DFT-Computation-Assisted EPR Study on Oxalate Anion-Radicals, Generated in γ-Irradiated Polycrystallites of H2C2O4·2H2O, Cs2C2O4, and K2C2O4·H2O

This report focuses on the oxalate anion radical (C2O4●−) formed during γ-radiolysis of polycrystalline oxalates: protonated oxalic acid (H2C2O4⋅2H2O), caesium oxalate (Cs2C2O4), and potassium oxalate monohydrate (K2C2O4⋅H2O). Irradiation at 77 K generates stable radical species, revealed by EPR spectroscopy and supported by DFT calculations. In H2C2O4⋅2H2O, the primary axial signal (gavg = 2.0035) is shown to arise from the structural relaxation of the HC2O4∙ radical into the intrinsically stable non-planar (D2d) conformation, resolving the symmetry conflict with the planar crystal precursor. Numerical deconvolution confirmed the co-existence of this radical with the secondary HCO2● species, exhibiting distinct relaxation characteristics. In Cs2C2O4, the broad isotropic signal (g ≈ 2.008) is attributed to the D2d form. Quantitative analysis proved a sharp, thermodynamically driven structural conversion (D2d→D2h) upon annealing above 220 K, where the D2h conformer (gavg ≈ 2.011) becomes the dominant species (≈73%). In K2C2O4⋅H2O, the C2O4●− radical undergoes rapid decomposition into the CO2●− radical (gavg ≈ 2.0007) due to the kinetic instability of the primary species in that matrix. Our findings underscore the crucial role of computational assistance and quantitative numerical fitting in EPR studies: DFT provided crucial assistance and yielded satisfactory agreement in most cases, while clarifying the structural and kinetic stability governed by the local cationic environment. The stability of the most resistant radical forms persists up to 430 K in the caesium salt.