The Lord of the Carbon Ring
Rings of 18 carbon atoms, held together by alternating triple and single bonds, represent a fundamentally new form of the element. To create them, researchers had to manually force individual atoms to react.
Carbon occurs in the earth's crust in two modifications: as graphite and as diamond. Football-shaped structures, the so-called fullerenes, which have been produced in the laboratory since 1985 thanks to the work of Harry Kroto, Richard Smalley and Robert Curl, can also be detected in space. As artificial versions of the element, we now also know graphene%nbsp;- a single layer of the graphite structure%nbsp;- as well as various types of carbon nanotubes, which can be thought of as rolled-up versions of a graphite sheet.
But that's not the end of the possibilities. The acetylene (ethyne, HCCH) can form long chains (polyynes) that only need a hydrogen atom at each end. If these chains could become infinitely long, then they too would be a modification of the pure element.
Another way to eliminate the hydrogen at the ends is to bring them together and make a ring. In principle, the quantum mechanical models of molecular orbitals prescribe that polyynes should be stretched out dead straight. However, according to calculations, there is a small margin to deviate from linearity, which could allow ring closure from a chain length of 18 carbon atoms. And there has been no lack of attempts to produce such a C18 species…