Exploration of pericyclic reaction transition structures by quantum mechanical methods: competing concerted and stepwise mechanisms

Density functional theory and multiconfigurational SCF calculations have been applied to a number of pericyclic reactions, including cycloadditions, electrocyclizations and sigmatropic shifts. Emphasis is on the competition between concerted and stepwise mechanisms, comparisons of computed and experimental activation energies and isotope effects, and the performance of MP2, CASSCF and DFT calculations. Various diradical processes, such as the vinylcyclopropane rearrangement and cyclobutane isomerizations, were also studied to test the performance of DFT with diradical processes. (C) 1997 Elsevier Science B.V.