Negative catalytic effect

In these equations, Dmax is the larger of the summed values of STERIMOL parameters, Bj, for the opposite pair 68). It expresses the maximum total width of substituents. The coefficients of the ct° terms in Eqs. 37 to 39 were virtually equal to that in Eq. 40. This means that the a° terms essentially represent the hydrolytic reactivity of an ester itself and are virtually independent of cyclodextrin catalysis. The catalytic effect of cyclodextrin is only involved in the Dmax term. Interestingly, the coefficient of Draax was negative in Eq. 37 and positive in Eq. 38. This fact indicates that bulky substituents at the meta position are favorable, while those at the para position unfavorable, for the rate acceleration in the (S-cyclodextrin catalysis. Similar results have been obtained for a-cyclodextrin catalysis, but not for (S-cyclodextrin catalysis, by Silipo and Hansch described above. Equation 39 suggests the existence of an optimum diameter for the proper fit of m-substituents in the cavity of a-cyclodextrin. The optimum Dmax value was estimated from Eq. 39 as 4.4 A, which is approximately equivalent to the diameter of the a-cyclodextrin cavity. The situation is shown in Fig. 8. A similar parabolic relationship would be obtained for (5-cyclodextrin catalysis, too, if the correlation analysis involved phenyl acetates with such bulky substituents that they cannot be included within the (5-cyclodextrin cavity. 

Cationic polyelectrolytes containing imidazole groups have been investigated by some researchers. Morawetz et alU4 first found that a cationic polymer, poly (l-vinyl-3-ethylimidazolium iodide), 65 (PQMelm), enhanced the hydrolyses of the negatively charged esters, i. e. NABA and 4-acetoxy-3-nitrobenzenesulfonate 66 (NABS). At intermediate pH, a large catalytic effect was observed and this was... 

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

The rate of an electron transfer from the reduced catalyst to the substrate is also important. If the rate is excessively high, the electron exchange will occur within the preelectrode space and the catalytic effect will not be achieved. If the rate is excessively low, a very high concentration of the catalyst will be needed. However, at high concentration, the anion-radicals of the catalyst will reduce the phenyl radicals. Naturally, this will be unfavorable for the chain process of the substitution. As catalysts, substances that can be reduced at potentials by 50 mV less negative than those of the substrates should be chosen. The optimal concentration of the catalyst must be an order lower than that of a substrate (Swartz and Stenzel 1984). 

As the burning rate increases in the high-pressure region, the formation of carbonaceous materials diminishes and hence the super-rate burning also diminishes and becomes plateau burning. This negative catalytic effect of lead compounds is considered to produce mesa burning. 

The rate profile (II) for the hydrolysis of l,3-dicarboxyphenyl-2-phosphate (DCPP) in the absence and presence of an equivalent amount of the bipyridine-Cu(II) chelate is shown in Figure 7. The catalytic effect of the metal chelate in this case is in marked contrast to the negative effect observed by the same chelate... 

For the preservation of hydrogen sulphide solutions a covering layer of paraffin oil or the addition of such substances as sugar, glycerol or salicylic acid has been suggested 3 in the case of the latter substances it is possible that their negative catalytic effect may be due to their rendering inactive traces of some otherwise powerful catalyst (compare p. 124). 

Hikita (Addnl Ref R) studied the thermal decompn of nitric esters such as NG and came to the conclusion that it is almost monomolecular in nature, but in the presence of even a small quantity of moisture, ionic catalysis becomes of increasing importance. Cu has a positive catalytic action for decompg NG at high temps, while gum has a markedly negative action at lower temps. The order of catalytic effect of various metal surfaces was observed to be as. 

The activation heats found are equal to 25,000 cal/mole (Mixture I) and 38,000 cal/mole (Mixture II). The dependence of the activation on the composition indicates that we are dealing with a complex chemical reaction. It is characteristic that admixture of the reaction products produces no specific catalytic effect (positive or negative) on the flame velocity, although in experiments on self-ignition such effects have been observed. The reason is that in a flame the reaction occurs in a zone where in all cases (including combustion of an undiluted mixture) a high concentration of the combustion products is attained. 

Only if the products do not exert a drastic—positive or negative—catalytic effect on the reaction rate. 

In addition to the ability of certain substances to combine with the products of the decomposition of nitrocellulose, it is possible that the same or other substances may have a positive or a negative catalytic effect and may hasten or retard the decomposition by their presence. But it has not yet been made clear what types of chemical substance hasten the decomposition or why they do so. Nitrogen dioxide hastens it. Pyridine hastens it, and a powder containing 2 or 3% of pyridine will inflame spontaneously if heated for half an hour at 110°. Powders containing tetryl are very unstable, while those containing 10% of trinitronaphthalene (which does- not react with the products of decomposition) are as... 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

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