Crystal Growth Using Low Temperature Gradient Sublimation in Vacuo
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A sublimation method based on the use of a conventional vacuum oven was developed for growing high quality single crystals of organic compounds suitable for structure determination. Key features of the method are the use of a low thermal gradient which favours the growth of large crystals and the use of vacuum which is ideal for air sensitive compounds. Crystals grown from the vapour are not affected by solvent effects and impurities have far less influence than when crystals are grown from solution. This low temperature gradient sublimation in vacuo technique was successfully applied to: compounds which give only solvated crystals from solution crystallization; fragile crystals which can be bent on the application of mechanical stress and systems whose crystal habit is influenced by the crystallization driving force. It was also possible to combine thermal transformation and crystal growth for some 1-[(1E,3E,5E)-4,6-bis(phenylsulfonyl)hexa-1,3,5-trien-1-yl]-4-benzenes which gave crystals of the corresponding 4-(phenylsulfonyl)biphenyls. Stanozolol and ethinylestradiol are two important steroids which before this work were known only as solvates but are shown here to have two polymorphs and just one crystal form respectively when crystallized from the vapour. Solution crystallization experiments run in parallel gave a wide range of solvated structures. All crystalline forms of stanozolol and ethinylestradiol were analysed using diffraction techniques (single crystal X-ray diffraction and powder X-ray diffraction) and thermal methods (differential scanning calorimetry and thermogravimetric analysis). Infrared spectroscopy was additionally used for the characterisation and identification of polymorphic forms. Crystal growth using the sublimation method also provides easy detection of the bending properties of molecular crystals. Crystal faces of naftazone and 1,3-dinitrobenzene when grown by sublimation tend to be curved in the bending direction. This effect is not observed for solution grown crystals. These bent crystals can be easily detected using optical microscopy. Crystal growth of 1,4-naphthaquinone, metal-free phthalocyanine and benzoic acid from the vapour was investigated under a range of crystallization driving forces. The results show that the crystal habit of planar molecules, which have stacked structures and no directional intermolecular interactions, strongly depends on crystallization driving force.
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