Ab Initio Structure Determination of Ammonium Hydrogen Alkyl Phosphates from X-ray Powder Diffraction Data

The structures of three ammonium hydrogen alkyl phosphates, NH₄PO₂(OH)(OR) [R = Me (1), Et (2), Prⁱ (3)], have been solved ab initio from X-ray powder diffraction data. The structure determinations were based on Patterson search methods combined with difference-Fourier techniques and the refinement was carried out using Rietveld methods. Powder diffraction data were first collected for all three compounds in the reflection mode using Cu K_α1+α2 radiation. A pronounced preferred orientation effect was found for all the samples, even by side-loading into a flat sample holder. A random orientation of the powder particles was obtained for (1) using a special suspension chamber for the sample preparation, while for (2) and (3) data were remeasured in capillaries using the transmission mode. Crystals of the ammonium hydrogen methyl phosphate (1) and the ammonium hydrogen ethyl phosphate (2) both belong to the monoclinic space group P2₁/c, Z = 4, with a = 9.9645 (4), b = 7.1801 (3), c = 7.8897 (3) Å and β = 96.243 (3)° for (1), and a = 12.6453 (4), b = 7.1502 (2), c = 7.9738 (2) Å and β = 108.875 (2)° for (2). The ammonium hydrogen isopropyl phosphate (3) crystallizes in space group An, Z = 4, with a = 6.2475 (2), b = 29.3825 (6), c = 4.6305 (1) Å and β = 100.904 (2)°. All three compounds are characterized by an extended hydrogen-bonding network involving the hydrogen phosphate units and the ammonium ions, forming double layers. The methyl (1), ethyl (2) and isopropyl groups (3), which point into the interlayer space, connect the layers by van der Waals interactions.

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