SECONDARY DEUTERIUM KINETIC ISOTOPE EFFECTS IN THE ROTATIONS OF ALKENES AND ALLYL RADICALS - THEORY AND EXPERIMENT

Secondary deuterium kinetic isotope effects (KIEs) associated with res tricted rotation about the C-C bonds of ethylene, propene, and allyl r adical substituted by H or D were calculated. Geometries and force con stants of ground states and transition states were obtained by ab init io MCSCF calculations with the 6-31G and/or 6-311G** basis sets. The KIEs were then calculated from the partition functions for the Bigelei sen 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 degrees C to 1.15-1.25 at 275 degrees C. P redicted values are compared with existing experimental results. As a test of our prediction that high-temperature alkene isomerizations sho uld display large KTEs, samples of cis-stilbene and alpha,alpha'-dideu terio-cis-stilbene were pyrolyzed; the measured KIE was a remarkable k (H)/k(D2) = 1.47 +/- 0.13(sigma(n)) at 287 degrees C.