Propriedades vibracionais da base de SCHIFF C20H21N3O2 e do derivado do ácido de Meldrum C12H11ClN2O4 por espectroscopias FT-RAMAN, FT-IR e cálculos de primeiros princípios

Abstract

In this work the vibrational properties of two organic substances were studied: Schiff base (4E)-4-(4-ethoxybenzylideneamino)-1,2-dihydro-2,3-dimethyl-1-phenylpyrazole-5-one of molecular formula C20H21N3O2 and the derivative of Meldrum’s acid 5-((5-chloropyridin-2ylamino)methylene)-2,2-dimethyl-1,3-dioxane-4,6-dione of molecular formula C12H11ClN2O4, (CYMM). Raman scattering measurements were performed at room temperature in the spectral region between 40 and 3500 cm-1 on the Schiff base C20H21N3O2 and in the region of 0 to 3375 cm-1 in the derivative of Meldrum’s acid CYMM and experiments of infrared spectroscopy in the intervals of 600 a 3500 cm-1 and 400 to 3400 cm-1, respectively. The vibrational modes and the wave numbers of the isolated molecule of the Schiff base C20H21N3O2 and of the derivative of Meldrum’s acid C12H11ClN2O4 were computed, both by density functional theory (DFT) calculations, implemented by the exchange and correlation function B3LYP and the basic function 6-31 G(d, p) with the aid of the Gaussian 09 program. The computational calculations reproduced the characteristics of the material in good agreement with the experimental spectrum. Based on this agreement, it was possible to associate the observed wave numbers with the atomic displacements in the molecules. In addition, (DPFT) calculations were performed on the crystal structure of CYMM, using the PWSCF and PHONON codes of the Quantum-Espresso software package, which allowed to obtain supplementary information about the network modes that are not observed in the calculations performed on an isolated molecule. Also for the two samples studied were realized potential energy distribution calculations PED, which made it possible to classify the normal modes of vibration with greater precision. In addition, Raman scattering experiments were performed as a function of temperature between 40 and 285 K in the Schiff base C20H21N3O2. No discontinuities or changes were observed in the measurements of wave numbers versus temperature, indicating that the monoclinic phase of the ambient temperature P21 / n (C2h5) is stable in this temperature range. A comparison of the behavior of the vibrational modes of the Meldrum acid derivative with those of other relevant organic substances is provided. In particular, a comparison is made between the modes associated with C=O stretching in CYMM and in three simple structures with carbon rings, in this case the amino acids L-phenylalanine, Ltyrosine and L-tryptophan. From this analysis, it was inferred that the band associated with oxygen binding with ring carbon provides sufficient intensity to be used as a probe in experiments with temperature and pressure variation.