Ab initio atomistic dynamical study of an excess electron in water

Small lithium ammonia clusters are model systems for the dissociation of metals into solvated cations and electrons in ammonia. Metal-ammonia solutions display a complex behavior with increasing metal concentration including a phase change from a paramagnetic to a metallic diamagnetic phase, and small clusters should be useful models in the low concentration regime, where one may expect the ammoniated electron to show a behavior similar to that of the hydrated electron. Yet, even in the low concentration regime the nature of the ammoniated electron is still controversial with cavity models supported by optical and density measurements whereas localized radical models have been invoked to explain magnetic measurements. Small clusters can shed light on these open questions, and in particular the Li-NH(3) tetramer represents the smallest cluster with a complete solvation shell for the Li(+) cation. In view of the controversies about the character of the excess electron, the first question investigated is whether different theoretical characterizations of the "excess electron" lead to different conclusions about it. Only small differences are found between orbital-based and spin density-based and between self-consistent-field and coupled-cluster-based methods. Natural orbitals from equation-of-motion coupled-cluster calculations are then used to analyze the excess electron's distribution of Li(NH(3))(4) with particular emphasis on the portion of the excess electron's density that is closely associated with the N atoms. Three different comparisons show that only about 6% of the excess electron's density are closely associated with the atoms, with about 1% being closely associated with any N atom, and that the electron is best characterized as a Rydberg-like electron of the whole cluster. Finally, it is shown that in spite of the small amount of density close to the N atoms, the spin-density at the N nuclei is substantial, and that the magnetic observations can plausibly be explained within the cavity model.

Section: Ab Initio Resultsmentioning

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Characterizing the excess electron of Li(NH3)4

Sommerfeld

Dreux

2012

“…I, magnetic resonance experiments in alkaline glasses at T = 77 K are interpreted in favor of a hexavalent coordination environment for e aq − , 3,4 which is consistent with some theoretical models of e aq − in liquid water. 40,55,62 An average electron-oxygen distance of ϳ2.0 Å has also been deduced. 61,62 If this value is characteristic of e aq − in liquid water under ambient conditions, then our solvation cavity is ϳ0.3 Å too small, whereas the TB model is about right, although it-like PEWP-2-predicts tetravalent coordination.…”

Section: B Structurementioning

confidence: 91%

A one-electron model for the aqueous electron that includes many-body electron-water polarization: Bulk equilibrium structure, vertical electron binding energy, and optical absorption spectrum

Jacobson

Herbert

2010

103

234

Previously, we reported an electron-water pseudopotential designed to be used in conjunction with a polarizable water model, in order to describe the hydrated electron [L. D. Jacobson et al., J. Chem. Phys. 130, 124115 (2009)]. Subsequently, we found this model to be inadequate for the aqueous electron in bulk water, and here we report a reparametrization of the model. Unlike the previous model, the current version is not fit directly to any observables; rather, we use an ab initio exchange-correlation potential, along with a repulsive potential that is fit to reproduce the density maximum of the excess electron's wave function within the static-exchange approximation. The new parametrization performs at least as well as the previous model, as compared to ab initio benchmarks for (H(2)O)(n) (-) clusters, and also predicts reasonable values for the diffusion coefficient, radius of gyration, and absorption maximum of the bulk species. The new model predicts a vertical electron binding energy of 3.7 eV in bulk water, which is 1.4 eV smaller than the value obtained using nonpolarizable models; the difference represents the solvent's electronic reorganization energy following electron detachment. We find that the electron's first solvation shell is quite loose, which may be responsible for the electron's large, positive entropy of hydration. Many-body polarization alters the electronic absorption line shape in a qualitative way, giving rise to a high-energy tail that is observed experimentally but is absent in previous simulations. In our model, this feature arises from spatially diffuse excited states that are bound only by electronic reorganization (i.e., solvent polarization) following electronic excitation.

“…The chemical physics literature is replete with electronwater interaction potentials, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] which have long been used ͑in conjunction with various methods of one-electron quantum mechanics͒ to examine the nature of the hydrated electron. As such, a person might reasonably question whether the chemical physics community genuinely needs yet another hydrated-electron model, especially in view of a study by Turi et al 10 that seems to validate certain assumptions that were made previously in the course of constructing electron-water pseudopotentials, such as the use of a local potential to model the exchange interaction.…”

Section: Introductionmentioning

confidence: 99%

The static-exchange electron-water pseudopotential, in conjunction with a polarizable water model: A new Hamiltonian for hydrated-electron simulations

Jacobson¹,

Williams²,

Herbert³

2009

Previously, Turi and Borgis [J. Chem. Phys. 117, 6186 (2002)] parametrized an electron-water interaction potential, intended for use in simulations of hydrated electrons, by considering H(2)O(-) in the "static exchange" (essentially, frozen-core Hartree-Fock) approximation, then applying an approximate Phillips-Kleinman procedure to construct a one-electron pseudopotential representing the electron-water interaction. To date, this pseudopotential has been used exclusively in conjunction with a simple point charge water model that is parametrized for bulk water and yields poor results for small, neutral water clusters. Here, we extend upon the work of Turi and Borgis by reparametrizing the electron-water pseudopotential for use with the AMOEBA water model, which performs well for neutral clusters. The result is a one-electron model Hamiltonian for (H(2)O)(n)(-), in which the one-electron wave function polarizes the water molecules, and vice versa, in a fully self-consistent fashion. The new model is fully variational and analytic energy gradients are available. We have implemented the new model using a modified Davidson algorithm to compute eigenstates, with the unpaired electron represented on a real-space grid. Comparison to ab initio electronic structure calculations for (H(2)O)(n)(-) cluster isomers ranging from n=2 to n=35 reveals that the new model is significantly more accurate than the Turi-Borgis model, for both relative isomer energies and for vertical electron detachment energies. Electron-water polarization interactions are found to be much more significant for cavity states of the unpaired electron than for surface states.