Lattice vibration spectra Part LVIII. OD(OH) frequency versus H-bond distance correlation diagrams: a case study for isomorphic M(SO 32·3H 2O (MMg, Mn, Fe, Co, Ni, Zn)

Abstract

The uncloupled ν OD band frequencies of matrix isolated HDO molecules in isostructural MSO 3·3H 2O (MMg, Mn, Fe, Co, Ni, Zn) have been assigned to the six different H positions in the structure by Raman single crystal techniques. The physical basis of this technique presented first in 1984 is described. The water molecules H 2O I and II are extremely asymmetric, i.e. the most asymmetric known so far, forming a very strong and a mediate H-bond each. The bifurcated H-bonds of H 2O III reveal a dynamic behaviour, as possibly common for bifurcated hydrogen bonds. The strength of the H-bonds present as well as the correlation between the OD(OH) band frequencies and the H⋯O hydrogen bond distances obtained from neutron diffraction studies are discussed in terms of the different acceptor strength (basicity) of the H-bond acceptor groups (SO 2− 3, H 2O), the synergetic effect, and the repulsive forces at the respective lattice sites. The increase of the H-bond donor strength (acidity) of the water molecules owing to metal—oxygen interactions (MOH 2) (synergetic effect) resembles that of the acid strengths of M(H 2O) 2+ 6 of the metal ions involved.