Structural Determination of Metallofullerene Sc_3C_(82) Revisited: A Surprising Finding

摘要

Endohedral metallofullerenes have attracted wide interest in recent years because of the promising material, catalytic, and biomedical applications. The structures and electronic properties of endohedral metallofullerenes have been extensively investigated both experimentally and theoretically. It is widely accepted that the maximum entropy method (MEM)/Rietveld analysis of synchrotron X-ray powder diffraction data is powerful for structural determination of endohedral metallofullerenes. Since the first application to Y@C_(82) in 1995, the endohedral structures of representative metallofullerenes, such as Sc@C_(82), La@C_(82), Eu@C_(82), Gd@C_(82), Sc_2@C_(66), Sc_2@C_(84), La_2@C_(80), Sc_2C_2@C_(84), and Sc_3@C_(82), have been determined by the MEM/Rietveld method. However, these structures do not always correspond to energy minima or most stable structures. For example, the MEM/Rietveld analysis of Sc_3@C_(82) has shown three Sc atoms encapsulated inside a C_(3v) isomer of C_(82) as a trimer with extremely short Sc-Sc distances of 2.33 A. On the other hand, Kobayashi and Nagase calculated that the endohedral structure is not an energy minimum; it is energetically much more favorable that three Sc atoms are encaged inside another C_(3v) isomer of C_(82) with much longer Sc-Sc distances of 3.6-4.0 A.