AN ELECTRONEGATIVITY MODEL FOR POLAR GROUND-STATE EFFECTS ON BOND-DISSOCIATION ENERGIES

Homolytic bond dissociation energies are a composite of the radical st abilization energies (RSE) of the product radicals and the polar groun d state stabilization energies (PSE) of the reactant molecules, Substi tuent effects on the PSE are rationalized in terms of changes in the d ifference of group electronegativities. Thus, the PSE is composed of a bond polarity term, which measures the contribution due to the change in the electronegativity difference between the atoms in the bond, wh ich is broken, and a polar relaxation term, which accounts for the sub stituent dependent group electronegativity changes in the remaining bo nds, A semi quantitative model based on Pauling's bonding theory is su itable to assess the direction and relative magnitude of such effects, For the cleavage of benzylic and related bonds, the polar relaxation energy can be neglected (one-bond approximation) to allow the use of c orrelation analyses and substituent a parameters for the interpretatio n of aryl substituent effects on the PSE, Accordingly, the plots of th e PSEs versus a substituent parameters should be linear, curved or par abolic depending on the electronegativity difference of the atoms in t he bond being broken (Delta EN); moreover, the slopes (rho values) sho uld increase linearly with Delta EN. The predicted dependences of the PSEs on aryl substituents are compared with known experimental results and with the data obtained from semiempirical calculations. (C) 1997 by John Wiley & Sons, Ltd.