ABINITIO CALCULATIONS OF THE STABILIZATION ENERGY OF PENTADIENYL RADICAL FROM ROTATIONAL BARRIERS AND FROM LOWERING OF BOND-DISSOCIATION ENERGIES

The energies of and barriers separating the E,E, E,Z and Z,Z geometries of pentadienyl radical have been calculated at the QCISD/6-31G*//UHF/6-31G* level, and the lowering of C-H bond dissociation energies (-DELTABDEs) by two vinyl groups and a 1,3-butadienyl group have also been computed at the QCISD/6-31G* level. The calculated barrier heights and -DELTABDEs are within about 2 kcal/mol of those that have been measured. In agreement with the results of recent experiments, the calculated rotational barriers are larger than the computed values of the comparable -DELTABDEs. The implications of this finding of a difference between these two different measures of the pentadienyl stabilization energy (SE) are discussed.