Hydrolysis and Dimerization of Th4+ Ion

Computational Explorations of Th4+ First Hydrolysis Reaction Constants: Insights from Ab Initio Molecular Dynamics and Density Functional Theory Calculations

The fundamental hydrolysis behavior of tetravalent actinide cations (An4+) with a high charge is crucial for understanding their solution chemistry, particularly in nuclear fuel reprocessing and environmental behavior. Using Th4+ as a reference of the An4+ series, this work employed both the periodic model and the cluster model to calculate the first hydrolysis reaction constant (pKa1) of the Th4+ aqua ion and conducted a detailed evaluation of these approaches. In the periodic model, ab initio molecular dynamics (AIMD) simulations of Th4+ in the explicit solvation environment are conducted, using metadynamics and constrained molecular dynamics to calculate pKa1 values. Metadynamics simulations with sufficient sampling yielded a value of 5.02, aligning with the experimental values (4.12-4.97). Moreover, AIMD results reveal further Grotthuss-type proton transfers and changes in the solvent structures, which are important for accurately modeling the hydrolysis process. In the cluster model, density functional theory calculations are performed on isolated hydrate clusters to obtain pKa1 values, describing solvation effects through the cluster-continuum model. Based on insights from the periodic models, particularly regarding further proton transfer, the cluster model was modified and tested using different functionals and similar cations (La3+and Ac3+). The pKa1 values obtained in the cluster model also show good agreement with the experimental values. The current computational approaches provide a comprehensive understanding of Th4+ hydrolysis and a reference framework for studying the hydrolysis of other lanthanide and actinide ions.

Hydrolysis Reactions of the High Oxidation State Dimers Th2O4, Pa2O5, U2O6, and Np2O6. A Computational Study

The energetics of the hydrolysis reactions for high oxidation states of the dimeric actinide species Th2IVO4, Pa2VO5, and U2VIO6 were calculated at the CCSD(T) level and those for triplet Np2VIO6 at the B3LYP level. Hydrolysis is initiated by the formation of a Lewis acid/base adduct with H2O (physisorbed product), followed by a proton transfer to form a dihydroxide molecule (chemisorbed product); this process was repeated until the initial actinide oxide is fully hydrolyzed. For Th2O4, hydrolysis (chemisorption) by the initial and subsequent H2O molecules prefers proton transfer to terminal oxo groups before the bridge oxo groups. The overall Th2O4 hydration pathway is exothermic with chemisorbed products preferred over the physisorption products, and the fully hydrolyzed Th2(OH)8 can form exothermically. Hydrolysis of Pa2O5 forms isomers of similar energies with no initial preference for bridge or terminal hydroxy groups. The most exothermic hydrolysis product for Pa is Pa2O(OH)8 and the most stable species is Pa2O(OH)8(H2O). Hydrolysis of U2O6 and Np2O6 with strong [O═An═O]2+ actinyl groups occurs first at the bridging oxygens rather than at the terminal oxo groups. The U2O6 and Np2O6 pathways predict hydrated products to be more favored than hydrolyzed products, as more H2O molecules are added. The stability of the U and Np clusters is predicted to decrease with increasing number of hydroxyl groups. The most stable species on the hydration reaction coordinate for U and Np is An2O3(OH)6(H2O).

Hydrolysis of Small Oxo/Hydroxo Molecules Containing High Oxidation State Actinides (Th, Pa, U, Np, Pu): A Computational Study

