Electron solvation in water-ammonia mixed clusters: Structure, energetics, and the nature of localization states of the excess electron.

The structure and energetics of water-ammonia mixed clusters with an excess electron, [(H2O)n(NH3)m]- with m=1, n=2-6 and m=2, n=2, and also the corresponding neutral clusters are investigated in detail by means of ab initio quantum chemical calculations. The authors focus on the localization structure of the excess electron with respect to its surface versus interiorlike states, its binding to ammonia versus water molecules, the spatial and orientational arrangement of solvent molecules around the excess electron, the changes of the overall hydrogen-bonded structure of the clusters as compared to those of the neutral ones and associated dipole moment changes, vertical detachment energies of the anionic clusters, and also the vertical attachment energies of the neutral clusters. It is found that the hydrogen-bonded structure of the anionic clusters are very different from those of the neutral clusters unlike the case of water-ammonia dimer anion, and these changes in structural arrangements lead to drastically different dipole moments of the anionic and the neutral clusters. The spatial distribution of the singly occupied molecular orbital holding the excess electron shows only surface states for the smaller clusters. However, for n=5 and 6, both surface and interiorlike binding states are found to exist for the excess electron. For the surface states, the excess electron can be bound to the dangling hydrogens of either an ammonia or a water molecule with different degrees of stability and vertical detachment energies. The interiorlike states, wherever they exist, are found to have a higher vertical detachment energy than any of the surface states of the same cluster. Also, for interiorlike states, the ammonia molecule with its dangling hydrogens is always found to stay on top or on a far side of the charge density of the excess electron without participating in the hydrogen bond network of the cluster; the intermolecular hydrogen bonds are formed by the water molecules only which add to the overall stability of these anionic clusters.