Unraveling the Potential Kinetic Stability of Uranyl Cation-Cation Interactions-Driven Dimers in Aqueous Solutions

Actinyl cations AnO₂ⁿ⁺ (n = 1 or 2) play a pivotal role in both nuclear fuel cycles and spent fuel reprocessing processes. The interaction between actinyl cations, known as cation-cation interaction (CCI), was first proposed by Goodenough in 1960. In this work, CCI in the dimer of uranyl ions (UO₂²⁺) in aqueous solutions is systematically investigated through integrated density functional theory (DFT), ab initio molecular dynamics (AIMD), and metadynamics simulations. AIMD simulations identify a metastable distorted T-shaped dimer [(H₂O)₅OU_DO_D-U_AO₂(H₂O)₄]⁴⁺. Free energy surface along θ(U_D-O_D-U_A) obtained from metadynamics simulations reveals a single local minimum at θ = 151.5°, indicative of the kinetic stability of the distorted T-shaped dimer. Chemical bond analysis reveals that the coordination bonding interaction between the two UO₂²⁺ cations demonstrates a predominant contribution, while hydrogen bonding interactions are also non-negligible in stabilizing the structure of the dimer. Thermodynamic calculations and energy decomposition analysis (EDA) of interfragment interactions in the dimerization of hydrated AnO₂ⁿ⁺ ions indicate that the CCI-mediated dimer formation between UO₂²⁺ cations in aqueous solutions is thermodynamically disfavored. We found that the coordination interactions and solvation gain in the T-shaped geometry cannot offset the initial desolvation penalty and intense electrostatic repulsion. In conclusion, our research first reveals the kinetic stability of UO₂²⁺ dimers in aqueous environments, while their thermodynamic instability accounts for the experimental challenges in observing such species, thereby providing a reference for subsequent studies on the CCI of actinyl ions.