Structure of a 50 mol kg–1aqueous solution of ammonium nitrate at 373 K by the isotopic difference method of neutron diffraction

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Abstract
A neutron diffraction study of a 50 mol kg–1 aqueous solution of deuterated ammonium nitrate [ca. 1 : 1 molecular ratio of D2O : N(1)D4N(2)O3 which is the saturation solubility at ca. 70 °C] was carried out at 100 °C. The first- and second-order isotopic difference methods of neutron diffraction were applied to the nitrogen nuclei of both the ammonium and nitrate ions. The results derived from the total and first-order difference diffraction functions show the existence of stable ND+ 4 and NO– 3 ions in the solution. Structural information concerning the ammonium ion–water and nitrate ion–water coordination was obtained from the first-order differences, and the results indicate a relatively weak hydration shell for these ions. A calculation of the second order difference function enabled the determination of the ‘cross’ pair radial distribution function gN(1)N(2)(r) from which it was concluded that the nearest-neighbour N(1)–N(2) distance increases significantly on the addition of D2O to ND4NO3.