| Summary: | Due to the enormous threat of protactinium to the environment and human health, its disposal and chemistry have long been important topics in nuclear science. [PaO(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> is proposed as the predominant species in hydrous and acidic solutions, but little is known about its formation mechanism. In this study, density functional theory (DFT) calculations demonstrate a water coordination-proton transfer-water dissociation mechanism for the formation of PaO<sup>3+</sup> in hydrous solutions. First, Pa(V) ion preferentially forms hydrated complexes with a coordination number of 10. Through hydrogen bonding, water molecules in the second coordination sphere easily capture two protons on the same coordinated H<sub>2</sub>O ligand to form [PaO(H<sub>2</sub>O)<sub>9</sub>]<sup>3+</sup>. Water dissociation then occurs to generate the final [PaO(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>, which is the thermodynamic product of Pa(V) in hydrous solutions.
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