Inverted potentials in two-electron processes in organic electrochemistry

Many molecules can accept or lose electrons in two sequential one-electron steps. Normally, gain or loss of the second electron occurs less readily than the first, which gives rise to two separate one-electron processes detected by voltammetry. In this instance, the intermediate (one-electron product) is stable with respect to disproportionation. There are cases known, however, in which the gain or loss of the second electron occurs more easily than the first, leading to a single two-electron voltammetric process. Here, the standard potentials are inverted with respect to their normal order and the one-electron intermediate is unstable with respect to disproportionation. Semiempirical molecular orbital calculations (AM1) have been used to compute disproportionation energies for a variety of aromatic hydrocarbons and the results were found to be remarkably similar to those calculated for charging spheres in vacuum. Experimental values of the disproportionation Gibbs energies in solution, calculated from the difference in potential for cases which show normal ordering, have been used to develop an empirical relation for the attenuation of the disproportionation energy on going from vacuum to solution. This relationship was then used to predict and/or rationalize cases where inversion or compression of potentials has been observed for hydrocarbons in the solution phase. A similar approach was used for other classes of molecules. Here, only a single model compound with normal ordering of potentials was used to predict the effect of solvation on the disproportionation energies for structurally related species. In general, the approach is quite successful in predicting and/or rationalizing the occurrence of inversion of potentials. The reduction of 3,6-dinitrodurene was predicted to occur with inversion and this was verified by cyclic voltammetric studies.