Clusters, carbon

By far the most important cluster structures are those composed of C atoms. The field of carbon clusters was bom with the discovery of a perfectly symmetric and exquisitely beautiful stmcture consisting of 60 C atoms [207]. The discoverers of this structure, R.R Curl, H.W. Kroto and R.E. Smalley, were awarded the 1996 Nobel prize for Chemistry. The subsequent production of this cluster in macroscopic quantities [208], made possible both its detailed study as weU as its use in practical applications such as the formation of C60 crystals. Following the discovery of C6o several other clusters composed of C atoms were found, as well as interesting variations of these structures. We will mention briefly these other clusters and stmctures, but will concentrate the discussion mostly on Ceo, which remains the center of attention in this field. 

The geometric features of this stmcture are very simple. The 60 C atoms form 12 pentagons and 20 hexagons, with each pentagon surrounded entirely by hexagons, as 

Vinci [209], Perhaps, instead of the misnomer that has prevailed in the literature, a more appropriate nickname for the Ceo cluster might have been archimedene . 

Six sets of five-fold axes of rotation, each axis going through the centers of two diametrically opposite pentagons there are 24 independent such operations. 

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An important class of molecule often described as clusters may better be referred to as micro-particles. This class includes metal, semiconductor and carbon clusters. Particularly interesting are the carbon clusters, C. ... 

Mass spectra from a carbon cluster ion source show strong magic numbers at C, and C d[136]. This led to the... 

Electron-Deficient Boron and Carbon Clusters (ed. with Wade and Williams), 1991. 

The field of fullerene chemistry expanded in an unexpected direction in 1991 when Sumio lijima of the NEC Fundamental Research Laboratories in Japan discovered fibrous carbon clusters in one of his fullerene preparations This led within a short time to substances of the type portrayed in Figure 11 7 called single-walled nanotubes The best way to think about this material IS as a stretched fullerene Take a molecule of Ceo cut it in half and place a cylindrical tube of fused six membered carbon rings between the two halves... 

Thus far the importance of carbon cluster chemistry has been in the discovery of new knowl edge Many scientists feel that the earliest industrial applications of the fullerenes will be based on their novel electrical properties Buckminsterfullerene is an insulator but has a high electron affinity and is a superconductor in its reduced form Nanotubes have aroused a great deal of interest for their electrical properties and as potential sources of carbon fibers of great strength... 

The question that began the fullerene story the possibility that carbon clusters are formed in stars moved closer to an answer in 2000 when a team... 

Buckminsterfullerene (Chapter 11 essay Carbon Clusters Fullerenes and Nanotubes ) Name given to the Cgo cluster with structure resembling the geodesic domes of R Buck minster Fuller see front cover... 

Benzene Dreams and Creative Thinking Carbon Clusters Fullerenes and Nanotubes... 

Acknowledgements—Japanese authors are indebted to M. S. Dresselhaus and G. Dresselhaus of MIT and to A. Oberlin of Laboratoire Marcel Mathieu (CNRS) for their useful discussions and suggestions. HWK thanks D. R. M. Walton for help and the Royal Society and the SERC (UK) for support. Part of the work by ME is supported by a grant-in-aid for scientific research in priority area carbon cluster from the Ministry of Education, Science and Culture, Japan. 

In particular, the laser melting experiment produced two well-differentiated populations of carbon clusters (a) spheroidal diamond particles with a radial texture... 

One of the main scientific issues of the discovery of the bucky-onions is the unresolved question of minimal energy configuration of carbon clusters (onion-... 

R. E. Williams, Coordination number-pattern recognition theory of carborane structures, Adv. Incrg. Chem. Radiochem. 18, 67-142 (1976). R. E. Williams, Chap. 2 in G. A. Olah, K. Wade and R. E. Williams (eds.). Electron Deficient Boron and Carbon Clusters, Wiley, New York, 1991, pp. 11-93. 

Klanberg, F., Askew, W. B. Guggenberger, L. J. (1968) Preparation and structure of Co3(CO)ioBH2N(C2H5)3, a new tricobalt enneacarbonyl carbon cluster, Inorg. Chem. 7, 2265-2271. 

The experimental work reported for carbon clusters ranges from the early investigations of Honig and Drowart on small clusters(1-3) to the recent studies in the groups of Smalley and Kaldor(10-16) on larger clusters with up to 200 atoms. 

Evidently, several aspects of this exciting area are difficult to study with experimental techniques. The different species are short-lived, reactive, and exist only under rather extreme conditions. These are conditions under which theoretical studies can contribute a lot to our understanding. Theoretical work has indeed been reported on smaller clusters with n-2 -10(42-50) as well as on some of the larger ones(25-41). The present work reports ab initio calculations for a number of large carbon clusters of relevance to the chemical problems addressed above. 

If the focus of interest is on the carbon clusters themselves, then of course no substitute system can be used. However, for studying the convergence of properties towards bulk values one can minimize the termination effects by saturating the dangling bonds in the simplest possible way, i.e. with hydrogen. By that approach one can both avoid the problem of handling an excessive number of open shells, and obtain a series of molecules that converge towards bulk properties more smoothly than the bare carbon clusters. 

The ionization potential (7.9 eV) falls right outside the bracket of experimental IP s reported for carbon clusters with 40 to 100 atoms (6.42 eV IP 7.87 eV, Ref. 11). Inclusion of correlation effects will lower the calculated ASCF IP by 0.25 to 0.50 eV, so that the corrected IP will be at the upper end of the experimental IP>bracket. Due to the diffuseness of the n orbital from which an electron is removed, the correlation error in the ASCF value will be smaller than in cases where an electron is removed from a well localized bond. In these cases a correction of 1 eV is usually applied. 

The fact that only ethylene and tetramethylethylene are evolved from exp-[8]rotane 168 and permethyl-exp-[6]rotane 173 upon thermal decomposition leads to the conclusion that the spirocyclopropane moieties in these expanded [n]rotanes fragment only externally and leave carbene moieties behind. Indeed, the MALDI-TOF mass spectra of several exp-[ ]rotanes show fragment ions with M minus 28. Thus, if this fragmentation in an exp-[n]rotane were to continue n times, a cyclic C carbon cluster would be left over. So far, however, a fragment ion with m/z = 480 corresponding to 182 has not been recorded in the mass spectrum of exp-[8]rotane 168 and it remains to be seen whether a Cgo cluster 183 will be detected in the mass spectrum of exp-[12]rotane 171 (Scheme 35). 

Scheme 35. Potential approaches to cyclo-Cio 182 and cyclo-C o 183 carbon clusters...

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