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首页> 外文期刊>Crystal growth & design >Understanding the Solid-State Phase Transitions of DL-Norleucine: An in Situ DSC, Microscopy, and Solid-State NMR Study
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Understanding the Solid-State Phase Transitions of DL-Norleucine: An in Situ DSC, Microscopy, and Solid-State NMR Study

机译：了解DL-正亮氨酸的固态相变：原位DSC，显微镜和固态NMR研究

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摘要

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.
机译：使用DSC，热台偏振显微镜和固态NMR研究了DL-正亮氨酸的α，β和γ形式之间的固态相变。由于晶体由2D氢键结合的双层组成，在连续的双层之间具有范德华相互作用，因此过渡以分层的方式发生，并具有传播的转变前沿。 390 K处的α->γ跃迁是一阶跃迁的明显例子，在多晶型物之间具有相对较大的焓差和较小的磁滞现象，表明该跃迁的动力学势垒相对较小。相反，在相似的晶体中无法重现α->β跃迁，并且焓差很小。 α和β多态性形式都可以在很大的温度范围内共存于“单晶”中，显然没有施加应力，而α->γ跃迁迅速传播以减轻体积和构象变化带来的压力。此外，单晶中的α->β跃迁的动力学比粉末中的快得多，这归因于缺陷对协同运动的抑制作用。估计α->β跃迁的热力学转变温度在253至268 K之间。这项工作还表明，传统的多晶型物筛选方法可能会忽略类似于DL-正亮氨酸中α->β跃迁的某些固态相变。

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