Tautomeric Equilibrium Revisited: proton-tautomerism in solvent and the fundamentals of molecular stability prediction.

Understanding the molecular stability is important for predicting the relative reactivity of chemical agents and the relative yields of desirable products. However, over decades, a consistent estimate of a particular chemical equilibrium (proton-tautomerism) has proven challenging. We revisit the molecular orbital analysis in the classic tautomeric oxo-hydroxy case, i.e., 2-pyridone/2-hydroxypyridine in gas-phase and solution, (Wong et al. 1992). Our results indicate the possibility of tuning the tautomeric equilibrium through directing groups. Our findings also reveal the lack of reproducibility of orbital energies as responsible for the remarkable contrast between the results of the wavefunction and density functional methods. Our proposal leads the correction in the estimation of relative stability in excellent agreement with experiments in gas-phase and solution. The analogous approach for different compounds corroborates the reliability of our description on the molecular stability and its potential application, e.g., a guide to estimate the relative stability of molecules, to measure the confidence of the proposed reaction mechanisms by different theoretical methods, development of the molecular switches and computer-aided drug design. A software tool for Gaussian 09 package, in the support information, is available on the author's ORCiD page.