Favorable Structures for Higher Fullerenes
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FAVORABLE STRUCTURES FOR HIGHER FULLERENES T. G. SCHMALZ, D. J. KLEIN, AND X. LIU* *Texas A&M University at Galveston, Department of Marine Sciences, Galveston, TX 77553- 1675
ABSTRACT Methods have been developed for the systematic generation of all fullerene isomers and for screening them to identify potentially stable structures. Some structural features associated with stable fullerenes are identified.
INTRODUCTION The suggestion in 1985 by the Rice group that a hollow truncated icosahedron of 60 carbon atoms might constitute a third stable allotrope of elemental carbon [1] set off a flurry of theoretical interest in the buckminsterfullerene structure which has been intensified by a continuing stream of experimental observations of the remarkable properties of this substance. By contrast, comparatively little theoretical attention has been directed towards other fullerene structures. However, it is now evident that the electric arc synthesis [2] of buckmninsterfullerene produces small amounts of higher fullerenes in addition to a substantial fraction of C 7 0 . Several of these larger fullerenes have already been separated in macroscopic quantities [3], and it seems likely that as synthetic techniques improve others will become available as well. These higher fullerenes are generally found to exist as a mixture of isomers [3], in contrast to the unique structures observed for C 6 0 and C 7 0 . While a variety of stable fullerenes can apparently be produced, it is also evident that these represent only a small fraction of the mathematically possible fuilerene structures. We have therefore been interested in developing theoretical methods capable of surveying fullerene structures and identifying those with special stability. Besides alding in the structural determination of new fullerenes, we hope in this way to be able to identify features common to stable fullerenes. Understanding the structural characteristics of the fullerenes, in turn, may provide insight into their mechanisms of formation, destruction, and chemical reactivity, though at the moment this remains a goal for future research. One of the striking features of buckminsterfullerene is its high symmetry, and it was initially suspected that its high symmetry was related to its stability. We now believe that symmetry is not the most important criterion of stability, and that the symmetry of buckminsterfullerene is more or less forced upon it by other more fundamental requirements. This means that any systematic search for stable fullerenes will have to include structures of all symmetries. While the number of highly symmetric fullerenes is manageable small, the number of possible fullerenes of all symmetries is much larger. For example, there are 1,812 isomers of C 6 0 fullerene [4] only five of which have a point group of order 12 or greater [5],and there are 8,112 isomers of C7 0 fullerene [6] only three of which have point groups of order 12 or greater [5]. Thus the direct use of ab initio quantum chemical methods to search for stable structures wo
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