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

Abstract

We report a photoelectron spectroscopy and computational study of hydrated N 3anion clusters, N 3 -(H 2 O) 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 N 3 -, 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 N 3solvated by one to five waters and showed that the first four waters interact directly with N 3and form the first solvation shell on one side of the solute. The fifth water does not directly solvate N 3and 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 N 3prefers interfacial solvation on water clusters and at the extended vacuum/water interface.

Enhanced Concentration of Polarizable Anions at the Liquid Water Surface: SHG Spectroscopy and MD Simulations of Sodium Thiocyanide

“…[1][2][3][4][5][6][7][8][9] These findings contradict the textbook description of ions being repelled from the interface and the outermost surface layer being devoid of ions. [10][11][12] This conventional wisdom is largely based on macroscopic measurements of the increasing surface tension of aqueous solutions with salt concentration.…”

Calculation of the properties of the S3− radical anion and its complexes with Cu+ in aqueous solution

“…Since this calculation is performed on a discrete cluster without 3D boundary conditions the polarizable S 3 À ion is actually more stable on the surface of the cluster. It is well established that weakly charged yet highly polarizable anions tend to adopt surface positions in water clusters (Yang et al, 2004). There are three modes of this larger cluster corresponding most closely to the S-S-S bend, the S-S symmetric stretch and the S-S antisymmetric stretch which occur at 240, 488 and 564 cm À1 , with the 564 peak being most intense.…”

Paradigms and paradoxes: why is the electron affinity of the azide radical, N3, so large?

“…They are: how actually large is the electron affinity of azide radical, and to which other species is the comparison being made? From contemporary gas phase ion energetics measurements [7][8][9], we may derive a consensus value of 2.71 eV (261 kJ mol -1 ). This value is almost as high as for the atomic halogens (F, Cl, Br, I), higher than the diatomic halogens (Cl 2 , Br 2 , I 2 , but not F 2 ), and higher than O, OH, O 2 , HO 2 , and O 3 [10].…”