The Cope Rearrangement Revisited

Using a CASSCF wave function and a 6–31G* basis set we have located several stationary points on the potential energy surface for the Cope rearrangement. At this level of theory we find both a synchronous path through a symmetrical aromatic transition state and a non-concerted path through a disymmetrical transition state followed by a symmetrical biradicaloid intermediate. After zero-point and thermal corrections both paths appear possible. A more elaborate description of electron correlation does not yield a clear preference for either path.