[email protected] Ceo LaigJCso center off-center

[email protected] Ceo LaigJCso center off-center

Fig. 12.4. Energy level diagram at the HOMO-LUMO gap region: for C«, (a), LatgQo with La at the center of the shell (b), and the La at an off-center position corresponding to the lowest energy state (c) [12.46]. In this diagram the solid lines denote filled states and the dashed lines denote empty states. The presence of the endohedral La splits the degenerate tlu and hu levels of C«,. The numbers 1, 3, 5 refer to orbital level degeneracy and the letter d denotes admixture of La 5 d wavefunction to the indicated fullerene levels.

Experimental photoemission experiments on the [email protected] cluster [12.48] support the calculated results (see Fig. 12.13), showing that two electrons are transferred to the carbon shell per alkaline earth dopant. In contrast, calculations of the electronic structure for [email protected] and [email protected] [12.43,49] do not show such charge transfer.

Motivated by the extraction and isolation of higher-mass endohedrally doped fullerenes, namely [email protected], [email protected], [email protected], SCjiaJC^, etc., from the soot (see §5.4) and by experimental EPR spectra in [email protected] and other metallofullerenes [12.50-54] (see §16.2.1), calculations of the electronic structure of metallofullerenes have been carried out, for specific fullerene isomers. The emphasis of the calculations has been directed to identifying the most stable isomers supporting endohedral doping [12.55,57,58,130], identifying the amount of charge transfer, and the location of the dopant species within the fullerene shell [12.46]. These calculations have also been directed toward obtaining the best agreement with experimental EPR spectra [12.59], photoemission data [12.48,60], and other physical measurements. Self-consistent calculations, for which the carbon shell atoms are allowed to relax in response to the endohedral dopant, show La3+ to be the stable charge state for forming [email protected] In contrast, La2+ is stable for exohedral doping, although endohedral doping yields a lower overall energy state. The bonding between the internal La3+ ion and the carbon cage atoms leads to extensive differences between the properties of the doped and undoped fullerenes [12.60]. These calculations also show significant differences in electronic properties, depending on whether the La is on an on-center or off-center position within the metallofullerene. Since La3+ donates three electrons to the fullerene shell, two electrons can be paired, but one remains unpaired, forming a singly occupied molecular orbital (SOMO) for the isolated [email protected] metallofullerene. It is reported [12.60] that in the solid phase relatively strong intermolecular interactions split the SOMO-derived band and the occupied portion of this band is centered 0.64 eV below EF, so that [email protected] is expected to be nonmetallic. The La 5d states do not strongly mix with the it-cage orbitals and the La 5d states remain localized and empty about 1.2 eV above the SOMO level [12.57]. However, the La 5p core states do hybridize with the 7r-cage orbitals. The intermolecular bonding for [email protected] is found to be considerably stronger than for C82 itself, leading to a greatly reduced vapor pressure of [email protected] [12.61]. Regarding the molecular distortions, the calculations (see Fig. 12.5) show that the largest displacements of the carbon distances on the C82 shell occur for carbon atoms close to the off-center La ion [12.60]. Because of the large amount of experimental activity in this area, the calculation of the electronic structure of various metallofullerene isomers is expected to remain an active field for some time.

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