Solvation of the Azide Anion (N3-) in Water Clusters and Aqueous Interfaces:  A Combined Investigation by Photoelectron Spectroscopy, Density Functional Calculations, and Molecular Dynamics Simulations

We report a photoelectron spectroscopy and computational study of hydrated N3- anion clusters, N3-(H2O)n (n = 0−16), in the gas phase. Photoelectron spectra of the solvated azide anions were observed to consist of a single peak, similar to that of the bare N3-, but the spectral width was observed to broaden as a function of cluster size due to solvent relaxation upon electron detachment. The adiabatic and vertical electron detachment energies were measured as a function of solvent number. The measured electron binding energies indicate that the first four solvent molecules have much stronger interactions with the solute anion, forming the first solvation shell. The spectral width levels off at n = 7, suggesting that three waters in the second solvation shell are sufficient to capture the second shell effect in the solvent relaxation. Density functional calculations were carried out for N3- solvated by one to five waters and showed that the first four waters interact directly with N3- and form the first solvation shell on one side of the solute. The fifth water does not directly solvate N3- and begins the second solvation shell, consistent with the observed photoelectron data. Molecular dynamics simulations on both solvated clusters and bulk interface revealed that the asymmetric solvation state in small clusters persist for larger systems and that N3- prefers interfacial solvation on water clusters and at the extended vacuum/water interface.