Metal–water interactions and hydrogen bonding in dittmarite-type compounds M′M″PO 4·H 2O (M′ = K +, NH 4+; M″ = Mn 2+, Co 2+, Ni 2+) : Correlations of IR spectroscopic and structural data

Abstract

The vibrational behavior of the uncoupled ν OD modes and of the water librations in dittmarite-type compounds M′M″PO 4·H 2O (M′ = K +, NH 4 +; M″ = Mn 2+, Co 2+, Ni 2+) is analyzed in terms of the influence of two types of metal−water interactions (M +−H 2O and M 2+−H 2O), the hydrogen bonding and the repulsion potential of the lattices. The M +−H 2O interaction is found to be the main factor, which influences ν OD. The strong K +−H 2O interaction weakens in a higher degree the intramolecular O−H bonds than the corresponding M 2+−H 2O interactions (M 2+ = Mn, Co, Ni). As a result ν OD is shifted to lower wavenumbers in the potassium series than in ammonium one, irrespective of the synergetic effect of M 2+, the hydrogen bond lengths and the repulsion potential of the lattices. The analysis of the spectroscopic data evidences for the dominating influence of the M 2+−H 2O interaction on the wagging mode. The blue shift of ν wag strictly follows the increasing synergetic effect of M 2+, i.e. ν M′/Mn < ν M′/Co < ν M′/Ni, in all cases, irrespective of the strength of the M +−H 2O interactions, the hydrogen bond lengths and the higher repulsion potential of the K-lattices. The rocking mode is insensitive to the replacement of the M 2+ ions. Some relations between the spectroscopic and structural data are given.