First-principles molecular dynamics study of stepwise hydrolysis reactions of Y3 + cations

First-principles molecular dynamics simulations are carried out to study the stepwise hydrolysis reactions of Y3 +-aqua in water. The simulation results reproduce the 8-fold coordinated hydration structure of Y3 +. A constrained molecular dynamics method is employed to investigate the hydrolysis reactions, and a thermodynamic integration technique is used to calculate the free-energy profiles. Thus, the stepwise pKa's are derived: pKa1 = 7.9, pKa2 = 9.1 and pKa3 = 10.3, while the fourth hydrolysis is found to be extremely unfavorable. Based on the dissociation constants, the hydrolyzing speciation of Y3 + is constructed. The simulation results also show that in the hydrolysis process, some H2O ligands leave the cation to become part of the bulk solution, which makes the hydration number of Y3 + cations decrease from 8 (Y(H2O)83 +) to 7 (Y(OH)(H2O)62 + and Y(OH)2(H2O)5+) and then to 6 (Y(OH)3(H2O)3).

► The hydrolyzing speciation of Y3 + is constituted with FPMD.
► The first three hydrolysis constants are predicted to be 7.9, 9.1 and 10.3 respectively.
► The fourth hydrolysis is very unfavorable.
► During hydrolysis reactions, some H2O ligands of Y3 + escape to the bulk.
► Y(H2O)83 +, Y(OH)(H2O)62 + and Y(OH)2(H2O)5+ are major forms in common pH range.