It is demonstrated that by neutron irradiation of polycrystalline C6o in an argon atmosphere or of C6o crystals containing absorbed or adsorbed argon, a stable endohedral argon compound is formed. The concentration of argon has been determined by instrumental neutron activation analysis via the short-lived 41mr isotope. It is supposed that the endohedral compound is formed by penetration of the argon atom into the C6o cages; a mechanism due to the high kinetic energy recoiling 41Ar atoms according to the 4Β°mr(n, ~)41Ar nuclear reaction seems the most plausible.
In a recent study [1] Saunders et al. have shown that buckminsterfullerene prepared via the arc combustion of graphite electrodes in a partial pressure of helium can entrap He atoms, resulting in a stable X@C60 compound (X = He). As this in situ entrapment is a relatively rare event, they demonstrated that helium and neon can also be endohedrally incorporated into pristine C60 by heating it in a He or Ne atmosphere at 600Β°C for 1 h at a pressure of 3 atm. For this thermally activated endohedral 'penetration' they proposed a mechanism involving the reversible breaking of one or more bonds in the C6o molecule temporarily opening a 'window' in the cage. Murry and Scuseria [2] presented convincing theoretical evidence for that 'window' opening mechanism. They also mentioned that the low efficiency of the incorporation of He and Ne observed by Saunders et al. [1] could come from the low efficiency of the thermal activation even when using high pressures [3]. Mention has been made [3] that other ways of Elsevier Science B.V.