All-carbon molecules: Evidence for the generation of cyclo[18]carbon from a stable organic precursor

The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclo[18]carbon, ab initio calculations predict a relatively stable, cyclic D_9h 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.