Enrichment, preferential mechanism

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

Tamura R, Takahashi H, Fujimoto D, Ushio T (2007) Mechanism and Scope of Preferential Enrichment, a Symmetry-Breaking Enantiomeric Resolution Phenomenon. 269 53-82 Tanaka H, see Matile S (2007) 277 219-250... 

This is the same mechanism as that given above for esters, in equation (42). The difference between esters and amides is apparent from a comparison of the two tetrahedral intermediates [5] and [17], The former contains three oxygens, any of which can be protonated, resulting in much lsO exchange being observed when the reaction takes place in 180-enriched water,275,276 but [17] contains a much more basic nitrogen, which will be protonated preferentially and lead to much less 180 exchange, as observed.274 277,278 Also, ammonium ion formation makes the overall reaction irreversible, unlike ester hydrolysis. The calculated solvent isotope effect for the Scheme 15 process is 1.00,280 exactly in accord with experimental observation.278,279... 

In summary, coal-fired power plants appear not to be the major source of most enriched elements on particles In urban areas, despite the great attention devoted to mechanisms by which those elements become preferentially attached to fine particles (e.g.. Refs. 18, 34). However, the detailed studies of processes In coal-fired plants are of considerable value, as the fundamentals should be applicable to other kinds of combustion sources. Furthermore, It may be necessary to use this fundamental approach to develop methods for predicting the source compositions for coal-fired power plants that have not been measured. Selenium Is much... 

To confirm the occurrence of the polymorphic transition predicted in the section 4 and to elucidate the mechanism, it is primarily necessary to clarify the enantiomeric assembly mode in the first-formed metastable crystal prior to the polymorphic transition and compare it with the stable crystal structure after the polymorphic transition with respect to a compound showing Preferential Enrichment. Since it is very possible that the stable molecular assembly structure in solution would be retained in the crystalline phase first-formed by crystallization from the same solvent,20 at first, we have investigated the enantiomeric association mode in solutions of the racemates showing Preferential Enrichment. Consequently, in our case, the variable temperature H NMR technique proved to be inapplicable to deciding which molecular association mode is more stable in solution, homochiral or heterochiral.21 Instead, the combined use of the solubility and supersolubility measurements under various conditions and the number-averaged molecular weight measurement by vapor pressure osmometry turned out to become a potent tool for this objective. 

Thus far three types of stable crystal structures, a-, 8- and e-forms, which are classified as a highly or fairly ordered racemic mixed crystal, have been obtained for the racemic samples of the compounds showing Preferential Enrichment. Among them, the 8-form crystal is most commonly found,10"19 while only one case is observed for each of the a- and e-forms (Table l).91618 It has been confirmed that with respect to ST the formation of the stable a-form crystal is not essential to the occurrence of Preferential Enrichment and is caused by further polymorphic transition of the once-formed 8-form polymorph.22 In this section, the crystal structure of the 8-form is described in detail, because this crystal structure provides a crucial information on the mechanism of the polymorphic transition causing Preferential Enrichment. Another important crystal structure of the e-form is mentioned in detail in the section 7.2. for the same reason. 

Observation and Mechanism of Polymorphic Transition Inducing Preferential Enrichment... 

The mechanism of Preferential Enrichment can be interpreted in terms of an interplay between (i) a polymorphic transition at the beginning of crystallization resulting in a considerable enantiomeric enrichment in the mother liquor and a slight enrichment of the opposite enantiomer in the deposited crystals and (ii) the chiral discrimination by the deposited crystals in the subsequent crystal growth process leading to a further enrichment of the same enantiomer in the mother liquor. 

Apart from the significance as a novel enantiomeric resolution phenomenon, the investigation on the mechanism of Preferential Enrichment has also shed light on the hitherto unknown mechanism of a polymorphic transition occurring during crystallization from a supersaturated solution. A combination of several techniques employed here would also be useful to elucidate the unknown mechanism of another type of polymorphic transition... 

Figure 26. Mechanism of Preferential Enrichment characterized by homochiral molecular association, polymorphic transition, followed by crystal disintegration, and chiral discrimination in the case of NNMe,. (Reprinted with permission from ref 18. Copyright 2003 American Chemical Society.)...

The enrichment in the fines and regolith breccias is thought to result from exposure on the lunar surface, resulting in either preferential removal of 12C or addition of 13C. A number of mechanisms have been proposed to explain these observations for the fines and must also be applicable to the regolith breccias. 

Fe and Mn oxides, respectively. Patterns of trace element enrichment in the nodules are determined by mineralogy, the controlling mechanisms being related to cation substitution in the crystal structure. Mn phases preferentially accumulate Cu, Ni, Mo and Zn. Alternatively, Co, Pb, Sn, Ti and V are enriched in Fe phases. 

Synaptic vesicle attachment or docking occurs in the active-zone region of the presynaptic terminal. This specificity for synaptic vesicle attachment implies a recogiution mechanism between the synaptic vesicle and the active zone. In this context, some or perhaps all molecules responsible for synaptic vesicle docking might be expected to exhibit preferential enrichment or exclusive localization to the active-zone region. Despite intense efforts and numerous candidate molecules, the... 

These small degree melts should also be extremely enriched in highly incompatible elements and would resemble alkali basalts rather than MORBs (unless the source was extremely depleted). This mechanism cannot be ruled out for some ocean island basalts (OlBs) (Sims et at, 1999 Elliott, 1997). However, it will not work for the significant degrees of melting (F s 10%) inferred for MORBs (Klein and Langmuir, 1987 McKenzie and Bickle, 1988 see Chapter 3.13). Moreover, to preserve the excesses produced by this mechanism requires preferential extraction of the enriched phase (e.g., the melt) and extraction on a timescale that is short compared to shortest-lived nuclide of interest. 

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