Design, Synthesis and Vibrational Spectroscopic Studies of the Nonlinear Optical Crystal l-histidine Potassium Chloride

L-Histidine Potassium Chloride (LHPC) crystals were grown by slow evaporation technique. The grown crystals were characterized by powder X-ray diffraction and analyzed by FT-IR and FT-Raman techniques. The geometry optimization, harmonic vibrational wavenumbers, natural bond orbital and thermodynamical analysis have been carried out using density functional theory (DFT) method at B3LYP/6-311++G(d,p) basis set. Optimized geometry values show a small deviation between the experimental and theoretical values, which reveals the inter- and intra-molecular hydrogen bonding in the crystal. Normal coordinate analysis has been performed on the basis of potential energy distribution analysis using MOLVIB program. The potential energy distribution analysis is useful to predict the vibrational modes, unambiguously. The natural bond orbital analysis has been performed in order to study the intramolecular bonding interactions among bonds and delocalization of unpaired electrons. HOMO and LUMO energies, molecular electrostatic potential and the first-order hyperpolarizability have been also studied. The scaled wavenumbers are in good agreement with the experimental wavenumbers. The first-order hyperpolarizability (β) of the title compound is found to be 9.50x10-30 e.s.u. The large hyperpolarizability and lowering of HOMO-LUMO energy gap supported the nonlinearility of the LHPC crystal. On the basis of vibrational analysis the thermodynamic properties of LHPC at different temperatures have been calculated, revealing the correlations between heat capacity, enthalpy and entropy.