Theoretical and experimental investigation of structural and vibrational properties of L-arginine·HClxBr1-x monohydrate crystals

Abstract In this work, five L-arginine·HClxBr1-x·H2O single crystals were grown through the slow evaporation method from a mixture of two aqueous solutions of L-arginine hydrochloride monohydrate and L-arginine hydrobromide monohydrate in proportions 0:1, 1:3, 1:1, 3:1, and 1:0, respectively. The crystals were grown by slow evaporation method from aqueous solutions at room temperature (25 °C). The grown single crystals were investigated by X-ray fluorescence (XRF) which presented: C6H14N4O2·HCl·H2O, C6H14N4O2·HCl0.14Br0.86·H2O, C6H14N4O2·HCl0.42Br0.58·H2O, C6H14N4O2·HCl0.63Br0.37·H2O and C6H14N4O2·HBr·H2O. X-ray diffraction (XRD), in association with Rietveld’s refinement, showed that all samples crystallized in the monoclinic system, with P21-space group. XRD experiments also evidenced that the unit cell volume of the LAHClxBr1-x crystals decreased as increasing Cl concentration. In addition, structure modeling and vibrational spectra calculations of the LAHClxBr1-x (x = 0.0, 0.5 and 1.0) crystals were performed at DFT/LDA level to be correlated with the experimental data. Raman spectroscopy experiments evidenced that bands associated with lattice modes undergo a blueshift as the Cl concentration was increased. This behavior can be explained by decreasing Cl mass as well as contraction of the unit cell towards LAHCl crystal, as observed by XRD data.