Polymorphism and Desmotropy in Heterocyclic Crystal Structures

There is some confusion about concepts such as polymorphism, tautomerism, tautomeric polymorphism, and desmotropy. This perspective review tries to clarify them by defining their boundaries. Most examples are from heterocyclic compounds and differentiate between NH/OH tautomers (fast prototropy) and CH ones (slow prototropy). In the case of “tautomeric polymorphism” and “desmotropy” it is strongly recommended to use desmotropy. The different possibilities of crystallization of tautomers are examined in the conclusion.