Vibrational behavior of matrix-isolated ions in Tutton compounds. I. Infrared spectroscopic study of NH_4+ and SO_4~2- ions included in magnesium sulfates and selenates

摘要

Infrared spectra of K_2Mg(SeO_4)_2.6H_2O and (NH_4)_2Mg(SeO_4)_2.6H_2O containing SO_4~2- guest ions and those of K_2Mg(SO_4)_2.6H_2O and K_2Mg(SeO_4)_2.6H_2O containing NH_4~+ guest ions are presented and discussed in the region of the stretching modes v_3 and v_1 of the sulfate ions and in the region of the asymmetric bend_ing modes v_4 of the NH_4~+ ions. The SO_4~2- ions matrix-isolated in the selenate matrices (approximately 5 mol%) exhibit three bands for v_3 and one band for v1 in agreement with the low site symmetry CI of the SeO_4~2- host anions. The NH_4~+ guest ions included in the potassium matrices are characterized also with three site symmetry components of v_4 (C_1 site symmetry of the K~+ cations). The spectral regions of v_4 and v_2 of the SO_4~2- guest ions as well as those of v_1, v_3 and v_2 of the NH_4~+ guest ions could not be analyzed precisely due to the overlapping of these motions with motions of other entities in the struc_tures (for example, the normal modes of water molecules and water librations). The extent of energetic distortion of the isomorphously included ions as deduced from the values of Δv_3 and Δv_4 (site-group splitting for the SO_4~2- and NH_4~+ guest ions, respectively) and Δv_(max) (the difference between the highest and the lowest components of the stretching modes for the sulfate ions) are commented. The spectroscopic experiments show that the degree of energetic dis_tortion of the guest ions is not affected by the guest ion concentration up to 20 mol%. It has been established that the SO_4~2- guest ions are stronger distorted in (NH_4)_2Mg(SeO_4)_2.6H_2O than in K_2Mg(SeO_4)_2.6H_2O (Δv_3 have values of 34 cm~-1 in the potassium matrix and 45 cm~-1 in the ammonium one; samples containing about 5 mol% sulfate ions, respectively). The formation of hydrogen bonds between the SO_4~2- guest ions and NH_4~+ host ions increases the electrostatic field strength at the lattice sites where the guest ions are located, thus leading to a larger extent of energetic distortion of the sulfate ions included in (NH_4)_2Mg(SeO_4)_2.6H_2O. The NH_4~+ guest ions are stronger distorted in K_2Mg(SO_4)_2.6H_2O than in K_2Mg(SeO_4)_2.6H_2O (Δv_4 have values of 73 cm-I in the former compound and 66 cm~-1 in the latter one) owing to the smaller unit-cell vol_ume of the sulfate than that of the selenate (repulsion potential of the lattice). 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 weaken on going from the neat potassium compounds to mixed crystals K_(1.8)(NH_4)_(o.2)Mg(SO_4)_2.6H_2O and K_(1_8)(NH_4)_(0.2)Mg(SeO_4)_2.6H_2O (the bands corresponding to water librations broaden and shift to lower requencies).