Endohedral compounds

Most of studies have been devoted to the CHA analysis by placing the nucleus at the centre of the spherical box. However, the confined atom may conceivably be located anywhere inside the spherical box. For atoms trapped in fullerene cages, in the theory of endohedral compounds, there exist experimental and theoretical studies that show that the confined atom is not at the centre of M C60 [72-78]. A similar problem has been studied in... 

The simulation of the EPR-spectrum of N Cgi(COOEt)2 (Fig. 39) taking into account a fine structure interaction is in nice accordance with the experimental spectrum. The simulation was carried out with the hyperfine interaction and g-factor of unmodified N Cgo and the fine structure interaction 0 2=2 G and E = 0.13 G (non-axial term). The shape of the extra lines definitely requires the inclusion of a non-axial term E, indicating that the molecular structure of the adduct induces some non-axiality which is not averaged out by fast axial rotation. The non-axial term was also observed at a measurement at 100°C showing that axial motional averaging does not take place even at this temperature. These results show that like He Cgo the new endohedral compound N Cgo can be used as a probe to monitor exohedral chemical addition reactions. Due to higher sensitivity, EPR requires less material than NMR.3He Cgo and N Cgo are complementary probes since different interactions are measured. 

As inferred from calculations, most light elements should behave similarly to atomic nitrogen inside C o and form unusual endohedral compounds [41b]. Their formation is one more hot topic of pursuit in current fuUerene research. 

In the case of the endohedral compound, the longest bond (2) does not possess the least favorable reaction energy. Hence, there is not an overall correlatiOTi between C-C bond distances and reaction energies, apart from the fact that the most reactive bonds do exhibit short C-C bond distances. 

Fig. 4.8 Representation of all non-equivalent bonds of the Ng2 C6o compound. The activation energies (in kcal mop ) corresponding to the Diels-Alder cycloaddition reaction between 1,3-butadiene and all non-equivalent bonds for all considered noble gas endohedral compounds. Ng2 Cjo has been represented on the right. A grey scale has been used to represent the different noble gases endohedral compounds black color is used to represent the helium-based fullerene, light grey for neon, medium grey for argon, dark grey for krypton, and white for xenon...

Once At2 and Kt2 are inserted inside Cgo, the reaction becomes substantially more exothermic (—32.2 and —39.9 kcal moP for bonds 1 and 2, respectively), and the activation barriers are largely reduced (to ca. 8 and 6 kcal moP for the At2 and Kt2 compounds, respectively). The addition to the [6,6] bond 3 is less favored, as the noble gas moiety is not totally reoriented to face the attacked bond. Of course, the larger the noble gas atom, the more impeded the rotation of the noble gas dimer inside the cage. Hence, for the larger noble gas endohedral compounds the addition is favored over those bonds situated close to the C5 axis where the dimer is initially contained. This lack of rotation leads to substantially less favored reaction and activation barriers. 

Fullerenes can also form compounds with atoms encapsulated within the cage structure, termed endohedral compounds and designated M C , where the symbol signifies that the M atom is encapsulated within the C cage, e.g., U C2g, Y Cgo, Y2 Cg2, La2 Cgo, La Cg2- Fullerene aggregates are bonded by van der Waals forces (c.f. graphite) and will permit the entrapment of alkali metal ions such as Li, Na, K, Rb and Cs [87]. 

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