Thermochemical parameters for organic radicals and radical ions. Part 4. The relationship between bond dissociation enthalpy and radical stability in benzenoid systems

It is shown that isodesmic reactions used to assess interaction energies between substituents (X and Y) in benzenoid compounds have not been corrected for X/H and Y/H interactions. Excellent linear Mulliken charge – interaction energy relationships are established for meta- and para-substituted toluenes. These studies illustrate the relative importance of σ- and π-effects on a quantitative basis. The ρ-values obtained show an equal importance of -π-effects in meta- and para-substituted toluenes. The signs of the ρ(π)-values confirm the presence of resonance type -π-interactions. The role of ring hydrogens in σ-type interactions cannot be assessed from this study. At present, BDH values in benzenoid systems cannot be corrected such that they can be used to define radical stability. 13C chemical shifts are found not to correlate with isodesmic interaction energies. The application of 13C chemical shifts is limited by the inability to independently relate these shifts to σ- and -π-effects.