Crystal structure prediction Subject

Although this aspect of crystal structure prediction is not the subject of a great degree of literature, it is not a trivial problem to decide whether two crystal structures are the same and care has to be taken that valid structures are not thrown away during this step. 

The subject of this study is particularly challenging for any crystal structure prediction methodology for several reasons. First of all the molecular entity in the solid state (a zwitterion) is different from that in the gas phase. This means that the transfer of molecular information from quantum mechanical... 

Are crystal structures predictable This question is still a Holy Grail for crys-tallographers, solid-state physicists and solid-state chemists, and has been the subject of controversial discussions for more than a decade. 

Crystal structure prediction The field of organic crystal structure prediction remains one of the best testing grounds for intermolecular potentials. Acciuades need not be as high as that needed for spectroscopic calculations, but the effects of molecular flexibility and many-body non-additivity need to be accounted for. See Price (2008, 2009) for recent reviews of this subject. For a description of dispersion-corrected DFT methods specially parametrized for organic crystals see Neumann and Perrin (2005). For a comprehensive examination of the role of detailed distributed multipole models in this field see Day et al. (2005). 

Applications of the CSD to advance the related areas of crystal engineering and crystal structure prediction are also increasing. Subjects of particular interest here are the topologies and relative frequencies of formation of the more common intermolecular interaction motifs observed in crystal structures. Etter et al. proposed a graph set description of motifs, a concept being taken forward by Bernstein et al., while Desiraju classifies motifs as supramolecular synthons (i.e., structure-directing influences) in crystal engineering applications. 

One can distinguish two phases in a crystal structure prediction, both quite difficult. The first phase is to make a list of structures that are not entirely unreasonable. This list was in older work rather short and obtained by hand, by comparing the molecule with related ones and guessing what kind of interactions could be possible and what kind of structures looked probable. A review of that type of work can be found in the book by Pertsin and Kitaigorodsky. In recent work this subjective method has been replaced by more or less well-defined algorithms, always based on energies calculated by an empirical force field. It turns out that lists obtained in that way can contain hundreds of possible structures. Two strategies are used random search versus systematic search. The... 

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. 

Asymmetric Diels-Alder reactions have been the subject of some of the more thorough mechanistic studies. Fairly reliable structural models for predicting the outcome of these reactions exist. In a review of the subject, it has been suggested that the stereochemical course of the reaction of a variety of chiral acrylates could be consistently predicted based on two models (Figures 39 and 40). Model A positions the complex in a (Z)-syn-s-trans conformation and presumes attack from the least-hindered face of the double bond. This model is consistent with almost all of the structural data for systems of this type (e.g. SnCWethyl cinnamate X-ray diffraction study). Contrapuntally, the large number of experimental observations that can be explained by this model support the assumption that the crystal structure conformation (26) is relevant to the course of these reactions. 

The behavior of real component powders may be estimated using a yield criterion (1), which requires a knowledge of the magnitudes of the directional response to stress of the particle and knowledge of the particle size distribution and particle orientation. Much of this information is not readily available for molecular organic solids and is the subject increasing attention of academic materials scientists. Even knowing the crystal structure, the dynamic response to mechanical stresses is not currently predictable without exhaustive effort (3). However, yield behavior may be measured and correlated to the... 

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