Abstract
The nootropic drug Piracetam is known to crystallize in three phases. In order to obtain their stability hierarchy from sublimation pressure inequalities, the drawing of a topologicalp,Tdiagram was attempted. For such a purpose and also for quality control, crystallographic and thermodynamic data were required. Powder X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) were used. Molecular energy calculations were performed. Phase I melts at 426K (ΔfusH(I) = +180 J·g−1). Phase II transforms into Phase I at 399K (Δ(II→I)H= +24 J·g−1). Phase III transforms into phase I at 392K (Δ(III→I)H= +28 J·g−1) or melts at 412K (ΔfusH(III) = +210 J·g−1). Thep,Tdiagram shows that phase I is stable at higher temperature and phase II at lower temperature, like phase III, which is stable under high pressure. At room temperature, phase II is the more stable form, and phase I the less stable one. This agrees with the spontaneous I → II transformation observed at 298K within a few hours, and with lattice energies, calculated previously. Molecular energy calculations and crystal structure comparison show how intermolecular hydrogen bonds and H-bonded dimers, in phases II and III, may stabilize conformations higher in energy than those of the isolated molecule and of phase I.
Published Version
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