Abstract: | The structure of (dimethylamino)methylene]bisphosphonic acid] dihydrate (C3H11NO6P2⋅2 H2O; 1 ) was solved ab initio from synchrotron powder X-ray diffraction data. The structure determination was based on direct methods combined with difference Fourier techniques, and the refinement was carried out using the Rietveld method. Using this high-quality diffraction pattern, it was possible to index a second phase which corresponds to the structure of the known (dimethylamino)methylene]bisphosphonic acid] monohydrate ( 2 ). (Dimethylamino)methylene]bisphosphonic acid] dihydrate ( 1 ) is monoclinic, space group P21/c, Z=4, with a=10.6644(1), b=9.1599(1), c=10.5213(1) Å, and β=98.353(1)°. The structure analysis indicates two non-equivalent P-atoms in the molecule of 1 which are also observed in the corresponding monohydrate 2 and unhydrated form 3 . All three compounds exhibit extended H-bonding networks which result in remarkably different 31P-NMR spectra. The (dimethylamino)methylene]bisphosphonic acids] 1 – 3 crystallize in the betaine-type structure which, therefore, contains two nonequivalent P-atoms. The −P(=O)(OH)2 and −P(=O)(OH)O− groups of 1 – 3 are involved in a number of strong H-bonds which can be characterized by the different 31P-NMR chemical shifts of the two P-atoms of 1 – 3 . |