Scientists Successfully Synthesize and Visualize Cyclo[18]Carbon

For the first time, researchers have stabilized and directly imaged a ring of 18 carbon atoms, confirming the elusive cyclo[18]carbon structure.

Carbon is one of the universe’s most abundant elements, existing in many allotropes that exhibit radically different properties—from the hardness of diamond, where each carbon is tetrahedrally bonded, to the planar sheets of graphite, graphene, nanotubes, and fullerenes, where each carbon is tricoordinate.

Among these, cyclocarbons—rings of carbon atoms each bonded to only two neighbors—have remained largely theoretical. For years, scientists debated whether the ring’s bonds were all identical (double bonds) or alternated between shorter and longer lengths (single–triple bond alternation). While gas‑phase evidence existed, the high reactivity of these molecules prevented isolation and detailed characterization—until now.

Atom Manipulation Enables Cyclocarbon Creation

Building on previous successes in imaging molecules with atomic force microscopy (AFM) and synthesizing molecules via atom manipulation, the University of Oxford and IBM Research teams set out to create, stabilize, and characterize cyclocarbon.

Figure 1: From left to right, precursor molecule C₂₄O₆, intermediates C₂₂O₄ and C₂₀O₂ and the final product cyclo[18]carbon C₁₈ created on a surface by dissociating CO masking groups using atom manipulation. The bottom row shows AFM data obtained with a CO‑functionalized tip on bilayer NaCl on a Cu single crystal.

Published today in Science, our approach involved generating cyclocarbon by atom manipulation on an inert surface at 5 K and probing it with high‑resolution AFM. The collaboration began three years ago with the shared goal of unlocking this long‑sought structure.

Our initial focus was on linear segments of two‑fold coordinated carbons, exploring routes to produce carbon‑rich materials via atom manipulation—triggering reactions by applying voltage pulses with the AFM tip. We discovered that such segments could be formed on a copper substrate coated with a chemically inert bilayer of NaCl (Nat. Chem. 10, 853‑858, 2018), preventing unwanted covalent bonding to the surface.

Following this success, the Oxford group synthesized a precursor to cyclo[18]carbon—C₂₄O₆, a triangular molecule featuring 18 carbon atoms plus six CO groups that stabilize the structure. The transformation from C₂₄O₆ to C₁₈ was first explored 30 years ago by François Diederich and Yves Rubin (J. Am. Chem. Soc. 1989, 111, 6870). With modern AFM techniques, we now see the product in atomic detail.

Using AFM, we located the C₂₄O₆ precursors prepared on the NaCl bilayer. By applying voltage pulses to the AFM tip, we systematically removed CO groups, observing intermediates with two and four CO groups removed before finally detaching all six and forming cyclo[18]carbon.

On the cold, inert surface, the molecules remained stable enough for detailed study. AFM images revealed nine bright lobes arranged in a circle, which, as the probe approached, resolved into the corners of a nonagon—consistent with the positions of triple bonds. Simulations confirmed that cyclo[18]carbon adopts a polyynic structure with alternating single and triple bonds, resolving the long‑standing debate.

These findings suggest exciting future possibilities. By fusing cyclocarbons or cyclic carbon oxides through atom manipulation, we can create larger, custom carbon‑rich molecules—potentially new allotropes—and pave the way for molecular electronics based on single‑electron transfer.

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An sp-hybridized molecular carbon allotrope, cyclo[18]carbon, Katharina Kaiser, Fabian Schulz, and Leo Gross (IBM Research – Zurich); Lorel M. Scriven, Przemyslaw Gawel, and Harry L. Anderson (Oxford University), Science Aug 15, 2019, doi/10.1126/science.aay1914, https://arxiv.org/abs/1908.05904