Comparison of valence-bond and molecular-orbital results for some ion-radicals

The results of valence-bond spin-density calculations for a number of positive (+) and negative (-) aromatic ion radicals are given. For naphthalene, it is found that the α and β position spin densities in the positive ions do not equal those in the negative ion; that is (ρ α(+)/ρα(-))=1.01 and (ρβ(+)/ρβ(-))=0.933. This result, which contrasts with that of molecular-orbital calculations, may provide a partial explanation of the small hyperfine splitting differences found in (+/-) ion pairs. It is noted that for a number of ion radicals, predominantly positive ions, the symmetries of the ground state, as predicted by the valence-bond and molecular-orbital methods, are not the same. Although these symmetry disagreements introduce only small changes in the spin densities of most of the molecules studied, the azulene positive-ion results may be sufficiently different to permit an ESR test of the theories. Other experiments and calculations which would also be important for evaluating the valence-bond method for ion radicals are discussed.