Potassium chloride , crystal structure

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

Fig. 4. Computer-generated crystal structure models nop row. left to right) Cuprite, zinc-blende, rutile, perovskite. iridymite (second row) Cristobalite. potassium dihydrogen phosphate, diamond, pyrites, arsenic (third rowt Cesium chloride, sodium chloride, wurtzite. copper, niccolite (fourth row) Spinel, graphite, beryllium, carbon dioxide, alpha i uanz. [AT T Bel Laboratories ...

Just as there are cation channels, there are also trans-membrane channels involved in the transport of biologically important anions such as Cl-. The crystal structure of the CIC chloride channel from Salmonella typhimurium was reported in 2002.3 Along with the determination of the Streptomyces lividans potassium channel structure, this work won a share of the 2003 Nobel prize in chemistry for Roderick MacKinnon (Howard Hughes Medical Institute, New York, USA). Chloride channels catalyse the flow of chloride across cell membranes and play a significant role in functions such as... 

From the product mixture of the reaction of diisobutylaluminum chloride with potassium, K2[A1,2(/Bu)i2] was recently isolated as a red crystalline solid by extraction with toluene. Its crystal structure was determined. The structure of K2[A1,2 (/Bu)i2] reveals an AI12 icosahedron with a net charge of minus two. Three slightly different Al-Al bond lengths (2.679(5), 2.680(4), and 2.696(5) A [89])were found within the icosahedron. 

Solids in which different kinds of atoms occupy structurally equivalent sites have also defect structures. Thus mixed crystals, say of sodium and potassium chlorides (Fig. 99), are examples of defect lattices of this kind. Lithium titanate, LigTiOg, has a rock-salt structure in which cation sites are... 

Potassium chloride occurs as odorless, colorless crystals or a white crystalline powder, with an unpleasant, saline taste. The crystal lattice is a face-centered cubic structure. 

A macrocyclic receptor (100) has also recently been prepared and its crystal structure was elucidated (220). In comparison with its acyclic analogue 101, an anion macrocyclic effect was observed, the stability constants for chloride complex formation [in DMSO] being K = 250 M (100) and K = 20 M (101). Receptor 102 was shown to act as a switchable cobaltocenium based chloridebinding host (221). The free receptor binds chloride anions, but on the addition of potassium ions, the binding is switched off. This effect is probably due to the ability of the potassium ion to form a sandwich complex with the two crown ether substituents, sterically hindering the anion-binding site. 

These crystal-structure analyses are concerned with the following molecules o-D-glucopyranose monohydrate/ D-xylopyranose/ methyl yS-D-xylopyranoside, di-jS-n-fructopyranose-strontium chloride trihydrate, cellobiose (independent determination), D-glucaric acid, D-galactonic acid, methyl a-D-lyxofuranoside, /3-D-lyxopyranose, methyl 3,4,6-tri-0 - acetyl - 2 - (chloromercuri) - 2 - deoxy - j3 - d - glucopyranoside, D-glu-copyranosyl (potassium sodium phosphate) tetrahydrate," and methyl 6-bromo-6-deoxy-a-D-galactopyranoside. ... 

The impurity concentration gradient theory assumes that the solution is more structured in the presence of a crystal. This increases the local supersaturation of the fluid near the crystal, which is the source of crystal nuclie. Changes in the structure of the solution near the crystal surface have been observed experimentally. Dissolved impurities in the solution are known to inhibit nucleation rates. Some of the impurities are incorporated into the crystal surface. Thus, a concentration gradient is formed that enhances the probability of nucleation. Experimental evidence of the theory was presented for the nucleation of potassium chloride in the presence of lead impurities. As expected, stirring the solution causes the impurity concentration gradient to disappear and hence, lower the nucleation rates (Denk 1970). 

Another physical property which has been used in the determination of molecular structure is the diffraction of X-rays by crystals. Reference has been made in Chapter 11 to the experiment of Friedrich and Knipping in 1912 in which they obtained a crude diffraction pattern from a crystal of copper sulphate. The pattern they obtained was too complicated to interpret and they turned to simpler materials. Their experiments were soon repeated by other workers, and within a few months W. L. Bragg had solved the first crystal structures by measuring and comparing the diffraction effects from sodium and potassium chloride. 

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