Estudo térmico, vibracional e cálculos teóricos em cristais de l-tirosina e haletos em altas temperaturas e altas pressões

Abstract

This thesis proposed to synthesize and characterize in a structural, thermal and vibrational way crystals of L-tyrosine and its halides and to make a comparison between them in order to understand. The crystals synthesized by the slow solvent evaporation technique had their experimental structural parameters confirmed by X-ray diffraction and show good agreement with those in the literature. DFT calculations show good accuracy in assigning vibrational modes. The study of high temperatures by Raman spectroscopy at low wavenumber in LT, LTHCl and LTHBr showed a connection between hydrogen bonds, phonon dynamics, thermal decomposition where the behavior of vibrational modes associated with torsion of the LT molecule is consistent with a weakening of these connections favoring the breaking of hydrogen bonds in the decomposition process. The high pressure Raman spectroscopy study on the LTHBr was carried out up to 8.1 GPa. Emergence of vibrational modes and high flexibility of L-tyrosine molecules are observed in the region of lattice modes. Furthermore, the change in slope (dɷ/dP) of these bands between pressures of 3.0 and 4.0 GPa reinforce the occurrence of a structural phase transition. The spectra in the region of internal modes show changes that reinforce the conformational phase transition. The stability of the crystal structure was investigated by high pressure Raman spectroscopy at LT up to 5.9 GPa and the changes in the spectra give evidence that the crystal underwent two phase transitions, the first at 2.1 GPa and the second at 3, 6 GPa. In both phase transitions, mode changes were observed in the low wavenumber region of the spectrum, in addition to other small changes in some internal modes. All materials showed reversible structural and conformational changes due to the similarity between the spectra before and after compression. There are strong indications that the presence or absence of halides influence the structural, thermal, and vibrational behavior of L-tyrosine according to changes observed at room temperature and when subjected to high pressures and high temperatures.