Hydrogen bond strength in some beryllium salts, BeXO 4·4H 2O and Me 2Be(XO 4) 2·2H 2O (X = S, Se; Me = K, Rb): Correlation of structural data and infrared spectra

Abstract

Infrared spectra of the title compounds are presented and discussed in the regions of the uncoupled O–D stretches of matrix-isolated HDO molecules (isotopically dilute samples). The strengths of the hydrogen bonds are analyzed in terms of the respective O w⋯O bond distances, the Be–OH 2 interactions (synergetic effect), the proton acceptor capabilities of the sulfate and selenate oxygen atoms as deduced from Brown's bond valence sums of the oxygen atoms, the anti-cooperative effect (proton acceptor and proton donor competitive effect). The infrared spectroscopic experiments reveal that comparatively strong hydrogen bonds are formed in the compounds under study, analogical to other hydrated beryllium salts owing to the large ionic potential of the small Be 2+ ions. The wavenumbers of ν OD show that the water molecules in BeSO 4·4H 2O and in the double salts are strongly energetically distorted, i.e. their local symmetries deviate remarkably from the C 2 v molecular symmetry (for example, Δ ν have values of 74 and 36 cm −1 for H 2O(1) and H 2O(2) in K 2Be(SO 4) 2·2H 2O, and 119 cm −1 in BeSO 4·4H 2O). The hydrogen bonds in K 2Be(SeO 4) 2·2H 2O are stronger than those in K 2Be(SO 4) 2·2H 2O due to the stronger proton acceptor capability of the SeO 4 2− ions. The proton donor strengths of the water molecules in K 2Be(SO 4) 2·2H 2O and K 2Be(SeO 4) 2·2H 2O are greater than those of the water molecules in BeSO 4·4H 2O and BeSeO 4·4H 2O (i.e. larger deviations from Mikenda's curve) due to the different compositions of the respective beryllium tetrahedra-Be(XO 4) 2(H 2O) 2 in the double salts and Be(H 2O) 4 in the simple ones (proton donor competitive effect). The intramolecular O–H bond lengths are derived from the ν OD vs. r OH correlation curve [H.D. Lutz, C. Jung, J. Mol. Struct. 404 (1997) 63].