Structure, vibrational characterization and SHG of the recrystallization product of L-phenylalanine from aqueous HCl solution

Direct recrystallization of l-phenylalanine from aqueous solution of HCl yields good quality single crystals of l-phenylalanine - l-phenylalaninium chloride (LPLP+Cl-). The compound crystallizes in the non-centrosymmetric space group P2(1) of the monoclinic system with two molecules in the asymmetric unit. This compound belongs to the ionic co-crystals. Both l-phenylalanine-l-phenylalaninium chloride molecules in the asymmetric unit are bound by a strong symmetric O-H center dot center dot center dot O hydrogen bond with an O center dot center dot center dot O distance of 2.43 angstrom. Such cationic dimers of (L-phenylalanine - l-phenylalanine)+ interact with chloride anions forming two dimensional layers parallel to (001) plane. Hirshfeld surface analysis and the 2D fingerprint plot were used to visualize the many types of intermolecular interactions that govern the formation of supramolecular architecture. The compound was also characterized by vibrational spectroscopy. Molecular structure, vibration frequencies and vibration band intensities were interpreted by structure optimization and normal coordinate force field calculations based on density functional theory (DFT). Second harmonic generation (SHG) efficiency of the crystals LPLP+Cl- is about 50 % of the standard KDP.