Solvent effect on the disproportionation of the tetrakis-p-methoxyphenylethylene cation radical. An experimental verification of theory

The disproportionation equilibrium, 2 TME+˙ [graphic omitted] TME2++ TME, where TME is tetrakis-p-methoxyphenylethylene, was observed to be profoundly influenced by the fraction of acetonitrile present in dichloromethane–acetonitrile mixtures. In going from pure acetonitrile to pure dichloromethane, K decreased from 1·1 to 0·03. In the mixed solvent system, peak voltammograms show increasing peak width with increasing fraction of dichloromethane. Differences in E0 for the two charge transfers were estimated from the voltammetric peak widths and disproportionation equilibrium constants were calculated. The plot of log K versus% acetonitrile was found to be linear over the entire range of solvent composition. Both the cation radical and dication were found to be stable in the solvent mixtures which had been passed over neutral alumina. Disproportionation equilibrium constants calculated from the visible absorption spectra of partially oxidized solutions of TME were nearly identical with the values predicted from analysis of the voltammetric peak widths.