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Variable-temperature and variable-pressure studies of small-molecule organic crystals

“…The three forms of glycine usually crystallize simultaneously from the same solution, 46,47 that is they can be classified as concomitant polymorphs. α Polymorph crystallizes spontaneously under almost any experimental conditions; it is the main polymorph obtained from pure aqueous solutions.…”

“…In dry NH 3 no polymorphous transitions take place, and any of the three polymorphs -α-, β-, or γ-can be preserved during an indefinitely long time. 46,47 The differences in the Gibbs energies of the glycine polymorphs ( Figure 12) are related to the differences in the weak intermolecular interactions. Therefore the heats of transitions between the polymorphs are rather low, and therefore the metastable forms can be obtained rather easily and can be preserved for a long time if the barriers required for a structural reorganization are much larger than the small potential energy gain resulting from the transformation.…”

“…At high enough temperatures the γ-form becomes less stable than the α-form, and one can expect the γ→α transformation on heating, what is really the case. Our study 46,47 has confirmed the hypothesis of Iitaka, 43 and of Sakai et al, 48 that "the γ -form may be a stable form at least at room temperature". The order of stability γ > α > β at ambient conditions found from our measurements correlates with the order of changes in the lattice energy calculated in 49 from the measured heats of dissolution of the three polymorphs in water.…”

“…This manifests itself in the calorimetric experiments (the difference in the thermodynamic functions measured for the polymorphs is within the experimental error even for the most accurate and precise measurements), in the experiments on crystal growth (concomitant polymorphs 45 ), in the mutual transformations of the polymorphs on storage under various conditions. All this holds for the polymorphs of glycine 46,47 .…”

“…46,47 The order of the stability of the glycine polymorphs at ambient temperature was shown to be γ > α > β. At 298.15 K, ∆G for the α-and the γ-forms was estimated as about 160 J/mol, predicting that the γ→α transition is thermodynamically forbidden at this temperature, whereas the reverse α→γ transition should be allowed.…”