Investigating the Resonance in Nitric Acid and the Nitrate Anion Based on a Modern Bonding Analysis

The nitrate anion, NO₃ ^-, is often regarded as a textbook example for the very fundamental concept of resonance. Usually, three equivalent resonance structures with one N-O double bond and two N-O single bonds are considered. Consequently, each of the three N-O bonds should have a partial double bond character. In this study, we analyse the resonance in NO₃ ^- in comparison with the related species HNO₃ and FNO₃ by applying a combination of the Quantum Theory of Atoms in Molecules (QTAIM), a natural bond orbital (NBO) analysis, the electron localizability indicator (ELI), and valence bond (VB) calculations. Despite the fundamental importance of nitrate salts and nitric acid for the environment, chemistry, and industry, a bonding analysis is absent from the literature so far. The classical resonance structures are clearly reflected by the bond analysis tools, but are not the only contributions to the bonding situation. The resonance in HNO₃ and FNO₃ is greatly perturbed by the hydrogen and fluorine atoms. In addition to theoretical calculations, experimental electron density and wave function refinements were carried out on a KNO₃ crystal.