Acidity of M(VI)O2(OH)2 for M = Group 6, 16, and U as Central Atoms

Abstract

Gas-phase acidities and aqueous solution pKa's are predicted for MO2(OH)2, where the center atom M is a main Group 6, 16, and U atom using the Feller-Peterson-Dixon approach based on coupled cluster CCSD(T) calculations with additional corrections. The gas-phase acidities of the MO2(OH)2 compounds are essentially the same for elements (M) of the same group, 304-310 kcal/mol at 298 K. All of the Group 6 compounds are 5-6 kcal/mol less acidic in the gas phase than H2SO4. The gas-phase acidity of UO2(OH)2 is calculated to be up to 338.0 kcal/mol, ∼10% less acidic in the gas phase than the other MO2(OH)2 acids. The most acidic molecule in aqueous solution is predicted to be H2SO4. Overall, for the Group 16 compounds, the pKa's increase going down the group, with H2PoO4 predicted to be slightly more acidic than nitric acid. H2CrO4 is the most acidic of the Group 6 transition metal compounds. The aqueous acidities of H2MoO4 and H2WO4 are comparable and about 3 pKa units less acidic than H2CrO4 and comparable in acidity to HNO3. H2UO4 is not acidic at all in aqueous solution with a pKa near 20 pKa units and is also not predicted to readily undergo hydrolysis reactions.