Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20

Fullerenes are graphitic cage structures incorporating exactly twelve pentagons(1). The smallest possible fullerene is thus C-20, which consists solely of pentagons. But the extreme curvature and reactivity of this structure have led to doubts about its existence and stability. Although theoretical calculations have identified, besides this cage, a bowl and a monocyclic ring isomer as low-energy members of the C-20 cluster family(2), only ring isomers of C-20 have been observed(3-6) so far. Here we show that the cage-structured fullerene C-20 can be produced from its perhydrogenated form (dodecahedrane C20H20) by replacing the hydrogen atoms bwith relatively weakly bound bromine atoms, followed by gasphase debromination. For comparison we have also produced the bowl isomer of C-20 using the same procedure. We characterize the generated C-20 clusters using mass-selective anion photoelectron spectroscopy; the observed electron affinities and vibrational structures of these two C-20 isomers differ significantly from each other, as well as from those of the known monocyclic isomer. We expect that these unique C-20 species will serve as a benchmark test for further theoretical studies.