Fullerene endohedral helium

C. G. Joslin, C. G. Gray, and S. Goldman, Chem. Phys. Lett., 227, 405 (1994). Infrared Rotation and Vibration-Rotation Bands of Endohedral Fullerene Complexes. Helium in C60-Derived Nanotubes. 

Keywoids Fullerene, Endohedral MetallofuUerene, Transition Metals, Helium, [2 + 2 + 2] Ring Opening. 

He C6o and He C7o The He NMR spectra of endohedral helium atoms were used to monitor the aromaticity of the fullerenes as a function of electron addition upon reduction. The reduction of He C6o and He C7o to their hexaan-ions was followed by a study of their He shifts [17]. The reduction was carried out with Li metal (in the presence of 2), and the NMR spectra of the anions were... 

The reaction of fullerene radical cation with helium, to produce endohedral He C6o, is a particular example of the last type of reaction, and is displayed in Figure 4.6 [27] ... 

NMR chemical shifts of the appropriate endohedral fullerenes (fullerenes that contain helium atoms)... 

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. 

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...

It might be mentioned that there is quite a large empty space in the interior of the fullerene molecule. It has proven possible to trap various rare-gas atoms and small molecules within the fullerene interior, and the subject has been studied in some detail. Helium and argon, for example, can be inserted into the fullerene cage at high temperatures and pressures. The resulting structures are referred to as endohedral complexes, and they can be identified by mass spectrometry. ... 

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... 

Helium ( He) (/= 1/2). The endohedral NMR chemical shifts of various known H2-H2O- and NH3-encapsulated fullerene compounds were calculated at the GIAO-B3LYP/3-21G and GIAO-HF/3-21G levels of theory with AMI- and PM3-optimised structures. The shift tendency of endohedral NMR chemical shifts were discussed and compared with that of endohedral He NMR chemical shifts.The equilibrium geometries and relative stabilities of the 19 possible isomers of C84O were studied... 

Fullerenes are hollow can cage atoms as large as xenon. The endohedral nature of a complex may be demonstrated by its stability. (HeCgg) , formed by collisions of C, with helium gas, survives... 

Second, is there any interaction between the helium and the fullerene host which would manifest itself in the chemical shifts and to what extent are the latter affected by the mobility of the endohedral noble gas Mixing between the orbitals of the Ceo cage and those of endohedral guests have long been found to be negligible. In the ab initio Hartree—Fock calculations, virtually the same result is obtained for (3(3He)ofHe C6o, 70 and for the chemical shift at the center of the pristine cages. It is only for the... 

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