Tautomeric Equilibria of Substituted 2-Pyridone/2-Hydroxypyridine in the Gas and Aqueous Phases

Heats of formation, entropies and Gibbs free energies for the twenty structures of substituted 2-pyridone and 2-hydroxypyridine were studied using semiempirical Austin Model (AM1) and Parametric Method 3 (PM3) calculations at the self-consistent field level, both in the gas and liquid phases, with full geometry optimization. It was revealed from the study that 2-hydroxypyridine is predominant in gas phase, while 2-pyridone in the liquid phase which agrees with the experimental and theoretical predictions. All substituents such as F, Cl, OH, CH3, NH2, NO2, CHO, CN, CF3 stabilize the 2-pyridone in the gas and liquid phases except F, Cl and NH2 in PM3 calculations in the gas phase. The substituents stabilization is more effective in liquid phase. This was also confirmed by thermodynamic calculations and isodesmic reactions.