INCORPORATION OF HELIUM INTO ENDOHEDRAL COMPLEXES OF C-60 AND C-70 CONTAINING NOBLE-GAS ATOMS - A TANDEM MASS-SPECTROMETRY STUDY

Endohedral complexes of fullerenes and noble-gas atoms were originally prepared by a mass spectrometric method involving high-energy collisi ons of fullerene ions and noble-gas targets and, following that, by a high-pressure, high-temperature method involving neutral fullerenes an d a noble gas as reactants. The latter method has made available suffi cient quantities of the endohedral complexes of C-60 and C-70 with the various noble gases (He, Ne, Ar, and Kr) that the tandem mass spectro metric method can be used to test the possibility of preparing a new c lass of endohedral complexes that contain two noble-gas atoms. These n ew capture complexes do indeed form upon activation of the radical cat ion of the noble-gas endohedral complex by high-energy collisions with helium atoms. These complexes then decompose to give equivalent paral lel losses of C-2 units accompanied by expulsion of none, one, or both of the captured noble-gas atoms. The capture event causes the gain in excess of 34 eV of additional internal energy, but the capture comple xes survive on the time scale (similar to 50 mu s) for the transit thr ough the tandem mass spectrometer. Their survival, along with their fr agmentation characteristics, is consistent with formation of an endohe dral complex containing both noble-gas atoms although the fullerene ca ge may now be distorted especially when the fullerene is C-60. By anal ogy with the properties of empty fullerene and endohedral metallofulle rene complexes, end points exist for the equivalent number of C-2 unit s that can be expelled to give a closed-structure product that contain s either one or two noble-gas atoms. These end points correlate direct ly with the sizes of the fullerene and the noble-gas atom(s), giving s upport to our conclusion that endohedral complexes containing two nobl e gases can be prepared.