Apparent stoichiometry of water in proton hydration and proton dehydration reactions in CH₃CN/H₂O solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH⁺ + n(H₂O) = X + (H ₂O)_nH⁺ was studied for strong mineral acids (negatively charged X, X = ClO₄Â, ClÂ, BrÂ, IÂ, CF₃SO₃Â) and for strong cationic acids (uncharged X, X = R*NH₂, H₂O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H⁺(H₂O)₂, remain almost unchanged at large n values up to at least 10 M of water.