Unveiling the Structure and Dynamics of Ac3+ Ion in Aqueous Solution: Insight From Relativistic Hybrid Forces Molecular Mechanics Molecular Dynamics Simulations

This work describes a molecular dynamics simulation study (MP2-DKH2/MM) that explores the structural and dynamical properties of hydrated Ac³⁺ ions in an aqueous solution. Simulation results indicate that the ion formed three hydration shells. The hydrated Ac³⁺ had a first hydration shell comprising 8–9 water molecules. It showed similar probabilities for both coordination numbers, showing a flexible first hydration shell with eight registered successful ligand exchanges during the simulation. The water molecules' mean residence times (MRT) in the first, second, and third hydration shells were 131.8, 6.46, and 2.67 ps, respectively. The complexes of octahydrate ([Ac(H₂O)₈]³⁺) and nonahydrate ([Ac(H₂O)₉]³⁺) were observed in the first hydration shell. The square antiprism (SA) geometry was adopted for octahydrate, while the gyroelongated square antiprism (GySA) geometry was adopted for nonahydrate. The simulations provided valuable insights into the ion-oxygen stretching frequencies. Specifically, the average stretching frequency for Ac³⁺ was found to be 404 cm⁻¹, which is in good agreement with the calculated value from the CCSD(T) calculation of 398.78 cm⁻¹. These findings indicate that including DKH2 relativistic approximation increases the accuracy of the simulation results and can contribute to understanding these actinide ions' behavior in aqueous environments, shedding light on hydrated systems' structural arrangements and dynamics.