By use of a perturbation approach, the valencehyphen;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 ofD2symmetry, which corresponds to parahyphen;disubstituted benzene, is treated in detail in terms of a basis set consisting of eighteen valencehyphen;bond functions. Previously given formulas are used in obtaining the secular equation, spin densities, and charge densities for these ionhyphen;radical systems.To test the usefulness of the method, spin densities of several of the methylhyphen;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 methylhyphen;benzene negative ions as well as the yet to be observed positive ions.
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