mode involving either addition/elimination or a direct SN2 like structure, but 1,2-modes have apparently never been previously proposed. A similar 1,1-substitution mechanism which avoids liberating the free cation C⩸N + (a species isoelectronic with C⩸C) may apply when cyanogen chloride or bromide (Cl-C≡N) reacts with benzene in the presence of e.g., aluminium chloride to produce benzonitrile, with benzene as nucleophile displacing the chloride nucleofuge directly at carbon. . The issue now is whether either of these alternative mechanisms have overall lower activation free energies than the previously mooted pathway generating C2 itself, whether unbound or as a "solvent-cage trapped" species. The ωB97XD/Def2-SVPD/SCRF=DCM model predicts the barriers for this bimolecular reaction (Eqn. 2 and Table , column 8) to be close to those obtained at the CCSD(T) levels and also that the differences between the more accurate Def2-TZVPPD and the computationally faster Def2-SVPD basis sets are acceptably small (<1 kcal mol -1 ). It was also possible to compare the free energy of the 1,2-substitution transition states (Eqn. 2) with that of free C2 + two Me-I molecules (Table , 28 column 7). If a correction of ~+28 kcal mol -1 noted above is applied to DG298 using the ωB97XD/Def2-SVPD/SCRF=DCM model, the 1,2-substitution reaction of Me-I + -C≡C -by Me-I as nucleophile still emerges as ~10 kcal mol -1 lower in energy than the pathway involving free C2. Replacing Me-I by phenoxyl radical as a better substituting nucleophile suggests that this 1,2-reaction is now ~21 kcal mol -1 lower in free energy than generation of free C2, and that the activation free energy itself (~21 kcal mol -1 ) is compatible with a facile room temperature reaction.