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

Calculations on the chair and boat Cope rearrangements have been perfo rmed at the CASPT2N level of theory, in which second-order perturbatio n theory is used to provide electron correlation beyond that included at the CASSCF level. Unlike previous CASSCF calculations with the 6-31 G basis set, CASPT2N calculations with 6-31G*, 6-311G**, and 6-311G(2 d,2p) all find a single stationary point of C-2h symmetry for the chai r and C-2v symmetry for the boat, which corresponds to the transition state for a reaction in which bond making and bond breaking are synchr onous. The CASPT2N energetics are in much better agreement with experi ment than are those computed at the CASSCF/ 6-31G level, as is the CA SPT2N finding that the Cope rearrangement is concerted and does not in volve the formation of a diradical intermediate.