CARBON CLUSTER CATIONS WITH UP TO 84 ATOMS - STRUCTURES, FORMATION MECHANISM, AND REACTIVITY

Carbon clusters are generated by laser desorption. Mass-selected beams are then pulse injected into an ion chromatography (IC) device. This device temporally and spatially separates the beam into its isomeric c omponents. Arrival time distributions (ATDs) are then measured at the detector. From these distributions, accurate mobilities are obtained f or each isomeric component, along with the fractional abundance of eac h isomer. Different isomer structures are calculated using quantum che mical methods. A Monte Carlo technique uses these structures to obtain accurate theoretical mobilities. Comparison of theory with experiment allows unambiguous structural assignment of the various families of i somers present in the cluster beam. The results indicate that, for car bon cluster cations, linear structures exist up to C-10+. Several fami lies of planar ring systems begin with monocyclic rings (ring I), whic h first appear at C-7+ and persist beyond C40+. Bicyclic rings (ring I I) are first observed at C21+ and persist beyond C40+, followed by tri cyclic rings (ring III, initiated at C30+) and tetracyclic rings (ring IV, initiated at C40+). A 3-dimensional family we label as 3-D rings begins at C29+, whose structure is not yet unambiguously assigned. Thi s family never exceeds 5% of the ions at any cluster size. Finally, th e first fullerene is observed at C30+, with this family dominating abo ve C50+. The growth pattern of carbon, beginning with C atoms, is show n to be linear --> monocyclic rings --> polycyclic rings --> fullerene s. No graphitic or ''cup'' shaped isomers are observed, eliminating th ese species as building blocks for fullerenes. Our structural data, wh en coupled with recently published annealing studies, indicate that fu llerenes are formed from isomerization of hot planar ring systems and that monocyclic rings and fullerenes are close in energy between C30and C36+ with fullerenes dominating above C36+. Reactions Of C-7+ to C -15+ with O2 and NO are reported and indicate that linear chains are g enerally much more reactive than rings. Finally, C60 and C70 are made with up to four positive charges but retain the fullerene cage structu re.