Interconversion Pathways of Chalcogen Rings. An Ab Initio MO Study of Model Reactions Involving Hypervalent Sulfur and Selenium Hydrides

Abstract

Chalcogen ring molecules are known to undergo facile interconversion reactions in organic solvents and in the melts, e.g. polymerization of liquid sulfur or the formation of Se₆ and Se₇ upon dissolving Se₈ in CS₂. These interconversion reactions have been suggested to proceed via the homolytic cleavage of the chalcogen-chalcogen bonds or through the formation of hypervalent intermediates. The experimental evidence on the actual mechanism, however, is rather sparse. The energetics of the various suggested interconversion pathways have been studied by ab initio MO techniques using suitable model reactions involving sulfur and selenium hydrides. Full geometry optimization has been performed for hypervalent chalcogen hydrides H₂EE and H₂E(EH)₂ as well as for the radicals HE and HEE (E = S or Se in the case of each chalcogen atom) utilizing the MIDI-4* basis sets. The energy changes in their formation from appropriate unbranched chalcogen hydrides HE_nH(n = 1,2 or 4) and HE_nE′_mH (m, n = 0–4; n + m = 2,4; E, E′ = S or Se) have been calculated including the corrections for the electron correlation by the second-order Møller-Plesset perturbation theory and for the zero-point vibrational energy. The formation of hypervalent interconversion intermediates is seen as an energetically plausible alternative for the homolytic cleavage of the chalcogen-chalcogen bonds. The present calculations help to understand why the interconversion of heterocyclic selenium sulfides proceeds principally with selenium-atom transfer.