Effects of cations upon absorption spectra. Part 5.—Charge-transfer-to-solvent spectrum of iodide and ion-pair formation

Abstract

Energies E_max and temperatures coefficients dE_max/dT of the first ultra-violet absorption maxima of various alkali metal and quaternary ammonium iodides have been measured for 27 solvents. The results are interpreted in terms of three different structures of ion-pairs in the solvent, and their relative concentrations in solution are correlated with the relative permittivity ε of the solvent. When 11 >ε>5 solvent-shared ion-pairs are the predominant species, and a simple electrostatic treatment supports this conclusion. When 23 >ε>11, solvent separated ion-pairs are also present, and when ε>23 solvent separated ion-pairs are at times detected. The presence of contact ion-pairs is deduced when ε<5. Some large shifts of E_max on addition of a common cation, in low concentrations (< 10^–1 M) in non-aqueous solvents, are reported.