Theoretical studies cluster compounds

Metal-metal (M-M) bonds, first noted in the early sixties, occur in several thousand transition-metal compounds [1]. Complex technetium compounds and compounds with M-M bonds (clusters) have been studied more extensively than many other classes of inorganic compounds. Increasing interest in technetium compounds is due to the practical uses of the "mTc isotope, which ranks first among radioactive isotopes used in nuclear medicine diagnostics [2-4]. On the other hand, technetium clusters are an interesting object for theoretical studies, because until recently, they were the only compounds in which the presence of these anomalous chemical bonds was thought possible. 

In comparison with theoretical studies of the complexes in silicon, very little work has been done in the compound semiconductors. We now summarize the theoretical treatments reported by Briddon and Jones (1989) using local-density cluster methods. 

DFT calculations on the model compound Cu6I6(SPhCH3)6 in its triplet state predict that the SHOMO localizes the spin density mainly on one Cu atom and its three iodo neighbors with the rest on one -SPh fragment, whereas it is rather delocalized over the Cu and I atoms in the HOMO (Fig. 32). The SHOMO is located at -0.042 a.u. above the HOMO (-0.125 a.u.). The simple arithmetic difference between the two levels suggests that the radiative relaxation should occur in the vicinity of 550 nm, which fits the experimentally observed maxima (Fig. 31). So the first theoretical study confirms a cluster-centered excited state as responsible for the observed emission, similar to other Cu4I4-containing species.146... 

The nature of the bonding in heteronuclear gold cluster compounds has been investigated primarily by means of semiempirical molecular orbital calculations and the results of these studies have allowed a number of structural generalizations to be made. These theoretical developments have been discussed in detail in a series of papers by Mingos et al. (210, 256, 279-282) and only a brief description of the results of this work will be given in this section. 

Extensive DFT and PP calculations have permitted the establishment of important trends in chemical bonding, stabilities of oxidation states, crystal-field and SO effects, complexing ability and other properties of the heaviest elements, as well as the role and magnitude of relativistic effects. It was shown that relativistic effects play a dominant role in the electronic structures of the elements of the 7 row and heavier, so that relativistic calculations in the region of the heaviest elements are indispensable. Straight-forward extrapolations of properties from lighter congeners may result in erroneous predictions. The molecular DFT calculations in combination with some physico-chemical models were successful in the application to systems and processes studied experimentally such as adsorption and extraction. For theoretical studies of adsorption processes on the quantum-mechanical level, embedded cluster calculations are under way. RECP were mostly applied to open-shell compounds at the end of the 6d series and the 7p series. Very accurate fully relativistic DFB ab initio methods were used for calculations of the electronic structures of model systems to study relativistic and correlation effects. These methods still need further development, as well as powerful supercomputers to be applied to heavy element systems in a routine manner. Presently, the RECP and DFT methods and their combination are the best way to study the theoretical chemistry of the heaviest elements. 

We have also studied the behavior of gas-phase radicals, such as the hy-droperoxyl radical (HO2) [62], in water clusters which is important in atmospheric science (Figure 16.4). The hydroperoxyl radical is a major species in the HOx chemical family [2] that affects the budgeting of many chemical systems in the atmosphere. The HOx system plays a central role (along with the OH radical) in oxidative chemistry in the troposphere and ultimately controls the production rate of tropospheric ozone [7,16]. It is hence considered significant in atmospheric [2,5] and combustion chemistry [184]. Recent theoretical studies [16,17] have indicated the HO2 radical to possess stable interactions with water clusters. Such stability provides an important sink for HOx compounds [16,17,61,185] in the troposphere. As a result, the structural and dynamical features of water clusters play a vital role on HO2 related chemistry. 

A very large number of theoretical studies have been performed on MgO and AI2O3. Only some of the early studies and some of the most recent will be described here, in order to give some idea of the extent of progress over the past two decades. Important advances have recently been made in the application of ionic models to such materials as well as in band-theory studies and embedded-cluster studies. After reviewing the early work, contemporary studies of structure, stability, phase relations, and dynamic properties will be described, followed by recent studies of spectral properties and characteristics of the electron-density distribution for each of these materials. Attention is then turned to Si02, the silica polymorphs, and various compounds and clusters that may be used to model tetrahedrally coordinated Si in silica and the silicates. 

Covering many areas of chemistry with boron at its centre, topics include applications to polyolefin catalysis, medicine, materials and polymers boron cluster chemistry, including carboranes and metal-containing clusters organic and inorganic chemistry of species containing only 1 or 2 boron atoms and theoretical studies of boron-containing compounds. New materials with novel optical and electronic properties are also discussed. 

We could show that the hypersilyl substituent is a very useful group in the synthesis of low valent gallium compounds. Not only the steric demand of this group but also its electronic properties, as theoretical studies have confirmed, contribute to its cluster stabilizing ability [18]. 

Carbon atoms crystallize in several forms. Graphite and diamond are well known carbon polymorphs. Fullerenes, which were discovered in the 1980 s, have also been well characterized. Carbon materials show a variety of different physical and chemical properties. Because of this the electronic structure of carbon materials has been investigated using a number of different experimental techniques, for example, XPS, UPS and XANES. Theoretical studies of carbon materials have been also performed. However, experimentally observed spectra are not always consistent with theoretical predictions. Recently, in order to understand the various kinds of observed electronic spectra, DV-Xa calculations have been performed on a small cluster model. [1] In the present paper, we report results of DV-Xa calculations performed on the carbon materials graphite, alkali graphite intercalation compounds (GIC), fullerene, and fluorinated fullerenes. 

A more complex situation was found for Na Pb. The unusual stability of this cluster was explained by comparison with the analogous Mg compound (Albert et al. 1995) and was found to originate in a larger charge transfer to the more electronegative lead and a larger polarizability of the Pb atom. In a combined experimental and theoretical study it was shown for the clusters Na Au and Cs Au that certain properties of the bulk are qualitatively present at the level of small clusters (Heiz et al. 1995). While the Na compounds show metallic behaviour, and the electronic structure can be described by means of the jellium model, in the Cs-Au clusters an ionic bond is most prominent. 

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