title: 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.
authors.affiliation: Dipartimento di Chimica G. Ciamician, Università degli studi di Bologna, Via Selmi 2, 40126 Bologna, Italy.
authors.forename: Dario
authors.initials: D
authors.lastname: Braga
authors.forename: Lucia
authors.initials: L
authors.lastname: Maini
authors.forename: Concezio
authors.initials: C
authors.lastname: Fagnano
authors.forename: Paola
authors.initials: P
authors.lastname: Taddei
authors.forename: Michele R
authors.initials: MR
authors.lastname: Chierotti
authors.forename: Roberto
authors.initials: R
authors.lastname: Gobetto