Clustering of alkali

Access to clusters of principally all elements in the periodic table has opened new possibilities for test of calculations and comparison with available experimental data and what is known from surface science. The geometry of the free clusters will, as discussed above, be determined by the electronic properties of the constituent atoms. This means that clusters of alkali and coinage metals will in a first approximation be determined by the free electrons while clusters of transition elements will be determined by the balance between the nd and (n -k l)s valence electrons. Noble gas atoms will behave as hard spheres, which under certain thermodynamic conditions can form larger clusters of icosahe-dral symmetry [134,135]. The geometry of these free clusters are quite different what one obtains if the cluster is constructed as a piece of the lattice known for the corresponding solid. 

Particularly exciting in cluster science has been how the simple jellium model has been able to in first order explain many quantities for clusters of alkali and noble metals as the appearance of magic numbers. Examples were given in Fig. 3. We have in connection with our studies of the reactivity of... 

Stable configurations of ionic clusters (M X-)n and (M +X 2)n have been calculated for a number of species on the basis of a simple Born-Mayer approach by minimizing the total energy. (42-44) The vibrational frequencies evaluated for small clusters of alkali halides compare well with the FIR data of matrix isolated species. (43)... 

It is well known from biological isolates, that buffer additives, e.g. borate, can completely inhibit the detection of ions. Another effect is clustering of alkali metal ions with the... 

B. Clusters of alkali atoms give shell closings... 

The mono-valent metals have also been found to show a certain type of periodicity dominated by an odd-even alternation. One example of the odd-even alternation is abundance spectra, where the even clusters are more abundant than the neighbouring odd ones. Ionization potentials have also been measured for clusters of Cu [99] to be higher for the even than for the odd sizes. Fig. 11. In addition to the odd-even alternation in the ionization energies, as shown in the figure there is an additional periodicity with high values for sizes 20, 34 and 40, which are the very numbers found for clusters of alkali elements. Values obtained are strongly dependent on the temperature of the cluster and a prerequisite for resolving this type of odd-even alternation is that the clusters are sufficiently cold. 

Despite the existence of basic sites in zeolites, the possibility of using zeolites as basic catalysts was forgotten for many years and it was realized only recently that they can also be successful in this field [56,57]. Two approaches have been used to prepare basic zeolites ion-exchange with alkali metal ions and generation within the pores of small clusters of alkali metals or oxides and alkaline earth oxides. Whereas simple ion-exchange with alkali metal ions produces relatively weakly basic sites, the presence of such clusters results in strongly basic sites. 

In previous work on clusters of alkali metal atoms, in addition to the very pronounced local concentration of charge at the origin in momentum-space (very laminar slow electrons), we have also observed topological features in A2Il(p) outside the momentum-space... 

Glendening, E. D. Natural energy decomposition analysis Explicit evaluation of electrostatic and polarization effects with application to aqueous clusters of alkali metal cations and neutrals. / Am. Chem. Soc., 118, 2473-2482, doi 10,1021/ja951834y (1996). 

Clusters of alkali metals and especially of sodium are the most studied of all. From the theoretical point of view, sodium is the one most amenable to treatments with simple models. The free-electron behavior known for the bulk phase has suggested that jellium-like models could also be suitable for small-size aggregates. By means of these models, in fact, a large variety of measurable properties have been calculated. This in turn has allowed the approximations used to be tested at several levels [121]. Two comprehensive and very instructive reviews have been dedicated to both experimental and theoretical approaches to simple metal clusters with an emphasis on phenomenological aspects and jellium or jellium-derived models [4, 5]. Here we shall report on DFT calculations that go beyond the assumption of a homogeneous, positively charged background. 

The thennal evaporation source was the earliest used to produce metal clusters in the gas phase [H, 12 and 13], mostly for clusters of the alkalis and other low melting point materials. In this technique, a bulk sample is simply... 

Herrmann A, Leutwyler S, Schumacher E and Woste L 1978 On metal-atom clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules Hel. Chim. Acta 61 453... 

Abstract—Clusters of and C70 coated with alkali or alkaline earth metals are investigated using photo-... 

By incorporation of alkali metal halides in the reaction mix, materials of composition M4-[MfiXig] can be produced in which each MgXn unit has a further six X atoms attached to its apices, so forming discrete clusters. 

Since the demonstration by Schumacher et al ) of the use of alkali metal vapor inclusion into a supersonic beam to produce clusters, there have been a number of attempts to generalize the approach. It has recently been recognized that instead of high temperature ovens, with their concommitant set of complex experimental problems, an intense pulsed laser beam focused on a target could be effectively used to produce metal atoms in the throat of a supersonic expansion valve. ) If these atoms are injected into a high pressure inert gas, such as helium, nucleation to produce clusters occurs. This development has as its most important result that clusters of virtually any material now can be produced and studied with relative ease. 

As already shown in Eqs. (3), (4) and (5), fragmentation of carbonyl cluster anions, on reaction with carbon monoxide becomes easier with increasing negative charge. It is therefore not surprising that reduction of tetranuclear clusters with alkali metals in excess, gives rise to simple mononuclear derivatives [M(CO)4]-, as found for Co4(CO)12 in liquid NH3 13, e Rh4(CO)12 in THF under C056) and Ir4(CO)12 in THF under CO 17S ... 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

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