Investigação teórica e experimental das propriedades estruturais, eletrônicas e térmicas do cristal de malonato de L-fenilalanina L-fenilalaninium

Abstract

In this work, the structural, electronic and vibrational properties of the crystalline environment of the L-phenylalanine L-phenylalaninium malonate salt were investigated. The synthesis of the compound occurred by the method of slow evaporation of the solvent with a 2: 1 ratio; the pH value measured was 2.61 under ambient conditions of temperature and pressure. The structural characterization by powder X-ray diffraction, combined with the Rietveld refinement method, confirmed that the material crystallized in a monoclinic system with space group P21, containing two molecules per unit cell Z = 2 and with the following network parameters : a = 14.08 Å, b = 5.53 Å, c = 14.65 Å and angle β = 107.39 º. The volume of the unit cell obtained after refinement was 1089.5 Å3 . The structural parameters, lengths and bond angles, analyzed via DFT were described satisfactorily with the LDA exchange and correlation functional. The vibrational properties were analyzed using the density functional theory (DFT) and FTIR and Raman spectroscopy techniques. In addition, to assign the vibrational bands we use the computational approach of calculations with the quantum_ESPRESSO package. Characterizations by thermal analysis Thermogravimetry (TGA), Derivative thermogravimetry (DTG), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) indicate decomposition of the molecules from 120 ° C and a large endothermic peak around 185 ° C regarding the decomposition of the material, and suggest that the material has five stages of decomposition. The main objective of this work is to investigate the structural and vibrational properties via DFT with those based on the functional B3LYP, B3LYP / PCM and LDA using the quantum-ESPRESSO package. The vibrational study at high temperatures, indicated that the material does not present a phase transition until the melting point, showing only small modifications for smaller wave numbers and a decrease in the relative intensity, showing the decomposition of the NH3 and CO2 groups.