Endohedral species

An important clue for the disappearance of many endohedral species in the process of solvent-extraction is given by Bandow et al.[Ba92] Handling the soot and performing the solvent extraction under anaerobic conditions, they observed LaCre and LaCs4 in the mass spectra of the solvent extract. Evidently, these species are susceptible to oxidation from the atmosphere and/or dissolved air, which either destroys or immobilizes them. This result calls for a careful reevaluation of their reactivity in an ion trap or similar experiment. 

IR were reported for the endohedral species H2 C60 186 IR data for Li Cgo gave evidence for two different species - one with much lower symmetry than the other. For Li C70, bands very similar to those of Ci40 were seen.187,188... 

As seen before, the inside of the fullerene cages is empty, so it is self-suggesting to try to fill this cavity with atoms or molecules. Indeed the isolation of so-called endohedral fullerenes succeeded quite soon after the discovery of fullerenes themselves. In endohedral species, single or even several atoms are situated inside the carbon cage, so it is purely topological circumstances that prevent them from breaking out of these compounds. 

Emichment of these endohedral species can be obtained by heating the intact solid fullerene under 3000 atm of a noble gas at 650°C. Under these conditions, helium, neon, argon, krypton, and even xenon can be forced into the cavity within Cso- This process is believed to occur via a window mechanism in which a C—C bond of the fullerene breaks to provide a window through which the noble gas atom enters the fullerene. Subsequent reformation of the C—C bond can trap the noble gas atom inside the cage. Recent work on creating orifices in through chemical modification produced an interesting model system in which entry and exit of both He and H2 can be monitored, as shown in Fig. 

Stabilization of C28 by an endohedral species is a fascinating topic especially as the C28 cage provides itself 28 electrons, and has the ability to accept four electrons in the LUMO allowing a route to a 32 electrons closed-shell M C2s system. Because of the quasi monoreference (94.3%) nature of the ground state of C28 [38] one can expect that the DFT is an appropriate theoretical framework for the study of the An C2s closed-shell complexes. 

As compared to the hypothetical non-interacting species the interaction between the Li+ cation and either the or Ceo cage leads to a moderate reduction of the diagonal polarizabilities and, in the case of [LiCeo]" also of the second hyperpolarizabilities. The reduction for a may be due to a contraction of the electron density caused by the attraction of the cation. Such an explanation will not suffice for 7 since the effect of the interaction on the diagonal components is quite different for Li C6o, and the second hyperpolarizability is not simply related to the size of the electron distribution. The first hyperpolarizabilities arise from asymmetry of the charge distribution and are, consequently, strongly enhanced in the endohedral species. 

Stabilization of highly reactive species in molecular bowls and capsules with an endohedral functionality 97LA2393. 

One particularly striking aspect of endohedral fullerene complexes is the surprising ability of the carbon cage to contain highly reactive species such as atomic nitrogen.1801 N C6o (54) has been prepared... 

While there are no studies of the toxicity of endohedral fullerenes in general, or gadofullerenes in particular, there are several preliminary studies regarding empty fullerenes and their derivatives. In initial steps toward characterizing the safety of water-soluble fullerene derivatives, a cytotoxicity study of various fullerene materials, including underivatized C60, C60[C(COOH)2]3, and two C.0(OH)r species, was conducted (Sayes et al., 2004). The results clearly showed that water-soluble derivatives were significantly less toxic than underivatized C60, and the trend was one of... 

The corresponding calculations on the endohedral and exohedral complexes of CgQ with H, F or the methyl radical as guest predict the same behavior [98]. In all cases the formation of endohedral covalent bonds is energetically unfavorable due to the analogous strain arguments, even if such reactive species as F atoms are exposed to the inner surface of Cjq. 

Table 14.3 Calculated endohedral He chemical shifts and symmetry of He C2o, He C6o and He Cgo species with different filling degree of the k electron system [107],...

Interaction-induced absorption by the vibrational or rotational motion of an atom, ion, or molecule trapped within a Ceo cage, so-called endohedral buckmin-sterfullerene, has excited considerable interest, especially in astrophysics. The induced bands of such species are unusual in the sense that they are discrete, not continuous they may also be quite intense [127]. Other carbon structures, such as endohedral carbon nanotubes, giant fullerenes, etc., should have similar induced band spectra [128], but current theoretical and computational research is very much in flux while little seems to be presently known from actual spectroscopic measurements of such induced bands. 

In the accepted molecular label given to an endohedral fullerene - such as Y C82 - the species before the sign denotes the content of the fullerene molecule, the latter being denoted by C ... 

Fullerenes can encapsulate various atoms within the cages, and these compounds have been referred to as endohedral fullerenes. For example, the symbolic representations La C6o and La2 Cso indicate that the fullerene cage encapsulates one and two lanthanum atom(s), respectively. The IUPAC description refers to these fullerenes species as incar-fullerenes, and the formulas are written as t LaCeo and tl Cso, (i is derived from incarcerane). Some metal endohedral fullerenes are listed in Table 14.2.1. The endohedral fullerenes are expected to have interesting and potentially very useful bulk properties as well as a fascinating chemistry. Some non-metallic elements, such as N, P, and noble gases, can be incarcerated into fullerenes to form N 0,0, P C6o, N C o, Sc3N C80, Ar Oo, etc. 

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