Effects of molecule stabilization energies on radical reactions: G3 and G3(MP2) model chemistries applied to benzylic systems.

We have carried out G3 and G3(MP2) calculations of the molecule stabilization energies (MSEs) brought about by 11 common substituent groups in the meta and para positions of benzyl fluoride. We find that MSE is a function of the tendency of the substituent to donate or to withdraw electrons, such that a classic Hammett plot can be drawn. We propose that, in general, the direction of the benzylic Z-X dipole of YC6H4ZX is the major factor controlling the sign of the slope of Hammett plots of benzylic atom abstractions by radicals. When the Z-X dipole is pointed away from the substituted ring, electron withdrawing substituents destabilize the molecule, contributing to a decrease of the Z-X bond dissociation energy, and electron donating substituents stabilize it. The reverse is true when the dipole is reversed. This proposal is supported by 13C NMR results and by a survey of relevant benzylic and quasi-benzylic hydrogen or halogen atom abstractions studied experimentally. Calculations at the G3 level of theory demonstrate an increase in the bond dissociation energy of p-YC6H4CH2-H with increasing electron withdrawing ability of Y, contrary to results of previous lower level calculations. MSE values of substituted benzyl fluorides (p-YC6H4CH2F) correlate well with allylic MSE (trans-YCH=CHCH2F) and quantify the relative efficacy of transmission of electronic effects by vinylogy.