The tautomers of four RS -stereosiomers of ascorbic acid were investigated at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels. Calculated equilibrium constants of the selected isomers predicted possible isomerization of tautomers, which is the major cause of racemization. Experimentally, specific rotation of vitamin C were monitored in water at room temperature and at 80-90 o C during 122 days. Optical activity of L-ascorbic acid was reduced to zero after 6 days in water at 80-90 o C.The optimized geometries and calculated vibrational frequencies were evaluated. The latter was compare with those of experimental values. The ? max values of L-ascorbic acid were found 259 (about pH=3) experimentally, 238 and 247 nm calculated at TD-DFT methods, respectively. The structures, stabilities, conformational analysis and electronic transitions of L-ascorbic acid anions (four stereoisomers) were studied theoretically. These anions are produced from the de-rotonation of C 4 –H and C 5 –H sites of L-ascorbic acid stereoisomers. It was observed that the de-protonation at C 5 site of two stereoisomers leads to the ring opening in both phases.Isomerization of the L-form to one of the D-form was observed during the optimization of the anions at C 5 . Conformational analysis (potential energy surface scan) of the opened ring anions was performed in search of energy minima and/or maxima. The absorption electronic transitions of the anions in the UV region were calculated using Time Dependent Density Functional (TD-DFT).
