First-principles molecular dynamics study of stepwise hydrolysis reactions of Y~(3+) cations

摘要

First-principles molecular dynamics simulations are carried out to study the stepwise hydrolysis reactions of Y~(3+)-aqua in water. The simulation results reproduce the 8-fold coordinated hydration structure of Y~(3+). 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 pK_a's are derived: pK_(a1)=7.9, pK_(a2)=9.1 and pK_(a3)=10.3, while the fourth hydrolysis is found to be extremely unfavorable. Based on the dissociation constants, the hydrolyzing speciation of Y~(3+) is constructed. The simulation results also show that in the hydrolysis process, some H_2O ligands leave the cation to become part of the bulk solution, which makes the hydration number of Y3+ cations decrease from 8 (Y(H_2O)_8 ~(3+)) to 7 (Y(OH)(H_2O)_6 ~(2+) and Y(OH)_2(H_2O)_5 ~+) and then to 6 (Y(OH)_3(H_2O)_3).