Hydrogen bond strength in chromates with kröhnkite-type chains, K2Me(CrO4)2·2H2O (Me = Mg, Co, Ni, Zn, Cd)

Infrared spectra of the title compounds with kröhnkite-type infinite octahedral-tetrahedral chains, K2Me(CrO4)2·2H2O (Me = Mg, Co, Ni, Zn, Cd), are presented in the regions of the uncoupled O-D stretching modes of matrix-isolated HDO molecules (isotopically dilute samples) and water librations. The strengths of the hydrogen bonds are discussed in terms of the respective Ow⋯O bond distances, the Me-water interactions (synergetic effect), the proton acceptor capability of the chromate oxygen atoms as deduced from Brown's bond valence sum of the oxygen atoms. The spectroscopic experiments reveal that hydrogen bonds of medium strength are formed in the chromates. The hydrogen bond strengths decrease in the order Cd > Zn > Ni > Co in agreement with the decreasing covalency of the respective Me-OH2 bonds in the same order, i.e. decreasing acidity of the water molecules. The infrared band positions corresponding to the water librations confirm the claim that the hydrogen bonds in K2Cd(CrO4)2·2H2O are stronger than those formed in K2Mg(CrO4)2·2H2O on one hand, and on the other-the hydrogen bonds in K2Ni(CrO4)2·2H2O are stronger than those in K2Co(CrO4)2·2H2O.