Polarizability of the nitrate anion and its solvation at the air/water interface

Abstract

The anisotropic molecular polarizability of the nitrate anion and its decomposition into atomic contributions is studied using ab initio quantum chemistry and the Atoms in Molecules theory. Aqueous solvation of NO₃⁻ in interfacial environments is investigated by a Car–Parrinello molecular dynamics simulation of a cluster, and classical molecular dynamics of an extended slab system with bulk interfaces using a polarizable force field based on the Atoms in Molecules analysis. Both in aqueous clusters and in systems with extended interfaces the nitrate anion clearly prefers interfacial over bulk solvation. This is primarily due to its large value of molecular polarizability, the gas phase value of which is reduced by only 5–10% in the aqueous environment. For polarizable force field simulations of ionic solvation, we recommend to cast the NO₃⁻ polarizability into three equal contributions of roughly 1.3 ų placed on the oxygen atoms of the anion.