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 E_i of the corresponding radicals and the solvation enthalpies ΔH_S^⊖ of the ions. Since the factors which increase E_i also diminish ΔH_S^⊖ the sum of E_i and ΔH_S^⊖, 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 – B}ΔE½ 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.