Structural Isomers of C 70
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STRUCTURAL ISOMERS OF C7 0 KRISHNAN RAGHAVACHARI AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT Alternative isomeric structures of C70 have been investigated using semiempirical and ab initio quantum chemical techniques. As in the case of C 60 , these isomers are characterized by the presence of pentagonal rings adjacent to each other. The lowest energy alternative isomer of C70 has only one pair of edge-sharing pentagons and lies = 1.4 eV higher in energy than the ground state. This energy difference is smaller than that for the lowest energy alternative isomer of C 60 which contains two pairs of adjacent pentagons and lies = 2.0 eV higher in energy than its ground state. INTRODUCTION The abundance of macroscopically isolated fullerenes such as C 60 and C70 [1] has been interpreted based on the "pentagon rule" which suggests that spheroidal structures containing adjacent pentagons are unstable [2-4]. Indeed, the stability of C60 and C 70 relative to the smaller carbon clusters can be understood from the fact that they are the two smallest fullerenes where it is possible to form "isolated pentagon" structures not containing edge-sharing pentagons. In recent studies [5,6], we have investigated the structures and stabilities of alternative structural isomers of C 60 . These isomers can be generated by means of a local transformation originally suggested by Stone and Wales [7]. The transformation involves the rotation of two of the atoms (and the bond connecting them) in a paracyclene-like unit by 90*, and interconnects the ground state structure with other isomers containing one or more pairs of adjacent pentagons. For C 60 itself, such a transformation generates an unique isomer containing two pairs of edgesharing pentagons ( two "defects"). This C2 , isomer lies = 2 eV higher in energy than the icosahedral ground state of C 60 [5,6,8]. Consideration of other isomers containing three and four such defects showed that their energy contributions are additive with each defect making the structure unstable by = 1 eV. Our studies [5,6] provided a quantitative foundation for the pentagon rule which has been qualitatively used previously. C 70 is the second most abundant fullerene whose structure and properties are well understood [9,10]. Recently, there has been some experimental evidence [11,12] that alternative isomers of C 60 and C 70 have been seen in chromatographic separations of fullerenes prepared in sooting flames. In this paper, we demonstrate that the lowest energy alternative isomer of C 70 is unique and lies = 1.4 eV higher in energy than the ground state of C 70 . We have optimized the structures of several different isomers using the MNDO semiempirical level of theory [13]. In our previous work on C 60 isomers [5], we found that the structures obtained at the MNDO level are very similar to the optimized Hartree-Fock (HF) [14] structures. Thus, in this work, we have used the MNDO geometries to perform single-point Hartree-Fock calculations with the valencedouble-zeta 3-21G basis set [14,15]. We expe
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