SPECIATION, STRUCTURAL CHARACTERISTICS AND PROTON DYNAMICS IN THE SYSTEMS NH4NO3-CENTER-DOT-1.5H2O AND NH4NO3-CENTER-DOT-1.5H2O- (HNO3,NH4F,NH3)-H2O AT 50-DEGREES-C

In order to obtain a good basis for exploring metal-ion complex format ion in molten NH4NO3.1.5H2O, some fundamental characteristics of the p ure hydrous melt and a number of compositions in the NH4NO3 . 1.5H2O-( HNO3, NH4F, NH3)-H2O system, have been investigated at 50-degrees-C. S everal aspects have been taken into consideration, e.g. thermodynamics of solvent autoprotolysis and HF formation, dynamics of proton exchan ge and structural properties. The acid dissociation constant of NH4+, K(a), and the equilibrium constant for formation of HF, K(HF), were ob tained from potentiometric measurements; K(a) = (2.2 +/- 0.2) x 10(-9) (mol kg-1)2 and K(HF) = 2160 +/- 40 (mol kg-1)-1. Results from F-19 N MR spectroscopy indicate that unprotonated fluoride, F-, probably exis ts as an H3NH+...F- ion pair in the solvent. The change in the F-19 ch emical shift with increasing HNO3 content in (NH4NO3-NH4F-HNO3) . 1.5H 2O verifies the conclusion from potentiometric data that HF is the onl y protonated fluoride species present. Raman spectroscopy and N-14 NMR experiments give clear evidence for an increased tendency to NH4+...N O3- ion-pair formation with decreasing water content in the systems NH 4NO3-H2O. However, no loss of degeneracy of the internal nu3 and nu4 n itrate bands at 1380 and 718 cm-1, respectively, was observed. The D3h symmetry of NO3- seems to be preserved in the NH4NO3 . 1.5H2O melt. R esults from Raman scattering, H-1 NMR and N-14 NMR experiments show si gnificant changes in the spectra upon acidification with HNO3. These o bservations suggest an increase in hydrogen-bonding ability with incre asing acidity. Results from large-angle X-ray scattering experiments o n NH4NO3 . 1.5H2O cannot be explained by a model comprising only inter actions between water molecules and ions. A residual contribution to t he overall radial electron density distribution at 1.8 angstrom is ten tatively assigned to remarkably short N(NH4)-O(NO3) distances. H-1 NMR spectroscopy shows a strong retardation of the proton exchange betwee n NH4+ 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, i s estimated at (4.3 +/- 1.5) x 10(7) s-1.