Reductive structural effects

It is difficult to attribute the capacity reduction which occurs above 700°C to structural effects because the structure of the samples is not changed significantly, as indicated by our X-ray diffraction measurements on the CRO samples. On the other hand, the hydrogen content of the samples is dramatically reduced over this temperature range. To investigate the importance of the hydrogen content, we made a series of cells from the other samples. 

Adzic RR, Tripkovic AV, Markovic NM. 1983. Structural effects in electrocatalysis oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms. J Electroanal Chem 150 79-88. 

Markovic NM, Adzic RR, Cahan BD, Yeager EB. 1994. Structural effects in electrocatalysis— Oxygen reduction on platinum low-index single-crystal surfaces in perchloric-acid solutions. J Electroanal Chem 377 249-259. 

Adzic RR, Markovic NM, Vesovic, VB. 1984. Structural effects in electrocatalysis Oxygen reduction on the Au(lOO) single crystal electrode. J Electroanal Chem 165 105-120. 

A point which emerges very clearly from the above discussion of structural effects on SN2 ring-closure reactions, is that there is a special effect, namely, a substantial reduction of non-bonded interactions in the transition states for closing of the smallest rings. This effect is held responsible for remarkably high kinetic EM s for the 3- and 4-membered rings in spite of their extremely low equilibrium EM s, and unusually high rate ratios of 5- vs 6-membered... 

E. Yeager, M. Razaq, D. Gervasio, A. Razak and A. D. Tryk, "The electrolyte factor in 02 reduction electrocatalysis Proc. of Structural Effects on Electrocatalysis and Oxygen Electrochemistry, Cleveland, OH, 1991. 

Whilst the mechanism of the process of size reduction is extremely complex, in recent years a number of attempts have been made at a more detailed analysis of the problem. If a single lump of material is subjected to a sudden impact, it will generally break so as to yield a few relatively large particles and a number of fine particles, with relatively few particles of intermediate size. If the energy in the blow is increased, the larger particles will be of a rather smaller size and more numerous and, whereas the number of fine particles will be appreciably increased, their size will not be much altered. It therefore appears that the size of the fine particles is closely connected with the internal structure of the material, and the size of the larger particles is more closely connected with the process by which the size reduction is effected. 

As an initial example let us consider the chemical reduction of cytochrome c (FW =12 400), a protein which plays a series of important roles in living organisms. The structural effects of the Fe(III)/Fe(II) redox process are known from crystallographic data, Figure 2.7a,b,c... 

It might be of interest to compare the observed structure effects on the hydrogenation rate with the parallel results concerning the noncatalytic reduction of ketones by some chemical reagent. Data on the reduction of... 

An opposing effect is possible under the severe conditions of a single extraction with HC1 some of the aluminum removed from the crystal structure may not be transported out of the catalyst particle. The resulting amorphous alumina, (after subsequent calcining) remaining in the particle would cause some reduction in effective diffusivity. Such amorphous alumina has been suggested by others (17,18). 

Kim OK, Little RC, Ting RY (1973) Polymer structural effects in turbulent drag reduction AIChE Symp Series 69 39... 

Pd and Ni catalysts with the structural effects on reductions with diimide (diazene) (ref. 6) and the equilibrium constants for the association of substituted ethylenes with a Ni(0) complex (ref. 7). These particular reactions were chosen because of our perception of their relation to the mechanisms of catalytic hydrogenation, and the insightful analysis of the relationship between structure and reactivity provided by the authors of these studies. 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

With the wide range of SSE s presently available, it should be possible to get an experimental value of Ein or E for almost any substrate, except possibly for those which are extremely difficult to reduce or oxidize or tend to form films. In the rare cases where an experimental value cannot be obtained, a reasonable value can often be inter- or extrapolated using known correlations between Hiickel MO parameters and oxidation or reduction potentials, or between gas phase ionization potentials and oxidation potentials 66 A very thorough discussion of structural effects on electrode reactions is available 24 as well as a comprehensive list of oxidation potentials of organic compounds 10 ... 

The discussed nonlinear phenomena during H2O2 reduction on Pt in acidic media constitute impressive examples of how small differences in the interfacial kinetics owing to structural effects, for example, or to minor changes in the composition of the electrolyte, can affect the dynamic response in a qualitative manner. 

Two perfectly reversible one-electron reduction steps are observed for Co.S ", at —0.53 V (Co +/+) and 1.205 V (Co+/ ). Comparison of the electrochemical properties of the cobalt catenate with the previously reported data for cobalt bpy or phen complexes [31, 32] shows here again a strong stabilization of the reduced states, Co and Co". Other sterically constrained polyimine ligands also lead to stabilized monovalent cobalt complexes [33-37]. Remarkable also is the drastic structural effect of the catenate on the Co +/-+ redox potential whereas bpy or phen complexes of cobalt(II) can easily be oxidized to octahedral cobalt(III) [36, 38], the redox potential value of the Co + -+ couple being close to 0 V, no oxidation peak is observed for Co.5 + prior to ligand oxidation (Ep > 1.6 V). The destabilization effect of Co " due to tetrahedral environment provided by the entwined and interlocked structure of 5 is thus very large (>1.5 V). 

To compute the reduction in effective sorption capacity that accompanies such blockages, Monte Carlo simulations were applied to lattice models [81,82]. For the l-dimensional channel case, the problem can also be evaluated analytically [81]. There is a precipitous decline in capacity even for relatively low faulting levels in the case of longer crystallites. For example, sorption experiments on polycrystalline gmelinites comprised of crystallites some 0.5 pm in length along the c-direction that have an observed factor of some 10 reduction in n-allmne capacity over that expected for the unblocked structure are indicative of a Suiting probability of only some 0.04. 

Anorectic drugs act mainly on the satiety centre in the hypothalamus (1). They also have metabohc effects involving fat and carbohydrate metaboUsm. Most of them are structurally related to amfetamine and increase physical activity. Their therapeutic effect tends to abate after some months, and part of this reduction in effect may be due to chemical alterations in the brain. Fenfluramine commonly produces drowsiness in normal doses, but has stimulaut effects in overdosage. Dexamfetamine, phenmetrazine, and benzfetamine all tend to cause euphoria, with a risk of addiction. Euphoria occasionally occurs with amfepramone (diethylpropion), phentermine, and chlorphentermine, but to a much lesser extent. Some adverse effects are due to sympathetic stimulation and gastrointestinal irritation these may necessitate withdrawal but are never serious. There are interactions with monoamine oxidase inhibitors and antihypertensive drugs. 

Arylaziridinium salts show a reverse structural effect on the polarographic halfwave potential, as electron-donating substituents make the reduction in aqueous solution easier and electron-attracting groups have the opposite effect [201]. A tetramerization of 1-benzylaziridine to a tetra-azacyclododecane may be induced anodically [202]. 

Immunotoxicological impacts include molecular and structural effects in immune tissues and organs, cellular pathology, reductions in immune cell numbers, retarded maturation of immune system cells, and altered immune system antibody production. These adverse effects are manifest by two types of reaction immunosuppression and immunostimulation. 

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