Solution-grown crystal Subject

Rh (DIPHOS ) (Z-methyl-a-acetamidocinnamate ) ]BF4. A sample of 0.2 g of [Rh2(DIPHOS)2][BF4]2 and 0.1 g of Z-methyl-a-acetamido-cinnamate were stirred in ca. 10 mL of warm methanol until a clear red solution was obtained. After cooling to room temperature, diethyl ether was added slowly until the solution turned slightly cloudy, followed by addition of just enough methanol to clarify the solution. Red crystals of [Rh( DIPHOS) (Z-methyl-a-acetamidocinnamate)][BF4] were grown by diflFusing diethyl ether into the solution over a period of ca. 20 hr and subjected to x-ray structure analysis. 

Routine measurements of d-d spectra are performed on solutions. If a suitable solvent cannot be found for a solid sample, a diffuse reflectance spectrum of a powdered sample can be taken. This is actually an absorption spectrum of the surface layers of the sample and is subject to a number of anomalies and artefacts. It is much better to study microscopic single crystals, preferably at low temperatures. Large crystals (if they can be grown) tend to absorb too strongly around band maxima small, thin (c. 0.01mm) plates are best. It is usually necessary to condense the incident beam by means of a lens in order to obtain detectable intensities of transmitted radiation. Thus the technique is more difficult and time-consuming than the familiar, routine solution measurement but it can provide much more information. 

The formation of mixed crystals by means of a solid-state reaction occurs under kinetic control and may be subject to different constraints than the formation of mixed crystals grown closer to equilibrium conditions from the melt or from solvents. However, whether or not mixed crystallization is possible, and to what extent, is still determined by the structural similarity of the reactant and the product. Kitaigorodskii suggested guidelines based on a volume analysis which indicate that chemically compatible compounds which have optimized overlapping volumes larger than ca. 75% are likely to form solid solutions [66],... 

Both are represented schematically in Figme 11.5, but the exact nature of the structure has been the subject of considerable controversy. Although the morphology of the single crystals grown from dilute solutions may be more regular and resemble the first model, for polymers that are crystallized from the melt (and this is by far... 

Crystals of 16a were grown by slowly cooling a saturated toluene solution of the complex under an ethylene atmosphere. X-ray diffiaction analysis at 120 K showed that each unit cell was made of two nonequivalent molecules of 16a - one of them being disordered - and one molecule of solvent. Figure 2 provides a graphic rendition of the molecular structure observed, together with selected structural parameters. It should be emphasized, however, that these data are subject to caution since it was not possible to come up with a R parameter lower than 0.16, even when data sets were collected at low temperature on freshly synthesized crystals. 

Extended-chain crystals have also been identified as tying together lamellae in bulk-crystallized polymers and forming the core ( skewed ) of the interesting shish kebab structures grown from dilute solutions subjected to shearing [10]. 

A continuous highly oriented fiber may be grown from dilute polyethylene solution subjected to shear flow in a Couette apparatus [70,74]. This device consists of a pair of coaxial cylinders, the inner one of which can be rotated rapidly. A dilute polyethylene solution introduced into the gap between the cylinders is subjected to shear flow. When the appropriate conditions of shear, temperature, and concentration are met, the polymer will crystallize on to a seed fiber held in the flow field. As the extended polyethylene fiber grows, it is wound up at a rate equal to its extensional growth rate, the tip of the growing fiber thus remaining at a fixed position. The resulting fiber consists of a bundle of shish... 

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