Physical Properties of Clusters

Owing in part to the relatively recent development of cluster chemistry, there is an almost complete lack of thermodynamic data for cluster compounds. The measurement of appearance potentials of positively charged ions by mass spectrometry has offered a technique for ready measurement of this parameter, but the high values obtained for ionization potentials and heats of formation of ions by this method cast some doubt on its accuracy (358). It would appear that ions are generated in excited states (22). It may be expected that there will be a considerable increase in the thermodynamic investigations of clusters in the near future, so as to provide quantitative data on which to base theoretical considerations. 

Apart from X-ray diffraction studies, the only physical properties of clusters which have been studied are the spectroscopic properties— the infrared and Raman, electronic, nuclear magnetic resonance, electron spin resonance, Mbssbauer, and mass spectra. 

Symmetry theory is in common use for the prediction of the number of fundamental vibrations active in the infrared and Raman spectra for a particular molecule (100). This has found particular application to the metal carbonyls, where the C-0-stretching vibration produces a very 

In view of these restrictions and the limitations of resolution, structural predictions based on this kind of spectroscopic data should be treated with caution. Overtones of the v(C-O) bands have been used to obtain more information about the fundamental vibrations and, hence, assist in prediction of structure 5, 49, 273). As for mononuclear carbonyls, the frequencies of C-0 stretching bands and force constants calculated from these by simplified force fields have been used as a relative measme of metal-carbon and carbon-oxygen bond strengths. 

The metal-metal vibrations of clusters are expected to produce absorptions in the region 250-100 cm as a result of the heavy atomic weights of the atoms and also because of the low force constants expected for these bonds 150). Until recently, no equipment was available which could detect energy absorptions in this region. New far-infrared spectro- 

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Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Clusters provide the bridge between the gas phase in which molecules are isolated and independent of each other, and the condensed phase, where intermolecular forces provide each liquid its unique properties. It is thus of considerable interest to probe the physical properties of clusters from dimers to very large clusters as they approach the properties of the bulk liquid. Some of these attributes, such as the ionization energy or electronic energy levels, can be defined and measured for the molecule as well as the clusters. Other properties, such as melting temperature and surface tension, are not defined for the isolated molecules but become important for clusters of some size. (Berry, 1990 Wales and Berry, 1990)... 

Ouster science is a relatively new area of research and lies at the borderline between physics and chemistry. Its interdisciplinary character follows simply from the fact that clusters are units of matter with properties intermediate between those of molecules and solids. This is also the main reason why the rationalization of the chemical and physical properties of clusters represents a formidable challenge for theory. In principle, one would like to be able to describe the transition from the molecular state to the bulk solid state within a single theoretical framework, using the same language and, possibly, the same degree of theoretical accuracy. 

In combination with TEM, thus probing on both integral as local (individual) level a deeper understanding of the interplay between geometry, electronic structure and consequently chemical and physical properties of clusters supported on various substrates is possible. 

The spherical shell model can only account for tire major shell closings. For open shell clusters, ellipsoidal distortions occur [47], leading to subshell closings which account for the fine stmctures in figure C1.1.2(a ). The electron shell model is one of tire most successful models emerging from cluster physics. The electron shell effects are observed in many physical properties of tire simple metal clusters, including tlieir ionization potentials, electron affinities, polarizabilities and collective excitations [34]. 

Thiel RC, Benfield RE, Zanoni R, Smit HHA, Dirken MW (1993) The Physical Properties of the Metal Cluster Compound Au55(PPh3) 12C16. 81 41-114 Thomas KR, see Chandrasekhar V (1993) 81 1-40... 

It should be noted that the above classification system of technetium cluster compounds is not the only possible one. In section 4 another classification is described, which is based on thermal stability and the mechanism of thermal decomposition. Section 2.2 is concerned with the classification based on methods of synthesizing cluster compounds. The classifications based on specific properties of clusters do not at all belittle the advantages of the basic structural classification they broaden the field of application of the latter, because for a better understanding and explanation of any chemical, physico-chemical and physical properties it is necessary to deal directly or indirectly with the molecular and/or electronic structures of the clusters. 

The next step in the direction of a deeper understanding of nanostructured materials depends on being able to isolate the individual structurally determined cluster units from the crystal lattice and then determine the physical properties of the single clusters in question. This long-term objective has been partially achieved in the gas phase investigation of a structurally determined Gai9R6 cluster [R = C(SiMe3)3] in an FT mass spectrometer (cf. Section 2.3.4.2.5, Ga clusters)... 

The Physical Properties of the Metal Cluster Compound Aus5(PPh3),2Cls... 

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