Protonated Hydronium Dication, H₄0²⁺. Hydrogen-Deuterium Exchange of D₂H¹⁷O⁺in HF:SbF₅ and DH₂¹⁷0⁺ in DF:SbF₅ and Theoretical Calculations^la

Isotopic hydronium ions D₂H¹⁷O⁺and DH₂¹⁷0⁺ in 1:1 (molar) HF:SbF₅/S0₂ and DF:SbF₅S0₂ solutions, respectively, at -15 °C undergo slow hydrogen-deuterium exchange as monitered by ¹⁷O NMR spectroscopy. The rate of such exchange increases with the increase in the acidity of the fluoroantimonic acid medium (1:2 molar composition). The previously observed lack of exchange of isotopic hydronium ions in the somewhat weaker Magic Acid, FSO₃H:SbF₅, medium (H₀≈ -21.5) suggests that in the stronger HF:SbF₅ medium (H₀ -25 to -28), the exchange occurs through the intermediacy of protonated hydronium dication. Consequently, the structure and stability of protonated hydronium dication has been probed by ab initio theory. The T_dsymmetry structure, 2, was found to be the minimum energy structure at the HF/6.31G level. Although 2 at the HF/6.31G level is thermodynamically unstable (dissociation preferred by 59.2 kcal/mol), it seems to have significant kinetic stability (deprotonation barrier 39.4 kcal/mol).