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 NO3− by distorted hydrogen bonds. In contrast to former studies on NO3−/H2O this configuration is found to be asymmetric at the highest theoretical level employed. The corresponding structure with C2v 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 NO3−/H2O 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.