 Firstly, an as yet un-identified mechanism recovers the energy. Is it possible that sufficient enthalpy can be recovered by say reorganisation of ionic lattice energies so that the resulting free energy barrier G298 ‡ could promote a sufficiently rapid (half-life <1 hour) reaction at room temperatures? This would allow C2 to be trapped by another species, but this would compete with ~barrierless reverse trapping by PhI. If so, by what type of mechanism could this recovered energy then be concentrated directly into the carbon-iodine bond in order to cleave it?  That the range of thermochemical values for the reaction obtained by the three models above is wrongly predicted to be too high by 20-25 kcal/mol.  Alternatively, one might consider that free C2 itself is not produced, but instead some other species which must account for the results of the trapping reactions. A possible check on the gas-phase trapping would be to condense whatever species emerges from the dry reaction flask onto a cold-finger at liquid helium temperatures in an argon matrix and subject this directly to spectroscopic (Raman or other) analysis as an alternative to chemical trapping.