Density functional theory calculations of the molecular force field of L-ascorbic acid, vitamin C.

We have studied L-ascorbic acid and characterized it by infrared spectroscopy in solid and aqueous solution phases. The density functional theory (DFT) method together with Pople's basis set show that three stable molecules for the compound have been theoretically determined in the gas phase, and that an average of only two more stable conformations are present in the solid phase, as it was experimentally observed. The harmonic vibrational wavenumbers for the optimized geometries of both structures were calculated at B3LYP/6-31G*and B3LYP/6-311++G** levels at the proximity of the isolated molecule. For a complete assignment of the vibrational spectra in the compound solid and aqueous solution phases, DFT calculations were combined with Pulay's scaled quantum mechanics force field methodology in order to fit the theoretical wavenumber values to the experimental ones. In this way, a complete assignment of all the observed bands in the infrared spectrum for l-ascorbic acid was performed. The natural bond orbital study reveals the characteristics of the electronic delocalization of the three structures while the corresponding topological properties of electronic charge density are analyzed by employing Bader's atoms-in-molecules theory.