Anions inhibitive

Little work has been carried out on the mechanism of inhibition of the corrosion. of copper in neutral solutions by anions. Inhibition occurs in solutions containing chromate , benzoate or nitrite ions. Chloride ions and sulphide ions act aggressively. There is evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies which permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

The anion plays a crucial role. BArF and other bulky fluorinated tetra-arylbo-rates or tetraalkoxyaluminates are the most suitable anions. Hexafluorophos-phate-containing catalysts display high reactivity in the initial phase of the reaction, but suffer deactivation before the reaction reaches completion. Tetrafluoro-borate, triflate or other more strongly coordinating anions inhibit the catalyst. 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

The nature of the counter anion was strongly related to the catalyst activity and corresponded to the sequence TfO >Pp6 =BPh4 3>BF4. Actually, the triflate allowed catalysis at room temperature and the BF4 anion inhibited the reaction, this inhibition being neutralized by the addition of HBF4. 

In general, anion inhibition is non-competitive below pH 7 and uncompetitive at pH 9 towards CO2 hydration, whereas it is competitive towards HCO3 dehydration at low and neutral pH (169,157c). Other inhibitors such as alkylcarbonates, acetates, alkoxides, alcohols (192a) and divalent metal ions like Cu(II), Hg(II) are also reported (160, 190,192b). 

In discussing Reaction (F), we remarked that other anions are observed to compete with OH " in the Stern layer. This sort of electrolyte inhibition is widely observed, and the dependence of the inhibition on both the size and charge of the ions generally corresponds to expectations. For example, in the base-catalyzed hydrolysis of carboxylic esters in the cationic micelles, anions inhibit the reaction in the order N03 > Br " > Cl > F. For acid-catalyzed ester... 

The common problems with those metallomicelles may be summarized as follows (1) Most of these complexes were prepared in situ and often were not isolated. Hence, the intended structures of the metallomicelles in solution or in the solid state were not verified. (2) The metal complexes in solution were not identified or characterized in rigorous thermodynamic senses by potentiometric pH titration, etc. The complexation constants and possible species distribution at various pH s were totally unknown. (3) Possible catalytically active species L-Mn+—OH were not identified by means of the thermodynamic pvalues. Those described were all obtained merely in kinetics. (4) The product (phosphate anion) inhibition was not determined. Accordingly, it often was not clear whether it was catalytic or not. (5) Often, the substrates studied were limited. (6) The kinetics was complex, probably as a result of the existence of various species in solution. Thus, in most of the cases only pseudo-first-order rates (e.g., with excess metal complexes) were given. No solid kinetic studies combined with thermodynamic studies have been presented. It is thus impossible to compare the catalytic efficiency of these metallomicelles with that of the natural system. Besides, different... 

A study of the kinetics and mechanism of peroxide disproportionation by the complexes of L3 (Figure 12) probed anion inhibition pathways [96], The disproportionation in water was found, as expected, to proceed by two consecutive two-electron steps ascribed to the MnnMnn MninMnm redox changes. 

Pulse electrolysis, which is often used to improve the quality of electrodeposits (see below), was found to have a strong effect on the structure of nickel deposits by modifying the interfacial sorption properties [6.62]. Molecular species such as H2 or Ni(OH)2 are easily desorbed during the relaxation time, improving the electrocrystallization process. Conversely, specifically adsorbed species such as Hads or anions inhibit the electrodeposition process of nickel. 

Other reports (27-29) have focused on the role of citric acid, as a source of carboxylate anions, during precipitation of calcium phosphates from electrolyte solutions. It has been found that citrate anions inhibit the ciystal growth of calcium phosphates and hinder their transformation into hydroxyapatites. This was attributed to the adsorption of citrate anions into the crystals and the displacement of an equivalent amount of phosphate anions. Interestingly, Rhee and Tanaka (30) found that the presence of a collagen membrane in the medium changed the behavior of citrate anions from being an inhibitor to becoming a promoter of calcification, provided that the molar ratios of calcium to citric acid were between 2 and 12. 

M. Frederickson, J. R. Coggin, and C. Abell, Vinyl fluoride as an isoelectronic replacement for an enolate anion Inhibition of type II dehydroquinases, Chem. Commun., (2002) 1886-1887. 

