Cyclocarbon: New form of synthesized carbon | Physical chemistry

A team of researchers from Oxford University and IBM Research-Zurich has, for the first time, synthesized and characterized a ring of 18 carbon atoms.

cyclo[18]carbon. Image credit: Kaiser et al.

Carbon, one of the most abundant elements of the universe, is capable of forming many different forms (allotropic) because of its valence.

Well known forms include diamond, graphite, amorphous carbon, carbon nanotubes, spherical and ellipsoidal fullerenes.

"Although it is well-studied forms of carbon, there are lesser known forms and one of them in particular was elusive: the cyclocarbons, where the carbon atoms do not exist. Have only two neighbors, arranged ring-shaped, "said the first author, Katharina Kaiser. from IBM Research-Zurich and his colleagues.

"Discussed for many years, the structure of cyclocarbons was unknown and two possibilities were debated, either with all links in the ring of the same length (double bonds only), or with shorter and longer alternating bonds ( alternation of single and triple bonds). . "

"The evidence of their existence was published in the gas phase, but because of their high reactivity, they could not be isolated and characterized – until now."

The team chose to generate cyclocarbon by manipulating atoms on an inert surface at low temperatures – about 5 K (minus 450 degrees Fahrenheit or minus 268 degrees Celsius) – and to study it using microscopy atomic force at high resolution.

"Initially, we focused on linear two-coordinate carbon segments, exploring possible ways to create such carbon-rich materials by manipulating atoms," the scientists explained.

"We triggered chemical reactions by applying voltage pulses with the tip of the atomic force microscope."

"We found that such segments could be formed on a copper substrate covered with a very thin layer of table salt."

"Because the salt layer is chemically very inert, the reactive molecules did not form covalent bonds to it."

"After the successful creation of linear carbon segments, we tried to create cyclocarbon on the same surface."

"To this end, we have synthesized a precursor of the cyclo[18]carbon which is a ring of 18 carbon atoms. "

The precursor of carbon monoxide, C₂₄O₆, has a triangular shape and contains 18 groups of carbon monoxide (CO) in addition to the 18 carbon atoms, increasing the stability of the molecule.

"Using atomic force microscopy, we located the precursor molecules, prepared on the thin salt film," said the researchers.

"By using voltage pulses applied at the tip of the atomic force microscope, we could remove pairs of CO groups from the precursor. We identified intermediates with two and four CO groups eliminated. "

Finally, the team was able to eliminate all six CO groups and form a cyclo[18]carbon.

"Future applications are suggested by the fact that we could merge cyclocarbons and / or cyclic carbon oxides by atom manipulation," the authors said.

"This ability to form larger, carbon-rich structures by fusing molecules with atom manipulation opens the door to creating more sophisticated carbon-rich molecules and new carbon allotropes."

"Eventually, custom molecular structures could be used as molecular electronics elements, based on the transfer of unique electrons."

The study was published online this week in the journal Science.

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Katharina Kaiser et al. 2019. An allotrope of hybrid molecular carbon sp, cyclo[18]carbon. Science, published online August 15, 2019; doi: 10.1126 / science.aay1914