Uranyl Affinity between Uranyl Cation and Different Kinds of Monovalent Anions: Density Functional Theory and...

In order to design effective extractants for uranium extraction from seawater, it is imperative to acquire a more comprehensive understanding of the bonding properties between the uranyl cation (UO₂²⁺) and various ligands. Therefore, we employed density functional theory to investigate the complexation reactions of UO₂²⁺ with 29 different monovalent anions (L^-1), exploring both mono- and bidentate coordination. We proposed a novel concept called "uranyl affinity" (E_ua) to facilitate the establishment of a standardized scale for assessing the ease or difficulty of coordination bond formation between UO₂²⁺ and diverse ligands. Furthermore, we conducted an in-depth investigation into the underlying mechanisms involved. During the process of uranyl complex [(UO₂L)⁺] formation, lone pair electrons from the coordinating atom in L^- are transferred to either the lowest unoccupied molecular degenerate orbitals 1ϕ_u or 1δ_u of the uranyl ion, which originate from the uranium atom's 5f unoccupied orbitals. In light of discussion concerning the mechanisms of coordination bond formation, quantitative structure-property relationship analyses were conducted to investigate the correlation between E_ua and various structural descriptors associated with the 29 ligands under investigation. This analysis revealed distinct patterns in E_ua values while identifying key influencing factors among the different ligands.