Role of topological charge stabilization in protomeric tautomerism

Protomeric tautomerism is analyzed in view of the topological charge stabilization rules. Based on Hückel molecular orbital considerations and modern DFT calculations, it was found that the branching of amino or hydroxyl groups significantly contributes to the stability of major species through the first- and second-order perturbations with respect to the isoelectronic hydrocarbon. While amino-imino tautomerism is almost completely dominated by topological charge stabilization, hydroxyl-oxo tautomerism is affected by changes in the resonance integral of C-O/C═O bonds. Nevertheless, apart from side effects such as hydrogen bonds or solvent effects, a quantitative preference rule for the prediction of the tautomeric stability can be developed using topological π-electron energetics. As well as the analyses of simple bases, applications to complex or extended systems are exemplified analyzing purine bases, polyguanide, and polyuret. The present approach can be useful in conjunction with chemical intuition that comes from conventional valence bond theory.