The generation of O2 from potassium superoxide was also applied to stop-flow procedures. In this method O2 was dissolved in dimethyl sulfoxide and stabilized in 18-crown-6-polyether. This method is useful for mechanistic studies indeed, McClune and Fee (1976) were able to obtain catalytic rate constants for bovine copper/zinc superoxide dismutase as a function of pH in various buffers. More recently the mechanism of catalysis and of anion inhibition of iron superoxide dismutase from E. coli have been examined by this method using a specially constructed stop-flow spectrophotometer (Bull and Fee, 1985). A limitation of the method is that the pre-equilibrium state cannot be properly investigated because of the time resolution of the stop-flow equipment (== 5 msec). 

Table II contains the IC, values obtained from data in Figure 5. As can be seen in this Table, of all the phenol ics tested, rosmarintc acid was the most efficient compound in deactivating the superoxide anion, inhibiting the superoxide dismutase-sensitive NBT reduction by 50 % with a concentration of 33 fiM.

The complexity of the inhibitory mechanisms of various anions was well demonstrated by Curzon and Speyer (186) who considered the inhibition of ceruloplasmin s oxidase activity by mixtures of various pairs of anions. Their data were presented in the form of inhibition isobols. Each point on an isobol representing, for example, 50% inhibition, is fixed by the concentrations of two anions at which 50% inhibition occurs. A straight line isobol indicates that both anions inhibit by binding to mutually exclusive sites. A curved, concave up, isobol indicates the inhibitors are binding to the same intermediate at different sites or, depending on the degree... 

The anion-sensitive ATPase can generally be inhibited by thiocyanate [4,5]. Since this anion inhibits gastric acid secretion, this finding has been used as an argument in favour of a role of the enzyme in proton transport. The concentration of thiocyanate required for maximal inhibition usually amounts to 5- 10 mM. The residual activity, which sometimes occurs, can be attributed to an anion-insensitive ATPase. The enzyme from brush border shows again a deviating behaviour in that it is relatively insensitive towards thiocyanate, the inhibition being less than 30% [17,37]. 

Erythrocytes do not contain mitochondria and any anion-sensitive ATPase activity in a particulate fraction must therefore be attributed to the plasma membrane. An Mg-ATPase, which could be stimulated by bicarbonate was found in rabbit erythrocyte membranes [31-33] and should thus be plasma-membrane bound. The properties of the enzyme in rabbit erythrocyte membranes are, however, completely different from those of the anion-sensitive ATPase from other tissues. Whereas sulfite is a good stimulant for the Mg-ATPase in mitochondrial and microsomal fractions of various tissues, it only slightly stimulates [32] or even inhibits [33] the erythrocyte enzyme. Other oxy-anions inhibit the ATPase of erythrocyte membranes [32]. The substrate dependence of the enzyme is greatly different from that of the enzyme from other tissues [32,33]. The half maximal inhibitory concentration of... 

E7.6. A set of experimental data relating critical activity of hydroxide anions inhibiting pitting of 18-8 stainless steel in FeCls solutions and chloride activity are hsted in Table E7.3 [13]. 

Several examples of competition between simple anions have been observed. Thus the perchlorate and the thiocyanate anion inhibit concentration of the iodide anion by the thyroid gland, without affecting the oxidative incorporation of the iodide anion into thyroxine (Stanbury and Wyngaarden, 1952). Likewise, the organism Nitrobacter, which oxidizes nitrite to nitrate, is inhibited by cyanate or chlorate (anions), an effect easily reversed by washing (Lees and Simpson, 1957). 

The effects of organic acids in the intestinal tract are two-fold. They reduce pH in the stomach and small intestine. Moreover, acid dissociation in the bacterial cell and the accumulation of salt anions inhibit microbial growth. 

The acceptor strength of the solvent may be equally important in reactions where the solvation stabilizes an electron pair donor molecule or an anion. Stronger solvation of the reactant ligand or anion inhibits the reaction and the rate will decrease. If the solvated anion is the activated complex or the reaction product, an increase in the stability of the solvate favours the reaction and increases its rate. Examples of each of the two types of effect are presented below. 

A Model with Zn -Macrocyclic Polyamines for Anion Inhibition of CA... 

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