The tetrahydridosulfonium dication, H₄S²⁺. Hydrogen-deuterium exchange of DH₂S⁺ in FSO₃D:SbF₅ and D₂HS⁺ in FSO₃H:SbF₅ and theoretical calculations

Isotopomeric hydridosulfonium ions undergo proton/deuterium exchange in (Magic Acid) FSO₃D·SbF₅ and FSO₃H-SbF₅-SO₂ solutions as monitored by ¹H NMR spectroscopy. The rate of exchange increases with the increase in acidity of the superacidic medium, indicating the involvement of isotopomeric tetrahydridosulfonium dications in the exchange process. The structure of tetrahydridosulfonium ion, H₄S²⁺, 2, was also probed by ab initio theory. The tetrahedral symmetry (T_d) structure of H₄S²⁺, 2, was found to be the minimum energy structure at the HF/6-31G* level. Although structure 2 is thermodynamically unstable (dissociation to H₃S⁺ + H⁺ is preferred by 25.2 kcal/mol) it appears to have significant kinetic stability (deprotonation barrier 59.2 kcal/mol).