THE COPE REARRANGEMENT REVISITED AGAIN - RESULTS OF AB-INITIO CALCULATIONS BEYOND THE CASSCF LEVEL

Calculations on the chair and boat Cope rearrangements have been performed at the CASPT2N level of theory, in which second-order perturbation theory is used to provide electron correlation beyond that included at the CASSCF level. Unlike previous CASSCF calculations with the 6-31G* basis set, CASPT2N calculations with 6-31G*, 6-311G**, and 6-311G(2d,2p) all find a single stationary point of C-2h symmetry for the chair and C-2v symmetry for the boat, which corresponds to the transition state for a reaction in which bond making and bond breaking are synchronous. The CASPT2N energetics are in much better agreement with experiment than are those computed at the CASSCF/ 6-31G* level, as is the CASPT2N finding that the Cope rearrangement is concerted and does not involve the formation of a diradical intermediate.