A theoretical treatment of solvent effects on the tautomeric equilibria of five-membered rings with two heteroatoms

The tautomeric equilibrium constants of nine oxo and hydroxy derivatives of five-membered heterocycles containing two ring heteroatoms (nitrogen or oxygen) as calculated by the AM1 quantum chemical model agree well with literature gas-phase data. The inclusion of solvent effects through a self-consistent reaction field technique into these calculations yields results in full agreement with experimental data obtained for these systems in aqueous solution. It is not possible to use the calculated energies for isolated molecules to predict the relative stabilities of these tautomers in solution.Solvent effects are specific, and differentially lower the energy of one tautomer over another demonstrating in some cases a strong solvent effect on tautomeric equilibria. In general, dielectric media favour charge separation and preferentially lower the energy of species with the greater degree of charge separation. In the species studied solvent reaction field effects favour ionic resonance forms of the carbonyl tautomers and hence a greater degree of aromaticity.