Endohedral effect

With gram quantities of fullerenes readily available thanks to the Kratschmer-Huffman arc vaporization technique, several properties of the Ceo cluster have been measured lately. Other data has been provided by recent ab initio calculations. In this section, we list the properties that are of relevance to the formation of endohedral complexes. These include molecular geometry (the bond lengths and the cage radius), ionization potential and electron affinity, and electric polarizability. The so-called endohedral effect is also discussed here. 

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... 

Fullerene generation by vaporization of graphite or by combustion of hydrocarbons is very effective and certainly unbeatable what facile production in large quantities is concerned. However, total synthesis approaches are attractive because (a) specific fullerenes could be made selectively and exclusively, (b) new endohedral fullerenes could be formed, (c) heterofullerenes and (d) other cluster modified fullerenes could be generated using related synthesis protocols. 

AC and AEC complexation is also effected efficiently by other macrocyclic ligands such as the spherands 13, cryptospherands 14 [2.9, 2.10], calixarenes [2.38, A.6, A.23], torands [2.39], etc., some of them, for instance the spherands displaying particularly high stabilities. A special case is represented by the endohedral complexes of fullerenes in which the cation (Sr2, Ba2+, lanthanides) is locked inside the closed carbon framework [2.40],... 

Chen, N., Zhang, E.Y., Tan, K. et al. (2007) Size effect of encaged clusters on the exohedral chemistry of endohedral fullerenes a case study on the pyrrolidino reaction of ScjGd3 xN Cgo (x = 0-3). Organic Letters, 9, 2011-2013. 

Our observation that the series of LiC6o-2n fragment ions terminates sooner than the corresponding Na-Cso -2n series is consistent with the endohedral binding picture. Just as Li can insert into Cm more easily than Na" ", it should be able to escape more readily. The large drop of ion signals toward bigger n clearly shows that Li loss competes effectively with C2 loss ... 

F. DE PROFT, C. VAN ALSENOY and P. GEELINGS, Ab initio study of the endohedral complexes of Ceo, Sieo and Geeo with monoatomic ions influence of electrostatic effects and hardness. J. Phys. Chem., 100, 7440 (1996). 

The behavior of CH4 (and that of ethane) adsorbed on purified, ultrasonicaUy cut bundles of SWNTs produced by laser ablation has been studied using NMR at room temperature and pressures below 1 MPa [38]. The nuclear H spectrum shows distinct features corresponding to methane (and ethane) in the gas phase vs methane and ethane adsorbed in the interior of the cut tubes. Adsorption at the interior of the cut nanotubes (endohedral adsorption) dominates the behavior at the pressures and temperatures studied. The effects of having O2 present during the adsorption measurements for methane and ethane were explored. It was determined that O2 does not affect endohedral adsorption for either gas because the binding energy of O2 is smaller than those of methane or ethane [38]. 

Besides the substances mentioned so far, functionalized fuUerenes like the simple Bingel adduct can be intercalated into nanotubes as well (Section 2.5.5.2). The formation of peapods has further been described for metallocenes (e.g., ferrocene), porphyrines (e.g., erbium phthalocyanine complex) and small fragments of nanotubes. The most important prerequisite for the feasibility of inclusion is always a suitable proportion of sizes of both the tube and the structure to be embedded. For example, this effect can be observed for the intercalation of different cobaltocene derivatives into SWNT. The endohedral functionalization only takes place at an internal diameter of 0.92nm or above (Figure 3.100). But there is also an upper limit to successful incorporation. When the diameter of the nanotube is too large, the embedded species can easily diffuse away again from the host. Only few molecules are consequently found inside such a wide tube. 

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