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

In this work a vibrational investigation of the equilibrium species in aque ous 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 m ain species: a totally protonated one, a zwitterionic form, and the nicotin ate anion. These species were also studied by ab initio (Hartree-Fock and s econd-order Moller-Plesset perturbation) and hybrid density functional theo ry calculations. The structures of these species were optimized and the vib rational frequencies calculated. These methods provided a good correlation between the calculated and the experimental vibrational frequencies. A grea t mixing of the ring vibrational modes, as well as between the ring and the substituent modes, was observed. (C) 2001 Elsevier Science B.V. All rights reserved.