Speciation, structural characteristics and proton dynamics in the systems NH4NO3· 1.5H2O and NH4NO3· 1.5H2O–(HNO3, NH4F, NH3)–H2O at 50 °C

Abstract

In order to obtain a good basis for exploring metal-ion complex formation in molten NH₄NO₃· 1.5H₂O, some fundamental characteristics of the pure hydrous melt and a number of compositions in the NH₄NO₃· 1.5H₂O–(HNO₃, NH₄F, NH₃)–H₂O system, have been investigated at 50 °C. Several aspects have been taken into consideration, e.g. thermodynamics of solvent autoprotolysis and HF formation, dynamics of proton exchange and structural properties. The acid dissociation constant of NH⁺₄, K_a, and the equilibrium constant for formation of HF, K_HF, were obtained from potentiometric measurements; K_a=(2.2 ± 0.2)× 10^–9(mol kg^–1)² and K_HF= 2160 ± 40 (mol kg^–1)^–1. Results from ¹⁹F NMR spectroscopy indicate that unprotonated fluoride, F^–, probably exists as an H₃NH⁺⋯F^– ion pair in the solvent. The change in the ¹⁹F chemical shift with increasing HNO₃ content in (NH₄NO₃–NH₄F–HNO₃)· 1.5H₂O verifies the conclusion from potentiometric data that HF is the only proteonated fluoride species present. Raman spectroscopy and ¹⁴N NMR experiments give clear evidence for an increased tendency to NH⁺₄⋯NO^–₃ ion-pair formation with decreasing water content in the systems NH₄NO₃–H₂O. However, no loss of degeneracy of the internal ν₃ and ν₄ nitrate bands at 1380 and 718 cm^–1, respectively, was observed. The D_3h symmetry of NO^–₃ seems to be preserved in the NH₄NO₃· 1.5H₂O melt. Results from Raman scattering, ¹H NMR and ¹⁴N NMR experiments show significant changes in the spectra upon acidification with HNO₃. These observations suggest an increase in hydrogen-bonding ability with increasing acidity. Results from large-angle X-ray scattering experiments on NH₄NO₃· 1.5H₂O cannot be explained by a model comprising only interactions between water molecules and ions. A residual contribution to the overall radial electron density distribution at 1.8 Å is tentatively assigned to remarkably short N(NH₄)–O(NO₃) distances. ¹H NMR spectroscopy shows a strong retardation of the proton exchange between NH⁺₄ and H₂O in the acidic region. The rate constant, k_H, for the proton-exchange step H₃N ·HOH(OH₂)_s–1+ H₂O → H₃N ·(OH₂)_s+HOH, is estimated at (4.3 ± 1.5)× 10⁷ s^–1.