SÍNTESE E ESTUDO TEÓRICO-EXPERIMENTAL DAS PROPRIEDADES ESTRUTURAIS E VIBRACIONAIS DO COCRISTAL DE L-VALINA E ÁCIDO FUMÁRICO

Abstract

Molecular solids, as multicomponent crystals, have received proper attention in the last decades, mainly, by their given potential to develop a new class of materials or improve a myriad of other properties of those already know. In this present work, a cocrystal of l-valine and fumaric acid was obtained in 2:1 stoichiometry by a slow evaporation method, and their vibrational properties are analyzed by experimental and theory, in terms of Density Functional Theory (DFT-B3LYP). X-ray diffraction revealed that LVAF crystal, at this ratio, crystallizes in a monoclinic system of C2 space group, with eight molecules on the unit cell. The structural parameters are: a = 22,224(2) Å, b = 7.268(5) Å e c = 5.344(5) Å; β = 91.78(4) ° and unit cell volume of 862.921(1) Å3, confirmed by powder x-ray diffraction and Rietveld’s refinement. The calculated optimized geometry shown a very close similarity to the experimental reported data and here recorded, acquired using free license software from Max Planck Institute ORCA, version 4.2.2. The simulated bond lengths and angles compared with experimental, has shown approximately 16% difference, mostly on groups associated with intermolecular bonds. Theoretical Raman bands are very close to the experimental one, with agreement to the structural geometry, where wavenumbers shift occurs on hydrogen-bonds groups. Room temperature Raman spectra are remarkable by zwitterion bands of L-valine molecule and different behavior on intensity relations of fumaric acid and valine modes. TG-DTA and DSC analysis, confirmed the fusion of the compound around 230 °C, followed by decomposition. The thermal analysis X-ray Diffraction and DSC curves suggested that crystals go to a phase transition (may conformational) before fusion point at 448 K (198 °C).