Tautomeric equilibria in relation to Pi-electron delocalization

By definition, tautomeric equilibria are associated with changes in π-electron distribution and are very important intramolecular proton-transfer reactions. Generally, π-electron delocalization plays a principal role in tautomeric systems and affects tautomeric preferences. However, other internal effects and external influences may change this general behavior. Numerical measures of π-electron delocalization are proposed. Among them, the geometric index (HOMA) describes the electron delocalization in both acyclic and cyclic compounds very well. It is emphasized that each parameter describing tautomeric preferences and electron delocalization should be analyzed taking various partial factors into account so that those of the greatest importance and strongest influence can be selected. Furthermore, only the simple tautomeric cyclic and acyclic systems and mainly those often used as models for biological molecules are discussed.