Three mechanism

Figure 3-4. a) The reaction site of an elimination reaction. The bonds to be broken are crossed through, and the bonds to be made are drawn with heavy lines, b) to d) The three mechanisms to achieve this reaction,... 

Now encounters between molecules, or between a molecule and the wall are accompanied by momentuin transfer. Thus if the wall acts as a diffuse reflector, molecules colliding wlch it lose all their axial momentum on average, so such encounters directly change the axial momentum of each species. In an intermolecuLar collision there is a lateral transfer of momentum to a different location in the cross-section, but there is also a net change in total momentum for species r if the molecule encountered belongs to a different species. Furthermore, chough the total momentum of a particular species is conserved in collisions between pairs of molecules of this same species, the successive lateral transfers of momentum associated with a sequence of collisions may terminate in momentum transfer to the wall. Thus there are three mechanisms by which a given species may lose momentum in the axial direction ... 

Clearly the general situation is very complicated, since all three mechanisms operate simultaneously and might be expected to interact in a complex manner. Indeed, this problem has never been solved rigorously, and the momentum transfer arguments we shall describe circumvent the difficulty by first considering three simple situations in which each of the three separate mechanisms in turn operates alone. In these circumstances Che relations between fluxes and composition and/or pressure gradients can be found without too much difficulty. Rules of combination, which are essea-... 

Equations (2.10), (2.18) and (2.24) provide the flux relations in situations where each of the three separate mechanisms of momentum transfer dominates. However, there remains the problem of finding the flux relations in "intermediate" situations where all three mechanisms may be of comparable importance. This has been discussed by Mason and Evans [7], who assumed first that the rates of momentum transfer due to mechanisms (i) and (ii) should be combined additively. If we write equation (2.10) in the form... 

Mechanisms of Filter Retention. In general, filtrative processes operate via three mechanisms inertial impaction, diffusional interception, and direct interception (2). Whereas these mechanisms operate concomitantly, the relative importance and role of each may vary. 

Other lesser mechanisms that result in aerosol removal by filters are (1) gravitational settling due to the difference in mass of the aerosol and the carrying gas, (2) thermal precipitation due to the temperature gradient between a hot gas stream and the cooler filter medium which causes the particles to be bombarded more vigorously by the gas molecules on the side away from the filter element, and (3) Brownian deposition as the particles are bombarded with gas molecules that may cause enough movement to permit the aerosol to come in contact with the filter element. Browruan motion may also cause some of the particles to miss the filter element because they are moved away from it as they pass by. For practical purposes, only the three mechanisms shown in Fig. 29-1 are normally considered for removal of aerosols from a gas stream. 

The case of intramolecular participation in ester hydrolysis has been extensively studied using acetylsalicylic acid (aspirin) and its derivatives. The kinetic data show that the anion is hydrolyzed more rapidly than the neutral species, indicating that the carboxylate group becomes involved in the reaction in some way. Three mechanisms can be considered ... 

It is interesting to note that all three mechanisms contributing to the attractive van der Waals interactions vary as the reciprocal of the separation distance to the sixth power. It is for this reason that the Lennard-Jones potential has been extensively used to model van der Waals forces. 

The principal use of acidity functions has been for the study of reaction mechanisms in acid-catalyzed reactions." We consider acid-catalyzed reactions in which a nucleophile, often water, may be a reactant. Three mechanisms are commonly considered ... 

Many everyday heat flows, such as those through windows and walls, involve all three heat transfer mechanisms—conduction, convection, and radiation. In these situations, engineers often approximate the calculation of these heat flows using the concept of R values, or resistance to heat flow. The R value combines the effects of all three mechanisms into a single coefficient. 

Productivity damage attributable to drilling or completion fluids results from three mechanisms ... 

Where monomers or radicals arc charged, readily ionizablc or capable of forming hydrogen bonds, mechanisms whereby the solvent could affect radical reactivity by disruption or involvement of hydrogen bonding may seem obvious. For other systems mechanisms are often still a matter of controversy even in the case of small radicals (Section 2.3.6.2). There are at least three mechanisms whereby the solvent might modify the outcome of a radical process ... 

In all three mechanisms, the reactions are performed at about 800°C and the oxygen-containing by-products (H20, and C02) are removed in the off-gas stream. 

Surface wear is defined as the deformation and loss of surface material as the result of a mechanical, thermal, or chemical action. These three mechanisms can act singly but are more often found in combination, which may make the wear process very difficult to analyze. Materials for wear protection have different responses to each of these wear mechanisms and, consequently, no universal wear material exists. To select the optimum material or combination of materials, it is essential to determine the cause and the mechanism of the wear as accurately as possible. The selection can then be made of the best and most cost-effective material. 

Fatigue during intermittent tetanic stimulation is caused by three mechanisms (a) decreased maximum Ca activated tension, (b) decreased sensitivity of the contractile proteins, and (c) decreased Ca release from the SR. 

It is evident that these three mechanisms are not easy to distinguish. All three give second-order kinetics, and two result in retention of configuration. In fact, although much work has been done on this question, there are few cases in which we can unequivocally say that one of these three and not another is actually taking place. 

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

At least three mechanisms have been proposed for electrophilic addition (these mechanisms are shown for attack by HX, but analogous mechanisms can be written for other electrophiles). 

The reaction is similar to the Diels-Alder (in action, not in scope), and if dienes are involved, the latter reaction may compete, though most alkenes react with a diene either entirely by 1,2 or entirely by 1,4 addition. Three mechanisms can be proposed analogous to those proposed for the Diels-Alder reaction. Mechanism... 

There is evidence that the reactions can take place by all three mechanisms, depending on the structure of the reactants. A thermal [ 2, + 2s] mechanism is ruled out for most of these substrates by the orbital symmetry rules, but a [ 2s + mechanism is allowed (p. 1072), and there is much evidence that ketenes and certain other linear molecules in which the steric hindrance to such an approach is... 

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

Ghanges in the availability of substrates are responsible for most changes in metabolism either directly or indirectly acting via changes in hormone secretion. Three mechanisms are responsible for regulating the activity of enzymes in carbohydrate metabolism (1) changes in the rate of enzyme synthesis, (2) covalent modification by reversible phosphorylation, and (3) allosteric effects. 

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