Ab Initio Molecular Dynamics Simulations of Solvated Electrons in Ammonia Clusters

Abstract

We investigated excess electron solvation dynamics in (NH 3 ) n – ammonia clusters in the n = 8–32 size range by performing finite temperature molecular dynamics simulations. In particular, we focused on three possible scenarios. The first case is designed to model electron attachment to small neutral ammonia clusters ( n ≤ ∼10) that form hydrogen-bonded chains. The excess electron is bound to the clusters via dipole bound states, and persists with a VDE of ∼160 meV at 100 K for the n = 8 cluster. The coupled nuclear and electronic relaxation is fast (within ∼100 fs) and takes place predominantly by intermolecular librational motions and the intramolecular umbrella mode. The second scenario illustrates the mechanism of excess electron attachment to cold compact neutral clusters of medium size (8 ≤ n ≤ 32). The neutral clusters show increasing tendency with size to bind the excess electron on the surface of the clusters in weakly bound, diffuse, and highly delocalized states. Anionic relaxation trajectories launched from these initial states provide no indication for excess electron stabilization for sizes n < 24. Excess electrons are likely to autodetach from these clusters. The two largest investigated clusters ( n = 24 and 32) can accommodate the excess electron in electronic states that are mainly localized on the surface, but they may be partly embedded in the cluster. In the last 500 fs of the simulated trajectories, the VDE of the solvated electron fluctuates around ∼200 meV for n = 24 and ∼500 meV for n = 32, consistent with the values extrapolated from the experimentally observed linear VDE– n –1/3 trend. In the third case, we prepared neutral ammonia cluster configurations, including an n = 48 cluster, that contain possible electron localization sites within the interior of the cluster. Excess electrons added to these clusters localize in cavities with high VDEs up to 1.9 eV for n = 48. The computed VDE values for larger clusters are considerably higher than the experimentally observed photoelectric threshold energy for the solvated electron in bulk ammonia (∼1.4 eV). Molecular dynamics simulations launched from these initial cavity states strongly indicate, however, that these cavity structures exist only for ∼200 fs. During the relaxation the electron leaves the cavity and becomes delocalized, while the cluster loses its initial compactness.

Single-Pole Polarization Models: Rapid Evaluation of Electron Affinities of Solvated-Electron and Superatomic Molecular Anionic States

“…The geometries of these clusters were obtained from a previous ab initio molecular dynamics (AIMD) study. 64 The st-COHSEX potential, similar to the case of water cluster studies, shows large deviations compared to the reference AC-GKS-spRPA method. The LRC-ωPBEh potential shows smaller errors (MAE = 65 meV), compared to the water clusters; however, the errors increase for larger cluster sizes.…”

“…The nuclear geometries are from an ab initio molecular dynamics (AIMD) study. 64 Figure 3. Radial probability density for the lowest virtual-energy eigenstate of one-particle Hamiltonians.…”

“…Here the optimized ω 0 value is 19.83 eV, which is close to that of the water clusters; the parametrization was done against the AC-GKS-spRPA EA of the (NH 3 ) 12 cluster. The geometries of these clusters were obtained from a previous ab initio molecular dynamics (AIMD) study . The st-COHSEX potential, similar to the case of water cluster studies, shows large deviations compared to the reference AC-GKS-spRPA method.…”

Stabilization of benzene radical anion in ammonia clusters

“…6 Further theoretical investigations studied weakly bound (NH 3 ) − n clusters and the dynamics of electron solvation. 7,8 Small ammonia cluster ions (NH 3 ) n H − and (NH 3 ) n NH − 2 with n = 1 and 2 were investigated by photoelectron spectroscopy. 9,10 Isolated ammonia molecules were also investigated with respect to the attachment of an excess electron.…”

“…6 Further theoretical investigations studied weakly bound (NH 3 ) n − clusters and the dynamics of electron solvation. 7,8…”