Infrared spectroscopic study of ions included in K2Me(CrO4)2·2H2O (Me = Mg, Cd) and crystal structure of K2Cd(CrO4)2·2H2O

The crystal structure of K2Cd(CrO4)2·2H2O was determined from single crystal X-ray diffraction data. It crystallizes triclinic (space group P 1 ¯ , a = 5.896(1) Å, b = 6.446(1) Å, c = 7.596(1) Å, α = 109.09(1)°, β = 96.80(1)°, γ = 109.29(1)°, V = 249.1 Å3, Z = 1, R1 = 0.023 for 2672 Fo > 4σ(Fo) and 79 variables), isotypic with the respective Mg-analogue. The structure is build up from CdO4(H2O)2 and CrO4 groups forming kröhnkite-type octahedral–tetrahedral chains which are linked by alkali cations and hydrogen bonds to layers and further to a three-dimensional framework. ional spectra (infrared and Raman) of K2Mg(CrO4)2·2H2O and K2Cd(CrO4)2·2H2O as well as infrared spectra of SO 4 2 - guest ions included in the chromate matrices are presented and discussed with respect to the CrO and SO stretching modes, respectively. The Raman spectra of the chromate compounds reveal that the symmetric stretching modes ν1 of the CrO4 tetrahedra appear at higher frequencies than the lowest wavenumbered components of the asymmetric stretches ν3. The uncoupled OD modes of matrix-isolated HDO molecules (isotopically dilute samples) shows that stronger hydrogen bonds are formed in K2Cd(CrO4)2·2H2O as compared to those formed in K2Mg(CrO4)2·2H2O owing to both the shorter hydrogen bond lengths Ow···O in the former compound and the covalent character of the CdOH2 bonds, irrespective of the larger CdOH2 bond distances than the respective MgOH2 bonds. The SO 4 2 - guest ions matrix-isolated in K2Mg(CrO4)2·2H2O and K2Cd(CrO4)2·2H2O exhibit three infrared bands corresponding to the asymmetric stretching and bending modes ν3 and ν4, respectively. Bands of small intensities corresponding to the symmetric stretches ν1 are also observed in the infrared spectra. All these observations are in good agreement with the low site symmetry of the CrO 4 2 - host ions (C1 site symmetry). The spectroscopic experiments reveals: (i) The SO 4 2 - guest ions undergo a slightly stronger energetic distortion in the cadmium compound as deduced from the values of Δν3 and Δνmax (the difference between the highest and the lowest wavenumbered components of the SO stretching modes), thus indicating a stronger lattice potential (static potential) at the respective site in the cadmium matrix (the respective values are 58 and 134 cm−1 for the former matrix and 36 and 131 cm−1 for the latter one). (ii) The SO 4 2 - guest ions are stronger distorted with respect to the SO bond lengths than to the OSO bond angles as deduced from the corresponding values of Δν4 and Δν3 (Δν4 have values of 10 cm−1 in both the matrices).