High-pressures study by Raman spectroscopy and DFT calculations of L-tyrosine hydrobromide crystal.

The high-pressure Raman spectra of L-tyrosine hydrobromide crystal (LTHBr) were obtained from 1.0 atm to 8.1 GPa in the 100-3200 cm-1 spectral region. The structural conformation and dimensions of the monoclinic unit cell were estimated using the powder X-ray diffraction (PXRD) method and Rietveld refinement using the GSAS program. At atmospheric pressure, the Raman spectrum was obtained in the spectral range of 100-3200 cm-1 and the assignment of the normal modes based on density functional theory calculations was provided. Large wavenumber shifts of modes at 106, 123, and 157 were observed, which were interpreted as the large displacement of the atoms, making the molecule a flexible structure. The change in the slope (dÉ· / dP) of these bands between the pressures of 3.0 and 4.0 GPa and the appearance of a mode of low wavenumber indicate the occurrence of a structural phase transition. A band initially observed at 181 cm-1 in the spectrum recorded at 0.7 GPa change the relative intensity with a band at 280 cm-1 (recorded at 5.8 GPa), indicating a conformational transition. In the region of the internal modes, the spectra show changes that reinforce the conformational phase transition since the bands initially at 1247 and 1264 cm-1 observed at 1.0 GPa have their intensities reversed, and at 3.0 GPa it is observed the fusion of the bands at 1264 and 1290 cm-1 (values recorded at ambient pressure). Thus, we can assume that the LTHBr crystal has undergone a structural phase transition and a conformational phase transition in the pressure range investigated.

High pressure Raman spectra of monoglycine nitrate single crystal.

Single crystal of monoglycine nitrate has been studied by Raman spectroscopy under high pressures up to 5.5 GPa. The results show changes in lattice modes in the pressure ranges of 1.1-1.6 GPa and 4.0-4.6 GPa. The first change occurs with appearance of bands related to the lattice modes as well as discontinuity in the slope of dΩ/dP of these modes. Moreover, bands associated with the skeleton of glycine suggest that the molecule undergoes conformational modifications. The appearance of a strong band at 55 cm(-1) point to a second phase transition associated with the lattice modes, while the internal modes remain unchanged. These anomalies are probably due to rearrangement of hydrogen bonds. Additionally, decompression to ambient pressure shows that the phase transitions are reversible. Finally, the results show that the nitrate anions play an important role on the stability of the monoglycine nitrate crystal.

Raman spectra of deuteriated taurine single crystals.

The polarized Raman spectra of partially deuteriated taurine [(ND3+)0.65(NH3+)0.35(CH2)2SO3-] crystals from x(zz)x and x(zy)x scattering geometries of the Ag and Bg irreducible representations of the factor group C2h are reported. The temperature-dependent Raman spectra of partially deuteriated taurine do not reveal any evidence of the structural phase transition undergone by normal taurine at about 250 K, but an anomaly observed in the 180 cm-1 band at approximately 120 K implies a different dynamic for this band (which is involved in a pressure-induced phase transition) in the deuteriated crystal.