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

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Abstract
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.