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M.M.H. Smets, S.J.T. Brugman, E.R.H. van Eck, J.A. van den Ende, H. Meekes, and H.M. Cuppen,

Understanding the solid-state phase transitions of dl-norleucine - an in-situ dsc, microscopy and solid-state nmr study,

Cryst. Growth Des. 15 (2015) 5157-5167

Abstract:

The solid-state phase transitions between the alpha, beta and gamma 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 alpha ->gamma 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 alpha ->beta transition is not reproducible in similar crystals and the enthalpy difference is very small. Both the alpha and beta polymorphic forms can coexist in a "single crystal" over a large temperature range, apparently without enforcing stress, while the alpha ->gamma transition propagates fast to relieve stress from the volume and conformational change. Moreover, the kinetics of the alpha ->beta 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 alpha ->beta 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 alpha ->beta transition in DL-norleucine.