Infrared and Raman studies of manganese dihydrogen phosphate dihydrate, Mn(H2PO4)2·2H2O. Part II: Region of the internal OH group vibrations

Infrared and Raman spectra of Mn(H2PO4)2·2H2O and of series of deuterated analogues recorded at room temperature (RT) and at the boiling temperature of liquid nitrogen (LNT) have been presented and analyzed with respect to the OH vibrations. In the OH stretching mode region ABC bands behaviour is observed in accordance with the structural data for presence of a short hydrogen bond (2.609 Å). It is presumed that the A band of the ABC trio most probably originates from the stretching OH vibrations of strongly hydrogen bonded POH(1) group, but the contribution of the second POH(2) group and the water molecule should be also considered. The strength of the four hydrogen bonds as deduced from the infrared wavenumbers of the isotopically isolated OD groups has been discussed in terms of the respective O···O distances, the hydrogen bond acceptor capability of the oxygen atoms, the hydrogen bond donor strength of the H2PO4 ions and water molecules and the hydrogen bond acceptor angles. The influence of the cooperative effect has been also analyzed.