First ring only made of carbon

Ring molecule of 18 carbon atoms for the first time reveals the structure of cyclocarbons

A ring made of pure carbon: Chemists have now created this form variant for the first time. © IBM Research
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Finally, after more than 50 years, chemists have for the first time made a ring out of carbon atoms - a long-sought molecule. For these cyclocarbons have never been studied or even produced. Their structure therefore remained enigmatic. The ring of 18 carbon atoms now created solves this mystery, because it reveals that the atoms in the ring are linked by alternating single and triple bonds, as the researchers report in the journal "Science".

Carbon is an extremely versatile element. Because it is not only a component of all organic molecules from methane gas to DNA, but can form many different structures even in pure form. The spectrum ranges from graphite and the single-layer "miracle material" graphene over the diamond to complex fullerenes - football-like molecules of 60 carbon atoms. The latter even occur in space.

Cyclocarbons have been too volatile and reactive to "pin it down". © IBM Research

The search for the ring

But a carbon variant proved to be elusive: Cyclocarbone. While ring-shaped hydrocarbons are ubiquitous, chemists have failed to produce ring-shaped molecules from carbon atoms. "Although there is evidence for the existence of cyclocarbons in the gas phase, this highly reactive species could not be structurally characterized nor investigated in condensed phases, " report Katharina Kaiser from the IBM Zzentrumrich Research Center and her colleagues.

The carbon rings have remained correspondingly enigmatic. Thus, it was not even clear whether the carbon atoms in these rings are linked via double bonds or whether triple bonds with single bonds alternate in the ring. Since carbon has four outer electrons that can participate in a covalent bond, theoretically both would be possible.

18 ring with carbon monoxide claw

Now, Kaiser and her team have succeeded in what chemists have been trying for more than 50 years: they first generated and investigated a cyclocarbon. For their experiment, they first prepared a suitable substrate from a sodium chloride layer on a copper substrate. On this substrate, they vaporized the ring-shaped carbon oxide compound C 24 O 6 . display

Now came the decisive step: They positioned the tip of an atomic force microscope (AFM) a few nanometers above the molecule and sent an increased electrical voltage through the ensemble for a few seconds. This procedure resulted in the removal of two, four, or six carbon monoxide units, "the researchers report.

When six CO molecules were removed from the ring, a pure carbon skeleton remained: a ring of 18 carbon atoms. Such a cyclo (18) carbon is considered to be the smallest thermodynamically stable form of these ring molecules.

Formation of the Cyclo (18) Carbone from the Precursor Molecule C24O6 and Transition Forms. Research IBM Research

Change of triple and single bonds

For the first time, researchers have succeeded in producing a pure carbon ring. Not only that, they also used the atomic force microscope to answer the question about the bonding structure of this cyclocarbone. "The resulting molecule showed a cyclic structure with nine light projections, " report Kaiser and her team. "Analogous to the precursor molecule, this can be explained by the fact that the light bulges are located above triple bonds."

This means that in this 18-membered ring, the carbon atoms are not connected to each other via double bonds, but through alternating single and triple bonds. Thus, the question of the structure of cyclocarbons, which has been discussed for decades, has finally been answered.

Extremely reactive

The experiments also showed that this cyclocarbon is extremely reactive. Even a slightly increased tension led to the fusion of two neighboring rings, as the researchers report. This makes these rings promising starting materials for more exotic carbon forms.

"The high reactivity of cyclo (18) carbon and its oxides paves the way for the synthesis of other carbon allotropes and carbon-rich materials, " explain Kaiser and her team. (Science, 2019; doi: 10.1126 / science.aay1914)

Source: Science

- Nadja Podbregar