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 .2.5 . 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
water 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 electronproton 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 rather controlled by a proton-induced
deformation of the excess electron's solvent shell. We demonstrate the necessary ingredients of a successful density functional
methodology for the hydrated electron that avoids potential pitfalls, such as self-interaction error, insufficient basis set, or lack of
dispersion interactions. We also benchmark the density functional theory methods and outline the path to faithful ab initio
simulations of dynamics and reactivity of electrons solvated in extended aqueous systems.