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

The infrared (IR) spectra of Mg(HCOO)(2). 2H(2)O and Mg1-xCux(HCOO)(2). 2H( 2)O mixed crystals (0 < x <less than or equal to> 0.5) have been recorded a nd the internal modes of the formate groups and the water molecules are rep orted. The number of the bands corresponding to the fundamental vibrations of the HCOO groups and the H2O molecules in the Mg(HCOO)(2). 2H(2)O spectru m reflects the existence of two crystallographically different formate grou ps and water molecules in coincidence with the structural data. The red-shi ft of the OD vibrations (matrix-isolated HDO molecules) at liquid nitrogen temperature as compared to ambient temperature (dv/dT > 0) evidences for th e formation of linear hydrogen bonds. The IR spectra of Mg1-xCux(HCOO)(2). 2H(2)O mixed crystals show that the regions of 3200-3400 cm(-1) (v(OH)), 23 00-2500 cm(-1) (v(OD) of matrix-isolated HDO), 500-950 cm(-1) (water librat ions) are mostly sensitive to the coordination environment of the metal ion s. So the analysis of the IR spectra reveals that the copper ions are local ized predominatingly at Me(1) sites in the mixed crystals, i.e. at sites wi th a six fold coordination through six oxygen atoms from the formate groups only. Mg0.26Cu0.74(HCOO)(2). 2H(2)O (new crystal phase) forms monoclinic c rystals with lattice parameters: a = 12.347(4); b = 7.240(4); c = 8.736(6) Angstrom beta = 103.08(5)degrees; SG P2(1)/c. The internal modes of the for mate groups and the water molecules in the new crystal phase are discussed by analogy to Mg(HCOO)(2). 2H(2)O and Cu(HCOO)(2). 2H(2)O, since the struct ural data are not available. The analysis of the spectra in the region of t he uncoupled OD vibrations shows that probably more than two water molecule s are expected to exist in the structure. It is assumed that the water mole cules bonded to the copper ions form stronger hydrogen bonds due to the str onger Cu-H2O interactions (synergetic effect) than those bonded to the magn esium ions. The librational modes of the water molecules in all the solid p hases studied are also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.