A.M.O. study of solvent effects in substituted benzene radical anions

Abstract

In this paper we report the results of a theoretical investigation dealing with the effect of the solvent upon the relative stability of the symmetric (ψ S) and antisymmetric (ψ A) electronic configurations of substituted benzene radical anions. The problem has been investigated using a modified SCF-MO INDO procedure, which includes the solvation effects in the way suggested by Constanciel and Tapia [1]. The computation have been carried out for a series of monosubstituted benzene radical anions (C 6H 5X) ⨪, with X = OH, CN, CH 3 and the energy differences ΔE = E S - E A between the energies of the symmetric (E S) and antisymmetric (E A) configurations computed for various values of the dielectric constant ϵ are listed i Table I, including also the gas phase situation (ϵ = 1). The substituents listed above have been chosen because they represent situations where the conjugative interactions are significantly different. We have already shown [2], in fact, that the conjugative effect is the dominant factor that determines the preferential stabilization of one of the two configurations ψ S and ψ A, that in the absence of the perturbation due to the substituent are degenerate. In all cases where the substituent has just the π-type lone pair (X = OH) or a filled-unfilled π-system characterized by a high energy HOMO and a low energy LUMO (X = CN), there is a largely dominant conjugative interaction that dictates the preferential stabilization. In these cases the solvent plays only a minor role, as shown by the results listed in Table I, since the solvent has not the effect to alter the dominant character of an interaction that is largely dominant in the gas phase. t001 Energy Differences ΔE = E S - E A (kcal/mole) for Various Monosubstituted Benzene Radical Anions at Various Values of the Dielectric Constant ϵ. Substituent ϵ = 1.0 ϵ = 5.0 ϵ = 20 ϵ = 80 OH 7.99 12.27 14.12 14.43 CN −13.86 −13.99 −14.54 −14.97 CH 3 −4.75 1.47 2.94 3.81 However, when there is not a largely dominant interaction as in the case X = CH 3, where the HOMO is at low energy and the LUMO at high energy, the effect of the solvent can become critical. In this case, our computations show that ψ A, that is less stable than ψ S in gas phase, becomes more stable when the medium has a dielectric constant greater than ∼3.5 and the trend becomes more pronounced with the increase of the dielectric constant. Radical anions are known for all the compounds we have examined, except phenol which may be compared with anisole [3]. The experimental results agree well with our computational results, not only for the radical anoins of phenol and benzonitrile [4], as previously observed [2], but also for the radical anion of toluene, where the experimental splitting constants determined by ESR in solution [5] denote a preferential stabilization of ψ A, in agreement with the result we obtain for a medium with ϵ ⩾ 3.5.