Ammonia dihydrate: Preparation, x-ray powder diffraction pattern and infrared spectrum of NH3⋅2H2O at 100 K

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Abstract
The dihydrate of ammonia has been isolated for the first time. Its x‐ray powder diffraction pattern can be indexed on a cubic unit cell a=7.118 Å with a 95% confidence limit of 0.015 Å at ∼105 K, and Z=4. The number of diffraction lines, neglect of the hydrogen atoms, and the assumption that the atoms are on sites of multiplicity 4 or 8 yield P213 or P4232 as possible space groups. The infrared spectra suggest O–H‐‐O distances of ∼2.76 Å, short O–H‐‐N distances of 2.74±0.02 Å, and N–H‐‐‐O distances of 3.1 Å or longer. A structure which fits these bond lengths well has been found in P213, with the oxygen and nitrogen atoms all on the 4a sites, of site group C3, with x=0.141 and 0.7111 for the oxygen atoms and x=0.3635 for nitrogen. The infrared spectra also show ν3(NH3) as a triplet, which means that the correct structure, including the hydrogen atoms, must have a lower symmetry than cubic, probably the orthorhombic P212121. That the crystal cannot be cubic is also seen from consideration of the apparent site symmetries of the water molecule under the cubic structures.