The $\ce{C18}$ allotrope cyclocarbon has been synthesized and imaged.[1] Science has most details behind a paywall, but this discussion includes an image:
In this octakaidecagonal molecule, each $\ce{\color{blue}{C}}$ is bonded viz. $\ce{C-\color{blue}{C}#C}$. Yet the above image looks like a nonagon. Why does every other carbon atom stand out as an obvious vertex while the others don't? My thoughts:
- The internal angles are close to $180^\circ$, making vertices hard to see. However, I wouldn't expect the internal angles of this octakaidecagon to differ much.
- I wonder if this molecule has a delocalised ring analogous to the one in benzene. On this idea, each of the nine visible edges could alternate between the states $\ce{C-C#C}$, $\ce{C#C-C}$. But this wouldn't explain why "odd" vertices have one appearance while "even" ones have another. I'm not convinced of this idea anyway, because it would average out to $\ce{C=C=C}$, unlike the $1.5$-bonds in benzene.
References:
- Kaiser, K.; Scriven, L. M.; Schulz, F.; Gawel, P.; Gross, L.; Anderson, H. L. An sp-hybridized molecular carbon allotrope, cyclo[18]carbon. Science 2019, eaay1914. DOI: 10.1126/science.aay1914.