Free Energies of Cavity and Noncavity Hydrated Electrons Near the Instantaneous Air/Water Interface

Abstract

The properties of the hydrated electron at the air/water interface are computed for both a cavity and a noncavity model using mixed quantum/classical molecular dynamics simulation. We take advantage of our recently developed formalism for umbrella sampling with a restrained quantum expectation value to calculate free-energy profiles of the hydrated electron's position relative to the water surface. We show that it is critical to use an instantaneous description of the air/water interface rather than the Gibbs' dividing surface to obtain accurate potentials of mean force. We find that noncavity electrons, which prefer to encompass several water molecules, avoid the interface where water molecules are scarce. In contrast, cavity models of the hydrated electron, which prefer to expel water, have a local free-energy minimum near the interface. When the cavity electron occupies this minimum, its absorption spectrum is quite red-shifted, its binding energy is significantly lowered, and its dynamics speed up quite a bit compared with the bulk, features that have not been found by experiment. The surface activity of the electron therefore serves as a useful test of cavity versus noncavity electron solvation.

Temperature dependence of the hydrated electron’s excited-state relaxation. I. Simulation predictions of resonance Raman and pump-probe transient absorption spectra of cavity and non-cavity models

“…15 We have chosen to taper the interactions for the simulations discussed here, however, both because Ewald summation is known to give a stronger finite size effect for this system, as we have discussed in Ref. 28, and because this choice is consistent with our previously published work, 14,22,23,41 allowing for direct comparison to the data presented below.…”

“…13,[19][20][21] Recently, however, both we 14,22,23 and others 17,18 have challenged this picture based on calculations that suggest that the excess electron's wavefunction encompasses several water molecules in a structure with only a small or even no central cavity. We note that our one-electron non-cavity pseudopotential has been criticized, [24][25][26] both for overbinding the electron energetically 15,27 and for predicting a negative molar solvation volume 28 when experiment suggests that this parameter should be positive. 29 Nevertheless, non-cavity hydrated electron models have been shown to account for 0021-9606/2017/147(7)/074503/14/$30.00…”

“…various experimental observations that the traditional cavity picture cannot explain, including the hydrated electron's resonance Raman spectral line shape, 22,23 behavior at the air/water interface, 28 and time-resolved photoelectron spectroscopy. 30 Another important feature of hydrated electrons is that their properties are temperature dependent.…”

Plasma physics of liquids—A focused review

“…Recently, multiple configurations have been proposed, 158,159 where the localized electrons can occupy a cavity or a non-cavity region with higher water density. 149,160,161 Next to that, a solvated electron might be stabilized by a neighboring positive counter-ion. 158 Remarkably, quasi-free electrons in amorphous ice have a similar lifetime as in liquid water, 162,163 suggesting that electron solvation might not be so different in liquids and amorphous solids after all.…”

Selectivity in Electron Attachment to Water Clusters

“…[1][2][3][4][5][6] While generally considered as a bulk solute, many of the roles of the hydrated electron are in fact associated with interfacial processes, but much less is known about the structure, let alone the reactivity, of excess electrons at aqueous interfaces. [7][8][9][10][11][12][13][14][15][16] To gain a fundamental understanding of electron solvation in water, isolated water cluster anions, (H2O)n − , have received much attention from both experiment 7,17-23 and theory. 9,[24][25][26][27][28][29] In these, the excess electron can adopt a number of binding motifs.…”