Exohedral Probes

Huang and Freiser (132, 133) were able to prepare exohedral metal C60 ions [MC60]+ by direct reaction of the bare metal ions Fe+, Ni+, Co+, Cu+, Rh+, and La+ with Cgo vapor produced from a heated probe. The [MC60]+ ions when subjected to low-energy collision-induced dissociation with argon all produced the Cg0 ion. These results show that the metal ions attach to the outer surface of C60. The exohedral metallofullerene ions differ from the endohedral metallofullerenes produced by laser ablation of metal oxide-graphite mixtures and support the observations of Smalley and co-workers (148) who found that endohedral metallofullerene ions dissociate by loss of C2 units. 

The simulation of the EPR-spectrum of N Cgi(COOEt)2 (Fig. 39) taking into account a fine structure interaction is in nice accordance with the experimental spectrum. The simulation was carried out with the hyperfine interaction and g-factor of unmodified N Cgo and the fine structure interaction 0 2=2 G and E = 0.13 G (non-axial term). The shape of the extra lines definitely requires the inclusion of a non-axial term E, indicating that the molecular structure of the adduct induces some non-axiality which is not averaged out by fast axial rotation. The non-axial term was also observed at a measurement at 100°C showing that axial motional averaging does not take place even at this temperature. These results show that like He Cgo the new endohedral compound N Cgo can be used as a probe to monitor exohedral chemical addition reactions. Due to higher sensitivity, EPR requires less material than NMR.3He Cgo and N Cgo are complementary probes since different interactions are measured. 

Compared to Ceo and C70, the chemistry of the higher fullerenes is still in its infancy and is essentially restricted to reduction and addition reactions of the most abundant isomers of C76, C78, and Cyclopropanation has emerged as a versatile means of derivatization, but no fulleroid-type derivatives have been reported so far. Even if these could be isolated eventually, assignment of the plethora of possible regioisomers will be extremely difficult. Thus, exohedral NMR probes will, at least in the near future, be of limited use for the study of ring currents in the higher fullerenes. 

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