ABSTRACT
In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a diatomic molecule and a bulk solid. At rst, people thought that clusters are simply small pieces of solids possessing the same structures like that of a corresponding solid with a lot of dangling bonds. The breakthrough in understanding peculiarities of clusters occurred in mid-1980s, when Smalley and coworkers realized that C60 is not a piece of graphite, diamond or carbine, but it had a so-called “buckyball” structure [1]. Thus, the C60 cluster was shown to be a high symmetry structure with no dangling bonds at all. Moreover, this discovery opened a new direction in chemistry, the so-called “Chemistry of Fullerenes.” Another spectacular example is the Au20 cluster which was shown to possess a beautiful tetrahedral structure [2] with unexpected chemical bonding based on 10 four center two electron bonds [3] located at the center of every small tetrahedra comprising Au20 cluster. For gold was considered as most inert metal over the centuries, it was used as coinage material as well as to make jewelry. Surprisingly, in 1987 Haruta et al. [4] discovered that nanoscale gold particles have unusual catalytic properties for selective oxidation of CO. Starting from this point it was clear that clusters are neither molecules nor crystals but unique chemical species with yet unknown structures, peculiar chemical bonding, and unexpected chemical reactivity. Since clusters are frequently composed of just a few atoms, their properties, such as structure, stability, and reactivity can be studied computationally using high levels of theory. Experimentally isolated clusters are generated in a molecular beam, where they are free from environmental inuence such as solvent, or counterions in a crystalline lattice. That allows one to study their
3.1 Introduction .................................................................................................... 55 3.2 Chemical Bonding of Metal Clusters ............................................................. 56 3.3 Extending Aromaticity from Clusters to Alloys ............................................. 58 3.4 Extending Aromaticity from Clusters to Solids .............................................60 3.5 Extending Aromaticity from Solids to Clusters ............................................. 62 3.6 Concluding Remarks ...................................................................................... 63 Acknowledgments ....................................................................................................64 References ................................................................................................................64
