Crystal structure prediction/control

Although concerned with the design and synthesis of one-, two-, and three-dimensional structures, crystal en,feineer-ing is not synonymous with crystal structure prediction, which is a far more precise and complex task. Crystal engineering at the current time also cannot seek to control, in anything but a very imprecise way. the factors involved in crystal nucleation and growth, though these obviously impact heavily upon the results obtained. 

Thus, there will be many practical uses for a reliable computational method of predicting the crystal structures of organic molecules. It is a fundamental challenge that underpins crystal engineering. We cannot claim to understand crystallization until we can at least predict the crystal structures that will be observed. It is probable that a reliable method of crystal structure prediction will have to represent in a quantitative way the factors that control crystallization. 

Several challenges in the area of polymorphism continue to be the focus of intensive research. These include polymorphic control in crystallization, structural elucidation of polymorphs available only as powders, and the central problem of crystal structure prediction by theoretical methods from a knowledge of the molecular structure only. Here, the essential features of these challenges are described, and some recent developments that attempt to address them are outlined. 

The polymer = 8.19 dlg in hexafluoro-2-propanol, HFIP, solution) in Figs 1 and 2 is prepared on photoirradiation by a 500 W super-high-pressure Hg lamp for several hours and subjected to the measurements without purification. The nmr peaks in Fig. 1 (5 9.36, 8.66 and 8.63, pyrazyl 7.35 and 7.23, phenylene 5.00, 4.93, 4.83 and 4.42, cyclobutane 4.05 and 1.10, ester) correspond precisely to the polymer structure which is predicted from the crystal structure of the monomer. The outstanding sharpness of all the peaks in this spectrum indicates that the photoproduct has few defects in its chemical structure. The X-ray patterns of the monomer and polymer in Fig. 2 show that they are nearly comparable to each other in crystallinity. These results indicate a strictly crystal-lattice controlled process for the four-centre-type photopolymerization of the [l OEt] crystal. 

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