STRUCTURES OF THE CLUSTERS PRODUCED BY LASER-DESORPTION OF FULLERENES- [2+2] CYCLOADDUCTS OF PRESHRUNK CAGES

The laser desorption of C-60 or C-70 fullerenes yields a distribution of ions with masses corresponding to an even number of carbon atoms ar ound integer multiples of 60 or 70. Clusters with 120 or 140 atoms hav e been characterized as large coalesced fullerenes and fullerene dimer s joined by a [2+2] cycloaddition. Several structures had been propose d for the species with less than an integer multiple of 60 or 70 atoms , but none of them has been validated experimentally. We have examined C-n (n = 112, 114, 116, 118, 136, and 138) cations and anions using h igh resolution ion mobility measurements and compared the results with trajectory calculations for a number of candidate geometries. We find that these species exist as near-spherical cages and [2+2] cycloadduc ts of smaller fullerenes derived from C60 Or C70 Quadruply bound C-116 , C-118, C-136, and C-138 isomers that would result from the stepwise elimination of Ct from the preformed [2+2] C-60 or C-70 dimers were no t observed. Hence the fullerene dimers lacking a few atoms are formed by coalescence of dissociation products rather than by dissociation of the products of coalescence. Plausible reasons why the [2+2] cycloadd ucts are the only dimers produced by the laser desorption of fullerene s are discussed.