The Cope Rearrangement Revisited with Multireference Perturbation Theory

Earlier calculations on the Cope rearrangement based on a CASSCF wave function for the six electrons in the rearranging bonds gave reasonable energies at most places on the potential surface except in the crucial region around the chair transition state. Recently developed MP2 methods for multireference wave functions yield greatly improved energies. Using our version of this method, we simultaneously find that the CASSCF wave function overestimates the diradical character of the wave function. When this error is corrected, the Dewar-type diradicaloid stable intermediate no longer occurs as a minimum in the potential surface and the aromatic transition state moves to shorter bond lengths.