Structure dustering

Veliah S, Xiang K H, Pandey R, Redo J M and Newsam J M 1998 Density functional study of chromium oxide dusters structure, bonding, vibrations, and stability Phys. Rev. B 102 1126... 

For this kind of toxicity specific structural prerequisites arc not required, thus leading to some spread of the compounds in stmctural space. About five of nine layers showed little tendency to duster, some of them with quite specifically acting... 

Tlie recognition of tlie imporiance of duster structure has resulted in a new un-detstanding of tlie role of a dummy ligand (Y) in tlie diemistry of mixed cuprates MeCulYiLi [145], As sliown in Sclieme 10.15 for tlie case of Y = ahty nyl, tlie... 

Figure 6-5. Excitation cncigics horn the ground stale to the two Davydov eoinponents (a and b ) as a function of die Th duster si/.e. The clusters are built with the crystal structure considering T(1 molecules lying within the same be layer.

Fig. 2. Single-particle electronic structures of (a) H4Si4Hi2, (b) Si5H12 (host), (c) Si4H12 (vacancy), and (d) Li4Si4H12, using the scattered-wave Xa duster method (Reprinted with permission from the American Physical Society, DeLeo, G.G., Fowler, W.B., Watkins, G.D. (1984). Phys. Rev. B 29, 1819.)...

In contrast to the polyhedral boranes B H +m there exist a number of neutral boron duster molecules B X (X = Cl, Br, I, NR2, R) all of them having closed deltahedral structures in spite of the fact that the number of bonding electron pairs is only n. For this reason these homonudear cluster compounds of boron are called hypercloso polyboranes. However, there also exist anions of type B X 2 which fit Wade s rules. 

The sizes of the dusters discussed are similar, and their diameters are slightly larger than that of a benzene ring. Therefore, it might be expected that they do not add too many additional steric demands to an organic molecule when they are covalently bound to structures such as porphyrins or sugars, or replace aromatic structures such as a phenyl ring in amino acids. 

With respect to the naked metal atoms, this is the largest metalloid duster that has ever been structurally determined by diffraction methods. The Ga2 unit in the center of the 64 naked Ga atoms is remarkable and unique in this entire field of chemistry [Figure 2.3-28(c)]. The Ga2 unit, which contains a bond that is almost as short (2.35 A) as the above-mentioned Ga-Ga triple bond (2.32 A) and resembles the Ga2 unit of a-Ga (2.45 A), is surrounded by a Ga32 shell in the form of a football with icosahedral caps [see d-Ga (Figure 2.3-17)]. The apex and base atoms of this Ga32 unit are naked and are oriented towards each other in the crystal in an unusual fashion (see below). The Ga2Ga32 unit is surrounded by a belt of 30 Ga atoms that are also naked . Finally the entire Ga framework is protected by 20 GaR groups. 

There is a series of metallaboranes of the type [SBgHjoM], [S2B7H7M], etc., where M seems to contribute two cluster electrons giving a nido-electron count. Nevertheless, these metallaboranes must be designated as arachno-dusters. Though the principal structure of the 10-vertex nido- and arachno-skeleton is the same, they differ structurally by the position of the bridging H atoms (which are the gunwale positions 5-10 and 7-8 in the arachno-case) and, moreover, in their physical properties, particularly in the sequence of the 11B NMR shift values. We will discuss the ar-10 clusters with formal ni-10 electron count later. 

The ni-11 dusters with their typical monocapped pentagonal-antiprismatic structure (Figure 3.4-9) have been comprehensively investigated. The group 15 and group 16 heteroatoms are generally found in the 4k positions of the pentagonal aperture. 

The bidentate oxazoline ligands 85 and 86 (and derivatives thereof) are excellent reporter ligands, and several studies have used NOEs to determine the nature of their chiral pockets [61, 113, 114, 126]. NOESY studies on the cations [Ir(l,5-COD)(86)]+ and several cationic tri-nudear Ir(iii)(hydrido) compounds [110], e. g. [Ir3(p3-H)(H)5(86)3] +, 87, in connection with their hydrogenation activity, allowed their 3-D solution structures to be determined. In addition to the ortho P-phenyl protons, the protons of the oxazoline alkyl group are helpful in assigning the 3-D structure of both the catalyst precursors and the inactive tri-nudear dusters. Specifically, for one of these tri-nudear Ir(iii) complexes, 87 [110], with terminal hydride ligands at d -17.84 and d -21.32 (and a triply bridging hydride at 5 -7.07), the P-phenyl and oxazoline reporters define their relative positions, as shown in Scheme 1.5. 

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

Future devdopment of SAM-based analytical technology requires expansion of the size and shape sdectivity of template structures, as well as introduction of advanced chemical and optical gating mechanisms. An important contribution of SAMs is in miniaturization of analytical instrumentation. This use may in turn have considerable importance in the biomedical analytical area, where miniature analytical probes will be introduced into the body and taiget-specific otgans or even cell dusters. Advances in high resolution spatial patterning of SAMs open the way for such technologies (268,352). 

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