Crystal structure, vibrational spectra and theoretical studies of l-histidinium dihydrogen phosphate-phosphoric acid

In this work, we report a combined experimental and theoretical study on molecular structure and vibrational spectra of l-histidinium dihydrogen phosphate-phosphoric acid, with particular emphasize on the correlation between the intermolecular hydrogen bonds and the hyperpolarizability. Single crystal of l-histidinium dihydrogen phosphate-phosphoric acid has been subjected to X-ray diffraction and Raman spectroscopy. The title compound crystallises in the non-centrosymmetric space group P21. Raman spectra have been recorded in the frequency range [150–3350 cm−1]. To obtain a more reliable assignment of the Raman and IR spectra, we have calculated the geometry and the frequencies using HF and DFT methods. All the experimental vibrational bands have been discussed and assigned to normal mode or to combinations and overtones on the basis of our calculations. The optimized geometric bond lengths and bond angles obtained by using HF and DFT (B3LYP and BLYP) are in well agreement with the experimental data. The results of DFT-B3LYP method have shown better fit to experimental ones than HF in evaluating vibrational frequencies. To investigate microscopic second order non-linear optical behaviour of the examined complex, the electric dipole μ, the polarizability α and the hyperpolarizability β were computed using DFT//B3LYP/6-31 G(d) method. According to our calculation, the title compound exhibits non-zero β value revealing microscopic second order NLO behaviour.