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

Abstract

The hydrated electronthe species that results from the addition of a single excess electron to liquid waterhas been the focus of much interest both because of its role in radiation chemistry and other chemical reactions, and because it provides for a deceptively simple system that can serve as a means to confront the predictions of quantum molecular dynamics simulations with experiment. Despite all this interest, there is still considerable debate over the molecular structure of the hydrated electron: does it occupy a cavity, have a significant number of interior water molecules, or have a structure somewhere in between? The reason for all this debate is that different computer simulations have produced each of these different structures, yet the predicted properties for these different structures are still in reasonable agreement with experiment. In this Feature Article, we explore the reasons underlying why different structures are produced when different pseudopotentials are used in quantum simulations of the hydrated electron. We also show that essentially all the different models for the hydrated electron, including those from fully ab initio calculations, have relatively little direct overlap of the electron’s wave function with the nearby water molecules. Thus, a non-cavity hydrated electron is better thought of as an “inverse plum pudding” model, with interior waters that locally expel the surrounding electron’s charge density. Finally, we also explore the agreement between different hydrated electron models and certain key experiments, such as resonance Raman spectroscopy and the temperature dependence and degree of homogeneous broadening of the optical absorption spectrum, in order to distinguish between the different simulated structures. Taken together, we conclude that the hydrated electron likely has a significant number of interior water molecules.

A Simple ab Initio Model for the Hydrated Electron That Matches Experiment

“…These one-electron pseudopotential models generally have been able to reproduce the room temperature optical spectrum, but significantly fail to produce the experimental shift with temperature. 8,24 …”

“…8,15 Schwartz and coworkers demonstrated that this new model does a much better job of explaining the optical properties and Raman spectrum than the older “cavity” models. 8 However, the LGS model 15 has been questioned particularly on the basis of its energetics and the expected very negative partial molar volume. 16-18 Jungwirth and coworkers 25 have recently performed large scale ab initio molecular dynamics simulations with a DFT water cluster embedded in a large classical MD water bath.…”

Direct observation of the collapse of the delocalized excess electron in water

“…[27] This picture is somewhat in between the consensus based on most of the pseudopotential calculations [26,28] and a rivaling new pseudopotential model, predicting a significantly larger size of a strongly delocalized hydrated electron even in the equilibrated ground state. [29,30] Note that an ab initio molecular dynamics approach by construct accounts for more quantum electronic structure effects than pseudopotentials, with the latter being also more prone to potential parametrization artifacts.…”

Hydration dynamics in water clusters via quantum molecular dynamics simulations

“…A notable problem concerns the structure of the hydrated electron. The more or less consensus, localized cavity structure 36,37,38 has been challenged by an alternative model, 39 a non-cavity type ("inverse-plum" 40 ) electron distribution for the bulk hydrated electron. As another structural issue, there is still no agreement on where the excess electron is localized relative to the nuclear frame in water clusters, i.e.…”

“…More recent simulations have been published with the LGS potential that predict results compatible with certain sets of experimental observations. 40,70 This situation makes it necessary to further investigate the problem and test the models. Since both the TB and the LGS models employ the same water-water classical potential (simple point charge (SPC) model with flexibility), 71 it is reasonable to compare these two models directly and identify possible dynamical signatures associated with the different electron-water molecule potentials.…”