Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite) and its isomorphous analogues - II. The O-H/N-H stretching region

The three investigated compounds-KMgPO(4)(.)6H(2)O (KMP), NH(4)MgPO(4)(.)6H(2)O (NMP) and NH(4)MgAsO(4)(.)6H(2)O (NMA)-are characterized by the existence, in their structure, of quite strong (OwO)-O-... hydrogen bonds (of which some are among the shortest of this type ever found in crystalline hydrates, the (OwO)-O-... distances being slightly above 260 pm). Reflecting the strength of the hydrogen bonds, a broad and structured feature extending from approximate to4000 down to approximate to2000 cm(-1) is found in the infrared spectra. Although not as rich in detail as the infrared spectra, the Raman ones are similar in appearance. Rather surprisingly, the spectral picture is not very different in the case of the two phosphate compounds (KMgPO(4)(.)6H(2)O and NH(4)MgPO(4)(.)6H(2)O) although N-H stretching bands are expected in the same region of the spectrum of NMP. Thus, obviously, the intensity of the feature in the O-H/N-H stretching region is due mainly to modes of the water molecules whereas the NH4 stretches contribute to a lesser degree to the shape and intensity. The comparison of the spectra in the O-D/N-D stretching region confirms this.