TRANSITION STRUCTURES OF PERICYCLIC-REACTIONS - ELECTRON CORRELATION AND BASIS SET EFFECTS ON THE TRANSITION STRUCTURE AND ACTIVATION-ENERGY OF THE ELECTROCYCLIZATION OF CYCLOBUTENE TO BUTADIENE

Cyclobutene (1), s-trans-butadiene (2), and the conrotatory transition structure 3 of the electrocyclic ring opening of 1 have been located at the ab initio RHF and MP2 levels of theory and with semiempirical techniques. Geometries of all three structures are reasonably insensitive to changes in basis set or inclusion of electron correlation. At the RHF level and with MNDO and MINDO/3, activation energies and heats of reaction are overestimated, while at the AM1, MP2/6-31G*, MP2/6-31G*//RHF/3-21G, and MP4SDTQ/6-31G*//MP2/6-31G* levels they are in good agreement with the experimental values.