Structural, mechanical and third-order non-linear optical properties of imidazolium hydrogen oxalate (IMHO) single crystals: insights from a theoretical and experimental approach

Imidazolium hydrogen oxalate (IMHO) single crystals were produced using the solvent evaporation method. Single crystal X-ray diffraction confirmed a monoclinic structure with space group P2₁/n and unit cell parameters: a = 5.83 Å, b = 17.92 Å, c = 6.85 Å, α = γ = 90°and β = 104.01°. A theoretical calculation using the DFT/B3LYP/6-31G(d) method yielded the optimized geometrical parameters, which were later compared to experimental values. Natural bond orbital (NBO) analysis is used to figure out charge delocalisation and hydrogen bonding interaction. The FT-IR analysis is used to identify functional groups. The UV–visible analysis showed 75% transmittance and an optical band gap of 4.77 eV. The emission behaviour was analysed by fluorescence spectral analysis. HOMO–LUMO analysis yielded an energy gap of 4.85 eV, supporting high chemical stability. Hirshfeld surface analysis clarifies the interactions seen in the created IMHO crystal. The mechanical characteristics is studied using Vickers microhardness analysis. Z-scan analysis confirmed third-order nonlinear optical activity with a nonlinear refractive index of 1.6 × 10⁻¹³ m²/W, absorption coefficient of 3.85 × 10⁻⁸ m/W, and third-order susceptibility χ⁽³⁾ of 5.765 × 10⁻¹⁰ esu. These results establish IMHO as a promising candidate for advanced NLO applications.