Competition between X···H···Y Intramolecular Hydrogen Bonds and X····Y (X = O, S, and Y = Se, Te) Chalcogen−Chalcogen Interactions

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 n O −σ* YH (or n S −σ* YH ) dative interaction. The proton-transfer processes in species with IHBs were also investigated.