Revealing multiple sources of a uranyl Raman fingerprint peak in nitric acid solution through first-principles calculations

Separation of uranium in nitric acid solution is a key issue in the solvent extraction of spent fuel. However, the structures of uranyl complexes in aqueous conditions containing nitric acid remain unclear. The Raman peak generated by uranyl at 870 cm-1 is considered an important internal standard for detecting uranyl complexes, which can be used for identifying the form of uranium in solution. In this work, first-principles calculations reveal multiple origins for this vibrational feature, attributing it to simultaneous contributions from [UO22+ · 5H2O], [UO2NO3+ · 3H2O], and [UO2NO3+ · 4H2O]. Crucially, all three species share pentacoordinate uranyl centers, with interaction mechanism analysis confirming bidentate nitrate coordination in the dominant [UO2NO3+ · 3H2O] configuration. Moreover, the quantitative agreement between computed spectra and experimental benchmarks validates our solvation-corrected theoretical model and method. These findings provide atomic-level insight into the structure of uranyl complexes in nitric acid solution, establishing a predictive framework for interpreting vibrational spectroscopic signatures in complex nuclear waste streams.