Clusters large

Among large metal clusters there are also some heteronuclear or mixed-metal clusters that have been mentioned as metal-alloy clusters . The bimetallic Au-Ag clusters (p-Tol3P)i2Aui8Ag2oCli4 and [(p-Tol3P)i2Aui8Agi9Brn], whose metal frameworks are illustrated in Fig. 2.39, belong to this class. A very 

Electron counting rules available for small and medium-sized clusters appear to be insufficient for very large clusters with coordinatively unsaturated surfaces. These types of clusters could be considered to be actually half way between complexes and metals. 

8 Quantitative Approaches to Delocalized Cluster Bonding Models 

The analogy between boranes and carboranes and transition metal clusters rests on the assumption that the fragments M(CO)x as well as the fragments BH in the boron derivatives contribute with three orbitals to the formation of the metal polyhedron. This analogy between electron-deficient boron species and the electron-rich metal clusters is fundamentally empiric. However there are some theoretical calculations for specific cluster species that make some clarity about the degree of validity of such an analogy. 

The HOMO as well as the LUMO in this cluster have metallic character. The HOMO-LUMO gap is about 80 000 cm so a configuration with more than 86 electrons would be rather unstable. 

Interest in clusters with nuclearity 10 continues, but progress is relatively slow as a result of the comparative difficulties of characterisation, together with the lack of general methods of preparation. In this account, the chemistry of the more interesting large complexes is gathered together in one place. 

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Kruger S, Birkenheuer U and Rdsch N 1994 Density functional approach to moderately large cluster embedding for infinite metal substrates J. Eleotron Speotroso. Relat Phenom. 69 31... 

The efficiency of separation of solvent from solute varies with their nature and the rate of flow of liquid from the HPLC into the interface. Volatile solvents like hexane can be evaporated quickly and tend not to form large clusters, and therefore rates of flow of about 1 ml/min can be accepted from the HPLC apparatus. For less-volatile solvents like water, evaporation is slower, clusters are less easily broken down, and maximum flow rates are about 0.1-0.5 ml/min. Because separation of solvent from solute depends on relative volatilities and rates of diffusion, the greater the molecular mass difference between them, the better is the efficiency of separation. Generally, HPLC is used for substances that are nonvolatile or are thermally labile, as they would otherwise be analyzed by the practically simpler GC method the nonvolatile substances usually have molecular masses considerably larger than those of commonly used HPLC solvents, so separation is good. 

These fibrous minerals share several properties which qualify them as asbestiform fibers (/) they are found in large clusters which can be easily separated from the host matrix or cleaved into thinner fibers (1) (2) the fibers exhibit high tensile strengths (1) (J) they show high length diameter ratios, from a minimum of 20 up to >1000 (1) (4) they are sufficiendy dexible to be spun and (5) macroscopicaHy, they resemble organic fibers such as cellulose (2). 

Annatto Extract. The aimatto tree (Bixa orelland) is a large, fast-growing shmb cultivated in tropical climates, including parts of South America, India, East Africa, and the Caribbean. The tree produces large clusters of brown or crimson capsular fmit containing seeds coated with a thin, highly... 

A very large cluster Au55(PPh3) 2Cl6 of as yet unknown structure has been reported. Physical measurements indicate the bonding to be substantially metallic in character [188]. 

The most difficult part of the theory lies in obtaining actual values for AF and v. For a large cluster of N molecules the extra surface tension due to the incremental surface area, edA, contributes an increase to the total free energy, whilst the bulk free energy per volume summed over the incremental volume, AF dV, gives a decrease to the total free energy. Hence, AF can be estimated as the maximum value of ad A — AF dV as a function of N. It is found that AF is proportional... 

Developments in transition metal cluster chemistry — the way to large clusters. G, Schmid, Struct. Bonding (Berlin), 1985,62, 51 (132). 

The first complex intermetallic compound found to have large clusters of atoms with local icosahedral symmetry was Mg32Al4, which has 162 atoms in a body-centred cubic unit17. The unit cube contains 98 icosahedra, 20 Friauf polyhedra and 44 others. 

Schmelcher PS, Cederbaum LS (1996) Two Interacting Charged Particles in Strong Static Fields A Variety of Two-Body Phenomena. 86 27-62 Schmid G (1985) Developments in Transition Metal Cluster Chemistry. The Way to Large Clusters. 62 51-85... 

We have discussed our theoretical calculations on metals ranging from very accurate ab initio studies of diatomic and triatomic systems to model studies of larger clusters. Recent improvements in the accuracy to which we can represent both the one-particle and n-particle spaces has significantly improved the reliability of theoretical calculations on small molecules. For example, we are able to predict definitively that AI2 has a Hu ground state even though the state lies within 200 cm . Calculations on clusters indicate that their geometry varies dramatically with cluster size, and that rather large clusters are required before the bulk structure becomes optimal. Since clusters are more... 

Large clusters of carbon atoms have been studied with ab initio calculations using basis sets of double-C quality. Planar, single-sheet graphite fragments with 6 - 54 atoms were investigated, as well as the spherical "Buckminsterfullerene ... 

Typical surfaces observed in Ising model simulations are illustrated in Fig. 2. The size and extent of adatom and vacancy clusters increases with the temperature. Above a transition temperature (T. 62 for the surface illustrated), the clusters percolate. That is, some of the clusters link up to produce a connected network over the entire surface. Above Tj, crystal growth can proceed without two-dimensional nucleation, since large clusters are an inherent part of the interface structure. Finite growth rates are expected at arbitrarily small values of the supersaturation. 

The drop in intensity can best be explained by assuming that the molybdenum sulfide form is sintering into large clusters. The model of Kerkhof and Moulijn (20) was used to interpret this data. 

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