Dynamics of Electron Solvation in Molecular Clusters

Abstract

S olvated electrons, and hydrated electrons in particular, are important species in condensed phase chemistry, physics, and biology. Many studies have examined the formation mechanism, reactivity, spectroscopy, and dynamics of electrons in aqueous solution and other solvents, leading to a fundamental understanding of the electron-solvent interaction. However, key aspects of solvated electrons remain controversial, and the interaction between hydrated electrons and water is of central interest. For example, although researchers generally accept that hydrated electrons, e aq -, occupy solvent cavities, another picture suggests that the electron resides in a diffuse orbital localized on a H 3 O radical. In addition, researchers have proposed two physically distinct models for the relaxation mechanism when the electron is excited. These models, formulated to interpret condensed phase experiments, have markedly different time scales for the internal conversion from the excited p state to the ground s state.Studies of negatively charged clusters, such as (H 2 O) n -and I -(H 2 O) n , offer a complementary perspective for studying aqueous electron solvation. In this Account, we use time-resolved photoelectron spectroscopy (TRPES), a femtosecond pump-probe technique in which mass-selected anions are electronically excited and then photodetached at a series of delay times, to focus on time-resolved dynamics in these and similar species. In (H 2 O) n -, TRPES gives evidence for ultrafast internal conversion in clusters up to n ) 100. Extrapolation of these results yields a p-state lifetime of 50 fs in the bulk limit. This is in good agreement with the nonadiabatic solvation model, one of the models proposed for relaxation of e aq -. Similarly, experiments on (MeOH) n -up to n ) 450 give an extrapolated p-state lifetime of 150 fs.TRPES investigations of I -(H 2 O) n and I -(CH 3 CN) n probe a different aspect of electron solvation dynamics. In these experiments, an ultraviolet pump pulse excites the cluster analog of the charge-transfer-to-solvent (CTTS) band, ejecting an electron from the iodide into the solvent network. The probe pulse then monitors the solvent response to this excess electron, specifically its stabilization via solvent rearrangement. In I -(H 2 O) n , the iodide sits outside the solvent network, as does the excess electron initially formed by CTTS excitation. However, the iodide in I -(CH 3 CN) n is internally solvated, and both experimental and theoretical evidence indicate that electrons in (CH 3 CN) n -are internally solvated. Hence, these experiments reflect the complex dynamics that ensue when the electron is photodetached within a highly confined solvent cavity.

Theoretical Studies of Spectroscopy and Dynamics of Hydrated Electrons

“…Despite the challenges, the debate surrounding the details of bulk electron hydration has motivated the extension of timeresolved measurements to hydrated electron clusters. 13,195,209,210,211 The experiments reveal that the excited state lifetimes are cluster size- Although there are no direct experimental data on the dynamics of hydrated electron cluster formation via electron attachment, theoretical studies of this process can suggest relevant microscopic details of cluster dynamics and offer insights into the process. A first step of such an analysis is the examination of potential electron localization sites of neutral water clusters.…”

On the applicability of one- and many-electron quantum chemistry models for hydrated electron clusters

“…Experimentally, this complexity is manifest in the variation of the vertical detachment energy (VDE), the energy needed to remove an electron from the clusters vertically, with cluster size. 3,4,5,6,7 The VDE-size diagrams exhibit various systematic patterns that may indicate common structural motifs within the groups. Based on early 8 and subsequent improved one-electron model quantum simulations, 9,10 and an allelectron density functional theory based molecular dynamics study, 11 tentative assignments of the different anionic groups have been proposed.…”

Hydration dynamics in water clusters via quantum molecular dynamics simulations

“…27,28,31,32,34 The kinetic energy of the detached electron (photoelectron) is then measured, and its binding energy to the cluster is determined. Our second computer experiment models the relaxation of neutral water clusters after removing the excess electron from anionic water clusters.…”

“…21,22,23 Despite the relative simplicity, water cluster anions still pose a challenging case for sophisticated theoretical 24,25,26 and experimental approaches. 27,28,29,30,31,32,33,34 Although molecular dynamics simulations have provided a rather consistent picture of the hydrated electron system, as have been reviewed recently, 35 a few unanswered, nonetheless disturbing questions remain open. A notable problem concerns the structure of the hydrated electron.…”