Abstract

Photoexcitation of halides dissolved in polar liquids results in charge-transfer-to-solvent (CTTS) states in which a halide valence electron has been transferred to a delocalised, solvent-supported orbital. Subsequent relaxation of CTTS excited solvated halides results in the formation of solvated electrons, ubiquitous species implicated in numerous chemical and biochemical transformations. Analogues of the CTTS excited states of solvated halides have also been observed in small iodide-polar solvent clusters, and the relaxation of CTTS excited iodide-polar solvent clusters, [I–(Solv)n]*, has attracted significant interest as a paradigm for investigating the
role of individual solvent molecules in trapping and solvating an excess electron.

In this work, a combination of high-level quantum chemical calculations and first-principles molecular dynamics simulations is employed to elucidate the relaxation mechanism of [I–(Solv)n]* (Solv = H2O, CH3CN and CH3OH) and to develop an in-depth understanding of the nature of the molecular motions and interactions involved in the associated electron solvation processes. A ‘two-level’ approach is employed, in which [I–(Solv)n]* trajectories are propagated on a potential energy surface computed with a relatively modest treatment of electron correlation
and a medium-sized basis set while electronic properties of cluster configurations sampled from the trajectories are computed with a much more rigorous quantum-chemical method and significantly larger basis sets. Results indicate that [I–(Solv)n]* relaxation involves rapid initial motion of the solvent molecules, leading to the separation of the excited electron from the iodine atom and a concomitant decrease in stability of the excited electron, followed by more gradual reorganisation of the cluster, which can have variable effects on the stability of the excited electron, depending on the type of solvent molecule in the cluster. In clusters with a strong network of solvent-solvent interactions, such as [I–(H2O)n]*, stabilisation of the excited electron occurs, while in clusters with a weaker network of solvent-solvent interactions, such as
[I–(CH3OH)n]*, solvent cluster fragmentation ultimately results in destabilisation of the excited electron. Subtle differences in the structural properties of the molecules within the cluster can thus heavily influence the electron solvation process in [I–(Solv)n]*, a reflection of the important role of individual molecules in supporting a solvated electron.