Decoding Oxyanion Aqueous Solvation Structure: A Potassium Nitrate Example at Saturation

Abstract

The ability to probe the structure of a salt solution at the atomic scale is fundamentally important for our understanding of many chemical reactions and their mechanisms. The capability of neutron diffraction to "see" hydrogen (or deuterium) and other light isotopes is exceptional for resolving the structural complexity around the dissolved solutes in aqueous electrolytes. We have made measurements using oxygen isotopes on aqueous nitrate to reveal a small hydrogen-bonded water coordination number (3.9 ± 1.2) around a nitrate oxyanion. This is compared to estimates made using the existing method of nitrogen isotope substitution and those of computational simulations (>5-6 water molecules). The low water coordination number, combined with a comparison to classical molecular dynamics simulations, suggests that ion-pair formation is significant. This insight demonstrates the utility of experimental diffraction data for benchmarking atomistic computer simulations, enabling the development of more accurate intermolecular potentials.

Parametrization and Molecular Dynamics Simulations of Nitrogen Oxyanions and Oxyacids for Applications in Atmospheric and Biomolecular Sciences

“…Our empirical charge set emphasized charge delocalization in NO 3 – and led to less structured coordination shells. The importance of this result lies in the fact that models based on point-charges and rigid solvent tend to produce excessively structured hydration shells …”

“…It is well known that NO 3 – has a poorly structured hydration shell. − Therefore, the NO 3 – model was assembled with the relatively large atom types “NR” and “OM”, whose Lennard-Jones interactions are described by large repulsive terms. In the GROMOS force field, Lennard-Jones interactions are defined in terms of atom types, with “OM” being the standard atom of choice for the description of anionic carboxylate groups.…”

“…However, even the description of bulk-phase solvation can be challenging. Neutron diffraction measurements , and ab initio calculations − have consistently revealed a poorly structured hydration shell around NO 3 – , while classical molecular-mechanics models tended to overestimate ion–solvent and even ion–ion interactions. It has been noted that the rigid structure of typical water models and the use of atomic point charges may produce coordination shells that are artificially well structured .…”

“…Neutron diffraction measurements , and ab initio calculations − have consistently revealed a poorly structured hydration shell around NO 3 – , while classical molecular-mechanics models tended to overestimate ion–solvent and even ion–ion interactions. It has been noted that the rigid structure of typical water models and the use of atomic point charges may produce coordination shells that are artificially well structured . This issue becomes critical in the case of NO 3 – and NO 2 – because of charge delocalization, from which one would rather expect smoother charge distributions and stronger polarizability.…”

A method to apply Zavitsas' aqueous electrolyte model to multicomponent solutions and its equivalence to Zdanovskii's rule

“…33 Diffraction experiments as well as various quantum mechanical and molecular dynamics studies indicate that K + , Cs + and nitrate bind water weakly, and the negative H d values reflect this. [34][35][36][37][38][39] Mathematically, what a negative H d means is that a mole of water in KNO 3 or CsNO 3 solution is more reactive than a mole of water in pure water.…”