The energetics of hydrolysis reactions for high oxidation states of oxo/hydroxo monomeric actinide species (Th^IVO₂, Pa^IVO₂, U^IVO₂, Pa^VO₂(OH), U^VO₂(OH), U^VIO₃, Np^VIO₃, Np^VIIO₃(OH), and Pu^VIIO₃(OH)) were calculated at the CCSD(T) level. The first step is the formation of a Lewis acid/base adduct with H₂O (hydration), followed by a proton transfer to form a dihydroxide molecule (hydrolysis); this process is repeated until all oxo groups are hydrolyzed. The physisorption (hydration) for each H₂O addition was predicted to be exothermic, ca. -20 kcal/mol. The hydrolysis products are preferred energetically over the hydration products for the +IV and +V oxidation states. The compounds with An^VI are a turning point in terms of favoring hydration over hydrolysis. For An^VIIO₃(OH), hydration products are preferred, and only two waters can bind; the complete hydrolysis process is now endothermic, and the oxidation state for the An in An(OH)₇ is +VI with two OH groups each having one-half an electron. The natural bond order charges and the reaction energies provide insights into the nature of the hydrolysis/hydration processes. The actinide charges and bond ionicity generally decrease across the period. The ionic character decreases as the oxidation state and coordination number increase so that covalency increases moving to the right in the actinide period.

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study

Hydrolysis of the Bi(III) aqua ion under a range of solution conditions has been studied by means of ab initio molecular dynamics simulations. While the Bi(III) aqua ion is stable in pure water, there is an increasing degree of hydrolysis with the number of hydroxide anions in the medium. This is accompanied by a monotonic decrease of the total coordination number to an asymptotic value of ∼6, reached under extreme basicity conditions. Comparison of the simulated Bi(III) hydrolyzed species with the experimental species distribution at different degrees of basicity suggests that, at the PBE/DFT level of theory here employed, liquid water shows an overly acidic character. Predictions of theoretical EXAFS and XANES spectra were generated from the AIMD trajectories for different Bi hydrolyzed species, [Bi(HO)_m(H₂O)_n]^3-m+, m = 0-3 and n = 7-2. Comparison with available experimental spectra is presented. Spectral features joined to the degree of hydrolysis and hydration are analyzed.

Structure, Stability, and Electronic Properties of Dimethyl Sulfoxide and Dimethyl Formammide Clusters Containing Th(4.)

By using accurate density functional theory calculations, we have studied the complexes of Th(4+) with dimethyl-sulfoxide (DMSO) and dimethyl-formammide (DMF) molecules. These solvents are prototypes for oxygen-donor organic environments in which the oxygen atom is connected to S and C atom, respectively. Extended structural, energetic, and electronic structure analysis has been performed to provide a complete picture of the physical properties at the basis of the interaction of Th(4+) with the two solvents. By using a cluster grow approach, we have found that, very likely, the first solvation shell contains nine molecules in the case of DMSO, while it contains eight molecules for DMF. The theoretical results shown here are in agreement with experimental data taken from the literature.

Exploring actinide materials through synchrotron radiation techniques

Synchrotron radiation (SR) based techniques have been utilized with increasing frequency in the past decade to explore the brilliant and challenging sciences of actinide-based materials. This trend is partially driven by the basic needs for multi-scale actinide speciation and bonding information and also the realistic needs for nuclear energy research. In this review, recent research progresses on actinide related materials by means of various SR techniques were selectively highlighted and summarized, with the emphasis on X-ray absorption spectroscopy, X-ray diffraction and scattering spectroscopy, which are powerful tools to characterize actinide materials. In addition, advanced SR techniques for exploring future advanced nuclear fuel cycles dealing with actinides are illustrated as well.

Structure and dynamics of the U4+ ion in aqueous solution: an ab initio quantum mechanical charge field molecular dynamics study

The structure and dynamics of the stable four-times positively charged uranium(IV) cation in aqueous solution have been investigated by ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation at the Hartree-Fock double-zeta quantum mechanical level. The QMCF-MD approach enables investigations with the accuracy of a quantum mechanics/molecular mechanics approach without the need for the construction of solute-solvent potentials. Angular distribution functions; radial distribution functions; coordination numbers of the first, second, and third shell (9, 19, and 44, respectively); coordination number distribution functions; tilt- and Theta-angle distribution functions; as well as local density corrected triangle distribution functions have been employed for the evaluation of the hydrated ion's structure. Special attention was paid to the determination of the geometry of the first hydration layer, and the results were compared to experimental large-angle X-ray scattering and extended X-ray absorption fine structure data. The solvent dynamics around the ion were also investigated using mean ligand residence times and related data and, resulting from the unavailability of any experimental data, were compared to ions with similar properties.