Aluminium clusters

The tetraanionic [H2O Al3(/r-OH)(/r-OR)3]" [R = C(CH2C02)2C02] moiety has been achieved by the combination of (H20)9 Al(N03)3 with citric acid [101], with the synthesis establishing the existence of polynuclear complexes of a type previously postulated [102, 103], Al NMR spectroscopic studies have established that the tris(aluminium) cluster (Section 3.2.5) is not fluxional in D2O but instead retains three distinct metal environments over a wide pH range (3.0-9.0) with acid hydrolysis occurring below pH 3.0 and the resultant spectrum revealing the presence of [(H2O), (RO)Al3] and/or [(H20) (ROH) A13] " and also [(HjOlg Al] "" [101]. 

Various refinements of the above model have been proposed for example, using alternative spherical potentials or allowing for nonspherical perturbations,and these can improve the agreement of the model with the abundance peaks observed in different experimental spectra. For small alkali metal clusters, the results are essentially equivalent to those obtained by TSH theory, for the simple reason that both approaches start from an assumption of zeroth-order spherical symmetry. This connection has been emphasized in two reviews,and also holds to some extent when considerations of symmetry breaking are applied. This aspect is discussed further below. The same shell structure is also observed in simple Hiickel calculations for alkali metals, again basically due to the symmetry of the systems considered. However, the developments of TSH theory, below, and the assumptions made in the jellium model itself, should make it clear that the latter approach is only likely to be successful for alkali and perhaps alkali earth metals. For example, recent results for aluminium clusters have led to the suggestion that symmetry-breaking effects are more important in these systems. ... 

Figure 14 Substituted galliinn and aluminium clusters [GagRg] (R = fluorenyl) and [Alni-Buu] (not all i-Bu groups shown)...

Fig. 3a-f. Structures of the most stable isomers calculated for aluminium clusters Al , n = 5-10 [44]... 

A contraction of the cluster volume with respect to that of an equivalent piece of bulk metal has also been predicted [96]. The calculated cluster radius is smaller than the radius assumed in the spherical jellium model, where the volume is the same as that of an equivalent piece cut out of a macroscopic metal. This global contraction seems to be a general feature of small metallic clusters, and is well documented experimentally [97]. The volume contraction explains the discrepancies between experimentally determined static polarizabilities of small aluminium clusters and those obtained from jellium calculations [98]. The measured polarizabilities of Aljv clusters with N < 40 are smaller than those predicted by a SJM calculation. The classical static polarizability (per atom) for... 

Other non-local density functional schemes have also been used to solve the problem of the LDA in relation to the electron affinity of atoms and clusters. One of those non-local schemes is the weighted-density-approximation (WDA), briefly mentioned in Sects. 2.2 and 2.3 above. In this scheme the large-r behavior of V,c(f) is given correctly (see Ref. [48] for details) and, as a consequence, electron affinity levels are bound. Figure 7 gives the calculated affinities of Aluminium clusters [49]. The spherical jellium model was used in this calculation and the affinities were obtained from the total energies of neutral and... 

A plot of experimental values of I — A versus is given in Fig. 9 for aluminium clusters [8]. The predicted linearity between the hardness and N ... 

There is interest in assembling solids from clusters because of the possibility of building synthetic solid materials with novel properties. One of the best known examples is the Ceo crystal. Recently, assemblies of mass selected gold nanoparticles passivated by a monolayer of thiol molecules have been prepared but much work remains to be done in developing such assemblies. For this purpose the clusters must retain much of their individual character after assembling, which is what happens to Ceo in the fullerite crystal. This condition is difficult to meet for metallic clusters because the interatomic interactions are more delocalized. Proposals for assembling doped aluminium clusters have been investigated by ab initio computer simulations ... 

Magic numbers in spectra of calcium and strontium clusters also indicate icosahedral growth, while aluminium clusters appear to adopt an fee structure with octahedral morphology. Magic numbers observed when metal atoms are grown around a cluster... 

Figure 8 Mass spectrum of aluminium cluster anions before (upper) and after (lower) reaction with oxygen. Al,3 is least reactive. After Leuohtner RE, Harms AC and Castleman Jr. AW (1989) Journal of Chemical Physics 91 2753.

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