Abstract.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 (videosupported thermomicroscopy, DSC, TGA), temperature-controlled FTIR microscopy and X-ray powder diffractometry, FT Raman and t3C NMR spectroscopy, as well as pycnometry. The thermodynamic relationships are illustrated in a semi-schematic energyjtemperature-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 ~ (transition point with rood. II 55 to 59 ~ Mod. I! (density 1.276 g cm-3) is thermodynamically metastabte in ambient conditions, but resistant. The transition point of mod. II with I (m.p. 105 ~ was determined to be 97.0 to 97.6 ~ Key words: pimelic acid, thermal analysis, polymorphism, energy/temperaturediagram, alkanedioic acid. [-31 and Delarbre [41 presupposed as known that alkanedioic acids with an odd number of carbon atoms generally are polymorphic and appear in two crystalline forms: the high-temperature form ~ and the lowtemperature form 3. Dupr6 la Tour [-1], Kay [2], RaoTwo authors have described the thermal behaviour of pimelic acid (pentanedicarboxylic acid) and reported a dimorphic relationship. Dupr6 la Tour [1] reported an enantiotropic transformation at 77 ~ In the other case, Cingolani [5] detected a transition at 96 ~ The aim of this work was to clear up these inconsistent results by means of new solid-state analytical methods in polymorphism research and to establish an energy/temperature diagram to illustrate the thermodynamic relationships of the modifications.
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