Intramolecular H Atom Transfer Reactions in Alkyl Radicals and the Ring Strain Energy in the Transition Structure

Ab initio calculations were performed on prototypical
intramolecular H atom transfer reactions in alkyl
radicals, namely, the identity reactions of 1,2-, 1,3-, 1,4-, and 1,5
intramolecular isomerizations in the ethyl, 1-propyl,
1-butyl, and 1-pentyl radicals, respectively. The equilibrium
geometries and the transition structures have been
optimized at the UHF/6-31G* and/or UMP2/6-311G** levels. The
activation energies at 0 K were calculated with
the MP-SAC2 and BAC-MP4 methods. The computed barrier heights
(41.1, 41.6, 24.6, 17.2 kcal mol-1 at the
MP-SAC2 and 43.1, 41.0, 25.1, and 18.8 kcal mol-1 at the
BAC-MP4 level of theory) follow the trend expected and
are in reasonable agreement with experimental data on related
reactions. The differences between the ab initio
barrier heights of alkyl isomerizations and that of the corresponding
bimolecular reaction are in good agreement
with the strain energies of the corresponding
cycloalkanes.