Structure of Large Nitrate-Water Clusters at Ambient Temperatures: Simulations with Effective Fragment Potentials and Force Fields with Implications for Atmospheric Chemistry

Structural properties of large NO3-center dot(H2O)(n) (n = 15-500) clusters are studied by Monte Carlo simulations using effective fragment potentials (EFPs) and by classical molecular dynamics simulations using a polarizable empirical force field. The simulation results are analyzed with a focus on the description of hydrogen bonding and solvation in the clusters. In addition, a comparison between the electronic structure based EFP and the classical force field description of the 32 water cluster system is presented. The EFP simulations, which focused on the cases of n = 15 and 32, show an internal, fully solvated structure and a surface adsorbed structure for the 32 water cluster at 300 K, with the latter configuration being more probable. The internal solvated structure and the surface adsorbed structure differ considerably in their hydrogen bonding coordination numbers. The force field based simulations agree qualitatively with these results, and the local geometry of NO3- and solvation at the surface-adsorbed site in the force field simulations are similar to those predicted using EFPs. Differences and similarities between the description of hydrogen bonding of the anion in the two approaches are discussed. Extensive classical force field based simulations at 250 K predict that long time scale stability of internal NO3-, which is characteristic of extended bulk aqueous interfaces, emerges only for n > 300. Ab initio Moller-Plesset perturbation theory is used to test the geometries of selected surface and interior anions for n = 32, and the results are compared to the EFP and MD simulations. Qualitatively, all approaches agree that surface structures are preferred over the interior structures for clusters of this size. The relatively large aqueous clusters of NO3- studied here are of comparable size to clusters that lead to new particle formation in air. Nitrate ions on the Surface of such clusters may have significantly different photochemistry than the internal species. The possible implications of surface-adsorbed nitrate ions for atmospheric chemistry are discussed.