SUBSTITUENT EFFECTS ON THE C-H BOND-DISSOCIATION ENERGY OF TOLUENE - A DENSITY-FUNCTIONAL STUDY

The bond dissociation energies of the benzylic C-H bond of a series of 16 para-substituted toluene compounds (p-X-C6H4CH3) have been calcula ted with the density functional method (BLYP/6-31G). The calculated s ubstituent effects correlate well with experimental rates of dimerizat ion of parasubstituted alpha,beta,beta-trifluorostyrenes and rearrange ment of methylenearylcyclopropanes. Both electron-donating and electro n-withdrawing groups reduce the bond dissociation energy (BDE) of the benzylic C-H bond because both groups cause spin delocalization from t he benzylic radical center. The calculated spin density variations at the benzylic radical centers correlate well with both the ESR hyperfin e coupling constants determined by Arnold et al. and the calculated ra dical effects of the substituents. The relative radical stabilities ar e mainly determined by the spin delocalization effect of the substitue nts, and polar effect of the substituents are not important in the cur rent situation. The ground state effect is also found to influence the C-H BDE.