Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A "window" mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000°C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.

It is demonstrated that fullerenes, prepared via the standard method (an arc between graphite electrodes in a partial pressure of helium), on heating to high temperatures release (4)He and (3)He. The amount corresponds to one (4)He for every 880,000 fullerene molecules. The (3)He/(4)He isotopic ratio is that of tank helium rather than that of atmospheric helium. These results convincingly show that the helium is inside and that there is no exchange with the atmosphere. The amount found corresponds with a prediction from a simple model based on the expected volume of the cavity. In addition, the temperature dependence for the release of helium implies a barrier about 80 kilocalories per mole. This is much lower than the barrier expected from theory for helium passing through one of the rings in the intact structure. Amechanism involving reversibly breaking one or more bonds to temporarily open a "window" in the cage is proposed. A predicted consequence of this mechanism is the incorporation of other gases while the "window" is open. This was demonstrated through the incorporation of (3)He and neon by heating fullerene in their presence.

In their first paper on Buckminsterfullerene,1 Kroto and Smalley et al. proposed a hollow structure. Interest in theinclusion of atoms into the cavity to form "endohedral complexesn has continued. Early work concentrated on incorporating metals? but noble gas atoms were obvious possibilities. Several studies reported that collisions of fullerene cation radicals with helium and neon in a mass spectrometer led to addition of the mass of the noble gas atoms to the ions.S7 Many theoretical studiesB-15 have appeared with estimates of the energies of fullerenes containing noble gas atoms, the dynamics of motion inside the cavity, and spectroscopic properties, even though none of these substances has been prepared in quantity.We have recently shown16 that fullerene prepared in the standard manner (a graphite arc in a low pressure of helium17) contains helium in about one in a million molecules. Heating fullerenes in several atmospheres of 3He or neon at 600 "C led to inclusion of these atoms at similar levels. We proposed that (1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163. (2) Heath, J. R.; OBrien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. J. Am. Chem. Soc. 1985,107,7779-7780. (3) Weiske, T.; Bdhme, D. K.; HruUk, J.; KrHtchmer, K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 884-886. (4) Schwarz, H.; Weiskc, T.; Bdhme, D. K.; HruUk, J. In Euckminrterfullerenes; Billups, W. E., Ciufolini, M. A,, Eds.; VCH Publishers: New (5) Ross, M. M.; Callahan, J. H. (7) Caldwcll, K. A.; Giblin, D. E.; Hsu, C. S.; Cox, D.; Gross, M. L. J. Am. (8) Breton, J.; Gonzalez-Platas, J.; Girardet, G. (IS) Cardini, 0.; Procacci, P.; Salvi, P. R.; Schettino, V. Chem. Phys. (16) Saunders, M.; JimCnez-Vhzqucz. H. A,; Cross, R. J.; Poreda, R. J. (17) KrHtchmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. 3734. 8362. 8741. 1864-7869.this process takes place by the reversible breaking of one or more bonds, opening a "window" large enough for entry of atoms. The noble gases were detected by heating samples in the modified mass spectrometer operated by one of us (R.J.P.). 16 We have now incorporated krypton using -15 atm at 600 OC for 2 h. An 18.5-mg sample of the resulting fullerene yielded 1.6 X 1 V cm3 of 84Kr on heating (1 atom in 410 OOO molecules), with a release pattern similar to that reported for helium and neon. Since several theoretical papers predicted that krypton and xenon would not fit into CM, we wanted to try to incorporate xenon.To investigate the spectroscopic and other properties of these interesting noble gas compounds, it is necessary to increase the fraction of fullerene molecules occupied. We have done this by carrying out the incorporation step at high pressures of the gas. One of us (S.M.) operates a facility with high-pressure steel vessels that can be heated behind appropriate shielding. Usually, a pump is employed to compress the gas into the vessel. Using expensive gases (e.g., 3He), this is a disadvantage since large amounts o...

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The reduction of 3He@C60 and 3He@C70 by lithium metal to give solutions of the corresponding hexaanions in THF-d 8 has been achieved under gentle conditions, at subambient temperatures, without sonication, by capitalizing on the ability of corannulene (1) to serve as an efficient electron carrier between the lithium metal and the solid fullerenes, which are virtually insoluble in the reaction medium. The 3He inside the C60 hexaanion is found to be more strongly shielded (by nearly 20 ppm!) than any previously reported 3He in a fullerene [δ(3He@C60 6-) = −48.7 ppm, relative to dissolved 3He gas in the solution], whereas the 3He inside the C70 hexaanion is actually deshielded [δ(3He@C70 6-) = +8.3 ppm], resonating at nearly 15 ppm lower field than any previously reported 3He in a fullerene. These results stand in complete accord with earlier predictions that the magnetic properties of C60 and C70 would be altered dramatically, and in opposite directions, by reduction of the fullerenes to their hexaanions. The phenomenal ability of C60 6- to shield an endohedral 3He from a powerful external magnetic field provides the most compelling evidence to date for the ability of electrons to move freely about the surface of a spheroidal π-system.