Computational studies on exothermic carbon–carbon bond cleavages

Structural aspects of single CC bond dissociation energies are examined and it is shown that in certain cases a negative bond dissociation energy (BDE) implies a very weak bond and an unstable species prone to bond breaking resulting in dissociation or structural rearrangement. It is proposed that, in such cases, a better quantitative indicator for the strength of the bond is the activation energy required for its fission. o-Phenelylene bis(nitrene) 1 is computed to have the most negative CC BDE for an observable species. Under cryogenic conditions, activation energy for the dissociation of this bond has been measured as only 2.8 kcal/mol, making it the weakest that we know of. An explanation based on the formation of two new bonds as responsible for this extremely weak bond is given.