Synthesis and Vibrational Identification of the Uranyl Peroxodicarbonato [UO2(O2)(CO3)2]4-: Insights into Peroxide-Carbonate Bonding from 18O Labeling, DFT and QTAIM analysis

In carbonate-rich aqueous environments relevant to spent nuclear fuel storage, radiolytic H₂O₂ and carbonate species promote the formation of uranyl peroxo-carbonate complexes under mild conditions. These complexes are poorly characterized spectroscopically, with many assignments remaining controversial. This work presents the first solution-phase identification of the peroxo-dicarbonato species Mx[UO₂(O₂)(CO₃)₂](4-x)- (M = Na⁺ or K⁺) achieved through Raman and ¹⁸O-labeled H₂O₂ Fourier-transform infrared (FTIR) spectroscopy, combined with density functional theory (DFT) calculations. Key vibrational signatures distinguish the peroxo-dicarbonato complex from those of the tricarbonato complex [UO₂(CO₃)₃]⁴⁻, including the νs(U-Oyl) stretch at 765-769 cm⁻¹, the ν₁(U-Operoxo) mode at 418-440 cm⁻¹, and the ν(O-O) stretch at 845-850 cm⁻¹, which shifts upon ¹⁸O substitution. FTIR data further confirm these assignments, providing the first infrared evidence of a uranyl peroxo-carbonato complex in aqueous solution. DFT calculations validate the observed frequencies and isotope shifts, clarifying the role of solvation and counter-cation effects. These results clarify uranyl peroxo-carbonato speciation, resolving long-standing ambiguities and establishing a robust spectroscopic framework for applications in uranium fuel dissolution, environmental mobility, and long-term waste stability.