Polarizability and concentration effects on the Raman spectra of picolinic acid species in aqueous solution

Abstract The Raman spectra of picolinic acid (2-pyridine carboxylic acid) in aqueous solution were obtained in the pH range 0.5–8.2, from 300 to 1800 cm −1 . The equilibrium involving the several species present in solution was investigated taking into account the frequency values and relative intensities of characteristic bands of each species at different pH values. The results thus obtained were compared with simulated curves of concentration versus pH, obtained from the equations that describe the relative concentrations of the different species in solution. It is shown that the significantly different intensities of some vibrational modes, present in the spectra of the species in equilibrium, not only reflect the differences in their relative concentrations, but also the differences in the relative Raman polarizabilities. This was observed mainly for modes involving the NH bending in their composition.