Polymorphism in crystalline cinchomeronic acid.
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.
Braga
Dario
D
Dipartimento di Chimica G. Ciamician, Università degli studi di Bologna, Via Selmi 2, 40126 Bologna, Italy.
Maini
Lucia
L
Fagnano
Concezio
C
Taddei
Paola
P
Chierotti
Michele R
MR
Gobetto
Roberto
R