Isolated clusters

Figure 19.4 Rear view of a bulk oil circuit breaker assembly showing single-break contacts, self aligning cluster Isolating contacts, terminal bushing and arc control pots (Courtesy ...

The Au SR clusters isolated above are treated as conventional chemical compounds. We have been studying the basic properties (e.g., optical [15-18,22,34], photophysical [16,35], chiroptical, magnetic [42]) and geometric structures [43] of Au SG clusters (1-9) as a prototypical system. As an... 

McCASKILL, D., GERSHENZON, J., CROTEAU, R., Morphology and monoterpene biosynthetic capabilities of secretory cell clusters isolated from glandular trichomes of peppermint (Mentha x piperita L.), Planta, 1992,187, 445-454. 

Bender, H., Siebert, R., Stadler-Szoke, A. (1982). Can cyclodextrin glycosyltransferase be useful for the investigation of the fine structure ofamylopectins Characterisation of highly branched clusters isolated from digests with potato and maize starches. Carbohydr. Res., 110,245-259. 

These are all changes which the theory of metal bonding would predict for the transition of metal - small cluster- -isolated atoms (94-97, 99-102, 136). However, before we ascribe the higher BE of small particles to bonding effects, we have to consider the following. 

The electrical conductivity of these alloys is metallic i.e., inverse proportional to the temperature between 4 to 300 K and is about 7 x 106 (Qm) 1 at room temperature. The crystal structure determination [7] shows the B atomic arrangement to consist of coplanar 4B clusters (isolated from one another) with three B atoms at the vertices and one B at the center of a triangle as shown in Fig. 5. This isolated 4B atomic arrangement has no precedent either in metal-borides [14] or boron hydrides [15]. 

The linear CO stretching frequency for the carbonylated platinum colloid while lower than that found for surface bound CO, is in the range reported for the platinum carbonyl clusters [Pt 3 (CO) 6 ] n / sind we find that the carbonylated colloid is easily transformed into the molecular cluster [Pt 12 (CO) 24 ] (10) reaction with water. The cluster was isolated in 50 yield based on platinum content of the precipitate by extraction with tetraethylammonium bromide in methanol from the aluminum hydroxide precipitated when water is added to the aluminoxane solution. The isolation of the platinum carbonyl cluster reveals nothing about the size or structure of the colloidal platinum particles, but merely emphasizes the high reactivity of metals in this highly dispersed state. The cluster isolated is presumably more a reflection of the stability of the [Pt3(CO)6]n family of clusters than a clue to the nuclearity of the colloidal metal particles - in a similar series of experiments with colloidal cobalt with a mean particle size of 20A carbonylation results in the direct formation of Co2(CO)8. 

These examples are only a few demonstrations of the general effort to use cluster reactions as a bridge between isolated binary systems and bulk reactions. The cluster isolated chemical reaction (CICR) technique elaborated in Chapter 2 of this Part is an important advance in that direction. 

Hence there is no gas-phase experiment yet which fully encompasses all aspects of an electron-transfer reaction in solution. In solution, the solvent acts first as a polarization medium, which affects the energetics of direct transfers from the donor to the acceptor. It can also act as a transport medium for indirect electron transfers. The first aspect has been addressed in various cluster experiments [276]. The second aspect was addressed more recently by considering the femtosecond dynamics of iodide-(water) anion clusters, as reviewed below [277]. Finally, clusters present the advantage of isolating one reaction pair free from secondary collisions, except those, which are desired, with the solvent molecules (or atoms). The latter consideration motivated the cluster isolated chemical reaction (CICR) technique reviewed in Section 2.8.3. 

The cluster isolated chemical reaction (CICR) technique was developed in Saclay to permit quantitative studies of heterogeneous chemical processes at a microscopic level. The ultimate goal is to document the coupling between a reaction coordinate... 

The fate of the reaction product in cluster isolated chemical reactions... 

Figure 9.26. Se NMR spectra of CdSe molecular clusters isolated by encapsulation with covalently-bonded Se-phenyl or Se-butyl moieties. The Se spectra relate to the internal portion of the cluster rather than to the organoselenide ligand. Note the change in relative proportions of the three-line fitted components, shifting towards the position of the bulk material with increasing cluster size. From Thayer et at. (1988), by permission of the copyright owner.

Recently, new spectroscopic techniques27 have allowed investigation of interactions between two, three or more molecules in order to understand the relative geometry and the kind of interactions which form clusters. Isolated clusters may serve as models for obtaining information on hydrogen bonds in solution as well as in biological systems too. 

The two clusters isolated from this reaction, HOs3(CO)10(NC9H8) and HOs3(CO)10(NC9H6), are regarded as intermediates (117). 

Fig, 3, Arrangements of FeOe octahedra in trimeric clusters isolated one from the other by... 

Ruan, X., Stassi, D., Lax, S. A., and Katz, L. (1997). A second type-1 PKS gene cluster isolated from Streptomyces hygroscopicus ATCC 29253, a rapamycin-producing strain. Gene 203, 1-9. 

To date the largest silver-tellurium cluster isolated and structurally characterized, albeit in moderate yields, is the complex [Ag48(ya3-TeBu")24Tei2(PEt3)i4] 6. Cluster 6 is obtained as the only crystalline product from the reaction of AgCl/PEt3/Te(Bu")SiMe3 (1 2 1) it is isolated as dark red crystals after several days (Scheme 2, Fig. 9). ... 

The largest copper selenide clusters isolated to date possess 140 or 146 copper atoms with 70 or 73 selenium ligands, respectively. The molecular stmctures of 34, 36 [23], and 37 are shown in Figure 3.52. 

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