Tautomerism

Tautomerism is a special, dynamic case of functional isomerism: rather than being two permanently separate compounds, a SINGLE compound exists as two structures -- its tautomers -- that continuously and readily interconvert into one another, differing in the position of at least one atomic nucleus (almost always a hydrogen atom migrating from one position to another). Two named systems cover how that migrating hydrogen moves.

In the dyad system, the hydrogen oscillates back and forth between two DIRECTLY LINKED polyvalent atoms. Hydrogen cyanide (H-C≡N) and hydrogen isocyanide (its tautomer, with the H instead bonded to nitrogen) is the textbook dyad pair -- the migrating hydrogen simply swaps between the carbon and the nitrogen it is directly bonded to. …