MP2 study of the proton-donating power of 100% fluorosulfonic and chlorosulfonic acids

The energies of gas-phase solvation of the FSO₃H ₂ ⁺ , and ClSO₃H ₂ ⁺ cations and FSO ₃ ^? , ClSO ₃ ^? anions by one molecule of the respective acid (FSO₃H or ClSO₃H) have been calculated by a quantum chemical method taking into account electron correlation in terms of MP2 theory with a 6-311++G(d,p) basis set. The energy of additional solvation of the resulting complexes by liquid acid has been estimated within the continuum model of solvation by the IPCM method, with the acid modeled as a continuum with a large dielectric constant. The calculated data have provided a quantitative estimate for the energy of self-ionization of liquid FSO₃H and ClSO₃H acids. The similarly calculated energy of solvation of protons in 100% fluorosulfonic and chlorosulfonic acids is lower than the heat of hydration of the proton in aqueous solution by 23.4 and 24.5 kcal/mol, respectively. These quantum chemical data explain why 100% liquid fluorosulfonic and chlorosulfonic acids exhibit the properties of a superacid.