Quantum chemical study of the interaction of nitrate anion with water

The interaction between nitrate anion and water has been investigated by Hartree–Fock calculations with the 6-311+G(d, p) basis set and by B3LYP density functional calculations with the aug-cc-pVTZ basis set. It is found that the global energy minimum is a planar configuration where both hydrogen atoms of water are coordinated to two oxygen atoms of NO<INF>3</INF>⁻ by distorted hydrogen bonds. In contrast to former studies on NO<INF>3</INF>⁻/H<INF>2</INF>O this configuration is found to be asymmetric at the highest theoretical level employed. The corresponding structure with C<INF>2v</INF> symmetry is a saddle point at slightly higher energy. A singly hydrogen-bonded configuration is still about 2.4 kcal/mol higher in energy. The shifts in the vibrational frequencies of water and nitrate upon complexation were calculated. A compact analytical potential function of NO<INF>3</INF>⁻/H<INF>2</INF>O for use in statistical thermodynamic simulations was obtained from 390 points of the energy surface and an intramolecular force field for the nitrate anion is presented. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 877–886, 1998