A free electron model for buckminsterfullerene and isolation and characterization of endohedral metallofullerenes.

Abstract

A free electron molecular orbital model for C$\sb{60}$ was developed within the independent electron approximation. Computational techniques were adapted from well-known crystal field formalisms to yield the energy levels and orbitals of a particle restricted to the surface of a sphere, perturbed by the 60 carbon nuclei. The free electron model provides a conceptually simple way of rationalizing the electronic energy levels and molecular orbitals of C$\sb{60}$. When the perturbing potential of the carbon atoms is written in an appropriate way, the high symmetry of C$\sb{60}$ can be used to greatly reduce the computational problem. The model is internally consistent, and is in substantial agreement with other models at a similar level of approximation. The qualitative predictions regarding the visible-UV spectrum of C$\sb{60}$ are reasonable. Fullerenes and endohedral metallofullerenes were produced in a modified carbon-arc reactor. The addition of helium streams blowing continuously through the arc allowed added control over the process and helped elucidate some factors governing fullerene yield. Metallofullerenes were shown to have significantly different solubility characteristics than empty fullerenes, allowing the production of highly enriched mixtures of metallofullerenes via serial extraction of the soot with different solvents. Metallofullerenes were efficiently isolated from the enriched extracts on HPLC columns packed with tetraphenylporphyrin-silica gel. Some versions of these columns were sufficiently selective to allow substantial purification of metallofullerenes directly from non-enriched extracts. Electron spin resonance studies of semi-purified extracts showed that while La@C$\sb{82}$ exists in two distinct forms, the two species are not in equilibrium, thus ruling out the possibility that there are two inequivalent sites within a single C$\sb{82}$ cage between which the lanthanum ion hops, and affirming the notion that the two species have different C$\sb{82}$ cage structures.