The characteristics of the intramolecular hydrogen bond (IMHB) for a series of 40 different enols of beta-diketones and their nitrogen counterparts have been systematically analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In some cases, two tautomers may exist which are interconnected by a hydrogen shift through the IMHB. In tautomer a the HB donor group (YH) is attached to the six-membered ring, while in tautomer b the HB acceptor (X) is the one that is attached to the six-membered ring. We found that changing an O to a N favors the a tautomer when the atom is endo and the contrary when it is exo, while the presence of a double bond favors the a tautomers. As expected, the OH group behaves as a better HB donor than the NH2 group and the C=NH group as a better HB acceptor than the C=O group, although the first effect clearly dominates. Accordingly, the expected IMHB strength follows the [donor, acceptor] trend: [OH, C=NH] > [OH, C=O] > [NH2, C=NH] > [NH2, C=O]. For all those compounds in which the functionality exhibiting the IMHB is unsaturated (I-type), the IMHB is much stronger than in their saturated counterparts (II-type). However, when the systems of the II-type subset, which are saturated, are constrained to have the HB donor and the HB acceptor lying in the same plane and at the same distance as in the corresponding unsaturated analogue, the IMHB is of similar or even larger strength. Hence, we conclude that, at least for this series of unsaturated compounds, the resonance-assisted hydrogen bond effect is not the primary reason behind the strength of their IMHBs, which is simply a consequence of the structure of the sigma-skeleton of the system that keeps the HB donor and the HB acceptor coplanar and closer to each other.

High-level G2(MP2) ab initio and B3LYP/6-311+G(3df,2p) density functional calculations have been carried out for a series of β-chalcogenovinylaldehydes, HC(X)−CHCH−CYH (X = O, S; Y = Se, Te). Our results indicate that for X = O, S and Y = Se, the O−H···Se and the S−H···Se intramolecular hydrogen bonds compete in strength with the O···Se and the S···Se interaction, while the opposite is found for the corresponding tellurium-containing analogues. The different strength of O−H···Se and O···H−Se intramolecular hydrogen bonds explains why the chelated enolic and keto forms of selenovinylaldehyde are very close in energy, although enol-tautomers are estimated to be about 10 kcal mol-1 more stable than keto-tautomers. The situation is qualitatively similar for selenothiovinylaldehyde, although the S−H···Se and S···H−Se intramolecular hydrogen bonds (IHBs) are weaker and much closer in strength, and the energy gap between enethiol- and thione-tautomers also smaller. The relative strengths of the X−H···Te and X···H−Te (X = O, S) IHBs, are very similar to those of the corresponding selenium analogues. However, there are dramatic differences as far as the X···Y (X = O, S; Y = Se, Te) interactions are concerned, which for Se-derivatives are rather small, while for Te-compounds are very strong. An analysis of these chalcogen-chalcogen interactions indicates that both, the electrostatic and the dative contributions are smaller for Se- than for Te-derivatives. In the latter, the electrostatic component clearly dominates when X = O, while the opposite is found for sulfur-containing derivatives. We have also shown that these two components are entangled in some manner, in the sense that strong electrostatic interactions favor the nO−σ*YH (or nS−σ*YH) dative interaction. The proton-transfer processes in species with IHBs were also investigated.

The complexes between BeX2 (X = H, F, Cl, OH) with different Lewis bases have been investigated through the use of B3LYP, MP2, and CCSD(T) approaches. This theoretical survey showed that these complexes are stabilized through the interaction between the Be atom and the basic center of the base, which are characterized by electron densities at the corresponding bond critical points larger than those found in conventional hydrogen bonds (HBs). Actually, all bonding indices indicate that, although these interactions that we named "beryllium bonds" are in general significantly stronger than HBs, they share many common features. Both interactions have a dominant electrostatic character but also some covalent contributions associated with a non-negligible electron transfer between the interacting subunits. This electron transfer, which in HBs takes place from the HB acceptor lone-pairs toward the σYH* antibonding orbital of the HB donor, in beryllium bonds goes from the lone pairs of the Lewis base toward the empty p orbital of Be and the σBeX* antibonding orbital. Accordingly, a significant distortion of the BeX2 subunit, which in the complex becomes nonlinear, takes place. Concomitantly, a significant red-shifting of the X-Be-X antisymmetric stretching frequencies and a significant lengthening of the X-Be bonds occur. The presence of the beryllium bond results in a significant blue-shifting of the X-Be-X symmetric stretch.