Endohedral fullerenes electron transfer

These examples demonstrate the well-known process of polymerization initiated by anion-radicals. Our next consideration is devoted to an unusual case of initiation. Intercalation of fullerenes by metals results in the formation of fullerene-metal derivatives. Paramagnetic metallofullerenes (anion-radicals) are the fullerenes doped with endohedral metal. According to calculations and structural studies, LaCs2, for example, contains La in the center of one hexagonal ring of the fuller-ene cage (Akasaka et al. 2000, Nishibori et al. 2000, Nomura et al. 1995). Intrafullerene electron transfer in metallofullerenes is possible (Okazaki et al. 2001). 

Additional electrons can be transferred to the C82 molecules via alkali-metal intercalation under standard intercalation conditions, for both systems saturation is reached at six K ions per endohedral fullerene molecule. 

Rare earth endohedral metallofullerenes are an interesting class of full-erenes because electron transfer from the encaged metal atom to the carbon cage has been known to occur and this dramatically alters electronic and magnetic properties of the fullerenes. 

Absorption spectra of endohedral mefallofullerenes in fhe ulfraviolet-visible-near IR (UV-Vis-NIR) region are unique as compared with those of empty fullerenes. Normally, the absorption spectra of mefallofullerenes have long fails fo fhe red down fo 1,500 nm or more. The absorption spectra of fhe major isomers of mono-mefallofullerenes M Cs2 (M = Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) are similar to each other and well represented by a sharp peak around 1,000 nm and a broad peak around 1,400 nm. These absorption peaks may be related to the intrafullerene electron transfers from the encaged metal atom to the carbon cage. 

The alkali metal fullerides, M3C60 (M = Na, K, Rb, and Cs), are a little different. The absence of the sign indicates that these are not endohedral fullerenes. Take K3C6O a representative example. It is prepared from stoichiometric amounts of solid C o potassium vapor. The three potassium atoms transfer their valence electrons to the fuUerene so that K3C60 more accurately written as (K )3(C o ) This special compound becomes a superconductor below the critical temperature, 7, of 19.3 K. (Below the 7, a superconductor is perfectly conducting—that is, it has zero resistance.) It has a face-centered cubic structure of C o anions with potassium cations in both the octahedral and tetrahedral holes as shown in Figure 15.9. (See p. 173 for more details on the positions and numbers of these holes.)... 

A first principles DFT investigation of the electronic structure of potassium doped C ) was carried out by Saito and Oshiyama [60]. Three different positions of the potassium atom were considered the endohedral center of C 0 and the tetrahedral and octahedral interstitial sites. The most stable position has been predicted for the tetrahedral site with a cohesion energy of 10 eV, followed by the octahedral site ( 8 eV) and the center of the fullerene molecule ( 6 eV). It was revealed that, in all cases, the 4s electron of the K transfers to the LUMO level of C60. Thus, in the frc.c. A3C60 phase three valence electrons of alkaline metals transfer to the lowest unoccupied level of each fullerene molecule and form the half filled conductivity band. In the case of b.c.c. AeCeo, the tiu electronic level of fullerene is completely filled by electrons, and the material is not conductive. 

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