Pure carbon is known to come in several configurations, called allotropes in chemistry. When each carbon atom is bonded to three other atoms to form hexagonal patterns, they either form single-atom-thick graphene sheets or, in bulk, soft graphite. When each carbon atom is added to four neighbouring atoms to form a pyramid-like pattern, it becomes diamond. Football-shaped buckyballs, or fullerenes, comprise 60-80 atoms of carbon combined together in pentagonal and hexagonal patterns. Hexagonal shapes also result in carbon nanotubes.
But just a ring of carbon, with each atom bonding with only two neighbouring atoms? Chemists Roald Hoffman at Cornell University and others have conjectured that such structures, which would be chains of pure carbon, should exist. Now, chemists from Oxford University and IBM Research Labs, Zurich, have achieved what seemed impossible, the first ring-shaped stable molecule of pure carbon, a circle of 18 atoms. The work was published in a recent issue of “Science”. Circular carbon molecules are called cyclocarbons, and the smallest such ring, comprising 18 atoms, is predicted to be stable.
According to Przemyslaw Gawel of Oxford University, who led the study, such a structure is chemically more reactive than graphene or diamond, and hence less stable, especially when bent. What the team has achieved is the structure with alternating double and triple bonds. The results suggested that long straight carbon chains might also be semiconductors. This could make them useful as components of future molecular-sized transistors and also produce superconductivity.
