AbstractDiatomic carbon (C2) is historically an elusive chemical species. It has long been believed that the generation of C2 requires extremely high physical energy, such as an electric carbon arc or multiple photon excitation, and so it has been the general consensus that the inherent nature of C2 in the ground state is experimentally inaccessible. Here, we present the chemical synthesis of C2 from a hypervalent alkynyl-λ3-iodane in a flask at room temperature or below, providing experimental evidence to support theoretical predictions that C2 has a singlet biradical character with a quadruple bond, thus settling a long-standing controversy between experimental and theoretical chemists, and that C2 serves as a molecular element in the bottom-up chemical synthesis of nanocarbons such as graphite, carbon nanotubes, and C60.

Trapping experiments were claimed to demonstrate the first chemical synthesis of the free diatomic species C at room temperatures, as generated by unimolecular fragmentation of an alkynyl iodonium salt precursor. Alternative mechanisms based on DFT energy calculations are reported here involving no free C , but which are instead bimolecular 1,1- or 1,2-iodobenzene displacement reactions from the zwitterionic intermediate by galvinoxyl radical, or by hydride transfer from 9,10-dihydroanthracene. These result in the same trapped products as observed experimentally, but unlike the mechanism involving unimolecular generation of free C , exhibit calculated free energy barriers commensurate with the reaction times observed at room temperatures. The relative energies of the transition states for 1,1 1,2 substitution provide a rationalisation for the observed isotopic substitution patterns and the same mechanism also provides an energetically facile path to polymerisation by extending the carbon chain attached to the iodonium group, eventually resulting in formation of species such as amorphous carbon and C .