Exploring americium and curium water clusters: gas-phase structures, solvation numbers, solvent and entropy effects

In this study, we theoretically investigate the global-minimum (GM) structures of trivalent actinide-water clusters An(H₂O)_n³⁺ (An = Am³⁺, Cm³⁺; n = 1-20) in the gas phase and aqueous solution using the DFT method. Additionally, the structures of large Am-water clusters Am(H₂O)_n³⁺ (n = 50, 100, 200, 300, 400, 500, and 600) are explored at the force field level. We conduct an in-depth analysis of the water solvent effect, entropy effect, infrared spectrum, binding energy, energy decomposition, and dynamic behavior of solvation shells. The structural differences between the gas phase and solution primarily stem from the high dielectric constant of aqueous solvent, a key physical property that stabilizes the polar interactions fundamental to the formation of water's characteristic hydrogen-bonding networks. In the gas phase, the GM structures of Am(H₂O)_n³⁺ and Cm(H₂O)_n³⁺ (n = 1-20) are highly analogous, with a preference for high-symmetry configurations when n ≤ 9. The maximum coordination numbers of An³⁺ in the first coordination shell are 9 in both the gas phase and solution. The average distances between An³⁺ and water molecules in the first and second solvation shells are calculated as r₁ (2.492-2.510 Å) and r₂ (4.619-4.722 Å), respectively. In the first solvation shell, the dominant interactions between An³⁺ ions and water molecules include donor-acceptor, charge-dipole, electrostatic, covalent orbital interactions, and Pauli repulsion; in the second solvation shell, hydrogen bonding and dipole-dipole interactions are the primary forces. For the large Am(H₂O)₆₀₀³⁺ cluster, the first solvation shell CN fluctuates between 9 and 10, and the second shell CN ranges from 25 to 29, providing new insights into the dynamic solvation behaviors of An³⁺ ions in bulk water.