Resonance stabilization energies in polycyclic aromatic hydrocarbon radicals

It is shown that relative values of ..pi..-bonding resonance stabilization energies for benzyl-type polycyclic aromatic radicals, as calculated by SCF-MO methods, closely correspond to energies calculated by the simple resonance theory approach of Herndon. The formula derived for the latter type of ..pi..-bonding resonance stabilization energy is E/sub ..pi..RSE/ (kcal mole/sup -1/) = 22.68 ln (CSC(R.)) -27.33 ln (CSC(RH)), where CSC(R.) and CSC(RH) are the corrected structure counts (number of stable Kekule isomers) for the radical and parent molecule, respectively. Two methods are then suggested for modifying this formula to estimate empirical radical stabilization energies for use in predicting standard heats of formation of stabilized hydrocarbon radicals.