Solution Structures and Acidity Constants of Molybdic Acid

Experiment has not been able to discriminate between the oxo (MoO3·(H2O)3) and oxyhydroxide (MoO2(OH)2·(H2O)2) forms of molybdic acid. Using first-principles molecular dynamics based pKa calculation techniques, we identify that MoO2(OH)2.(H2O)2 is the true solution structure and its OH ligands are the acidic site. Simulations at elevated temperatures up to 573 K show an encouraging agreement between calculated and experimental pKa’s, which validates our method of prediction of subtle pH-dependent speciation in hydrothermal solutions. We find that molybdate species have highly volatile pH- and temperature-dependent coordinations, which is related with the experimentally observed variability in Mo coordination of polyoxometalates (POMs). These results form a physical basis for understanding the properties of Mo in numerous lab and natural processes ranging from formation of POMs to transport and deposition mechanisms in crustal fluids.