One- and Two-Dimensional Carbon Clusters: Isomers, Structures and Isomer Abundances.
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One- and Two-Dimensional Carbon Clusters: Isomers, Structures and Isomer Abundances. Gert von Helden, Ming-Teh Hsu, Paul R.Kemper, Michael T.Bowers Department of Chemistry, University of California, Santa Barbara,CA 93106 Abstract
The new technique of ion structural chromatography is applied to carbon clusters. The results indicate that C5+ and C6+ are purely linear but C7+, C8 +, C9+ and C1O+ have both linear and monocyclic ring structures. From C11+ to C20 ÷ only monocyclic ring structures are observed. At C21+, a new family of planar ring structures appears. The first 3 dimensional structure occurs at C29 + and the first fullerene at C30÷. Isomer structure is verified by the comparison of experimental mobilities with those derived from theory for the various structures. For C20+ only the monocyclic ring is observed experimentally but electronic structure calculations suggest more compact structures might be lower in energy. The results are discussed in terms of possible growth mechanisms for C60.
Introduction During the last ten years, the structures and properties of metal and semiconductor clusters have been the objects of an intense research effort. Most of their chemical and physical properties are dramatically different from either the isolated atom or the bulk. As a consequence, there is currently an intense interest in determining how these properties evolve as a function of cluster size. Carbon clusters are especially interesting due to their possible importance in combustion and pyrolysis processes,, their importance in astrophysical processes, and the recent synthesis of macroscopic amounts of "fullerene" materials 1 . Due to the directional bonding of carbon, carbon clusters are expected to exhibit complex structures with the possible existence of structural isomers. With the exception of the fullerenes, most carbon clusters, due to their high reactivity, can not be isolated, investigated or analysed by standard means. Structures of carbon clusters other than the fullerenes have been deduced from size distributions and metastable reactions in mass spectra 2 , photoelectron spectroscopy 3, reactivity studies 4 and various computational methods 5 . For some clusters, spectroscopic data is available and the reader is referred to an excellent
Mat. Res. Soc. Symp. Proc. Vol. 270. @1992 Materials Research Society
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review acticle 6 . The reactivity studies of Parent and McElevany 7 indicate unambiguously the presence of different isomeric forms for C7÷, C8+ and C9 +. Photoelectron studies3 on negatively charged ions suggest a structural transition near C-O which the authors ascribe to a transition from linear forms to monocyclic rings. Several high level ab-initio calculations have been undertaken. Results for neutral clusters show that the ground state, in the size range between 6 and 10 atoms, can be either linear or cyclic5h. In some cases, the energies of the linear or cyclic configurations are relatively close and the presence of structural isomers is therefore not unlikely. Recent ab-initio ca
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