No Free C2 Is Involved in the DFT-Computed Mechanistic Model for the Reported Room-Temperature Chemical Synthesis of C2.
Trapping experiments were claimed to demonstrate the first chemical synthesis of the free diatomic species C2 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 C2, but which are instead bimolecular 1,1- or 1,2-iodobenzene displacement reactions from the zwitterionic intermediate 11 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 C2, exhibit calculated free energy barriers commensurate with the reaction times observed at room temperatures. The relative energies of the transition states for 1,1 vs 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 C60.