Stable Compounds of Helium and Neon: He@C ₆₀ and Ne@C ₆₀

Abstract

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.

Direct detection and quantitation of He@C60 by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry

“…The results also demonstrate the feasibility of determining the extent of He incorporation from the FT-ICR mass spectral peak heights. The present measurements are in agreement with those obtained by the pyrolysis method [1][2][3]. Although limited in sensitivity, the mass spectral method is faster and easier than pyrolysis.…”

“…Endohedral compounds of all the noble gases can be prepared by heating fullerenes under high noble gas pressure [1][2][3]20]. Initially, yields of incorporation were approximately 0.1% for He, Ne, Ar and Kr and 0.03% for Xe.…”

“…The 3 He/ 4 He ratio in those fullerenes was very high, ϳ150 times greater than in the earth's atmosphere and similar to the value estimated for the early solar system. In the above studies, the noble gases were detected and quantified by pyrolyzing the material to release the gas, which was detected in either a quadrupole mass spectrometer [1,3,25] or a linear time-of-flight mass spectrometer [24]. It is possible, with sufficient labeling, for the larger noble gas endohedral fullerenes to be detected in standard mass spectra [2].…”

“…Another technique for the preparation of endohedral fullerene compounds is that of ion implantation (the endohedral species is neutralized on implantation). Endohedral species containing lithium and other alkali metals [15][16][17], the noble gases, He and Ne [18], and nitrogen [19] have been produced by that method.Endohedral compounds of all the noble gases can be prepared by heating fullerenes under high noble gas pressure [1][2][3]20]. Initially, yields of incorporation were approximately 0.1% for He, Ne, Ar and Kr and 0.03% for Xe.…”

“…Recent results have shown that the presence of KCN catalyzes the incorporation of noble gas, giving higher yields of incorporation (ϳ1%) in a single labeling experiment [23]. It is also known that He is trapped inside fullerenes (at about 1 ppm of empty fullerene) prepared by the Krätschmer-Huffman procedure [1,8]. In a recent finding, it was shown that fullerenes extracted from the Allende and Murchison meteorites contained endohedral helium [24].…”