Chemical Science International Journal

Infrared spectra of ammonium doped rubidium and cesium sulfate and selenate matrices, M'₂M''(XO₄)₂∙6H₂O (M' = Rb, Cs; M'' = Mg, Co, Zn; X = S, Se), are presented and discussed in the region of  both the asymmetric bending modes n₄ of the ammonium ions and the water librations. The ammonium ions included in the sulfate matrices display three bands corresponding to n₄ in agreement with the low site symmetry C₁ of the host cations, while those included in the selenate matrices exhibit four bands for n₄. A hypothesis is made that the reason for this phenomenon is the formation of polyfurcate hydrogen bonds between the ammonium guest ions and the selenate host ions, thus leading probably to a disorder of the ammonium ions. The degree of energetic distortion of the ammonium ions with respect to the bond angles H−N−N as deduced from the values of Dn₄/n_c depends on the repulsion potential of the matrices and the strength of the hydrogen bonds. The strength of the hydrogen bonds formed in the rubidium and cesium salts containing isomorphously included ammonium ions as deduced from the wave numbers of the water librations is discussed. The analysis of the spectra reveals that weaker hydrogen bonds are formed in the mixed crystals M'_1.85(NH₄)_0.15M''(XO₄)₂·6H₂O (M' = Rb, Cs; M'' = Mg, Co, Zn; X = S, Se) as compared to those formed in the neat rubidium and cesium compounds. These spectroscopic findings are owing to the decrease in the proton acceptor strength of the SO₄^2- and SeO₄^2- ions as a result of the formation of hydrogen bonds between the host anions and the ammonium guest cations (anti-cooperative or proton acceptor competitive effect).