Abstract
This chapter reviews the substituent tautomerism of six-membered heterocycles that has appeared since 1976. The most significant development since 1976 is the enormous emphasis on the computational studies. Recent advances in computer technology in combination with increasingly accurate quantum-chemical methods, allow studies of systems of up to 50 atoms on standard personal computers at relatively high levels. Computational chemistry has become increasingly important in traditional heterocyclic chemistry. It discusses the generalized approach to the tautomerism phenomenon and its studies using theoretical, physical spectroscopic, non-spectroscopic, and chemical methods. The chapter also reviews the effect of introduction of an additional heteroatom; inductive substituents, mesomeric substituents, and the effect of benzannulation on the position of the tautomeric equilibrium of azines with various functional groups were investigated by PMO semiempirical calculations.