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

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.