Study of the tautomeric equilibrium of pyridoxine in 1,4-dioxane/water mixtures by 13C nuclear magnetic resonance. Thermodynamic characterization and solvent effects.

A significant temperature dependence has been found for the (13)C NMR chemical shifts of pyridoxine in 10%, 20%, 30%, 40%, 50%, and 60% v/v 1,4-dioxane/water mixtures (pH = 7.0). The nuclei most sensitive to the temperature effect were C-3 and C-6 in all of the mixtures. This dependence has been explained on the basis of a thermally induced tautomeric equilibrium shift between the neutral and the dipolar forms of the pyridoxine molecule. The thermodynamic characterization of this tautomeric equilibrium, which interconverts quickly on the NMR time scale, has been achieved by considering the observed average (13)C NMR chemical shifts at different temperatures through fitting the experimental data to a theoretical curve. The fitting accuracy is greatly improved on using linear correlations between the average chemical shifts obtained from different nuclei at the same temperature. The methodology outlined above allows the DeltaH degrees value to be calculated for the tautomeric process and the chemical shifts of the pure extreme forms, i.e., neutral and dipolar, to be deduced. These values have been used to calculate the thermodynamic parameters of the tautomerization equilibrium in each dioxane/water mixture. The effect of solvent on the tautomeric equilibrium and the averaged chemical shift has been explained in terms of a multiparameter equation developed by Kamlet and Taft. The overall solvent effect is the sum of two different effects: the dipolarity and polarizability of the solvent and the ability of the solvent to act as a hydrogen-bond donor toward a solute.