Vibrational behavior of matrix-isolated ions in Tutton compounds. III: Infrared spectroscopic study of NH 4+ and SO 42− ions included in cobalt sulfates and selenates

Abstract

Infrared spectra of sulfate doped K 2Co(SeO 4) 2·6H 2O and (NH 4) 2Co(SeO 4) 2·6H 2O and those of ammonium doped K 2Co(SO 4) 2·6H 2O and K 2Co(SeO 4) 2·6H 2O are presented and discussed in the region of the stretching modes ν 3 and ν 1 of the sulfate ions and in the region of the asymmetric bending modes ν 4 of the NH 4 + ions, respectively. The SO 4 2− ions matrix-isolated in the selenate matrices (samples K 2Co(SeO 4) 1.98(SO 4) 0.02·6H 2O and (NH 4) 2Co(SeO 4) 1.98(SO 4) 0.02·6H 2O) exhibit three bands for ν 3 and one band for ν 1 in agreement with the low site symmetry C 1 of the SeO 4 2− host anions. The NH 4 + guest ions included in the potassium sulfate matrix are characterized also with three site symmetry components of ν 4 ( C 1 site symmetry of the K + cations). However, the NH 4 + guest ions included in the potassium selenate matrix display four bands corresponding to ν 4 instead of three bands expected due probably to some kind of disorder of the ammonium ions. The extent of energetic distortion of the isomorphously included sulfate ions as deduced from the values of Δ ν 3 (site group splitting), Δ ν max (the difference between the highest and the lowest components of the stretching modes for the sulfate ions) and Δ ν 3/ ν c ( ν c is the centro-frequency of the asymmetric stretching modes) are commented. The spectroscopic experiments reveal that the SO 4 2− guest ions are less distorted in the selenate matrices as compared to the same ions in the neat sulfates due to the larger unit-cell volumes of the selenate compounds, i.e. to the smaller repulsion potential at the lattice sites where the guest ions are located. The analysis of the spectra reveals that the band positions of the water librations in the host compounds are affected by the included NH 4 + guest ions. The formation of hydrogen bonds between the NH 4 + guest ions and the XO 4 2− host ions leads to a decrease in the proton acceptor capabilities of the anions ( anti-cooperative or proton acceptor competitive effect) and as a result the hydrogen bonds formed by the water molecules weaken on going from the neat potassium compounds to the mixed crystals (the bands corresponding to water librations broaden and shift to lower frequencies). This effect is more pronounced in the case of the selenate matrix due to the stronger proton acceptor capability of the selenate ions than that of the sulfate ones.