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 (g_avg = 2.0035) is shown to arise from the structural relaxation of the HC₂O₄∙ radical into the intrinsically stable non-planar (D_2d) 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 D_2d form. Quantitative analysis proved a sharp, thermodynamically driven structural conversion (D_2d→D_2h) upon annealing above 220 K, where the D₂h conformer (g_avg ≈ 2.011) becomes the dominant species (≈73%). In K₂C₂O₄⋅H₂O, the C₂O₄^●− radical undergoes rapid decomposition into the CO₂^●− radical (g_avg ≈ 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.