Isotope effects in BeH2⋯HCN

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.

Intermolecular HH vibrations of dihydrogen bonded complexes H3EH(-)...HOR in the low-frequency region: theory and IR spectra

The results of DFT calculations of harmonic and anharmonic frequencies of the dihydrogen bonded (DHB) complexes H 3EH (-)...HOR (E = B, Al, Ga and HOR = CH 3OH, CF 3CH 2OH) in gas phase and in low polar medium (by CPCM model) in comparison with the partners are presented. Normal coordinate analysis of the low-frequency modes was carried out to assign the new vibrations induced by DHB formation by the potential energy distribution values. Among them, the intermolecular H...H stretching vibrations only have individual modes. The influence of central atom mass and isotope and the strength of the proton donor effects were determined. The systems convenient for IR studies were chosen from the calculation predictions. The spectral investigation was made on the BH 4 (-)/ROH complexes (ROH = CH 2FCH 2OH (MFE), CF 3CH 2OH (TFE), (CF 3) 2CHOH (HFIP)). The results of temperature dependence, isotope substitution, and influence of the proton-donor strength studies agree with the theoretical conclusions. Combination of experimental and theoretical approaches allowed determining for the first time the intermolecular stretching mode characterizing intrinsic DHB vibrations.

The Properties of Weak and Strong Dihydrogen-Bonded DH⋅⋅⋅HA Complexes

The properties of six dihydrogen-bonded (DHB) dimers with the BeH2 molecule as a proton acceptor were calculated by MP2, CCSD(T) and B3LYP methods. The structural, energetic and spectroscopic parameters are presented and analyzed in terms of their possible correlation with the interaction energy and the intermolecular H...H separation. The symmetry-adapted perturbation theory (SAPT) calculations were performed to gain more insight into the nature of the H...H interactions. The studied complexes are divided into three groups based on the calculated intermolecular distances and the interaction energies which range from approximately -1 to -42 kJ mol(-1). The analysis of the interaction energy components indicates that, in contrast to conventional hydrogen bonds, the induction energy is the most important term in the BeH2NH4+ complex. On the other hand, there is no sharp boundary between the DHB complexes classified as hydrogen bonded and van der Waals systems. The complexation-induced changes in vibrational frequencies and in proton shielding constants show a relationship with the interaction energy. The values of the 2hJXH and 3hJBeX coupling constants correlate well with the interaction energy and with the intermolecular distance.