THEORETICAL SECONDARY KINETIC ISOTOPE EFFECTS AND THE INTERPRETATION OF TRANSITION-STATE GEOMETRIES .1. THE COPE REARRANGEMENT

Theoretical secondary kinetic and equilibrium deuterium isotope effects for the Cope rearrangement of 1,5-hexadiene were calculated using RHF/3-21G, RHF/6-31G*, UHF/6-31G*, CASSCF/3-21G, and MP2/6-31G* levels of theory and the Bigeleisen-Mayer method. Isotope effects for transition structures, corresponding to a concerted process of bond reorganization, several different cyclohexane-1,4-diyl-like species, and two allyl radicals, were examined and compared with the experimentally determined values. The theoretical isotope effects calculated for the concerted pathway are in best agreement with experiment. Similar calculations were used to examine the 1,5-heptadiene and 3-methyl-1,5-hexadiene Cope rearrangements. The transition structure with methyl in the equatorial position is 1-2 kcal/mol more stable than that with the axial methyl. These results are analyzed in the framework of Gajewski's More O'Ferrall-Jencks diagrams for the Cope rearrangement, correlating kinetic isotope effects and bond order.