Dimerization Pathway, Structure, and Acidity of Binuclear Mo(VI) Species in Aqueous Solution from First-Principles Molecular Dynamics

Understanding the early condensation of aqueous Mo(VI) species is important for improving polyoxomolybdate speciation models, particularly in the highly acidic environments relevant to high-level liquid waste (HLLW) processes. Here, we carried out first-principles molecular dynamics (FPMD) simulations to investigate the dimerization of monomeric MoO(OH)3(H2O)2+ and characterize the structure and acidity of the resulting binuclear species. Constrained molecular dynamics simulations show that dimerization is initiated by a hydroxyl–hydroxyl H-bonded precursor formed between two monomers, followed by proton transfer, nascent water formation, dissociation, and μ2-oxo bridge formation. The overall free energy change (−19.2 kcal/mol) indicates that the dimer [Mo2O3(OH)4(H2O)4]2+ is thermodynamically stable. For this dimer, each Mo center adopts a distorted octahedral coordination environment with one terminal oxo ligand, one μ2-oxo bridge, two hydroxyl groups, and two coordinated water molecules. pKa calculations using the FPMD-based vertical energy gap method further indicate that deprotonation preferentially occurs at the hydroxyl groups rather than at the coordinated water. These results suggest hydroxyl-assisted dehydration as a key elementary step in Mo(VI) dimerization and provide a molecular basis for understanding the early growth of polyoxomolybdate clusters in aqueous solution.