Ionization Potentials of Iodide, Bromide, and Thiocyanate Anions in Solvents: Solvent–Solvent Interactions in the Solvation Sphere

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Abstract
The solution-phase ionization potentials of I−, Br−, and SCN− ions in 16 solvents including water have been measured by a photoelectron emission spectroscopic technique. The values obtained as the threshold energy Et for halides are found to be correlated well with the Mayer–Gutmann acceptor number and better with the Riddle–Fowkes modified acceptor number of the solvent indicating that the electronic states of the halides are predominantly stabilized through the Lewis acid–base interaction. The reorganization energies ΔGr of solvent after the photoionization of the halides are obtained from the Et values. The experimental ΔGr values are reproduced by using a simple model which incorporates the dipole–dipole repulsion and the bond formation energies in the first solvation layer. The solvation mechanism of SCN− is somewhat different from those of the halides because its Et depends less on the acceptor number and its ΔGr values in alcohols are about half of those for the halides.