Estudo de espalhamento Raman em cristais de L-leucina submetidos a altas temperaturas e a altas pressões

Abstract

In this work we performed polarized Raman scattering and infrared absortion measurements in L-leucine amino acid crystal at room temperature with the objective to determine the vibrational normal modes of the material. For this purpose, it was utilized the classification of vibrational modes available for three other aliphatic protein amino acids, L-alanine, L-valine and L-isoleucine. In the second part of the work we have studied the behavior of the vibrational modes of L-leucine crystal in the temperature range 290 – 413 K. At room temperature the L-leucine crystallizes in a monoclinic structure C2 2 with four molecules per unit cell. The analysis of the behavior of normal modes in the same temperature range has revealed changes in the relative intensity of bands related to internal modes, like those observed around 800 and 920 cm-1, giving indications of a structural phase transition undergone by L-leucine crystal at 353 K. The structural transition has been confirmed by the observation of vanishing of a lattice modes at 110 cm-1 in the same temperature value. As a suggestion to explain the structural change one argues that during the phase transition the point group of the crystal changes from C2 to Cs, a fact that would allow the vanishing of LO modes below the transition temperature. Finally, in the third part of this Thesis, studies of Raman scattering of L-leucine crystal as function of pressure are presented. It was found that the L-leucine crystal exhibits two structural phase transition between 0 and 4.0 GPa. The first of them involves CH and CH3 units, as well as parts of the L-leucine molecule related to hydrogen bonds. The results also suggest that the first transition 0-0.46 GPa changes the Lleucine crystal to a very different structure from the original, since a new band rises in a region where usually are observed only shifts of frequencies. In the second phase transition, between 0.80 and 1.46 GPa, it was observed (i) the vanishing of lattice modes, (ii) the rising of various internal modes and (iii) the shifting of high energy modes. Again, it is supposed that the units CH and CH3 are involved as well as some hydrogen bonds. The change of slope dω / dP for some peaks to pressures above 4 GPa can indicate the occurrence of a third phase transistion for L-leucine crystal.