Excess electron solvation in ammonia clusters

Abstract

We performed a combination of quantum chemical calculations and molecular dynamics simulations to assess the stability of various size NH3n− ammonia cluster anions up to n = 32 monomers. In the n = 3–8 size range, cluster anions are optimized and the vertical detachment energy of the excess electron (VDE) from increasing size clusters is computed using various level methods including density functional theory, MP2, and coupled-cluster singles doubles with perturbative triples. These clusters bind the electrons in nonbranched hydrogen bonding chains in dipole bound states. The VDE increases with size from a few millielectron volt up to ∼200 meV. The electron binding energy is weaker than that in water clusters but comparable to small methanol cluster VDEs. We located the first branched hydrogen bonding cluster that binds the excess electron at n = 7. For larger (n = 8–32) clusters, we generated cold, neutral clusters by semiempirical and ab initio molecular dynamics simulations and added an extra electron to selected neutral configurations. VDE calculations on the adiabatic and the relaxed anionic structures suggest that the n = 12–32 neutral clusters weakly bind the excess electron. Electron binding energies for these clusters (∼100 meV) appear to be significantly weaker than those extrapolated from experimental data. The observed excess electron states are diffuse and localized outside the molecular frame (surface states) with minor (∼1%) penetration to the nitrogen frontier orbitals. Stable minima with excess electron states surrounded by solvent molecules (cavity states) were not found in this size regime.

Ab Initio Molecular Dynamics Simulations of Solvated Electrons in Ammonia Clusters

“… 24 Most recently, we also performed ab initio and DFT calculations on ammonia cluster anions, extending the investigated systems up to 32 monomers. 23 We found that long extended chains of ammonia cluster anions localize less than 1% of the excess electron density in the frontier orbitals of the nitrogen atoms, a minor effect that largely supports Sommerfeld’s view. 24 Although this argument seems to be valid for chains of dipole bound anions, one has to realize that physical properties observed for finite size systems are not necessarily transferable to the bulk.…”

“…We performed ab initio molecular dynamics simulations to model excess electron solvation in finite size ammonia clusters. Since the applied methods are similar to those employed in a previous work, 23 we outline the simulation techniques only briefly here, but provide more discussion on a few specific key details.…”

“…Previously, we reported a combined quantum mechanical and molecular dynamics study, a first step in this direction. 23 First, we examined the electron binding properties of optimized linear hydrogen-bonding chains along with several other branched configurations. These small clusters in the n = 3–8 range bind the electron weakly in dipole bound states with VDEs up to ∼200 meV.…”

“…We also generated intermediate size configurations with n = 8–32 monomers using a different sampling procedure, finite temperature classical and ab initio molecular dynamics. 23 In these simulations, neutral ammonia cluster equilibrium trajectories were generated, and selected configurations along the trajectories were tested for excess electron binding. We note that this simulation procedure can be thought of as a simplified model of the first step of an electron attachment scenario, where a zero kinetic energy electron is attached to a neutral cluster.…”

“… 34 Subsequent quantum mechanical calculations along the neutral trajectories indicated that initially the excess electron is always attached to the surface of the ammonia clusters for the investigated cluster sizes. 23 …”