Polymorphism in Crystalline Cinchomeronic Acid

Abstract The structural relationship between the two crystal forms of cinchomeronic acid ( CA 3,4‐dicarboxypyridine) has been investigated by single crystal X‐ray diffraction, IR and Raman spectroscopy and solid state NMR spectroscopy, showing that the two polymorphs form a monotropic system, with the orthorhombic form I being the thermodynamically stable form, while the monoclinic form II is unstable. In both forms CA crystallizes as a zwitterion and decomposes before melting. The crystal structure and spectroscopic analysis indicate that the difference in stability can be ascribed to the strength of the hydrogen‐bonding patterns established by the protonated N‐atom and the carboxylic/carboxylate O‐atoms.