Quantitative structure–spectrum relationship in uranyl complexes: Density functional theory and Raman insights into sodium ions-modulated coordination evolution

Uranium extraction from seawater is a promising strategy to address terrestrial uranium depletion. However, the complex marine environment induces diverse uranium speciation and coordination structures, creating substantial challenges for uranium detection and extraction. Herein, we establish the quantitative structure-spectrum relationship through correlating the uranyl-oxygen bond lengths and symmetric stretching frequencies in typical uranyl complexes by employing density functional theory calculations. Moreover, the structural evolution of uranyl in sodium carbonate solutions under acidic to weakly alkaline conditions is investigated in combination with Raman spectroscopy experiments, demonstrating a linear correlation between the uranyl-oxygen bond lengths and vibrational frequencies in sodium-containing complexes, which verifies and extends the applicability of Badger's rule in uranyl systems. Atomic-level analyses further reveal that sodium ions modulate ligand charge distributions via strong electrostatic interaction, resulting in weakened uranium-oxygen bonds and enhanced structural stability of the uranyl complexes. These findings deepen the understanding of the structure-spectrum relationship in complex solutions and provide insights for uranium resource exploration in seawater.