Doping endohedral

On the basis of the very small net XRC obtained in the London calculations for Ceo, a very small value for (5rc has been predicted, ca. 0.5 ppm, and a more pronounced shielding in the center of C7o. ° The development of techniques to produce macroscopic amounts of fullerenes doped endohedrally with He, an excellent NMR nucleus, eventually enabled M. Saunders and associates to test this prediction. For Fle C6o and Fle C7o. (5( He) values of —6.3 and —28.8 ppm, respectively, were measured relative to free Fle. The predicted (5rc and experimental (5( He) in the case of Ceo did not agree in a quantitative sense, but qualitatively, the large difference in the Fle chemical shifts between Ceo and C70 was confirmed. This difference also reflects the susceptibility exaltation of the latter over the former. 

Y2 Cg2 denotes two Y atoms inside a Cg2 fullerene [19], Thus far, only small quantities of endohedrally-doped fullerenes have been prepared and only limited investigations of endohedrally-doped crystalline materials have been reported but steady progress is being made both in synthesis and in properties measurements [20]. 

Walter, M. and Hakkinen, H. (2006) A hollow tetrahedral cage of hexadecagold dianion provides a robust backbone for a tuneable sub-nanometer oxidation and reduction agent via endohedral doping. Physical Chemistry Chemical Physics, 8, 5407-5411. 

In contrast to carbon, which forms structures derived from both sp2 and sp3 bonds, silicon is unable to form sp2 related structures. Since one out of four sp3 bonds of a given atom is pointing out of the cage, the most stable fullerene-like structure in this case is a network of connected cages. This kind of network is realized in alkali metal doped silicon clathrate (19), which were identified to have a connected fullerene-like structure (20). In these compounds, Si polyhe-dra of 12 five-fold rings and 2 or 4 more six-fold rings share faces, and form a network of hollow cage structures, which can accommodate endohedral metal atoms. Recently, the clathrate compound (Na,Ba), has been synthesized and demonstrated a transition into a superconductor at 4 K (21). The electronic structure of these compounds is drastically different from that of sp3 Si solid (22). 

Endohedral doping (encapsulation) of other materials within carbon nanostructures can be carried out by nano-capillary effects or during synthesis (Fig. 4.3(b)). A great variety of halides, oxides, metals and alloys have been encapsulated within CNTs [36-41]. When transition metals are encapsulated, the entire sample can exhibit high magnetic coercivities ca. 0.22 T [42,43]. The encapsulation of C60 molecules can also be accomplished and if the material is heat treated at high temperatures... 

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). 

Soot containing small quantities of the endohedral metaUofullerenes can be simply prepared by vaporizing graphite doped with a source of the appropriate metal, typically the metal oxide. The characterization of these materials is discussed in Section 6.3. 

In particular, the Ceo molecule is often a system of choice for manipulation experiments [3-8] because of its compact nearly spherical shape, which makes it easy to image and ensures that its manipulation trajectories are simple enough. Besides its ability to roll, C6o is a prototype molecule for endohedral metal-doped fiillerenes, which are a promising material for quantum computers [9]. Manipulation of C o has been performed using scanning tunneling microscope (STM) on copper [3], gold [4] and silicon [5-8] surfaces. 

Fig. 1.49. Scanning tunneling spectra of endohedrally doped Ceo- (a) La Ceo, (b) Ce C60- Note the band gap around OV bias voltage in the case of Ce Ceo (b) indicative of nomnetaUic behavior [235]...

Tiirker, L. andErkof, S. (2003). AMI treatment of endohedrally hydrogen doped fuUerene, nHjfglCgg.J. Mol. Struct. (Theochem), 638, 37-40. 

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. 

Fig. 34.11. Ground triplet (d D2 ) and excited singlet (s C2a) states of (Cgo) with two views (for the alkali cation doped exohedral fullerenes), and spin-containing endohedral fullerenes [128,148].

Since their discovery, fullerenes and their derivatives have been the subject of very extensive research. One of the topics investigated intensively are the linear and nonlinear optical (NLO) properties, owing to a variety of possible applications. Here we review some of the recent work of our group in this area, which is concerned with the ab-initio calculation of molecular NLO properties of two different kinds of fullerene derivatives, a) substituted 1,2-dihydro fullerenes and b) fullerenes endohedrally doped with atoms or small molecules. Apart from the purely electronic response, we also focus on the vibrational contributions to the NLO response, that is, to the response of the nuclei to the external electric fields. 

Laser ablation was the first method used to synthesize endohedral fiillerenes. To synthesize La Cg2, a graphite rod doped with La203 was vaporized with a Nd YAG laser under argon at a temperature of 800 °C. Laser ablation is now mainly used to study the formation of endohedral fullerenes rather than used for actually synthesizing them. 

The DC arc technique is carried out in a water-cooled chamber with a graphite cathode and a graphite anode that has been doped with the desired metal and a catalyst in the presence of a gas, usually helium. One reason for the lack of transition-metal endohedral fullerenes can be attributed to the difficulty of current DC arc-discharge methods to cause encapsulation of transition metals in fullerenes, for which the reason remains unclear. For this reason, transition metals are... 

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