Ab initio study of the structural properties of ascorbic acid (vitamin C)

Geometries of the neutral and ionic tautomeric species of ascorbic acid were optimized at the density functional theory (DFT) level using the B3LYP functional. The radical species were evaluated using the unrestricted B3LYP method. Single-point energy calculations were also performed using the Moller-Plesset (MP2) and unrestricted MP2 (UMP2) methods for the closed-shell and open-shell systems, respectively. The effects of aqueous solution were evaluated using the conducting polarized continuum model (CPCM) and polarized continuum model (PCM). The geometries of most stable radicals in the respective groups were also optimized in the water solution using the CPCM model at the LJB3LYP level. All calculation were performed using the 6-311++G(d,p) basis set. The nature of stationary points on the gas phase potential energy surfaces (PESs) was evaluated using vibrational frequency calculations; all geometries characterize local minima. The species obtained by the deprotonation of the O-3 site is the most stable monoanion of ascorbic acid. For the radical species, the structure obtained by the dehydrogenation of the O-3 site is the most stable monoradical. Among the radical anions, the species obtained by the deprotonation of the O-3 site and subsequent dehydrogenation of the O-2 site is the most stable in the gas phase and in an aqueous medium. The computed isotropic hyperfine coupling constants of this species were found to be in good agreement with the experimental data. Our investigation also supports the earlier findings that the oxidized species of ascorbic acid in water solution by the OH center dot radical is radical anion of the AAO(3)(center dot)O(2)(-) form. The spin densities and molecular electrostatic potentials are also discussed. (C) 2006 Wiley Periodicals, Inc.