Fig xA Lattice parameters of stearic acid in the C

The analysis of the Raman bands in the low-wavenumber region is interesting mainly because they are associated with the lattice vibrations. The behavior of these Raman bands can give important information related to the structural and/or symmetry changes in the unit cell [41], [46], [47] and [48]. In reality, the analysis of the modifications of certain modes may indicate that molecular changes occur in the crystal. Such molecular changes can be understood in two different ways: (i) phase transition undergone by the crystal and (ii) rotation or twisting of the molecule in the unit cell (conformational phase transition originated by the rearrangement of the hydrogen bonds between the COOH groups of the dimers). In the first case, the occurrence of structural phase transition, it NU 7026 is necessary to observe the change of the modes associated with both, the internal modes and the lattice modes, as verified in several organic crystals studied either with temperature [27], [49] and [50] or pressure [15] and [22] changes. On the flip side, as in a study performed on the taurine crystal, some changes in the internal modes can be interpreted as a rotation and twisting of zwitterions in the crystal structure [34]. At this point, we also remember a study performed on the stearic acid C form crystals under high pressure, has revealed several modifications in the Raman spectrum. These modifications, such as discontinuities in the lattice modes and the appearance, as well as the disappearance of bands, suggested that the crystal has undergone three phase transitions [22]. Furthermore, a previous investigation on the stearic acid by Raman and infrared vibrational techniques with temperature variations between 300 and 90 K was performed and no evidence of the occurrence of phase transition was mentioned [23].