The relation between reduction potential and solvation energy for some arylmethylium ions

Abstract

The polarographic half-wave potentials, E½, of arylmethylium ions in MeSO3H range from –0.275 V for Ph2HC+ to –0.955 V for (4-McOC6H4)3C+, against the Hg/HgSO4/98% H2SO4 electrode. The narrowness of this span, equivalent to ca. 70 kJ mol–1, is explained in terms of the ionization potentials Ei of the corresponding radicals and the solvation enthalpies ΔHS⊖ of the ions. Since the factors which increase Ei also diminish ΔHS⊖ the sum of Ei and ΔHS⊖, which determines E½, does not change much from one extreme of the array of ions to the other, although both terms may vary over a much greater range. A comparison of E½ for the three triarylmethylium ions in McSO3H and CH2Cl2 shows that (a) as the charge becomes more diffuse, the difference between E½ for any one ion in the two solvents I and II,I–IIΔE½(R+), becomes smaller; and (b) the difference in E½ between two ions, A – BΔE½ becomes smaller for a given solvent, the more polar that solvent. The value of E½ obtained in different solvents can be correlated by a new kind of diagram in which the origin of the axes represents the E½ of Ph3C+. For each of three very different ions the E½ values are almost identical in CH2Cl2 and MeCN; this can be attributed to the fact that E½ is not that of a free cation, but of a cation which is part of an ion pair or a higher aggregate, formed from the abundant supporting electrolyte.