Crystal Six

The selection of the right crystal holder thus plays an important role in all measurements with quartz oscillators. Various crystal holder designs are recommended for the different applications with or without shutter, bakeable for UHV, double crystal holder or crystal six as well as special versions for sputter applications. In addition to these important and more mechanical aspects, the advances in measuring and control technology and equipment features will be discussed in the follovi/ing. 

Three structurally isomeric forms have been established for the six-carbon saccharinic acids. In the order of their discovery, these are the sac-charinic or 2-C -methylpentonic acids, the isosaccharinic or 3-deoxy-2-C -(hydroxymethyl)-pentonic acids, and the metasaccharinic or 3-deoxy-hexonic acids. Although none of these six-carbon, deoxyaldonic acids has been crystallized, six are known in the form of crystalline lactones (saccharins). All the possible metasaccharinic acids of less than six-carbon content have been obtained, in the form of crystalline derivatives, by the sugar-alkali reaction. Only one example of a branched-chain deoxyaldonic acid (the racemic, five-carbon isosaccharinic acid) of other than six-carbon content has been so obtained. The formation of saccharinic acids containing more than six carbon atoms remains to be explored. 

In spite of its rarity as an equilibrium species in aqueous media, many salts of the octamolybdate anion have been crystallized. Six different structural forms of the anion have been characterized (Table 2). These differ in the linkage and types of MoO coordination polyhedra. In 1976 Klemperer and Shum had proposed a nondissociative process for the interconversion of the a- and /2-isomers 23 An intermediate in this mechanism was a 7-structure, which was later isolated by Cruywagen and co-workers.24 Other potential intermediates, the so-called a-y and (3-y structures, are closely related to the observed -structure. The final structures listed in Table 2, e and , have been isolated from hydrothermal processes and appear to be controlled by the coordination demands of the lattice or the countercations. 

Poly(ethylene glycol) (PEG) McPherson [35] has advocated the use of PEG for protein crystallisation. Out of 22 proteins chosen to test PEG as a precipitant, 13 yielded crystals, six for the first time. The general formula of PEG is... 

Cortisone in the form of the acetate ester, administrated as an anti-inflammatory agent, exists in polymorphic and pseudopolymorphic forms (solvate crystals). Six crystalline forms were distinctly found by the analysis of solid-state C NMR spectra of nine various samples [36]. Crystal structures were known for three forms and, therefore some correlation between NMR and XRD results could be made. Larger chemical shifts of C3=0 are related with the presence of hydrogen bonding other chemical shift differences between solid forms appear in the side chain and the conformations of ring A. 

Samples of hen egg-white lysozyme were crystallized six times. Lysozyme was dissolved in D2O (Diaprep) at a concentration of 180 mg ml" and adjusted to pH 4.75. No correction was made for deuterium effect in the pH reading. 

The word is also used to denote a unit in a solid crystal of an electrovalent compound such as NaCl in which each Na is electrically attracted by the surrounding six Cl" and each Cl" is electrically attracted by the surrounding six Na. The structure of such crystals is termed ionic to indicate that the crystal is not an aggregate of independent molecules. 

Pu (86 years) is formed from Np. Pu is separated by selective oxidation and solvent extraction. The metal is formed by reduction of PuF with calcium there are six crystal forms. Pu is used in nuclear weapons and reactors Pu is used as a nuclear power source (e.g. in space exploration). The ionizing radiation of plutonium can be a health hazard if the material is inhaled. 

One of the main uses of these wet cells is to investigate surface electrochemistry [94, 95]. In these experiments, a single-crystal surface is prepared by UFIV teclmiqiies and then transferred into an electrochemical cell. An electrochemical reaction is then run and characterized using cyclic voltaimnetry, with the sample itself being one of the electrodes. In order to be sure that the electrochemical measurements all involved the same crystal face, for some experiments a single-crystal cube was actually oriented and polished on all six sides Following surface modification by electrochemistry, the sample is returned to UFIV for... 

The otiier type of noncrystalline solid was discovered in the 1980s in certain rapidly cooled alloy systems. D Shechtman and coworkers [15] observed electron diffraction patterns with sharp spots with fivefold rotational synnnetry, a syimnetry that had been, until that time, assumed to be impossible. It is easy to show that it is impossible to fill two- or tliree-dimensional space with identical objects that have rotational symmetries of orders other than two, tliree, four or six, and it had been assumed that the long-range periodicity necessary to produce a diffraction pattern with sharp spots could only exist in materials made by the stacking of identical unit cells. The materials that produced these diffraction patterns, but clearly could not be crystals, became known as quasicrystals. 

Figure Bl.8.4. Two of the crystal structures first solved by W L Bragg. On the left is the stnicture of zincblende, ZnS. Each sulphur atom (large grey spheres) is surrounded by four zinc atoms (small black spheres) at the vertices of a regular tetrahedron, and each zinc atom is surrounded by four sulphur atoms. On the right is tire stnicture of sodium chloride. Each chlorine atom (grey spheres) is sunounded by six sodium atoms (black spheres) at the vertices of a regular octahedron, and each sodium atom is sunounded by six chlorine atoms.

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

Eijck B P, W T M Mooij and J Kroon 1995. Attempted Prediction of the Crystal Structures of Six Monosaccharides. Acta Crystallographica B51 99-103. 

FIGURE 1 4 An ionic bond IS the force of attrac tion between oppositely charged ions Each Na ion (yellow) in the crystal lattice of solid NaCI IS involved in ionic bonding to each of six surrounding Cl ions (green) and vice versa... 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

Tris(2,4-pentanedionato)iron(III) [14024-18-1], Fe(C H202)3 or Fe(acac)3, forms mby red rhombic crystals that melt at 184°C. This high spin complex is obtained by reaction of iron(III) hydroxide and excess ligand. It is only slightly soluble in water, but is soluble in alcohol, acetone, chloroform, or benzene. The stmcture has a near-octahedral arrangement of the six oxygen atoms. Related complexes can be formed with other P-diketones by either direct synthesis or exchange of the diketone into Fe(acac)3. The complex is used as a catalyst in oxidation and polymerization reactions. 

Discotic Phases. Molecules which are disk-shaped rather than elongated also form thermotropic Hquid crystal phases. Usually these molecules have aromatic cores and six lateral substituents, although the predominance of six lateral substituents is solely historical molecules with four lateral substituents also can form Hquid crystal phases. Although the flatness of these molecules creates a steric effect promoting alignment of the normal to the disks, the fact that disordered side chains are also necessary for the formation of these phases (as is often the case for Hquid crystallinity in elongated molecules) should not be ignored. 

Eor example, bis-(T- -heptyloxyben2yHdene)-l,4-phenylenediamine [24679-01-4] exhibits seven Hquid crystal phases the nematic and six smectic phases (19). The order of appearance of the phases as the temperature is increased is usually consistent with a decrease in the long-range positional and orientational order of the molecules. Eor example, ethyl 4-(4 -ethoxyben2yHdeneamino)cinnamate [2863-94-7] (3) has three Hquid crystal phases (20). 

---