"The Hund's rule ground state is denoted by (g).

"The Hund's rule ground state is denoted by (g).

[12.19], yielding the level ordering shown in Fig. 12.3(b), which closely relates to that calculated for C^ in Fig. 12.3(a). The breakdown of the simple symmetry arguments for C70 and given in §4.4.1 indicates that detailed calculations are needed to specify the level ordering for the lower symmetry fullerenes.

12.6.2. Metallofullerenes

Calculations of the electronic structure for the endohedral metallofullerenes [email protected] and CsiaiQo [12.42-44] show charge transfer of one electron, from the endohedral metal atom to the C60 shell. More detailed calculations for a variety of metallofullerenes show that the ions tend to lie off-center and the negative charge on the fullerene cage is delocalized

[12.45.46] (see §5.4 and §8.2). The amount of off-center displacement has been shown to depend on the charge transfer and the size of the ion. The dependence on the ion size is estimated by comparing the equilibrium positions of Na and K in C50 and La and Y in C60 [12.46]. Electronic energy level calculations for [email protected] show a single electron charge transfer from the A1 to C60 and very little off-center displacement of Al+ within the fullerene cage. The corresponding results for LaigiQo (see Fig. 12.4) show a lifting of the degeneracy of the HOMO and LUMO levels of C60 and a significant lowering of the energy of the LUMO levels as the La shifts to an off-center position. A similar result was reported for [email protected]

[12.46.47], although for [email protected] the d levels of the Y are not close to the Qo LUMO levels, as is the case for [email protected], shown in Fig. 12.4.

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