Synthesis and electronic structure studies of a novel nonlinear optical crystal L-leucinium squarate monohydrate: A spectroscopic view

Nonlinear optical active L-leucinium squarate monohydrate single crystals were grown using the slow evaporation technique. The crystal structure was determined using the single crystal X-ray diffraction technique. FT-IR and FT-Raman spectra were recorded and analyzed. Meanwhile, the thermal characteristics of the sample were analyzed using TGA-DTA techniques. In addition, the equilibrium geometry, vibrational wavenumbers and first-order hyperpolarizability were calculated with the aid of the density functional theoretical method. The red-shift of NH3+ stretching wavenumbers indicates the formation of inter- and intra-molecular hydrogen bonding in the molecule. Similarly, the blue-shift in the C-H stretching wavenumbers is due to the presence of C-H⋯O hydrogen bonding. The natural bond orbital analysis confirms the occurrence of a strong intermolecular hydrogen bonding that is responsible for the stabilization of the molecule. Molecular electrostatic potential maps help to predict the electrophilicity and nucleophilicity groups in the molecule. The HOMO-LUMO energy gap value suggests the possibility of charge transfer within the molecule. The SHG conversion efficiency was investigated using the Kurtz and Perry method and was found to be higher than that of urea. The intercontacts in the crystal structure were analyzed using the Hirshfeld surfaces analysis method. The thermodynamic properties at different temperatures were also calculated.