Excited States of Iodide Anions in Water: A Comparison of the Electronic Structure in Clusters and in Bulk Solution

Abstract

A new computational approach for calculating charger-transfer-to-solvent (CTTS) states of anions in polar solvents is presented. This is applied to the prototypical aqueous iodide system when the anion is placed in the interior or at the gas−liquid interface of a bulk water solution or hydrated in small gas phase clusters. The experimental vertical detachment energies and CTTS transition energies are quantitatively reproduced without any adjustable parameters. The representative shapes of bulk CTTS wave functions are shown for the first time and compared with cluster excited states. The calculations start with an equilibrium classical molecular dynamics simulation of the solvated anion, allowing for an extended sampling of initial configurations. In the next step, ab initio calculations at the MP2 level employing an extended diffuse basis set are performed for the anionic ground and lowest triplet state, as well as for the corresponding neutral system. It is argued that due to the small singlet−triplet splitting, the triplet state is a good model for the experimental CTTS state. The present calculations on aqueous iodide ion are made computationally feasible by replacing all water molecules (or all waters except for the first solvation shell) by fractional point charges. It is concluded that the bulk wave function is mainly defined by the instantaneous location of voids in the first solvation shell, which arise due to thermal disorder in liquid water. The key ingredient to CTTS binding in the bulk is the long-range electrostatic field due to the preexisting polarization of water molecules by the ground state iodide ion. This is very different from the situation in small water clusters, where the CTTS state is an order of magnitude more fragile due to the lack of long-range polarization. Therefore, it is argued that the electronic structure of small halide clusters cannot be directly extrapolated to the bulk.