Large crystals

Many scientifically and teclmologically important substances caimot be prepared as single-crystals large enough to be studied by single crystal diffraction of x-rays and, especially, neutrons. If a sample composed of... 

Liquid crystals Large Some non-zero but very small... 

A major limitation of diffraction techniques has been the need to obtain crystalline samples. If scientists could learn how to crystallize large molecules in a routine manner, a breakthrough would result. In the biological area, this limitation is keenly experienced for membrane-bound proteins, which are important in many biological functions. Scientists are now devising techniques and strategies to crystallize these proteins—if not in three-dimensional, then in two-dimensional lattices. 

The cloud point test is one of the most commonly used methods to evaluate the low-temperature characteristics of distillate fuel. The cloud point temperature identifies the point when wax begins to form into crystals large enough to become visible in the fuel. At this temperature, wax can settle from fuel, deposit onto fuel filters, and interfere with the flow of fuel through small tubes and pipes. During cold weather months, distillate fuels with lower cloud point values are refined and blended to minimize the low-temperature problems associated with wax. 

As described in Chapter 11, bond valences can play a role in modelling but, since most crystal structures can still not be predicted ab initio, diffraction methods remain the most common and reliable technique for determining the structures of those compounds that can be prepared as single crystals large enough for study by X-ray or neutron diffraction. 

Bis(2-mercaptoethyl)suIphone (BMS) M 186.3, m 57-58°. Recrystd from hexane as white fluffy crystals. Large amounts are best recrystd from de-oxygenated H20 (charcoal). It is a good alternative to dithiothrcitol and has pKa25 values of 7.9 and 9.0 in H20. Its IR (film) has V 2995, 2657, 1306, 1248, 1124... 

Yellow crystals Large yellow crystals Pale-yellow crystals... 

If the spacings of the arcs on a powder photograph do not lead to identification, the determination of unit cell dimensions from the powder photograph may be attempted the methods are described in Chapter VI. If crystals large enough to be handled individually can be picked out of the specimen, single-crystal rotation photographs may be taken and used for identification this also is dealt with in Chapter VI. 

Fig. 12-10.—The arrangement of atoms in a layer of the boric acid crystal. Large circles represent oxygen atoms and small circles boron atoms. The double lines represent hydrogen bonds.

In liquid crystals or LC-glasses one looks for orientational order and an absence of three-dimensional, long-range, positional order. In liquid crystals, large scale molecular motion is possible. In LC-glasses the molecules are fixed in position. The orientational order can be molecular or supermolecular. If the order rests with a supermolecular structure, as in soap micelles and certain microphase separated block copolymers, the molecular motion and geometry have only an indirect influence on the overall structure of the material. 

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