IR study of solid phases formed in the Mg(HCOO)2–Cu(HCOO)2–H2O system

Abstract

The infrared (IR) spectra of Mg(HCOO)2·2H2O and Mg1−xCux(HCOO)2·2H2O mixed crystals (0<x≤0.5) have been recorded and the internal modes of the formate groups and the water molecules are reported. The number of the bands corresponding to the fundamental vibrations of the HCOO groups and the H2O molecules in the Mg(HCOO)2·2H2O spectrum reflects the existence of two crystallographically different formate groups and water molecules in coincidence with the structural data. The red-shift of the OD vibrations (matrix-isolated HDO molecules) at liquid nitrogen temperature as compared to ambient temperature (dv/dT>0) evidences for the formation of linear hydrogen bonds. The IR spectra of Mg1−xCux(HCOO)2·2H2O mixed crystals show that the regions of 3200–3400cm−1 (νOH), 2300–2500cm−1 (νOD of matrix-isolated HDO), 500–950cm−1 (water librations) are mostly sensitive to the coordination environment of the metal ions. So the analysis of the IR spectra reveals that the copper ions are localized predominatingly at Me(1) sites in the mixed crystals, i.e. at sites with a six-fold coordination through six oxygen atoms from the formate groups only. Mg0.26Cu0.74(HCOO)2·2H2O (new crystal phase) forms monoclinic crystals with lattice parameters: a=12.347(4); b=7.240(4); c=8.736(6)Åβ=103.08(5)°; SG P21/c. The internal modes of the formate groups and the water molecules in the new crystal phase are discussed by analogy to Mg(HCOO)2·2H2O and Cu(HCOO)2·2H2O, since the structutal data are not available. The analysis of the spectra in the region of the uncoupled OD vibrations shows that probably more than two water molecules are expected to exist in the structure. It is assumed that the water molecules bonded to the copper ions form stronger hydrogen bonds due to the stronger Cu–H2O interactions (synergetic effect) than those bonded to the magnesium ions. The librational modes of the water molecules in all the solid phases studied are also discussed.