Zinc cations

Consider acetic acid, HC2H302, and the hydrated zinc cation,... 

Propenyl)-1,3-dithiane, after lithiation and addition of zinc chloride, reacts with ethyl 2-oxopropanoate to give preferentially the. vvn-adduct37, which is an intermediate in the synthesis of racemic /ra .s-tetrahydro-2,3-dimethyl-5-oxo-2-furancarboxylic acid. It is assumed, that the ethoxycarbonyl group is brought to a pseudoaxial position in the cyclic transition state by the chelating zinc cation. 

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. 

We incorporate an ammonium salt to immobilize the Zn2+ ions NH4CI is prepared as a paste, and forms a partially soluble complex with zinc cations produced at the... 

Figure 19.2 Metal-binding sites in the channel aligned on the three-fold symmetry axis shows binding to three zinc atoms and their symmetrically related subunits the first is in the entrance of the funnel-shaped channel (in cyan), the second is in an alternative position (in blue) and the third is aligned on the three-fold axis (in grey). The overall stoichiometry is seven zinc cations per channel, i.e. 56 zinc cations per molecule. The two representations are with two different orientations part a is aligned on the three-fold axis and part b is perpendicular to the axis. (From Toussaint et al., 2006. Copyright 2006, with permission from Elsevier.)...

Strategy. We will assume that the only solutes present are ZnS04 and H2SO4, and first calculate the ionic strength of the solution. (For the purposes of this calculation, we can safely assume that the amounts of zinc sulfate are negligible when compared with the amounts of sulfuric acid.) We will then calculate y for the zinc cation from I (and, for simplicity, obtain y from the Debye-Huckel limiting law). 

Knowing (Zn ) from the Nemst equation and the activity coefficient y for the zinc cation from the Debye-Huckel law, we can therefore calculate the concentration, [ZnS04], from equation (3.10). 

Dewi et al. used a cationic polysulfonium membrane as a separator in a zinc—air cell. They showed that the polysulfonium membranes were highly effective in preventing zinc cation permeation from the anode to the cathode compared to the commercially available PP separators. 

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. 

The zinc that is produced today starts as the zinc sulfide (ZnS) minerals zinc blende or sphalerite or from zinc carbonate (ZnCO ) known as smithsonite or calamine. In the electrolytic process, these minerals are dissolved in water to form the electrolyte in the cell where the zinc cations are attracted and collected at the cathode and deposited as a dull, brittle type of zinc. 

An example of the above mentioned cascade complexation of carboxylates by macrocyclic receptors containing metal ionic centers is the inclusion of oxalate by the dien dicobalt complex 9 (Martell, Mitsokaitis) [12]. Similarly, the -cyclodextrin (jS-CD) 10, modified with a zinc cation bound by a triamine side chain, encapsulates anions like 1-adamantylcarboxylate in its cavity, fixing them by combined electrostatic and hydrophobic interactions [13], Zinc s group achieved the enantioselective transport of the potassium salts of N-protected amino acids and dipeptides by making use of the cation affinity of... 

Check for the complete extraction of the zinc cations from the cation exchanger. For this end, put a drop of the solution flowing out on a watch glass, add two drops of a 4 Af solution of hydrochloric acid and one drop of a potassium hexacyanoferrate(Il) solution. If zinc ions are present, a white precipitate of zinc hexacyanofer-rate(II) forms. After extraction of the zinc, wash the resin with 20 ml of water. Dispose of the washing water. 

Fig. 18.7 Chemical structures of water-soluble viologen-linked zinc cationic porphyrins (ZnP(C V)4).

The zinc cation gives by far the most active catalyst. Iron, cobalt, and nickel cations also gave salts with considerable catalytic activity. Cadmium, because of its chemical similarity to zinc, and aluminum, because of its use in other epoxide polymerization catalysts, were considered as likely candidates to give active catalysts. However, complexes of the salts of these cations were only slightly catalytic. The salts used as cation sources in catalyst preparations also affected catalytic activity. Zinc salts, especially zinc chloride and zinc bromide, were retained in considerable amounts in the finished complexes, and the use of these salts gave the most active catalysts. 

Many artificial systems have been designed recently to imitate the function and behaviour of native enzymes - biomimetic chemistry [27]. Among them, calixarene-based receptors bearing one, two or three Zn(II) complexes on the upper rim were prepared as a model for phosphoesterases [28-31]. Dinuclear receptor 25 was reported to enhance the rate of transesterification of the RNA model substrate 2-hydroxypropyl-p-nitrophenyl phosphate more than 20,000 times compared with the non-catalysed reaction. The complexation mode for the phosphate anion can be described as cascade complexation where the anion is coordinated within the cavity formed by two zinc cations. 

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