On the polymorphism of dicarboxylic acids: I pimelic acid

The results of solid-state analytical studies on pimelic acid demonstrate that the compound occurs in three polymorphic modifications. The modifications have been characterized by means of thermal analysis (video-supported thermomicroscopy, DSC, TGA), temperature-controlled FTIR microscopy and X-ray powder diffractometry, FT Raman and C-13 NMR spectroscopy, as well as pycnometry. The thermodynamic relationships are illustrated in a semi-schematic energy/temperature-diagram, which gives information about the relative stabilities and physical properties of the three modifications. Mod. III (commercial product, density 1.315 g cm(-3)) is the thermodynamically stable form between absolute zero and nearly 57 degrees C (transition point with mod. II 55 to 59 degrees C). Mod. II (density 1.276 g cm(-3)) is thermodynamically metastable in ambient conditions, but resistant. The transition point of mod. II with I (m.p. 105 degrees C) was determined to be 97.0 to 97.6 degrees C.