Understanding the Solid-State Phase Transitions of dl-Norleucine: An in Situ DSC, Microscopy, and Solid-State NMR Study

Abstract

The solid-state phase transitions between the α, β and γ forms of DL-norleucine were studied using DSC, thermal stage polarization microscopy and solid-state NMR. Since the crystals consist of 2D hydrogen-bonded bilayers with van der Waals interactions between consecutive bilayers, the transitions occur in a layer-wise fashion with a propagating transformation front. The α↔γ transition at 390 K is a clear example of a first order transition with a relatively large enthalpy difference between the polymorphs and a small hysteresis, indicating the kinetic barrier for this transition is relatively small. In contrast, the α↔β transition is not reproducible in similar crystals and the enthalpy difference is very small. Both the α and β polymorphic forms can coexist in a "single crystal" over a large temperature range, apparently without enforcing stress, while the α↔γ transition propagates fast to relieve stress from the volume and conformational change. Moreover, the kinetics of the α↔β transition are much faster in single crystals than in powders, which is attributed to the inhibitory effect of defects on cooperative motion. The thermodynamic transition temperature of the α↔β transition is estimated between 253 and 268 K. This work also shows that traditional methods of polymorph screening might overlook some solid-state phase transitions similar to the α↔β transition in DL-norleucine.

Temperature-induced order–disorder structural phase transitions of two-dimensional isostructural hexamethylenetetramine co-crystals

“…The DSC measurement revealed two sharp endothermic peaks at T c = 260.56 and 260.49 K on the first and second heatings, respectively, and a sharp exothermic peak at 255.89 K on cooling (Fig. 2), which indicated a reversible first-order transition similar to other reported phase transitions (Chen et al, 2015;Khan et al, 2015;Smets et al, 2015;Takanabe et al, 2017;Zeb et al, 2017). The observed enthalpy change and heat hysteresis are 4.34 J g À1 and 4.64 K. The average entropy change, ÁS, measured from the DSC curve is 0.0168 J g À1 K À1 or 6.4584 J mol À1 K À1 , and provided ÁS ¼ R ln N, then the possible configurations, N, equals to 2.17, indicating the two-site order-disorder characteristic of the studied transition (Asghar et al, 2016;Besara et al, 2011;Jain et al, 2009;Mą czka et al, 2014).…”

Energy barriers and mechanisms in solid–solid polymorphic transitions exhibiting cooperative motion

“…This small number might explain why experimentally part of the crystal is found not to transform over the course of several hours. 21 For these cases, the onset of a possible transition might require more than 180 molecules to move concertedly. One could imagine that the amount of molecules required is dictated by the local defect density.…”

“…This is in accordance with experimental thermal stage microscopic observation where in some cases the crystal is found to "wobble" during the transition, but the cell parameters before and after the transition are again the same. 14,21 The dashed line in Fig. 5 shows the fit with only the prefactor as a free parameter and .…”

Investigations into an Intramolecular Proton Transfer and Solvent Dependent Acid‐Base Equilibria in 2,6‐Pyridine Diacetic Acid

“…Being cognizant that the restricted C−C bond rotations would limit the formation of a 6‐membered planar ring (between the exchanging atomic centers), the occurrence of an intramolecular proton exchange in the solid state of PDA⋅H 2 O would be improbable. As such, temperature‐dependent SCXRD experiments were carried out to study the dynamicity of the solution mediated proton transfer in triggering possible solid‐state phase transitions . Through detailed temperature dependent SCXRD studies and DSC measurements, it was revealed that a reversible phase transition (Figure ) occurred, from the monoclinic system ( C 2/ c ) at 298 K to the triclinic one ( ), at 168 K (Figure : Table S2); this phenomenon was distinct from the unchanged (P2 1 2 1 2 1 ), orthorhombic system, which was observed for dipic monohydrate …”