Secondary Isotope Effects in the Chemically Activated sec-Butyl-d and -d8 Systems and sec-Butyl-d1 and -d9 Systems

The decomposition of chemically activated sec-butyl-d8 and -d9 radicals, produced by H- and D-atom addition to cis-butene-2-d8, has been studied at 195° and 300°K over a range of pressures. These measurements, as well as internal comparison experiments with cis-butene-2-d0, reveal a large normal secondary intermolecular kinetic-isotope ratio which varies with the pressure and temperature, and whose minimum value is a factor of 4 and whose maximum value averages around 7. The absolute magnitude of the ratio, and its temperature, pressure, and energy dependence are in satisfactory agreement with the theory of these chemical-activation systems; some aspects of this dependence are discussed. Evidence against a significant mechanistic isotope effect on the energetics is presented. Calculated isotope effects for all of the possible sec-butyl radical systems, to be formed by H- and D-atom addition to butene-d0 and -d8 isomers, are given.