complexes obtained at the M06-2X/aug-cc-pVTZ level were analyzed using the MBIE energy decomposition formalism, in parallel with QTAIM, ELF, NCIPLOT and AdNDP analyses of their electron density. For n = 1 the interaction enthalpy for the different families of monomers, Be (Mg) hydrides and Be (Mg) fluorides, follows the same trend as the intrinsic basicity of the base that interacts with them. This interaction is greatly reinforced after the deprotonation of the base, resulting in a significant enhancement of the intrinsic acidity of the corresponding MX2-Base complex. For (MX2)2 clusters a further reinforcement of the interaction with the base is observed, this reinforcement being again larger for the deprotonated complexes. However, the concomitant increase of their intrinsic acidity is one order of magnitude larger for hydrides than for fluorides. Unexpectedly, the cyclic conformers (MX2)3, which are more unstable than the linear ones, become the global minima after association with the base and the same is true for the deprotonated complex. Accordingly, a further increase of the intrinsic acidity of the (MX2)3-Base complexes with respect to the (MX2)2-Base ones is observed. This effect is maximum for (MgF2)3 clusters, to the point that the (MgF2)3-Base complexes become more acidic than nitric acid, the extreme case being the cluster (MgF2)3-NCH, whose acidity is higher than that of perchloric acid.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

Theoretical study of intermolecular interactions in CB4H8-HOX (X=F, Cl, Br, I) complexes

The molecular aggregation based on intermolecular interactions between CB4H8 and HOX (X=F, Cl, Br and I) with particular emphasis on their bonding characteristics have been investigated using second order Moller-Plesset perturbation (MP2) method with aug-cc-pVDZ basis set. Different kinds of interactions including hydrogen bond (HB; H⋯O, XH; H⋯X), dihydrogen bond (DiHB; H⋯H) and non-classical B-B⋯H interactions were found between CB4H8 and HOX molecules. The structures of complexes have been analyzed using AIM and natural bond orbital methodologies. Good correlations have been found between the interaction energies (SE), the second-order perturbation energies E((2)), and the charge transfer qCT in the studied systems.

A comparative interplay between small heterorings and hypofluorous acids

Through the B3LYP/6-311++G(d,p) calculations, theoretical studies of structural parameters, electronic properties, infrared vibration modes, and charge density topologies on the C2H4O∙∙∙HX and C2H5N∙∙∙HX (X = F or O-F) heterocylic hydrogen complexes are presented. The H-bond distances and high energies point out strong contacts and stable interactions in these complexes, and the relationships between the frequency shifts on the H-F and H-O bonds as well as O-F σ-holes with the interaction strength are the benchmarks of this current work. The computations of charge transfer amounts in light of the ChelpG and NBO approaches revealed a separation of charge density on the O-F σ-holes, whose statement is reinforced by the QTAIM descriptors. Despite that O∙∙∙H and N∙∙∙H H-bonds have been characterized as closed-shell interactions, qualitatively the appearance of a partial covalent profile also was unveiled by the QTAIM protocol.

The covalence and infrared spectra of cationic hydrogen bonds and dihydrogen bonds

A theoretical study of hydrogen bonds and dihydrogen bonds formed by ethyl cation, hydrocarbons and magnesium hydride is presented with calculations performed at the BHandHLYP/6-31G(d,p) level of theory. The structural results and IR analyses demonstrated great insights, mainly the strengthening and weakness of the CC bond of the ethyl cation and π or pseudo-π bonds, respectively. The interaction strength was measured through the supermolecule as well as by means of additional approaches. The QTAIM calculations were applied to characterize not only the intermolecular interactions but specifically the covalent character in the H + ⋯ π, H + ⋯ pseudo-π and H + ⋯ H contacts. The NBO calculations were useful to interpret the polarization on the CC bond and whether this effect is related with the bond length reduction as well as increase of charge density and frequency shifts.

