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 NH3 n - 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.