Synthesis and Vibrational Identification of the Uranyl Peroxodicarbonate [UO2(O2)(CO3)3]4-: Insights into Peroxo-Carbonate Bonding from 18O Labeling, DFT and QTAIM analysis

In carbonate-rich aqueous environments relevant to spent nuclear fuel storage, the coexistence of carbonate species and radiolytically generated H₂O₂ is expected to promote the formation of uranyl peroxo-carbonate complexes under mild conditions. Despite their importance, reliable vibrational markers for uranyl peroxo-carbonate complexes remain scarce, and several reported spectral assignments remain controversial. Here, we report the first unambiguous solution-phase characterization of the peroxo-dicarbonate species Mx[UO₂(O₂)(CO₃)₂](4-x)- (M = Na⁺ or K⁺) by combining Raman spectroscopy with ¹⁸O-labeled H₂O₂, Fourier-transform infrared (FTIR) spectroscopy, and density functional theory (DFT) calculations. Distinct vibrational fingerprints are identified for Mx[UO₂(O₂)(CO₃)₂](4-x) and are clearly differentiated from those of the triscarbonate complex [UO₂(CO₃)₃]⁴⁻. Key Raman bands of the peroxo-dicarbonate complex include the νs(U-Oyl) stretch at 765-769 cm⁻¹, the ν₁(U-Operoxo) mode at 418-440 cm⁻¹, and the ν₁(O-O) stretch at 845-850 cm⁻¹, the latter exhibiting a pronounced isotopic shift upon ¹⁸O substitution. Complementary FTIR measurements reveal weak absorptions near 425, 765 and 865 cm⁻¹ attributed to ν₁(U-Operoxo), νs(U-Oyl) and ν₁(O-O), respectively, providing the first infrared evidence of a uranyl peroxo-carbonate complex in aqueous solution. DFT calculations reproduce the observed frequencies and isotope shifts, clarifying the role of solvation and counter-cation effects. These results resolve long-standing ambiguities in uranyl peroxo-carbonate speciation and establish a robust spectroscopic framework relevant to uranium fuel dissolution, environmental mobility, and long-term waste stability.