Tautomeric Polymorphism of 4‑Hydroxynicotinic
Acid
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Abstract
4-Hydroxynicotinic
acid (4-HNA) was discovered to exist in the
solid state as either 4-HNA or its tautomer 4-oxo-1,4-dihydropyridine-3-carboxylic
acid (4-ODHPCA) in three polymorphs and two hydrates. Packing motifs
differ as each of the three oxygen atoms acts as the hydrogen-bond
acceptor, respectively, in the anhydrate forms, while in the hydrate
forms, water molecules participate in hydrogen bonding with 4-HNA.
Phase behaviors of the forms were characterized by differential scanning
calorimetry (DSC), hot-stage microscopy (HSM), and thermogravimetric
analysis (TGA). It was found that anhydrates I and II converted into
III during heating; the two hydrate forms dehydrated at different
temperatures and eventually transformed into anhydrate III, and sublimation
of all five forms led to form III when the crystals were heated. Quantum
mechanical calculations were performed providing further insight into
the polymorphism