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
kH/kD 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
kH/kD2 = 1.47 ±
0.13
(σn) at 287 °C.