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 M x [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 v1(U-O peroxo) mode at 418-440 cm−1, and the v1(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 v1(U-Operoxo), νs(U-Oyl) and v1(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.