Hydrolysis and dimerization of Th4+ ion

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Aqueous Geochemistry

Background:

• Thorium(IV) (Th(4+)) hydrolysis is crucial for understanding its behavior in aqueous environments.
• Previous studies have explored Th(4+) speciation, but detailed structural insights into hydrolysis products remain limited.
• Computational methods offer a powerful tool to investigate complex solution chemistry.

Purpose of the Study:

• To investigate the hydrolysis of Thorium(IV) in aqueous solution using Density Functional Theory (DFT) calculations.
• To determine the stable coordination numbers (CNs) of Th(4+) hydrolysis products.
• To elucidate the dimerization behavior of Th(4+) hydrolysis species and its implications for speciation.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model Th(4+) hydrolysis.
• Systematic investigation of stable coordination numbers for various hydrolysis products.
• Analysis of reaction energetics, including Gibbs free energy changes, for dimerization reactions.
• Comparison of calculated structures and properties with existing experimental data for Th(4+) aquo and disulfato complexes.

Main Results:

• Coordination number of Th(4+) significantly decreases with stepwise hydrolysis.
• The fourth hydrolysis product, Th(OH)4(0), exhibits a coordination number of 6 (octahedral).
• Th(OH)4(OH2)2(0) readily forms a dimer, Th2(OH)8(0), via Th-OH-Th bridging with a Gibbs energy change of -24.0 kJ/mol.
• Dimerization inhibits the stable existence of Th(OH)4(0) in aqueous solution, consistent with the absence of direct evidence for this species and the presumed presence of oligomers like Th4(OH)16(0).
• Calculations for Th(4+) disulfato complexes show minimal changes in CN and Th-O distance compared to the aquo ion, aligning with experimental observations.

Conclusions:

• Stepwise hydrolysis leads to a reduction in the coordination number of Th(4+).
• Dimerization of Th(OH)4(0) is thermodynamically favorable, explaining its instability as a monomeric species in solution.
• The DFT findings support the existence of higher-order thorium oligomers in aqueous solutions.
• Computational modeling provides valuable insights into Th(4+) speciation and coordination chemistry, complementing experimental data.