The C₇H₁₀ Potential Energy Landscape:  Concerted Transition States and Diradical Intermediates for the Retro-Diels−Alder Reaction and [1,3] Sigmatropic Shifts of Norbornene

The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels−Alder reaction of norbornene have been examined, and the activation parameters and ¹³C primary kinetic isotope effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.