THEORETICAL-STUDIES OF THE SOLVENT EFFECT ON THE STABILITY OF THE ASCORBIC-ACID TAUTOMERS

AM1 calculations on three conformers of each of the four tautomeric forms of ascorbic acid are reported. According to the results for isolated molecules, tautomer 3 should be lowest in energy, whereas experimentally only tautomer 1 is observed in the crystal as well as in solution. Solvent effects on the relative stability of the various structures are assessed by applying a combined quantum chemical and force field approach. It turned out to be very important to fully optimize both the water environment and the solute, which was achieved iteratively by means of a suitably modified AMI Hamiltonian taking into account the charge polarization due to the solvent. Inclusion of the solvent effects reduces appreciably the energy difference between tautomers 1 and 3, but does not change their energetic ordering. CI calculations indicate that the AMI method tends to underestimate the bond delocalization effects in the enone substructure of 1 so that correlation effects are important to describe the relative stability of 1 and 3.