Hydration properties determining the reactivity of nitrite in aqueous solution

Abstract

The knowledge of the hydration properties of the nitrite ion is key to understanding its reaction mechanism controlled by solvent effects. Here, ab initio quantum mechanical charge field molecular dynamics was performed to obtain the structural and dynamical properties of the hydration shell in an aqueous solution of nitrite ions, elucidated by data analysis using a molecular approach and an extended quantitative analysis of all superimposed trajectories with three-dimensional alignment (density map). The pattern of the power spectra corresponded to the experimental data, indicating the suitability of the Hartree-Fock method coupled with double-ζ plus polarization and diffuse functional basis sets to study this system. The density maps revealed the structure of the hydration shell, that presented a higher density in the N-O bond direction than in the axis vertical to the molecular plane, whereas the atomic and molecular radial distribution functions provided vague information. The number of actual contacts indicated 4.6 water molecules interacting with a nitrite ion, and 1.5 extra water molecules located in the molecular hydration shell, forming a H-bonding network with the bulk water. The mean residence times for the water ligands designated the strength of the hydration spheres for the oxygen sites, whilst the results for the nitrogen sites over-estimated the number of water molecules from other sites and indicated a weak structure. These results show the influence of the water molecules surrounding the nitrite ion creating an anisotropic hydration shell, suggesting that the reactive sites are situated above and below the molecular plane with a lower water density.

Properties of aqueous nitrate and nitrite from x-ray absorption spectroscopy

“…The hydration of each anion was found to be consistent between the small and large simulation boxes and across interaction lengths, with nitrate and nitrite hydrated by an average of 11.8 and 11.9 water molecules, respectively, in the first shell. These values are somewhat larger than previously reported hydration numbers from ab initio simulations 40,41 and classical MD simulations. 9 Some of this discrepancy is likely explained by differences in the method by which hydration number is calculated.…”

A Review of Sodium Nitrite Solubility in Water and Physical Properties of the Saturated Solutions

“…Extensive diffraction and computational studies indicate that Na + usually has approximately six water molecules coordinating it in an aqueous solution. , Kamada et al’s neutron diffraction data in D 2 O indicate that there are ∼3.7 water molecules coordinating NO 2 – . Computational studies, however, indicate that six is a more appropriate coordination number for NO 2 – , a coordination number including all of the waters surrounding NO 2 – . , If both Na + and NO 2 – were to have their coordination sphere full of water, they would have to share water molecules at saturation because there are less than six water molecules available per ion. Even when sharing waters, only a limited number of geometries could accommodate both ion’s complete hydration sphere.…”

“…The average lifetime of a water molecule in magnesium’s hydration sphere is ∼200 ps but only 34 ps for Na + , and they attributed the jamming mechanism in magnesium salts to the long lifetime of water molecules in the Mg 2+ coordination sphere . Computational studies indicate the average residence time in the NO 2 – hydration sphere is even less than in the Na + hydration sphere, 15 to 43 ps . Consequently, the water in the Na + and NO 2 – coordination spheres is more mobile than for electrolytes like MgCl 2 that behave like soft colloids.…”

“…66,67 − , a coordination number including all of the waters surrounding NO 2 − . 69,70 If both Na + and NO 2 − were to have their coordination sphere full of water, they would have to share water molecules at saturation because there are less than six water molecules available per ion. Even when sharing waters, only a limited number of geometries could accommodate both ion's complete hydration sphere.…”

Salt Solubilities in Aqueous Solutions of NaNO3, NaNO2, NaCl, and NaOH: A Hofmeister-like Series for Understanding Alkaline Nuclear Waste

“… 59 − 66 These coordinating water molecules form hydrogen bonds with the ions. 63 − 68 With 55.5 mol of water per kilogram and six water molecules per ion, a solution with more than 9.25 mol of ions per kilogram water (or 4.63 mol of fully dissociated monovalent electrolyte) does not have enough water to provide each ion with six water molecules without sharing. This means that in a solution with greater than 4.63 m of 1:1 electrolyte (or 5.05 m for NaOH), the ions are either not fully dissociated, or the ions must be sharing at least some water molecules with each other.…”