Spin-coupled description of aromaticity in the retro Diels−Alder reaction of norbornene

The electronic rearrangements along the lowest-energy path for the gas-phase retro Diels−Alder reaction of norbornene are monitored using spin-coupled theory. We find that the most dramatic changes to the electronic structure occur in a relatively narrow interval in which the system passes through a geometry at which it can be considered to be significantly aromatic. We provide an estimate of the vertical resonance energy. Our results are consistent with the anticipated synchronous “aromatic” nature of this reaction, but we find that the key changes occur a little before the actual transition state is reached