Theoretical analysis of equilibrium geometries and barriers of rotation in molecules H2X2, with X=O, S, Se, and Te

A model analysis is presented of equilibrium geometries and rotational barriers in molecules of composition H2X2, with X=O, S, Se, and Te, applying exchange perturbation theory. The basis of the model is an assumed analogy with the phenomenon of ‘‘superexchange’’ (indirect exchange) in solids with paramagnetic cations. A diamagnetic center in the superexchange model is formed by two spin-paired electrons in an ‘‘effective’’ 1s Slater orbital determined from the valence-shell diamagnetic susceptibility of the X atom. The model is conceptually related to the so-called ‘‘through-space’’ (i.e., direct) and ‘‘through-bond’’ (i.e., indirect) interactions, of recent interest in organic chemistry. A survey of the literature is given concerning alternative interpretations of equilibrium geometries and barriers (cis and trans), in particular with respect to the possible influence of lone-pair orbitals. The results of the model are found to be comparable to those of large-scale ab initio calculations.