Polymorphism, Phase Transitions, and Thermal Stability of l-Pyroglutamic Acid

Abstract

l-Pyroglutamic acid undergoes a phase transition, α → β, at ∼68 °C on heating, which is reversible with hysteresis depending on the cooling rate, before melting at ∼162 °C. On cooling, a further reversible transition, α → α′, which shows martensitic characteristics, occurs at ∼ −140 °C. The structural changes at the transitions were studied by variable temperature X-ray powder diffraction and Raman spectroscopy and compared with reports of single crystal structure determinations. Differences in Raman spectra were attributed to differences in intermolecular N−H···O interactions in the three enantiotropic polymorphs. By optical microscopy, crystals frequently jumped around the microscope slide on passing through the α/β transition; this is attributed to discontinuous changes in the unit cell dimensions leading to a spring effect in the crystals.

Dual Stress and Thermally Driven Mechanical Properties of the Same Organic Crystal: 2,6-Dichlorobenzylidene-4-fluoro-3-nitroaniline

“…The small temperature window between the phase transition and the melting temperature indicates that the phase transition is associated with abrupt molecular motion with new molecular orientations in the martensite phase. 41,[47][48][49][50][51][52] The new structure leads to accumulated strain energy which is subsequently released at an interface leading to splitting. Further heating causes the broken parts to undergo scissoring motion after which the crystal melts.…”

“…29 This has been attributed to first order displacive solid-solid transition that occurs with Bain distortion. [38][39][40][41] Elastic properties in molecular solids are largely determined by the isotropy of crystal packing. 8,18 Recent studies by Ghosh and Reddy on caffeine co-crystals have established that bending of elastic organic crystals is related to isotropic packing with the presence of relatively weak and dispersive interactions in all directions.…”

Perpetually Self-Propelling Chiral Single Crystals

“…Structure Elucidation of the Unknown Phases. While the structures of forms α and α′ of L-PGA are known, 27 those of forms α and α′ of D-PGA and of forms β of both L-PGA and D-PGA were not determined prior to this study. To establish the complete mechanism of the two thermosalient phase transitions, we determined the structures of forms α and α′ L-PGA and D-PGA at various temperatures and also determined for the first time the structure of the β forms of both isomers (for crystallographic details, see Tables S1−S12, Supporting Information).…”

“…X-ray diffraction analysis of forms α and α′ confirmed the reported unit cells. 27,31 The fortuitous occurrence of two phase transitions and both strong (HT) and weak (LT) thermosalient effects in a single system provides insight into the driving force of crystal motility. Figure 7 shows the crystallographic relationships between the structures of the three forms, and Table 1 contains the relevant crystallographic details (see also Figures S5−S10 and Tables S1−S3 and S12, Supporting Information).…”

Polymorphism and Solvatomorphism 2010

“…160 The 2010 literature also contained a number of additional reports regarding the phase transformations among polymorphs and solvatomorphs of other organic compounds. Although some of these substances may have only limited pharmaceutical significance, the references summarized in Table 3, [161][162][163][164][165][166][167][168][169][170][171][172][173][174][175][176][177] are collected to pique the interests of appropriately directed investigators.…”