16-carbon ring is doubly antiaromatic

By manipulating atoms with a high-powered microscope, chemists have coaxed into being a doubly antiaromatic carbon allotrope. Although the molecule, known as cyclo[16]carbon or C₁₆, may exist in interstellar space, its existence on Earth is fleeting. That’s because its antiaromatic nature makes it highly reactive—a property that could make it useful for creating other novel carbon compounds.

Cyclo[n]carbons have haunted University of Oxford chemistry professor Harry L. Anderson for about 30 years. His first project as a postdoctoral researcher, which he abandoned after deeming it too difficult, was to make C₁₈ —a ring composed of 18 carbon atoms joined by alternating single and triple bonds. In 2019, Anderson and IBM Research-Zurich’s Leo Gross led a team that made the 18-carbon allotrope.

C₁₈ proved to be exceptionally stable for its size because it is doubly aromatic, with 18 π electrons running parallel and 18 π electrons running perpendicular to the ring. Aromatic compounds, which possess 4n + 2 π electrons, have enhanced stability thanks to electron delocalization.

Antiaromatic compounds, which have 4n π electrons, aren’t delocalized and tend to be unstable. “The difficulties in synthesizing antiaromatic cyclocarbons are mainly due to their reduced stability compared to aromatic ones,” Wei Xu, who studies cyclo[n]carbons at Tongji University and was not involved in the work, says in an email.

That’s true of C₁₆ —the newest carbon allotrope from Anderson and Gross’s team—which is doubly antiaromatic. The researchers used an atomic force microscopy (AFM) and scanning tunneling microscopy (STM) tip to generate C₁₆, which appears circular, as well as negatively charged C₁₆^-, which adopts an oval shape (Nature 2023, DOI: 10.1038/s41586-023-06566-8).

AFM and STM studies revealed that C₁₆’s bond lengths differ significantly. This difference in bond length confirms that there is no delocalization and the compound is antiaromatic. Next, Anderson and Gross say, they want to create other elusive, carbon-rich antiaromatic molecules.