The structures of heterocyclic complexes ruled by hydrogen bonds and halogen interactions: interaction strength and IR modes

In this work, the existence of multiple interactions in heterocyclic complexes of C2H4O⋯nHCCl3 and C2H4S⋯nHCCl3 with n=2 and 3 was unveiled at the B3LYP/6-311++G(d,p) level of theory. The forward analyses of the vibrational spectra revealed the appearing of red-shifts in the H-C bond. In agreement with this and through the optimized geometries of these systems, an increase in the H-C bond length was also observed. Besides O⋯H and S⋯H, other hydrogen bonds formed between chlorine⋯hydrogen and mainly the halogen interactions formed by chlorine⋯chlorine were identified. Thereby, the vibration spectra of the heterocyclic complexes were reanalyzed with the purpose to locate new red-shifts, although only those characterized in H-C have been detected up to then. In addition to the correlation between the frequencies shifted to downward values followed by increases in the bond lengths, the interpretation of the red-shifts was conducted by means of the Bent rule of the hybridization theory. The interaction strength was examined in several viewpoints, and one of them was the relationship between the H-bond energies and the intermolecular electronic density computed by means of the Quantum Theory of Atoms in Molecules (QTAIM). Moreover, the prediction of the interaction strength was also made through the combination between vibration modes (red-shifts) and variation of topological parameters, such as the electronic density and Laplacian of the proton donor bond (C-H).

Modulating weak intramolecular interactions through the formation of beryllium bonds: complexes between squaric acid and BeH2

The electronic structure of the two most stable isomers of squaric acid and their complexes with BeH2 were investigated at the B3LYP/6-311 + G(3df,2p)// B3LYP/6-31 + G(d,p) level of theory. Squaric acid forms rather strong beryllium bonds with BeH2, with binding energies of the order of 60 kJ mol(-1). The preferential sites for BeH2 attachment are the carbonyl oxygen atoms, but the global minima of the potential energy surfaces of both EZ and ZZ isomers are extra-stabilized through the formation of a BeH···HO dihydrogen bond. More importantly, analysis of the electron density of these complexes shows the existence of significant cooperative effects between the beryllium bond and the dihydrogen bond, with both becoming significantly reinforced. The charge transfer involved in the formation of the beryllium bond induces a significant electron density redistribution within the squaric acid subunit, affecting not only the carbonyl group interacting with the BeH2 moiety but significantly increasing the electron delocalization within the four membered ring. Accordingly the intrinsic properties of squaric acid become perturbed, as reflected in its ability to self-associate.

Bonding topology, hydrogen bond strength, and vibrational chemical shifts on hetero-ring hydrogen-bonded complexes — Theoretical insights revisited

This work presents a theoretical study about the interaction strength of the hydrogen-bonded complexes C2H4O···HF, C3H6O···HF, C2H4O···HCF3, and C3H6O···HCF3 at the B3LYP/6–311++G(d,p) level. The structures, hydrogen bond energies, charge transfers, and dipole moments of these complexes were analyzed in accordance with routine spectroscopy events, such as the red- and blue-shifts on the stretch frequencies of the proton donors (HF and HCF3). The ChelpG atomic charges were used to quantify the charge-transfer fluxes from electron donor (O) towards to acceptors (HF or HCF3). Moreover, the topological calculations on the basis of the quantum theory of atoms in molecules (QTAIM) approach were also used to unveil the hydrogen bond strength (O···H), mainly in the determination of their electronic densities and Laplacian shapes.

Theoretical aspects of binary and ternary complexes of aziridine···ammonia ruled by hydrogen bond strength

B3LYP calculations, ChelpG atomic charges, and quantum theory of atoms in molecules (QTAIM) integrations were used to investigate the binary (1:1) and ternary (1:2) hydrogen-bonded complexes formed by aziridine (1) and ammonia (2). In a series of analysis, geometry data, electronic parameters, vibrational oscillators, and topological descriptors were used to evaluate hydrogen bond strength, and additionally to determine the more prominent molecular deformations upon the formation of C(2)H(5)N···NH(3) (1:1) and C(2)H(5)N···2NH(3) (1:2) systems. Taking a spectroscopic viewpoint, results obtained from analysis of the harmonic infrared spectrum were examined. From these, new vibrational modes and red- and blue-shifts related to the stretch frequencies of either donors or acceptors of protons were identified. Furthermore, the molecular topology of the electronic density modeled in accord with QTAIM was absolutely critical in defining bond critical points (BCP) and ring critical points (RCP) on the heterocyclic structures. Taking all the results together allowed us to identify and characterize all the N···H hydrogen bonds, as well as the strain ring of the aziridine and its stability.