Dissociation of Strong Acid Revisited: X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations of HNO₃ in Water

Abstract

Molecular-level insight into the dissociation of nitric acid in water is obtained from X-ray photoelectron spectroscopy and first-principles molecular dynamics (MD) simulations. Our combined studies reveal surprisingly abrupt changes in solvation configurations of undissociated nitric acid at approximately 4 M concentration. Experimentally, this is inferred from shifts of the N1s binding energy of HNO(3)(aq) as a function of concentration and is associated with variations in the local electronic structure of the nitrogen atom. It also shows up as a discontinuity in the degree of dissociation as a function of concentration, determined here from the N1s photoelectron signal intensity, which can be separately quantified for undissociated HNO(3)(aq) and dissociated NO(3)(-)(aq). Intermolecular interactions within the nitric acid solution are discussed on the basis of MD simulations, which reveal that molecular HNO(3) interacts remarkably weakly with solvating water molecules at low concentration; around 4 M there is a turnover to a more structured solvation shell, accompanied by an increase in hydrogen bonding between HNO(3) and water. We suggest that the driving force behind the more structured solvent configuration of HNO(3) is the overlap of nitric acid solvent shells that sets in around 4 M concentration.