Ion radicals of methyl-substituted benzenes: Temperature dependence of spin and charge densities

Abstract

The spin and charge densities for the monomethyl and dimethyl benzene negative ions are calculated with the valence-bond perturbation procedure developed previously. Because the difference in energy (ΔE) between the two lowest eigenvalues of these systems is found to be small, thermal averaging is included in the determination of the theoretical spin densities. Meta-xylene negative-ion data are used to evaluate the required valence-bond parameters. Comparison of the measured and calculated hyperfine constant values, including the available temperature-dependence data, shows good agreement in most cases. However, a number of discrepancies appear to exist and further investigation is required. The experimental and theoretical difficulties in obtaining good ΔE values are also discussed. The formulation is used to predict spin densities for some unknown methyl-substituted benzene negative ions and for the methyl-substituted benzene positive ions. Also, values are obtained for the perturbation parameter (ΔCx-) for the other alkyl-substituted benzene negative ions which have been studied experimentally. Some discussion is given concerning the extension of the treatment to systems other than substituted benzene ion-radicals. The reliability of the method is considered, and comparisons are made with available molecule-orbital calculations.