Magnetic circular dichroism in the charge-transfer-to-solvent excited states of iodide ion in aqueous solution

Magnetic circular dichroism (MCD) spectra of iodide ions in solution are measured in the two resolved ultraviolet absorption bands that have been attributed to the charge-transfer-to-solvent (CTTS) excitation resulting in the iodine atom in 2P1/2 and 2P3/2 states. The relative absorption intensities and Faraday parameters of the twin bands in aqueous solution are theoretically reproduced with values of exchange interaction K = 500–600 cm-1 and spin-orbit coupling constant ζ = 5000 cm-1. A discrete variational (DV)-Xα molecular-orbital calculation is carried out for the [I(H2O)6]- ion, which is surrounded by a polarized dielectric continuum. This calculation reproduces the observed spectral parameters and leads to the conclusion that the lowest allowed transition of hydrated iodide ions in aqueous solution is to be ascribed to a Rydberg-type 5p → 6s excitation within the iodide ion, which gives rise to a charge transfer beyond the first solvation sphere (6·8a 0). However, the excitation yields no appreciable charge transfer far beyond the layer of oriented water molecules in immediate contact with [I(H2O)6]-.