Higher Fullerenes: Structure and Properties
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ABSTRACT The structures of higher fullerenes are described based on 13C NMR measurements of isolated and purified samples. The most interesting aspect deduced from the present structural work on the higher fullerenes up to C90 is that among 19 kinds of fullerenes with different sizes and isomers, 18 fullerenes commonly have at least one C2 symmetry axis in their molecular frame. Only one exception is C90 fullerene with a Ci symmetry. The spectral feature of UV/Visible absorption obtained for the HPLC-isolated C120 fullerene gives, on the other hand, a strong indication that the numbers of co-existing isomer of very large fullerenes are extremely limited, probably one or two. Considering the huge numbers of IPR-(isolated pentagon rule) satisfying isomer of C120 (10,774), these experimental evidences may suggest the presence of very strong selectivity for the formation of stable higher fullerenes, which, in turn, is closely associated with the unknown growth process of a fullerene cage network.
INTRODUCTION Soon after the discovery of large-scale preparation of C60 fullerenel, the presence of much larger all carbon molecules has been found and identified in carbon soot. From mass spectrometric characterization, it has been suggested that these all carbon molecules possess a similar molecular structure and property to C60 and C70. Actually, soon later, several kinds of stable higher fullerenes were isolated and characterized by means of 13C NMR spectroscopy, revealing the presence of a cage structure with a fiveand six-membered ring system 2 -5 . The studies of higher fullerenes, particularly those placed attention to the structure are very important to understand not only the general properties of a novel molecular system consisting of a five- and six membered ring network but also the unknown growth processes of fullerene networks. From mass spectrometric points of view, very large fullerenes up to over 1000 carbon atoms have been detected in the carbon raw soot or solvent extract 6 . However, the structural study of higher fullerenes has still been very limited , and C84 fullerene is the biggest size whose molecular structure was well identified. Among many intriguing aspects of the research of the higher fullerenes, of particular interest is what is the general feature of large fullerenes; is a spherical shape more preferable? or tube like structure? Furthermore, very recent research has suggested that there exists many kinds of new fullerene family, in which single or multiple metal atoms are Mat. Res. Soc. Symp. Proc. Vol. 359 ©1995 Materials Research Society
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thought to be trapped in the fullerene cages 7 . In the most of the cases of metallofullerene formation, the cages of higher fullerenes play an important role on the stabilization of metallofullerenes. In this sense, t
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