Polarized Raman and Infrared Spectroscopy and ab Initio Calculation of Palmitic and Stearic Acids in the Bm and C Forms.

A complete experimental study on the vibrational properties of palmitic and stearic acids crystallized in the Bm and C forms, both belonging to the monoclinic system with the P21/a (C2h5) space group, through polarized Raman and infrared spectroscopy, is reported in this paper. Density functional theory calculations were also performed to assign the normal modes and to help in the interpretation of the experimental data. The different polarizations were compared and their influence on the spectral profiles, in both the lattice and the internal mode regions, was discussed. In general, the Raman and infrared spectra exhibit accentuated differences among the polymorphic forms, which are associated with the different molecular modifications, defined as gauche and all-trans conformations. Insights about interaction among different groups are also furnished.

The temperature-dependent single-crystal Raman spectroscopy of a model dipeptide: L-Alanyl-L-alanine.

A single-crystal of peptide L-alanyl-L-alanine (C6H12N2O3) was studied by Raman spectroscopy at low-temperature, and a tentative assignment of the normal modes was given. Evidence of a second order structural phase transition was found through Raman spectroscopy between the temperatures of 80K and 60K. Group theory considerations suggest that the transition leads the sample from the tetragonal to a monoclinic structure. Additionally, our study suggests that the mechanism for the structural phase transition is governed by the occupation of non-equivalent C1 local symmetry sites by the CH3 molecular groups. Analysis based on group theory suggests L-alanyl-L-alanine presents C2 symmetry at low temperatures.

Structural and vibrational properties of K(3)Fe(MoO(4))(2)(Mo(2)O(7))-a novel layered molybdate.

The new compound K(3)Fe(MoO(4))(2)(Mo(2)O(7)) was synthesized and characterized by a single-crystal x-ray structure determination, and IR and Raman spectroscopic studies. The crystal structure at room temperature and ambient pressure is monoclinic, space group C 2/c, with the unit cell dimensions a = 32.885(7), b = 5.7220(11), c = 15.852(3) Å, ß = 91.11°, Z = 8. The FeO(6) octahedra are joined by corners with MoO(4)(2-) tetrahedra and Mo(2)O(7)(2-) units. Some of the K(+) ions form layers in the b × c-plane. The origin of various Raman and IR vibrational modes is discussed. These results indicate that a clear energy gap exists between the stretching and remaining modes. High-pressure Raman scattering studies were also performed. These studies showed the onset of two reversible first-order phase transitions near 1.2 and 7.4 GPa, which are associated with strong distortion of the MoO(4)(2-) and Mo(2)O(7)(2-) units.

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.