Abstract
The investigation of mechanical properties in molecular crystals is emerging as a novel area of interest in crystal engineering. Indeed, good mechanical properties are required to manufacture pharmaceutical and technologically relevant substances into usable products. In such endeavour, bendable single crystals help to correlate microscopic structure to macroscopic properties for potential design. The hydrate forms of two anticonvulsant zwitterionic drugs, Pregabalin and Gabapentin, are two examples of crystalline materials that show macroscopic plasticity. The direct comparison of these structures with those of their anhydrous counterparts, which are brittle, suggests that the presence of water is critical for plasticity. In contrast, structural features such as molecular packing and anisotropic distribution of strong and weak interactions seem less important.
Highlights
In recent times, the classic idea of crystals as a brittle objects has been progressively abandoned (Dunitz, 1984)
It was suggested that bending requires the anisotropic hierarchical distribution of strong and weak supramolecular interactions along perpendicular directions
An increasing number of studies report that, at the microscopic level, hydrated forms exhibit higher plasticity than the anhydrous equivalents. In such phases, increased plasticity does not seem to correlate to a particular structural feature
Summary
The classic idea of crystals as a brittle objects has been progressively abandoned (Dunitz, 1984). The discovery of plastic bending in the (quasi)-isotropic structures of dimethyl sulfone proves that alternative mechanisms might exist (Thomas et al, 2017) Due to their relevance in manufacturing and processing, many studies of mechanical properties involve APIs (active pharmaceutical ingredients), which suggest that hydrate forms might have higher plasticity than their corresponding anhydrates (Sun & Grant, 2004; Liu et al, 2018; Fucke et al, 2012; Chang & Sun, 2017). Despite the similar crystal packing, the hydrate forms show superior plasticity, which results in macroscopic bending of single crystals whereas the anhydrous forms are brittle
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