The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclocarbon, ab initio calculations predict a relatively stable, cyclic D9h ground state geometry with alternating C-C (1.36 angstroms) and C≡C (1.20 angstroms) bonds. The synthesis and x-ray crystal structure of a direct precursor to C 18 are described. The analysis of laser flash heating experiments on this precursor by time-of-night mass spectroscopy shows a sequence of retro-Diels-Alder reactions leading to C18 as the predominant fragmentation pattern. Structural evidence is provided for the generation of an all-carbon molecule from a well-characterized organic precursor.
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