Vibrational analysis of nicotinic acid species based on ab initio molecular orbital calculations

Abstract

In this work a vibrational investigation of the equilibrium species in aqueous solution of nicotinic acid was performed, by both experimental as well as theoretical methods. The Raman spectra of its aqueous solutions at some pH values were obtained and analyzed considering the equilibrium of three main species: a totally protonated one, a zwitterionic form, and the nicotinate anion. These species were also studied by ab initio (Hartree–Fock and second-order Møller–Plesset perturbation) and hybrid density functional theory calculations. The structures of these species were optimized and the vibrational frequencies calculated. These methods provided a good correlation between the calculated and the experimental vibrational frequencies. A great mixing of the ring vibrational modes, as well as between the ring and the substituent modes, was observed.