Structure, Dynamics, and Reactivity of Hydrated Electrons by Ab Initio Molecular Dynamics

Abstract

Understanding the properties of hydrated electrons, which were first observed using pulse radiolysis of water in 1962, is crucial because they are key species in many radiation chemistry processes. Although time-resolved spectroscopic studies and molecular simulations have shown that an electron in water (prepared, for example, by water photoionization) relaxes quickly to a localized, cavity-like structure similar to 2.5 angstrom in radius, this picture has recently been questioned. In another experimental approach, negatively charged water clusters of increasing size were studied with photoelectron and IR spectroscopies. Although small clusters can bind an excess electron, their character is very different from bulk hydrated species. As data on electron binding in liquid water have become directly accessible experimentally, the cluster-to-bulk extrapolations have become a topic of lively debate. Quantum electronic structure calculations addressing experimental measurables have, until recently, been largely limited to small clusters; extended systems were approached mainly with pseudopotential calculations combining a classical description of water with a quantum mechanical treatment of the excess electron. In this Account, we discuss our investigations of electrons solvated in water by means of ab initio molecular dynamics simulations. This approach, applied to a model system of a negatively charged cluster of 32 water molecules, allows us to characterize structural, dynamical, and reactive aspects of the hydrated electron using all of the system's valence electrons. We show that under ambient conditions, the electron localizes into a cavity close to the surface of the liquid cluster. This cavity is, however, more flexible and accessible to water molecules than an analogous area around negatively charged ions. The dynamical process of electron attachment to a neutral water cluster is strongly temperature dependent. Under ambient conditions, the electron relaxes in the liquid cluster and becomes indistinguishable from an equilibrated, solvated electron on a picosecond time scale. In contrast, for solid, cryogenic systems, the electron only partially localizes outside of the cluster, being trapped in a metastable, weakly bound "cushion-like" state. Strongly bound states under cryogenic conditions could only be prepared by cooling equilibrated, liquid, negatively charged clusters. These calculations allow us to rationalize how different isomers of electrons in cryogenic clusters can be observed experimentally. Our results also bring into question the direct extrapolation of properties of cryogenic, negatively charged water clusters to those of electrons in the bulk liquid. Ab initio molecular dynamics represents a unique computational tool for investigating the reactivity of the solvated electron in water. As a prototype, the electron proton reaction was followed in the 32-water cluster. In accord with experiment, the molecular mechanism is a proton transfer process that is not diffusion limited, but rat...

To Be or Not to Be in a Cavity: The Hydrated Electron Dilemma

“…15 This places ab initio calculations at the limit of what is computationally feasible for density functional theory (DFT) and out of range for more accurate quantum chemistry methods. We note that when the limits are pushed, however, recent cluster calculations based on DFT have shown water density penetrating deep into the electron's center, even for room temperature water (e.g., see Figure 2 of ref 31). And when extending such calculations to bulk liquid water, Uhlig, Marsalek, and Jungwirth (UMJ) have found a hydrated electron that has both cavity and non-cavity characteristics.…”

Hydrated Electrons in Water Clusters: Inside or Outside, Cavity or Noncavity?

“…The renewed (or better to say, continuous) research interest in the species is fueled by experiments employing the most sophisticated laser techniques to explore hydrated electron properties in the femtosecond 4,5,6,7,8,9,10,11 or even in the attosecond time regime, 12 and by theoretical attempts to provide a consistent molecular level picture underlying the experimental measurements. 13,14,15 Molecular dynamics (MD) simulations have proved to be very efficient at rationalizing experimental observations and have converged during the years to a rather satisfying picture of the hydrated electron, as have been reviewed recently. 16 Molecular dynamics on hydrated electron has been predominantly applied in the one-electron approximation, treating the electron quantum mechanically, while modeling the solvent environment by classical force fields 17,18,19,20 in a mixed quantum-classical MD (QCMD) framework.…”

Charge transfer to solvent dynamics in iodide aqueous solution studied at ionization threshold

“…This view is supported by most recent theoretical calculations showing that an excess electron cannot be stabilized at the liquid water surface [41]. The theoretical description of the solvated electron is based on Density Functional Theory (DFT) and provides a faithful description of solvated electrons in large water clusters and in bulk water [42].…”