Hydrogen bonding systems in metal(II) formate dihydrates, M(HCOO)2•2H2O (M = Mg, Mn, Co, Ni, Cu, and Zn).: Double matrix infrared spectroscopy

The hydrogen bond strengths in the title isomorphous compounds as deduced from the infrared wavenumbers of the respective uncoupled OD stretches of matrix-isolated HDO molecules are discussed in terms of O-w center dot center dot center dot O hydrogen bond distances, metal-water interactions (synergetic effect) and unit-cell volumes (repulsion potential of the lattice). The VOD stretching modes caused by guest ions (Cu2+ and M2+ guest ions matrix-isolated in M(HCOO)(2)center dot 2H(2)O and Cu(HCOO)(2)center dot 2H(2)O, respectively) are shifted at higher frequencies as compared to those in the host and the parent compounds, respectively. The values of these shifts (Delta v) for the M-OH2 center dot center dot center dot OCHO-Cu hydrogen bond systems formed when Cu2+ guest ions are included in the structures of M(HCOO)(2)center dot 2H(2)O are considerably larger (i.e., weaker hydrogen bonds) as compared to those of the same type formed when M2+ guest ions are included in the structure of Cu(HCOO)(2)center dot 2H(2)O. A variant of double matrix infrared spectroscopy (M(HCOO)(2)center dot 2H(2)O, M = Mg, Co, and Ni) with incorporated Cu2+ ions up to 50 mol% additionally to matrix-isolated HDO molecules is proposed in order to deduce the cation distribution in mixed formates. The analysis of the infrared spectra reveals that the included Cu2+ ions are localized at the two available lattice positions in the M(HCOO)(2)center dot 2H(2)O (M = Co and Ni) host lattices and preferentially at the M(1) sites (sites with a coordination through six formate oxygen atom) in the Mg(HCOO)(2)center dot 2H(2)O host lattice. (c) 2006 Elsevier B.V. All rights reserved.