Abstract:

The radioactivity and toxicity of actinides impede experimental investigation into their chemical properties in the condensed phase. The rapid development of computational methods and computational facilities allows for alternative experimental methods, including the use of a molecular force field, to gain insight into the coordination chemistry and dynamics of actinides. The key to this method is the force fieild parameters. In the present work, we report the development and validation of the AMBER (Assisted Model Building with Energy Refinement) force field parameters for Np⁴⁺, Am³⁺, and Cm³⁺ based on the experimentally determined ion-oxygen distance (IOD). The parameter set, together with that reported for Th⁴⁺, U⁴⁺, and Pu⁴⁺, was then applied to investigate the coordination chemistry and dynamics of these six actinide ions in the aqueous phase, in the absence and presence of counterions Cl^-, NO₃^-, and CO₃^2-. The simulations showed a shorter An-O_w coordination length for An⁴⁺ than for An³⁺, and for higher atomic numbers of ions in the same valence state. Th⁴⁺ preferentially existed in a 10-coordinated state, adopting a BCASP (bicapped square antiprism) conformation, while the other ions tended to be 9-coordinated with a CASP (capped square antiprism) conformation. The only exception was Cm³⁺, which adopted a TCTP (tricapped trigonal prism) conformation. The results also showed that the water molecules around An⁴⁺ were more ordered than those around An³⁺, as indicated by the smaller angles between the An-O_w vector and the dipole direction of the water ligand. This highly ordered structure of coordinated water affected their translation and rotation, i.e., the diffusion coefficient and rotational relaxation time of the water molecules around An⁴⁺ were smaller than those in the case of An³⁺ due to the stronger electrostatic interaction between An⁴⁺ and ligating water. The hydration free energies of the targeted actinide ions were also calculated by the FEP (free energy perturbation) method. An⁴⁺ underwent a greater degree of stabilization than did An³⁺ upon hydration; among the ions in the same oxidation states, those with a higher atomic number were better stabilized. In summary, the results of the simulations were consistent with the literature data in terms of the hydration structure, coordination of counterions, and hydration free energy of the actinide ions. The ability of the parameter set to describe the dynamics of water in the vicinity of actinides remains to be verified due to the lack of reference data. We tentatively propose that it may be used to investigate the coordination chemistry of actinides both in conformational analysis and binding strength, while special care should be taken when studying the kinetics of the solvated system. This work is expected to enrich our understanding of the solution behavior of An³⁺/An⁴⁺ at the force field level.

Key words: An³⁺, An⁴⁺, AMBER force field