AB-INITIO CALCULATIONS OF THE RELATIVE RESONANCE STABILIZATION ENERGIES OF ALLYL AND BENZYL RADICALS

Ab initio calculations have been carried out at various levels of theory on the relative resonance stabilization energies (RSEs) of allyl and benzyl radicals, based on both the computed difference between the methyl C-H bond dissociation energies (BDEs) of toluene and propene and on the calculated barriers to methylene rotation in allyl and benzyl radicals. Comparison of the results with the 6-31G** basis set at the CASSCF, CASPT2N, and CASCISD levels gives estimated RSEs for benzyl that are 2.0-2.5 kcal/mol less than those in allyl. Based on a methyl C-H BDE (DH298) = 87.2 +/- 0.7 kcal/mol for propene, a value of DH298 = 89.5 +/- 1 kcal/mol is obtained for the methyl C-H BDE of toluene. The rotational barrier in benzyl radical at 298 K is calculated to be 12.5 +/- 1.5 kcal/mol. Comparison of the results of calculations based on UHF and CASSCF reference wave functions shows that, except for geometry optimizations, the latter an to be preferred for radicals like benzyl.