Solvation spectra. Part 60.—Specific solvation of iodide ions

The solvation of iodide ions has been studied using ultraviolet and infrared spectroscopy. Addition of protic solvents to solutions of iodides in methyl cyanide resulted in loss of a c.t.t.s. band at 40 700 cm^–1 and growth of new bands at higher energy, with the generation of isosbestic points. These new bands are assigned to iodide ions hydrogen bonded to a single hydroxyl proton. This assignment was confirmed by studies of the O—H stretching band for the protic solvent, the equilibrium constants derived therefrom being equal, within experimental error, to those derived from the ultraviolet (c.t.t.s.) spectra. Further addition of protic solvent resulted in a more continuous high-energy shift in the c.t.t.s. spectrum for iodide, and no further well defined species could be detected. In the case of ethylene glycol strong evidence for chelation to iodide was adduced.

Isosbestic points in the c.t.t.s. spectrum for iodide ions were also generated when certain solvents were added to aqueous solutions, in the 0–0.05 mole fraction cosolvent region. The new band was always shifted to high energies, and was markedly narrower than that for aqueous iodide ions. This effect is not thought to be due to solvation by the cosolvent, but rather is interpreted in terms of loss of one water molecule from the primary solvation shell of solvent-rich iodide ions. Similar results were obtained on the addition of tetra-alkylammonium chlorides and are again interpreted in terms of a desolvation of iodide ions.