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The chemical composition, vibrational spectroscopy and DFT studies of the essential oils from Lippia sidoides constituents were performed in this study. The harvesting time periods greatly affect the yields of the essential oil extracted from the leaves of the Lippia sidoides plant material. The chemical composition of the essential oil by gas chromatography coupled to mass spectrometry (GC MS) showed thymol as the main constituent of the essential oil. Through Fourier transform Raman (FT-Raman) and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy measurements exhibited a good agreement of the thymol component from the essential oil and the polycrystalline thymol. The Density Functional Theory (DFT) calculations were performed over the molecular structure of the thymol and predicted its Raman and infrared spectra. A good correlation between the experimental spectra of the essential oil and the thymol was observed. Complete assignments of the normal modes were presented for both Raman and infrared spectra of the thymol.

In this study, two 4′-aminochalcones with structural similarity were structurally characterized using the NMR technique of 1H and 13C. Polycrystalline samples of these chalcones were analyzed by FT-Raman and ATR-FTIR spectroscopy at room temperature in the regions 40-4000 cm-1 and 400-4000 cm-1, respectively. The DFT calculations using the B3LYP functional with 6-311 G(d,p) basis set were performed, revealing the optimized molecular geometry and the vibrational wavenumbers for these chalcones. Conformational analysis was done from the optimized structures to obtain different conformers. The calculated harmonic vibrational wavenumbers were scaled to improve the agreement between theoretical and experimental wavenumbers. A complete vibrational assignment using Potential Energy Distributions (PED) was provided for the observed Raman and infrared spectra. The scaled wavenumbers were found to be in good agreement with experimental wavenumbers. Furthermore, a comparative structural and vibrational analysis was performed. The structural analysis allowed us concluding that in addition to differences in the position 4′ – where one chalcone has a methoxy group and the other, an ethoxy group – the molecular conformation of them is significantly different, especially regarding the torsion angles linking the enone chains with the rings of these chalcones. The vibrational analysis allowed us to identify all Raman and infrared bands of the two chalcones, highlighting the main differences and correspondences between them.