| Noble gas atoms are incorporated into the fullerenes using high temperature and pressure. Only 1 in 1000 molecules of these carbon structures contains the atom—He, Ne, Ar or Kr—with less than 0.03% in the case of Xe. Subjecting the material to the same process produced a linear but only modest increase in the yield of labeled fullerenes, an increase far from the 10% equilibrium incorporation predicted by theory. A recent calculation found a high-energy barrier for the process, implying no incorporation would occur; clearly not the case. However, the incorporation was significantly cut down when the experiment was carried out with sublimed C60. Taken together, these observations suggest the participation of catalytic agents, present in an unsublimed sample, that aid in lowering the activation barrier for the incorporation. The belief is that these promoter molecules, most probably in the form of radicals, add to the fullerene; weaken a C-C bond at the site which subsequently breaks to form an opening or “window” on the fullerene, allowing entry of the noble gas atom. The identity of these promoter agents remains a mystery. However, noticeable increments in the incorporation were detected when a small amount of ethane or oxygen was introduced. Still, they are insufficient to overcome backgrounds from the largely unlabeled molecules, precluding IR, UV and 13C NMR determinations. 3He-labeled fullerenes are the exceptions. 3He NMR spectroscopy allows sensitive detection of those material containing 3He. Solid state 3He NMR studies of 3He@60 provided information on the magnetic anisotropies in the fullerene crystal as well as relaxation process for the trapped 3He; it has a long T 1. For the other noble gas-labeled fullerenes, spectroscopy would require isolation or, at least, enrichment of these molecules. Column chromatography was effective. Serial HPLC afforded over 90% Kr@C60. The Kr@C 60 content can also be enriched kinetically via a photochemical reaction which appeared to discriminate the labeled fullerenes. Incorporation of tritium was achieved via hot-atom chemistry. High tritium activities were detected using 3He as the tritium precursor in the nuclear reaction. |
