Resolving the odd-number cyclo[13]carbon

Cyclocarbons, which are rings of carbon atoms, are elusive molecular allotropes of carbon. They are excellent benchmarking systems for testing quantum chemical theoretical methods, and potential precursors to other carbon-rich materials, such as such as graphyne, graphdiyne, carbyne, and elusive molecular allotropes of carbon. On first sight, cyclocarbons are structurally simple, and their electronic structure resembles that of a particle on a ring. They can be rationalized with textbook physics. However, various structural distortions can happen, which result in differing electronic states. This makes calculations extremely challenging calling for verification by experiments.

The monocyclic structures of cyclocarbons make them interesting systems in the context of aromaticity. Recently, we ^{1, 2} and the group led by Wei Xu from Tongji University,^{3, 4} generated even-number cyclocarbons, comprising 10 to 20 carbon atoms in the ring, being highly aromatic (C₁₀, C₁₄, C₁₈) or highly anti-aromatic (C₁₂, You can learn more about our work resolving the first anti-aromatic carbon allotrope here.C₁₆, C₂₀).

Odd-number cyclocarbons, which have been elusive to date, are predicted to be even less stable than even-number ones. Today, we’re reporting the first on-surface synthesis of an odd-number cyclocarbon, which was published in Science. The cyclocarbon in question is C₁₃, a ring of 13 carbon atoms, which we obtained by manipulation of a precursor molecule with a scanning probe microscope’s tip.

Custom precursor molecules were synthesized by a group led by Harry Anderson at the University of Oxford. We deposited them on a thin film of table salt (NaCl) and generated C₁₃ by atom manipulation techniques in a low-temperature scanning probe microscope at the IBM Zurich Research Laboratory. To this end, we removed ten chlorine atoms from the precursor and cleaved two carbon-carbon bonds, obtaining cyclo[13]carbon.

We characterized the structure of C₁₃ using ultra-high resolution atomic force microscopy (AFM) with CO functionalized tips, a technique we developed 15 years ago at IBM,⁵ and we studied the electronic structure of C₁₃ with scanning tunneling microscopy (STM).

While even-numbered cyclocarbons adopt closed-shell configurations where all electrons are paired up in orbitals, we found that C₁₃ adopts an open-shell configuration. C₁₃ has two singly occupied molecular orbitals, and their unpaired electrons align parallel creating a triplet ground state rendering the molecule magnetic. We observed that C₁₃ molecules have a kinked geometry, which shows different extents of distortion and carbene localization, depending on the molecular environment. Measured molecular orbital densities showed to agree with calculations of the orbitals using quantum computing algorithms. Our calculations also indicated that cyclo[13]carbon is anti-aromatic.

Finally, we could also employ the high reactivity of the compounds. Using atom manipulation we prepared the C₁₃ dimer cyclo[26]carbon, demonstrating the potential of cyclocarbons and their precursors as building blocks for novel carbon allotropes.