Espectroscopia vibracional do Ala-Ala policristalino em função da temperatura

Abstract

Infrared and Raman spectra of polycrystalline L-alanyl-L-alanine (Ala-Ala) were studied as a function of temperature. For the infrared measurements the temperature varied between 83 K and 293 K in regular intervals of 10 K. No qualitative changes were observed in the infrared spectra between these temperatures, indicating that the crystalline structure of Ala-Ala is stable for 83 K. Identifications were made for several of the normal modes of vibration due to the molecular groups that constitute the molecule of Ala-Ala and that was compared with the literature. For Raman scattering measurements the temperature varied from 20 K to 300 K in regular intervals of 20 K. Similarly to the infrared measurements, no qualitative changes were observed in the Raman spectra between 80 K and 300 K. Only quantitative changes in the normal modes of vibration of the polycrystalline Ala-Ala inherent to the temperature variation of 220 K were observed. For temperatures lower than 80 K, gradual qualitative changes were also observed in the Raman spectra of polycrystalline Ala-Ala, evidence of a second order structural phase transition. From the group theory and Raman spectra analysis, the Ala-Ala crystal must go from a tetragonal structure with factor group C4 with four molecules in the primitive cell to a monoclinic structure with factor group C2, with molecular ions occupying local symmetry sites C1 and maintaining four molecules in the primitive cell. A detailed analysis was made of the behavior of the frequencies of the normal mode vibrations active in Raman with temperatures in the interval between 20 K and 300 K. The frequencies of most of the studied modes present a linear behavior with temperature which is normal for any type of crystal when submitted to temperature variations of the order of 300 K. From the analysis of the graphs of the dependence of ωj with T, it is observed that the Ala-Ala crystal exhibits a structural phase transition for temperatures between 80 K and 60 K. The identification of the normal mode vibrations of Ala-Ala for temperatures of 300 K, 80 K and 20K indicates that the mechanism for the structural phase transition is governed by the occupation of the non-equivalent C1 local symmetry sites by the CH3 molecular ions.