Disulfuric acid dissociated by two water molecules: ab initio and density functional theory calculations

Abstract

We have studied geometries, energies and vibrational spectra of disulfuric acid (H₂S₂O₇) and its anion (HS₂O₇⁻) hydrated by a few water molecules, using density functional theory (M062X) and ab initio theory (SCS-MP2 and CCSD(T)). The most noteworthy result is found in H₂S₂O₇(H₂O)₂ in which the lowest energy conformer shows deprotonated H₂S₂O₇. Thus, H₂S₂O₇ requires only two water molecules, the fewest number of water molecules for deprotonation among various hydrated monomeric acids reported so far. Even the second deprotonation of the first deprotonated species HS₂O₇⁻ needs only four water molecules. The deprotonation is supported by vibration spectra, in which acid O–H stretching peaks disappear and specific three O–H stretching peaks for H₃O⁺ (eigen structure) appear. We have also kept track of variations in several geometrical parameters, atomic charges, and hybrid orbital characters upon addition of water. As the number of water molecules added increases, the S–O bond weakens in the case of H₂S₂O₇, but strengthens in the case of HS₂O₇⁻. It implies that the decomposition leading to H₂SO₄ and SO₃ hardly occurs prior to the 2nd deprotonation at low temperatures.