The results for increasingly complete models of the galvinoxyl trap are shown in Tables 1 for both the 1,1-and 1,2-substitution mechanisms. The reaction between 2 and phenoxyl radical has a slightly lower barrier (ΔG ‡ ~16.9-18.8 kcal mol -1 ) in the gas phase than in dichloromethane solution (~19.4-19.6), due to solvent stabilization of the ionic 2. The free energies of the 1,1and 1,2-substitutions tend to be similar but not identical, which would account for the small variations in isotopic ratios of the final product. Such a model no longer requires stipulating fast radical pairing in a solvent cage to account for unequal isotope ratios in solutions. For the full galvinoxyl model, 1,2-substitution resulting in a-labelled C-product is computed as slightly lower in free energy, in accord with observation. These transition states also have slightly different dipole moments (1,2 isomer 12.2D, vs 1,1-isomer 11.0D), which suggests that such differences may explain the changes in isotope ratios as a function of solvent observed in the original experiments (cf Figure ). The height of the dichloromethane solution free energy barrier (25.9 kcal mol -1 ) is now perhaps 3-4 kcal/mol higher than expected for a facile room temperature reaction, but the size of the system has thus far precluded full conformational optimisation to identify any conformers with lower energy barriers. The energies of both the 1,1-and 1,2-substitution transition states are lower than the computed combined free energies of the trapping species + free C2 + iodobenzene by ~12.3 and 14.3 kcal mol -1 respectively. Bimolecular reaction between free C2 and any trap would augment that free energy difference because of an additional free energy barrier induced by loss of entropy (see e.g. the entry in Table for the reaction between free C2 and dichloromethane), reinforcing the conclusion that the route involving bound rather than free C2 is the more probable mechanism. The transition state for reaction of 2 with 9,10-dihydroanthracene shows much greater discrimination between 1,1-and 1,2-substitution, with the latter being clearly favoured (Figure and Table ). The former has a small degree of biradicaloid character (<S 2 >= 0.4129) and is highly asynchronous, tending towards formation of HC≡C• and 9,10-dihydroanthracen-9-yl radical as a "hidden intermediate", but which eventually results in hydrogen abstraction from the latter by the former to give the final trapped products. The more stable 1,2-isomer has no biradicaloid character at the equally asynchronous transition state and at this point approximately corresponds to hydride abstraction to form to a 9,10-dihydroanthracen-9-ylium cation and a HC≡C -hidden-intermediate ion-pair instead, which then collapses to final observed products. Importantly, a thermally accessible barrier is computed for this reaction (ΔG ‡ ~23.8 kcal mol -1 ), which again is lower than the combined (corrected) energies of the species involved in unbound C2 by ~19.4 kcal mol -1 plus any additional entropic barrier for bimolecular reaction of C2 (see above). The next mechanism to be addressed here relates to the observation 10,11 that along with trapping of assumed unbound C2 itself, other major products are clearly carbon oligomers, including the formation of C60. Can these too arise without the intermediacy of free/unbound C2? The reaction of 2 with itself to form a new C-C bond provides an obvious route for such a process (Figures and). A 1,2-transition state is clearly lower than the 1,1-mode and hence provides a facile thermal route to formation of a bound C4 species (ΔG ‡ 15.4 kcal mol -1 ). The geometry of the former has a novel aspect in having two-fold (C2h) symmetry, with each molecule of 2 acting as the nucleophile attacking the other and both iodobenzene units apparently acting as the nucleofuge. An IRC (Figure ) reveals that this symmetry is initially maintained following the transition state, with apparent elimination of a free C4 unit, but eventually the energy landscape breaks symmetry to bifurcate and the unit of C4 is "frustrated" by recombining with one PhI only to form PhIC4. Such a bifurcating potential energy surface is reminiscent of the dimerization of cyclopentadiene. Further low barrier reactions between this product and more of 2 extends the carbon chain to six, this time favouring 1,1-substitution. The process can be repeated to form longer linear or even branched carbon chains (Table , footnotes d and e).