Crystalline packing arrangement

Chin, D. N., Palmore, G. T. R. and Whitesides, G. M. (1999fl). Predicting crystalline packing arrangements of molecules that form hydrogen-bonded tapes. J. Am. Chem. Soc., 121, 2115-22. [185]... 

The transparency and refractive power of the lenses of our eyes depend on a smooth gradient of refractive index for visible light. This is achieved partly by a regular packing arrangement of the cells in the lens and partly by a smoothly changing concentration gradient of lens-specific proteins, the crystallins. 

FIG. 5 Schematic representation of packing arrangements of natural amphipathic double-chain lipids with different headgroup size in crystalline bilayers. The small filled circles indicate the accommodation of spacer molecules, such as water or ions. (Reprinted by permission from Ref 14, copyright 1992, Elsevier Science.)... 

Figure 3. The structure of crystalline SiHjF. Silyl iodide has a similar packing arrangement, but the chains are much more acutely bent at iodine.

The packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

Crystallographic data for the material tabulated here has been deposited with the CCDC as supplementary publication CCDC-136762 CSD WISVOJ. For the manganese-molybdenum cluster the checkers also obtained a second crystalline modification with an alternative unit cell, namely, Ri, a = ll. ilS (5) A, c = 44.25 (2) A, V = 11570 (5) A, featuring a different packing arrangement of the cluster units. 

It remained to investigate why some crystalline anils of saiicyl-aldehyde are phototropic while others are not. We decided to look for a possible distinction on the basis of crystallographic factors, and have made a survey of the crystallographic constants of a number of anils. The compounds chosen do not have any strongly polar substituents (apart from the —OH) which would impose major distortions on any possible characteristic packing arrangements. The results of the survey are listed in Table 1. 

Figure 26. Packing arrangement of 7-chlorocoumarin in the crystalline state. Note the presence of two pairs of reactive 7-chlorocoumarin. In one pair the molecules are related by a centrosymmetry and in the other by a mirror symmetry. On this diagram mirror symmetric molecules appear one on top of another. Centrosymmetric molecules appear to be displaced with respect to the other.

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