Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite) and its isomorphous analogues. III. Spectra of protiated and partially deuterated magnesium ammonium phosphate hexahydrate

Magnesium ammonium phosphate hexahydrate, MgNH4PO4·6H2O (synthetic struvite) is a well-known biomineral, its major biological importance being related to its presence in human urinary sediments and vesical and renal calculi. The Fourier transform infrared and Raman spectra of magnesium ammonium phosphate hexahydrate were recorded and analyzed from room temperature (RT) down to the boiling temperature of liquid nitrogen (LNT). Also recorded and analyzed were the spectra of its partially deuterated analogues. The recorded spectra were compared with the corresponding ones of the previously studied potassium analogue. On the basis of such a comparison it was concluded that the main contribution to the intensity of the broad and structured feature in the O-H/N-H stretching region comes from the bands originating from the H-O-H stretches. The location of at least some of the stretching vibrations of the ammonium ions (albeit one of its deuterated forms) is possible in the spectrum of the sample containing ≈2% deuterium. The bands at 2326 and 2277 cm−1 (and probably, at least partially, that at 2393 cm−1) can be assigned with certainty to N-D stretching vibrations of isotopically isolated NH3D+ ions. The LNT Raman bands at 1702 and 1685 cm−1 are attributed to the ν2NH4+ mode and those at 1477 and 1442 cm−1 are observed are attributed to the ν4 mode. The relatively high frequencies (1302 and 1295 cm−1) of some of the bands due to the ND bending vibrations of isotopically isolated NH3D+ ions are in line with the existence of quite strong hydrogen bonds formed by ammonium ions. The librations of the deuterated forms of water molecules may be coupled with the components of the phosphate ν4 vibration.