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 beta-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...HSe 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 = 0, 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)-sigma*(YH) (or n(S)-sigma*(YH)) dative interaction. The proton-transfer processes in species with IHBs were also investigated.