Electronic structure of endohedral Sc@C82;: an ab initio Hartree-Fock analysis

The electronic structure of endohedral Sc@C-82 has been studied by different computational techniques. To optimize the geometry of Sc@C-82 and the ban C-82 cage, ab initio Hartree-Fock (HF) calculations in the minimal STO 3G basis have been performed. The split-valence 3-21G* basis set has been used to derive the Sc binding energy and atomic net charges. The optimized position of Sc is strongly off-center. The coordination number of the endohedral atom is six. Sc is located above the 6-6 bond of a pyracylene unit in the pocket region of C-82, a geometrical arrangement which minimizes the Sc-C contacts to the carbon atoms of the second coordination sphere. The spatial reorganization process of the carbon atoms in the transition from C-82 to Sc@C-82 an compared with spatial reorganization processes encountered in the transition from pyracylene C14H8 to the corresponding dianion C14H82- and from icosahedral C-60 to the dodeca anion C-60(12-). The excess electrons in the corresponding systems lead to a suppression of the bond-length alternation in the pentagon fragments. The pi electronic structure of the different systems has been described by simple 'resonance' structures. Furthermore, we have discussed the C-13 and Sc-45 hyperfine interaction (hfs) in Sc@C-82. The C-13 hfs data have been simulated by local spin density and INDO calculations. The present findings are compared with experimental observations and the results of other computational studies. (C) 1998 Elsevier Science B.V.