An analysis is made of the spin distribution and hyperfine splittings of the toluene anion and cation radicals. Considered are the importance and magnitude of the splitting of the benzene-ion degeneracy by the interaction between the ring and the methyl group, the effects of spin delocalization into the methyl group by different mechanisms, and the consequences of vibrational and thermal coupling of the near-degenerate levels. The spin delocalization mechanism in the toluene ions is shown to be in agreement with a perturbation model for the Ċ–B = A system (here Ċ corresponds to the aromatic ring). The vibronic calculation is based on an ASMO–CI treatment of the electronic energy levels and their variation with bond lengths and angles; an attempt is made to include all significant vibrational contributions. The previously neglected alteration of the neutral-molecule vibrational functions due to the addition or removal of a pi-electron is found to be of particular importance. Although the exact quantitative results are dependent on the electronic parameters, the lowest anion vibronic state consists of about 90% of the antisymmetric and 10% of the symmetric electronic state. Comparison of the calculated and experimental temperature dependence of the anion hyperfine splittings shows that the vibronic treatment gives reasonable results.

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The vibronic wavefunctions and energy levels of the benzene, benzene−1−d, benzene−2,6−d2, benzene−1,4−d2, and benzene−1,3,5−d3 anion radicals have been obtained and used to determine the effect of deuterium substitution on the degenerate energy levels and spin densities of the benzene anion. It is shown that a dynamic Jahn−Teller effect treatment is required for these systems to obtain a quantitative interpretation of the consequences of deuterium substitution. The ground vibronic states of benzene− and benzene−1,3,5−d3− are degenerate. In benzene−2,6−d2−, the symmetric state (A1 under C2v) is 22 cm−1 below the antisymmetric state (B1 under C2v). In benzene−1−d− and benzene−1,4−d2−, the antisymmetric states are lower by 9 and 39 cm−1, respectively. The benzene−2,6−d2− and benzene−1,4−d2− hyperfine coupling constants and their temperature dependence calculated using the vibronic wavefunctions and energy splittings are in very good agreement with the experimental values. For benzene−1−d, the experimental splitting is estimated to be 20 cm−1, about twice as large as the calculated value, and agreement between the experimental and calculated hyperfine constants is less satisfactory. The vibronic energy splitting are interpreted in terms of the effect of deuterium substitution on the vibrational potential energies corresponding to the symmetric and antisymmetric electronic states. Calculations of the expectation values of the unpaired electron energy over the neutral molecule and the two anion Born−Oppenheimer state vibrational functions show that changes in the average values of the out−of−plane bending coordinates upon deuteration lead to a splitting of the vibrational potential energies. To a first approximation, the differences among the anions are a function of the unpaired orbital bond orders at the deuterated positions in the two electronic states. If the substituted bond order is large and negative, deuterium substitution increases the vibrational potential energy, but if it is approximately zero, the potential energy is essentially unchanged.