Ultraviolet–visible spectrophotometric study of N-alkylpyridinium iodides in non-aqueous solvents. Evidence for the formation of solvent-shared ion-pairs

Abstract

The u.v.–visible absorption spectra of solutions of various alkylpyridinium iodides have been studied at various temperatures and in binary-solvent mixtures. The results are interpreted as due to the existence of an equilibrium between the contact ion pair and solvent-shared ion pair in a solution in non-dissociating solvents. The band at ca. 42 000 cm^–1 has been attributed to a modified charge-transfer-to-solvent transition of the iodide ion within a solvent-shared ion pair. The thermodynamic parameters (ΔG^°, ΔH^° and ΔS^°) for the interconversion of two forms of ion pairs have also been determined in various solvents.