Computational study of Th(4+) and Np(4+) hydration and hydrolysis of Th(4+) from first principles

Parametrization of Trivalent and Tetravalent Metal Ions for the OPC3, OPC, TIP3P-FB, and TIP4P-FB Water Models

Commonly seen in rare-earth chemistry and materials science, highly charged metal ions play key roles in many chemical processes. Computer simulations have become an important tool for scientific research nowadays. Meaningful simulations require reliable parameters. In the present work, we parametrized 18 M(III) and 6 M(IV) metal ions for four new water models (OPC3, OPC, TIP3P-FB, TIP4P-FB) in conjunction with each of the 12-6 and 12-6-4 nonbonded models. Similar to what was observed previously, issues with the 12-6 model can be fixed by using the 12-6-4 model. Moreover, the four new water models showed comparable performance or considerable improvement over the previous water models (TIP3P, SPC/E, and TIP4P_EW) in the same category (3-point or 4-point water models, respectively). Finally, we reported a study of a metalloprotein system demonstrating the capability of the 12-6-4 model to model metalloproteins. The reported parameters will facilitate accurate simulations of highly charged metal ions in aqueous solution.

Hydration Structure and Hydrolysis of U(IV) and Np(IV) Ions: A Comparative Density Functional Study Using a Modified Continuum Solvation Approach

We studied the hydration and the first hydrolysis reaction of U(IV) and Np(IV) ions in an aqueous environment, applying a relativistic density functional method together with a recently proposed variant of a continuum solvation model where the solute cavities are constructed with effective atomic radii, based on charge-dependent scaling factors. In this way, one obtains improved solvation energies of charged species. We demonstrate that solute cavities, constructed with scaled atomic radii as described, permit one to calculate hydrolysis constants of acceptable accuracy. As a consequence, one is also able to estimate free hydration energies of U(IV) and Np(IV) in adequate agreement with empirical data. According to the model calculations, U(IV) is coordinated by eight to nine water molecules, while the preferred coordination number of Np(IV) is 8. For the highly charged ions under study, the modified solvation model simultaneously yields improved geometries, hydration energies, and hydrolysis constants.

Density Functional Theory Investigations on the Mechanism of Formation of Pa(V) Ion in Hydrous Solutions

Due to the enormous threat of protactinium to the environment and human health, its disposal and chemistry have long been important topics in nuclear science. [PaO(H₂O)₆]3+ is proposed as the predominant species in hydrous and acidic solutions, but little is known about its formation mechanism. In this study, density functional theory (DFT) calculations demonstrate a water coordination-proton transfer-water dissociation mechanism for the formation of PaO3+ in hydrous solutions. First, Pa(V) ion preferentially forms hydrated complexes with a coordination number of 10. Through hydrogen bonding, water molecules in the second coordination sphere easily capture two protons on the same coordinated H₂O ligand to form [PaO(H₂O)₉]3+. Water dissociation then occurs to generate the final [PaO(H₂O)₆]3+, which is the thermodynamic product of Pa(V) in hydrous solutions.

Mononuclear thorium halide clusters ThX4 (X = F, Cl): gas-phase hydrolysis reactions

Density functional theory (DFT) calculations have been performed to explore the gas-phase hydrolysis reaction of mononuclear thorium halide clusters ThX4 (X = F, Cl). We have found that the hydrolysis of ThCl4 is easier than that of ThF4. Furthermore, their hydrolysis reactions favor pathways of direct dehydration of Th(OH)4 instead of further hydrolysis of ThOX2. There are some differences between the hydrolysis of ThCl4 and that of MCl4 (M = Ti, Zr and Hf). The X-HY (X = F, Cl; Y = F, Cl and OH) hydrogen bonds play an important role in the hydrogen transfer process of the hydrolysis reaction. The differences in the steric effects and bonding may be important factors that are related to the disparities in the hydrolysis of the above-mentioned metal halides.