Proton solvation and self-dissociation of 100% sulfuric acid: The quantum-chemical analysis

Abstract

Ab initio quantum-chemical calculations of solvation energies of H₃SO₄⁺ cation and HSO₄⁻ anion by one or two molecules of sulfuric acid in the gas phase have been performed at the Hartree–Fock (HF) level using the 6-31++G** basis set. Additional contributions to the solvation energy of the acid arising from electrostatic interactions with the surrounding molecules in the liquid were estimated in the framework of the polarizable continuum models (PCM or IPCM models). Together with the experimental values for the energies of protonation and deprotonation of sulfuric acid in the gas phase the calculated solvation energies were then used to estimate the heats of self-dissociation of the liquid sulfuric acid and of proton solvation by the anhydrous acid. An almost quantitative agreement of the calculated heat of self-dissociation with the experimental value of +15 kJ mol⁻¹ indicates a rather high accuracy of the calculations. A remarkable feature of the anhydrous sulfuric acid is a low value of solvation energy of protons that is about 85 kJ mol⁻¹ smaller than in the aqueous solution. This result explains why anhydrous sulfuric acid, despite the low value of dissociation constant, behaves as a superacid.