Spin Resonance Spectra of Substituted Aromatic Ions: Perturbation Model

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Abstract
By use of a perturbation approach, the valence‐bond method is applied to weakly substituted benzene positive and negative ion radicals. Small changes in the Coulomb integrals are introduced to remove the degeneracy present in the unsubstituted case. The case of D2 symmetry, which corresponds to para‐disubstituted benzene, is treated in detail in terms of a basis set consisting of eighteen valence‐bond functions. Previously given formulas are used in obtaining the secular equation, spin densities, and charge densities for these ion‐radical systems. To test the usefulness of the method, spin densities of several of the methyl‐benzene negative ions are compared with hyperfine constants determined from the ESR spectra of these ions. The theory accounts qualitatively for all of the experimental results, and in most cases the agreement between the measured and the calculated hyperfine constants is semiquantitative. Still better agreement may come with improved knowledge of the molecular integrals involved and of the degree of mixing of nearly degenerative states. Predictions are made for the spin densities in other methyl‐benzene negative ions as well as the yet to be observed positive ions.