Fourier Transform Vibrational Spectra of Magnesium Hydrogenphosphate Trihydrate. I. The O-H Stretching Region∗

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Abstract
The Fourier transform (FT) infrared and Raman spectra of newberyite, MgHPH4 - 3H2O are studied in the region where the stretching vibrations of the water molecules (protiated and deuterated) and the O-H/O-D stretches of the hydrogenphosphate anions are expected to appear. The O-H stretching vibrations give rise to a complex feature known as the A,B,C trio. Since neither of the maxima found below 3000 cm−1 represents a true band arising from a given fundamental, it is pointless to correlate their frequencies with the observed O…O distances. In the water stretching region, the two bands with highest frequencies undoubtedly correspond to the anti symmetric and symmetric stretch of one type of the water molecules. The stretching vibrations of one of the remaining two types of H2O molecules are clearly uncoupled and the O-H oscillator involved in the weaker hydrogen bond is responsible for a band at 3376 cm−1 whereas the rest of the water stretchings are apparently overlapped yielding the complex band below 3320 cm−1. Thus the situation is again complicated and the correlations between the frequencies and the Ow…O distances are inappropriate. The two bands at highest frequencies (3522 and 3483 cm−1 at RT) exhibit a positive temperature coefficient.