Secondary Deuterium Kinetic Isotope Effects in the Rotations of Alkenes and Allyl Radicals: Theory and Experiment

Secondary deuterium kinetic isotope effects (KIEs) associated with restricted rotation about the C−C bonds of ethylene, propene, and allyl radical substituted by H or D were calculated. Geometries and force constants of ground states and transition states were obtained by ab initio MCSCF calculations with the 6-31G* and/or 6-311G** basis sets. The KIEs were then calculated from the partition functions for the Bigeleisen equation. The “rotational” KIEs are predicted to be larger than typical secondary KIEs. For simple alkenes, predicted k H / k D values range from 1.32−1.46 at 25 °C to 1.15−1.25 at 275 °C. Predicted values are compared with existing experimental results. As a test of our prediction that high-temperature alkene isomerizations should display large KIEs, samples of cis -stilbene and α,α‘-dideuterio- cis -stilbene were pyrolyzed; the measured KIE was a remarkable k H / k D2 = 1.47 ± 0.13 (σ n ) at 287 °C.