Zinc cation ordering

Equilibria considerations on solution-grown zinc chalcogenide compounds have been put forward by Chaparro [28] who examined the chemical and electrochemical reactivity of solutions appropriate for deposition of ZnS, ZnSe, ZnTe (and the oxide ZnO) in order to explain the results of recipes normally used for the growth of such thin films. The author compared different reaction possibilities and analyzed the composition of solutions containing zinc cations, ammonia, hydrazine, chalcogen anions, and dissolved oxygen, at 25 °C, by means of thermodynamic diagrams, applicable for concentrations usually employed in most studies. 

However, significant stabilization is also contributed by nN—>-szn donor-acceptor interactions, each with estimated second-order interaction energy 49.4 kcal mol-1, as depicted in Fig. 4.52. Each ammine ligand thereby donates about 0.061 e to the zinc cation, primarily to the vacant 4s orbital which acquires about 0.371 e total occupancy. As before, the high formal hypervalency at the metal center is achieved within the limits of the duodectet rule, i.e., without significant involvement of extravalent metal p orbitals. 

The possibility that Mn generally favors tetrahedral coordination as its valence approaches +2 (i.e., d ) is unlikely given that MnO has a rock-salt structure not zinc blende or some other structure with Mntet. Instead, the driving force for Mn movement out of the octahedral sites of 7-Lii/2Mn02 into neighboring Li layer tetrahedral sites appears to be due to the unique cationic ordering and associated cationic interactions that are present in 7-Lii/2Mn02. 

The first example in Figure 11.1 shows a pyrimidine trione compound bound to the active site of stromelysin. In order to get the interaction of one of the acidic pyrimidine nitrogens with the zinc cation correctly, the deprotonated negatively charged isomer has to be considered. 

It is not sufficient to show that the active protein contains zinc, in order to classify the enzyme to be a metalloenzyme. It must be proved that the zinc in the metalloprotein is functional. This point was examined27 with preparations similar to the final products listed in Table IX. Although the enzyme was completely resistant to the action of EDTA at pH 8 (see Table VI p. 414), the preparations were still inactivated by the chelating agent at pH 5. Activity was restored by Zn2+, but by no other cation examined. 

Local description of the arrangement around cations concludes unambiguously for [Cu-Cr-Cl] in a cationic ordering. The evidence of similar ordering for [Zn-Cr-Cl] was only obtained by a combined EXAFS and UV-Vis study of the formation of this LDH in solution [18], The structural pathway so-reported involves the heterocondensation between hexa-aquo zinc(II) complexes and deprotonated chromium monomers. 

Figure 4.3 Example of an activity cliff illustrated by closely related adenosine deaminase inhibitors having dramatic potency differences. The introduction of a hydroxyl group that coordinates a zinc cation in the active site of the enzyme adds several orders of magnitude to the potency of an inhibitor.

There is an increase in the importance of electrophilic catalysis by zinc cation relative to acetic acid for deprotonation of the a-carbonyl carbons of hydroxyace-tone, a substrate which provides a second stabilizing chelate interaction between the hydroxy group at the substrate and the metal dication that is expressed at transition state for proton transfer [19]. For example, the third-order rate constants kx for the Zn +-assisted acetate-ion-promoted deprotonation of the a-CHs and a-CH20H groups of hydroxyacetone are 32-fold and 770-fold larger, respectively, than the corresponding second-order rate constants kAco for proton transfer to acetate anion assisted by solvent water that is present at 55 M (Scheme 1.12). This shows that Zn + stabilizes the transition state for proton transfer from the a-CHs... 

Reduction of radiation fields by treatment of surfaces is an active research area at the present time. A number of techniques are being studied, including electropolishing, mechanical polishing, pre-oxidation and application of surface coatings. Also, deliberate addition of cations (eg zinc) in order to modify surface oxidise to reduce activity uptake is being considered. 

