Combined effects of temperature and pressure upon the charge transfer to solvent spectrum of iodide in water and non-aqueous solvents. Local structural effects and ion pair formation

Abstract

The energies of the first absorption band maxima for iodide in water, methanol, isopropanol, n-butanol and propylene carbonate do not shift linearly to higher energies when the pressure is increased from atmospheric to 2000 p.s.i. in contrast to previous suggestions. The shift is linear in methyl cyanide but less than that found previously at higher pressures. The temperature shift (dE_max/dT) decreases slightly with increasing pressure as expected. A normalising pressure is proposed, above which a linear relationship occurs. Spectral behaviour at sub-normalising pressures should thus facilitate understanding the nature of the solvation of ions in aqueous and non-aqueous media, and that of the bulk liquid. At high temperatures a liquid under pressure has a considerably reduced dielectric constant and a computer program was used to resolve a new band arising in methyl cyanide solutions. The presence of a solvent shared ion pair was deduced. Indirect evidence for a similar situation in isopropanol was also obtained.