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The well-known food dye E102 could be detected in aqueous solutions by means of regular Raman and surface-enhanced Raman scattering (SERS) spectroscopy at micromolar and nanomolar levels, respectively. The changes observed in the profile of the band at 1365 cm −1 , characteristic of the n(-N N-) mode of the azo chromophore group, allowed us to establish the present species in the Raman and SERS solutions at different concentrations or pH values. Protonation at the azo group of the molecule was detected for acidic pH values <3. In the pH range 3-8, no changes in the molecular species were observed. Density functional theory (DFT)-calculated geometries, harmonic vibrational modes and Raman scattering activities for E102 were in good agreement with the experimental data, a complete vibrational assignment being proposed. A strong chemical interaction of E102 with colloidal particles was evidenced and an adsorbed geometry was proposed.

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FTIR and FT-Raman spectra of 5-bromouracil in the powder form were recorded in the region 400-4000 cm −1 and 50-4000 cm −1 , respectively. The observed wavenumbers were analysed and assigned to different normal modes of vibration of the molecule. Quantum chemical calculations were performed to support the assignments of the observed wavenumbers. The performance of the B3LYP hybrid density functional (DFT) method was compared with other methods. With the 6-31 G * * and 6-311+G(2d,p) basis sets, the calculated geometry, dipole moments and harmonic vibrations were determined. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted wavenumbers of 5-bromouracil. The total atomic charges and thermodynamic parameters were calculated, and are discussed briefly. Structure and harmonic vibrations of 5-bromouracil were also calculated in the presence of water within a simple model with one molecule. It is observed that the bromine atom at position 5 exhibits smaller inductive effects than the fluorine atom, producing a small distortion of the electrostatic potential around the ring and a reduction of the molecular dipole moment.