The interaction between nitrate anion and the cations Li+, Mg2+, Ca2+: Frequencies and binding sites

The interaction between the nitrate anion, NO, and the cations Li+, Mg2+, and Ca2+ was investigated by means of ab initio calculations. Vibrational frequencies, binding sites, and intramolecular deformations of the anion are calculated. It is found that for all three cations the only stable binding site is situated on the bisection of the ONO angle. This differs from reported binding sites for protonation and predictions from the electrostatic potential. The other possible binding sites, at one oxygen and above/below the molecular plane, do not constitute local minima of the unrestricted energy surface; they are less than ∼ 20% higher in energy. The changes of the vibrational frequencies of nitrate anion upon ion-pair formation are analyzed.