Controlling the Reacttvtty of C 60 : A Theoretical Analysis of the Electronic States Modulated by Substitutional Chemist
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ABSTRACT The availability of large amounts of Buckminsterfullerene has allowed a plethora of experimental investigations on fullerenes. The chemical and physical studies have focussed on synthesis, isomerism, magnetism, spectroscopy and high temperature superconductivity in doped materials. The chemical reactivities of fullerenes have been defined and most of the studies are dominated by C60 isomers. Some of the observed activities of fullerenes parll d" of alkeies. In our previous studies, the reactivity of the 6-6' bond with respect to eco- addition was described. Current studies have exploited the olefinic nature of the 6-6' bond and analyzed the effect of the addition on°tluster stability and frontier character. In this work, we describe the mechanisms of simple substitution and analyze stability and orbital effects for the addition chemistry of Cw with multiple species. Evolving changes in orbital frontier character are analyzed with respect to site directed chemistry exhibited by C60. INTRODUCTION The availability of large quantities of fullerenes has aroused increasin research i the cage chemistry of C,'. Multiple endo- and exo- hedrally substituted fullerenes have beon prepared24. In most of these objectives def-inte patterns in reactivity and regio-chemisay hav beon observeds'. For example, metal substitution occurs at the 6-6 site, promoted or stabilized by back bonding', while other substitutional chemistry is more complex. In the present work, the electronic effects induced by substitution are analyzed yielding information about the evolution of the frontier chemistry during the course of the addition. For addition, the stability of the isomers is described in terms of seemingly competing effects: frontier orbital chemistry and fuilerene cage stability. Here, we describe electronic effects and stability factors that drive the regiochemistry for a variety of groups ranging from simple halogem, to nitro sad amine groups to perhalomethyl substituents. The analysis of the frontier orbital character"' and isoawm preference information""'3 proves very important for analysis of reaction chemistry and spectral characterization of fullerene materials. COMPUTATIONAL METHOD AND MODELS The C6 geometry utilized in the calculations was obtained from X-ray crystallographic data'. The structures of the substituted species were obtained from molecular mechanics optimization using the MM2 force field. Four possible substitutional sites were analyzed as shown in Figure 1. These include 1-3 (meta), 1-4 (para), 1-2 (ortho, 5-6 and 6-6' sites). The 5-6' and 6-6' sites are distinct due to die threefold symmetry observed in the icosohedron.
Figure 1. Two types of bonds of C,. 211
Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
The electronic structure calculations have relied on applications of the extended HOckel method has already demonstrated utility in describing hydrocarbon and C. chemistry"'. The method developed by Hoffmann et al". describes relative energy trends extremely well and gives an exc
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