Energetics of tert-Butoxyl Addition Reaction to Norbornadiene: A Method for Estimating the π-Bond Strength of a Carbon−Carbon Double Bond

The energetics of tert-butoxyl radical addition reaction to norbornadiene was investigated by time-resolved photoacoustic calorimetry (TR-PAC). The result, together with the C−O bond dissociation enthalpy (BDE) in the addition product, allowed us to calculate the π-bond dissociation enthalpy in norbornadiene. Quantum chemistry (QC) methods were also used to obtain several enthalpies of reaction of the addition of oxygen-centered radicals to alkenes. The π-bond dissociation enthalpies in these molecules were calculated by a procedure similar to that used in the case of norbornadiene and were compared with the π-BDE values obtained by the method proposed by Benson. These two different approaches yield similar values for the π-BDEs in alkenes, indicating that the addition method proposed in the present study is a valid way to derive that quantity. The influence of strain in the π-BDEs of cyclic alkenes was investigated and allowed us to justify the difference between the π-BDE in norbornene and norbornadiene. Finally, the thermochemistry of the addition and abstraction reactions involving these two molecules and tert-butoxyl radical was analyzed.