Ring mechanism

A substitutional atom (indicated by shading in Fig. 8.1) may jump and replace an adjacent nearest-neighbor substitutional atom. In the ring mechanism, the substitutional atom exchanges places with a neighboring atom by a cooperative ringlike rotational movement. 

Dislocations must also be taken into consideration as possible high mobility paths for particles. The density Pe 2ind the spatial arrangement of dislocations are very difficult to control. Thus, even for single crystals, transport coefficients can be structure-sensitive at temperatures less than about half the melting temperature where volume diffusion no longer predominates. 

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In dry compressors, shaft end seals are generally one of five type.s. These are labyrinth, restrictive ring, mechanical contact, liquid film, and dry gas seal. The labyrinth type is the most simple but has the highest leakage. The labyrinth seal is generally ported at an axial point between the seals in order to use an eductor or ejector to control leakage and direct it to the suction or a suitable disposal area. Alternatively, a buffer gas is used to prevent the loss of process gas. Appendix D presents a calculation method for use with labyrinth seals. 

Llorga, O. et al. The sequential allosteric ring mechanism in the eukaryotic chaperonin-assisted folding of actin and tubulin. Embo J 2001, 20, 4065-75. 

Transfer of the hydrogen atom by a four-membered ring mechanism is an oversimplification. Deuterium labeling showed that three-, five-, and six-membered rings are also involved in longer chain compounds with relative dominance dependent on the compound. See Djerassi, C., and Fenselau, C. J. Am. Chem. Soc. 87, 5747 (1965) McLafferty, F. W. and Turecek, F. Interpretation of Mass Spectra, 4th ed. Mill Valley, CA University Science Books, 1993, pp. 261-262. 

Figure 8.1 Ring mechanism for diffusion of substitutional atoms.

These reactions are reversible. This dual-site occupancy leads to complicated solute diffusion behavior and has been described for several solute species in Si [4, 6. 8]. There is no compelling evidence that the ring mechanism in Fig. 8.1 contributes significantly to diffusion in any material. 

The ring mechanism in Section 8.1.1 is an example of a direct mechanism. 

Because of the six-membered ring mechanism for the addition, lithium enolates don t usually do conjugate additions. For enol equivalents that do, see Chapter 29. 

In alkali hydroxides extended ring mechanism for proton motion can occur leading to a tracer propagation but not to any charge transport. The Haven ratio (H) amounts to several thousand in such cases.216... 

Benzene ring (mechanical treatment) Carter 1976, Allinger 1983a, Beckhaus 1983, Andersson 1984, Kao 1987, Liljefors 1987, Pettersson 1987, Fukazawa 1988, Tsuzuki 1990a, Broeker 1991b, Brant 1992... 

The mechanism for halohydrin formation is similar to the mechanism for halogenation addition of the electrophile X (from X2) to form a bridged halonium ion, followed by nucleophilic attack by H2O from the back side on the three-membered ring (Mechanism 10.4). Even though X is formed in Step [1] of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X" is the nucleophile. 

Ethylene formation is fastidiously controlled in plants (as are other hormone productions) and the oxidative conversion of sp -hybridized C(3) and C(4) of methionine to sp -hybridized ethylene (139) via the (now you see it, now you don t) three-membered ring mechanism is a pretty act of metabolic functional group conjuring. 

Fujii, T., Itaya, T., Saito, T. Base-catalyzed ring opening and reclosure of the adenine ring mechanism, substituent effects, and synthetic utility. Symp, Heterocycl, [Pap,] 1977,129-134. 

The convenient mechanism for the interconversion of 13 to 27 could be involved with the breaking and reassembling of one of the rings at Pd-N linkage in order to thread the other ring (mechanism A in Scheme 1) The studies on equilibrium... 

FIGURE 8.3 Simplified schemes for the main mechanisms of transport of atoms in solids. (A) Interstitial (B) vacancy (C) interstitially (D) crowed-ion (E) ring mechanisms. 

Ring mechanism (Figure 8.3E). The atoms can exchange places with their neighbors, and the deformation caused by the rotation depends on the number of atoms that are able to rotate. 

This analysis indicates that an equatorial-entry mechanism (D) is a possible rationale of the observed epimerization reactions. The diequatorial ring mechanism is unlikely. Definitive proof lies in a proper kinetic analysis of epimerization and comparison of rates with acyclic analogues. The rate of epimerization must be shown to be consistent with a rate retardation corresponding to a relative increase in AG of ca. 22kcalmol ... 

Ring mechanism (cooperative motion possible but not demonstrated)... 

There is another type of lattice diffusion mechanism, called direct exchange or ring mechanism. As shown schematically in Fig. 5.8d, during this diffusion, atoms exchange their positions through rotation in a circle way. There are no any defects involved in this diffusion. This is a multiatomic action. However, this mechanism is... 

Fig. 5.8 Lattice diffusion by a vacancy mechanism, b interstitial mechanism, c interstitialcy mechanism, and d ring mechanism. Reproduced with permission from [1]. Copyright 2003, CRC Press...

Understanding Coffee Rings Mechanism and Theoretical Models... 

In general, the existence of a Kirkendall effect in the sense of a displacement of markers does not permit a conclusion to be drawn as to a definite diffusion mechanism as, for example, a vacancy diffusion mechanism. To give a very improbable example - even if a ring mechanism is the predominant diffusion mechanism, a displacement of markers in the external system could nevertheless occur because lattice sites can be locally created or destroyed at sites of repeatable growth as a result of the fact that the concentration of vacancies, which are always present, depends upon the composition of the alloy. 

Figure 7.11 Lattice diffusion by (a) the vacancy mechanism, (b) the interstitial mechanism, (c) the interstitialcy mechanism, and (d) the ring mechanism.

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