Resolving the first anti-aromatic carbon allotrope

Synthetic carbon allotropes such as graphene, carbon nanotubes, and fullerenes advanced our understanding of fundamental physics and chemistry. They have revolutionized materials science and led to new technologies. Now, we have succeeded in synthesizing and resolving another carbon allotrope, the doubly antiaromatic cyclo[16]carbon, which was reported this week in Nature.¹

A few years ago, we synthesized and studied in detail for the first time the properties of an elusive carbon allotrope that is a ring of carbon atoms, called cyclocarbon. Cyclo[N]carbons are molecular rings consisting of N carbon atoms. This first resolved cyclocarbon was cyclo[18]carbon, or short C₁₈, which we achieved in collaboration with a group from the University of Oxford, led by Harry Anderson.² We obtained it using atom manipulation techniques where we deliberately dissociated bonds of a precursor molecule using voltage pulses from the tip of a scanning probe microscope. The precursor molecule was custom designed by our collaborators at Oxford. We could then study the formed C₁₈ molecule on a sodium chloride (NaCl) surface using atomic force microscopy (AFM) with bond-resolved contrast, a technique we invented at IBM.³

Prior to our research, the structure of C₁₈ had been debated to have either 18 bonds of equal length between the carbon atoms (a cumulenic structure), or to have alternating short and long bonds (a polyynic structure). Our experiments revealed it to have a polyynic structure. Recently, a team from university of Tongji University in China succeeded in characterizing two more members of the cyclocarbon family, C₁₀ and C₁₄. Interestingly, C₁₀ exhibits a no bond-length alternation, cumulenic structure — in contrast to C₁₈.

The reason that the numbers of carbon atoms resolved in the rings described so far, those being 10, 14 and 18, comes down to aromaticity. Because the numbers belong to the series $(4n + 2)$, with $n$ being an integer, these cyclocarbons are all doubly aromatic. The aromaticity helps stabilizing these molecules that are in general very reactive and unstable.

This raised the question if also a possibly antiaromatic cyclo[N]carbon — one with $4n$ carbon atoms — could be stable and characterized.

We teamed up again with Anderson’s group from Oxford, this time with the goal of creating cyclo[16]carbon, C₁₆, a possibly antiaromatic carbon allotrope. At Oxford, they synthesized the precursor molecules, which contained CO (carbon monoxide) and Br (bromine) masking groups. In our first attempts, we learned that using solely one sort of masking group, either CO or Br, as we did for C₁₈, did not work for C₁₆. However, with the combination of CO and Br masking groups in one precursor, we did finally succeed in obtaining C₁₆.

To this end, we dissociated the masking groups by atom manipulation and obtained C₁₆ on a NaCl surface. For C₁₆, we observed bond-length alternation by AFM. In addition, we also obtained images of orbital densities, which were not obtained for the previously characterized cyclocarbons, and we investigated C₁₆ in two different charge states. Our results corroborate the doubly antiaromatic character of C₁₆. The relatively simple structure of C₁₆ renders it an interesting model system for studying the limits of aromaticity, and its high reactivity makes it a promising precursor to additional novel carbon allotropes.

This is fundamental research, aimed to increase general understanding in chemistry and physics. We built upon the atom manipulation techniques pioneered by IBM Fellow Don Eigler, who spelled out “IBM” with atoms in 1990. We advanced these techniques to control the formation and dissociation of bonds to create novel molecules that could not be studied in detail before. The molecules we can create now, such as the cyclocarbons, are interesting model systems. Although they are small and highly symmetrical, they are extremely challenging to be described in theory.

Our experimental results provide benchmarks for theory. As a result, our experiments also offered a unique opportunity to validate our quantum computing algorithms for electronic structure calculations. In particular, our hybrid embedding quantum algorithm showed a great level of accuracy comparable with the most accurate classical reference calculations, while requiring a modest number of resources (qubits). These results demonstrate the potential of our quantum computing software, motivating our effort in developing a quantum ecosystem capable of performing these calculations on quantum hardware.