Zinc is electrodeposited from the sodium zincate electrolyte during charge. As in the zinc/bromine battery, two separate electrolytes loops ("posilyte" and "nega-lyte") are required. The only difference is the quality of the separator The zinc/ bromine system works with a microporous foil made from sintered polymer powder, but the zinc/ferricyanide battery needs a cation exchange membrane in order to obtain acceptable coulombic efficiencies. The occasional transfer of solid sodium ferrocya-nide from the negative to the positive tank, to correct for the slow transport of complex cyanide through the membrane, is proposed [54],... 

Analyses of rate measurements for the decomposition of a large number of basic halides of Cd, Cu and Zn did not always identify obedience to a single kinetic expression [623—625], though in many instances a satisfactory fit to the first-order equation was found. Observations for the pyrolysis of lead salts were interpreted as indications of diffusion control. More recent work [625] has been concerned with the double salts jcM(OH)2 yMeCl2 where M is Cd or Cu and Me is Ca, Cd, Co, Cu, Mg, Mn, Ni or Zn. In the M = Cd series, with the single exception of the zinc salt, reaction was dehydroxylation with concomitant metathesis and the first-order equation was obeyed. Copper (=M) salts underwent a similar change but kinetic characteristics were more diverse and examples of obedience to the first order, the phase boundary and the Avrami—Erofe ev equations [eqns. (7) and (6)] were found for salts containing the various cations (=Me). 

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

Another characteristic of 2 1 clays is isomorphous substitution, where iso means same and morphous means shape. During the formation of clay, cations other than aluminum and silicon become incorporated into the structure. In order for this to work and still ensure a stable clay, the cation must be about the same size as either aluminum or silicon, hence the term isomorphous. There are a limited number of cations that satisfy this requirement. For silicon, aluminum as Al3+ and iron as Fe3+ will tit without causing too much distortion of the clay structure. For aluminum, iron as Fe3+, magnesium as Mg2+, zinc as Zn2+, and iron as Fe2+ will fit without causing too much structural distortion (see Figure 3.4). 

The use of other metal cations such as those derived from zinc, lithium, or aluminium proved less effective (136). Treatment of allyl alcohol with diethyl zinc in the presence of a catalytic amount of diisopropyl (/ ,/ )-(+ )-tartrate (DIPT) in 1,4-dioxane, however, afforded the corresponding (5/f)-2-isoxazolines with excellent selectivity er >92 8) (178). Addition of dioxane was necessary in order to avoid precipitation of the complex of zinc salts containing the DIPT moiety. Without this solvent, lower stereoselectivity was found, probably due to the precipitation mentioned above, which prevents the favorable catalytic cycle proposed (Scheme 6.32) (178). 

Since Zn2+ ion does not hydrolyze as readily as cations of higher charge, bases need to be added in order to initiate precipitation by forced hydrolysis. Particles of different morphologies, including spheroids and ellipsoids, formed in the presence of different weak bases on heating solutions of zinc salts (Figure 1.1.16) (110). Dispersions of narrow size distributions resulted under specific reactant concentrations as shown in Figure 1.1.17 (110). X-ray diffraction analysis showed all particles of different shapes to be composed of zincite, ZnO. 

The change in conformation seen here (both in the crystal and in solution) may not be due to the zinc so much as to the anion. The X-ray study showed that the change in zinc coordination took place with uptake of an anion in a very hydrophobic pocket. This is consistent with the binding strength order observed SCbT > I" > CD > SO4". In the solution studies two thiocyanate anions are required for the overall change. Apart from the dependence on cation and anion concentration, the equilibrium position between the conformations in solution is temperature and pH dependent. 

The temperatures reported in the kinetic studies range from 260 to 350°C. In most of the investigations, the hydration rates were found to be of the first order with respect to acetylene [300,302—304]. With zinc phosphate [303], cadmium—calcium phosphate [300] and cation-exchanged zeolites [304], the rates were independent of the concentration of water. Thus the simple kinetic equation... 

A dissolution mechanism for zinc, cadmium and mercury in their molten halides has been proposed on the basis of experimental and literature data.949 Dissolution occurs at the metal-salt phase boundary. Adsorbed M24- cations are reduced to M+ ions which then migrate into the salt phase where Mi4" dimers form. The stability of the M2+ ions was found to increase in the order Zn < Cd Hg (the latter is so stable that Hg+ is undetectable in solution). 

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