UV-VIS, NMR AND FT-IR SPECTRA OF TAUTOMERS OF VITAMIN C. EXPERIMENTAL AND DFT CALCULATIONS

Experimental and computational analysis of spectroscopic parameters of L-ascorbic acid (vitamin C) and its tautomers was investigated by Density Functional Theory (DFT) using B3LYP method. The optimized geometries and calculated vibrational frequencies were evaluated. The latter was compare with those of experimental values. The lambda(max) values of L-ascorbic acid were found 259 (at pH = 3) experimentally, 238 and 247 nm calculated in the gas and water phase at TD-DFT methods, respectively. The experimental order of upsilon(O-H) modes was obtained at upsilon(O-3-H-3) > (O-6-Ho-6) > (O-2-H-2) > (O-5-Ho-5). The theoretical orders were calculated at upsilon)O-6-Ho-6) > (O-2-H-2) > (O-5-Ho-5) > (O-3-H-3( and upsilon(O-3-H-3) > (O-5-Ho-5) > (O-6-Ho-6) > (O-2-H-2), respectively, in the gas phase and aqueous phase. The highest wave number of tautomers (S,Z)-3,5,6-trihydroxy-2-oxohex-3-enoic acid (2), (2Z,4E)-2,3,5,6-tetrahydroxyhexa-2,4-dienoic acid (3), (R)-2-((S)-1,2-dihydroxyethyl)-4,5-dihydroxyfuran-3(2H)-one (4) was calculated for the stretching modes O-6-Ho-6, O-3-H-3, O-6-Ho-6 bonds, respectively, in the gas phase. The experimental and theoretical calculations of proton NMR predicted H-3 as the most deshilded proton.