Hydrolysis and dimerization of Th⁴⁺ ion

Hydrolysis of Th⁴⁺ in aqueous solution was studied by density functional theory (DFT) calculations. First, stable coordination numbers (CNs) of Th⁴⁺ hydrolysis products were studied systematically, and it was found that the CN significantly decreases as a stepwise hydrolysis reaction proceeds. The fourth hydrolysis product Th(OH)₄ ⁰ has CN 6 with an octahedron coordination. Th(OH)₄ ⁰ can readily form a dimer complex Th₂(OH)₈ ⁰ via a Th-OH-Th bridging through an exergonic reaction with a Gibbs energy change of -24.0 kJ/mol. Consequently, dimerization inhibits Th(OH)₄ ⁰ to stay as stable aqueous species. The calculated result is in agreement with the fact that there is no direct evidence to confirm the presence of Th(OH)₄ ⁰ while oligomeric species such as Th₄(OH)₁₆ ⁰ are presumably present. Similar calculations on the Th⁴⁺ disulfato complex reveal that the CN and the average Th-O distance of Th(SO₄)₂ ⁰ remain almost the same as those in the Th⁴⁺ aquo ion, which is also in agreement with experimental data.