Infrared study of νOD modes in isotopically dilute (HDO) kieserite-type compounds MXO4·H2O (M=Mn, Co, Ni, Zn, and X=S, Se) with matrix-isolated M′ and X′O4 guest ions

The hydrogen bond strength in kieserite-type sulfate and selenate monohydrates has been studied by the method of double-matrix spectroscopy. The infrared spectra of isotopically dilute (matrix-isolated HDO molecules) kieserite-type compounds MXO₄·H₂O (M=Mn, Co, Ni, Zn, and X=S, Se) with matrix-isolated X′O₄²⁻ and M′²⁺ guest ions are presented and discussed in the region of the OD stretching modes. The OD frequencies indicate that the compounds under investigation form comparatively strong hydrogen bonds. The frequency shifts of the uncoupled OD stretching modes of the HDO molecules within the isostructural series and those influenced by the guest ions, and the strength of the hydrogen bonds formed, are discussed in terms of the respective O_wO distances, which hint at stronger hydrogen bonds for the sulfate series than for the selenate one by mistake, the larger hydrogen bond acceptor capability of SeO₄²⁻ ions compared to SO₄²⁻ ones, the different metal–water interactions and repulsion potentials of the lattice, and the reorientation of the water molecules caused by the guest ions. The shifts of the OD stretches of the ODOSe′O₃ bonds (Se′O₄²⁻ matrix isolated) to the lower wavenumbers as compared to the parent selenates are caused by the reorientation of the hydrate water molecules and strengthening the hydrogen bond to the stronger acceptor and vice versa. When smaller metal ions having smaller M–OH₂ bond lengths and, hence, stronger synergetic effect replace larger ones, the OD stretches are shifted to lower wavenumbers as compared to those due to the host M–O_wO bonds and vice versa.