Infrared study of the vibrational behavior of CrO 42− guest ions matrix-isolated in metal (II) sulfates (Me=Ca, Sr, Ba, Pb)

Abstract

Infrared spectra of matrix-isolated CrO 4 2− guest ions in host sulfate matrices - CaSO 4·2H 2O, SrSO 4, BaSO 4 and PbSO 4 are reported and discussed with respect to the Cr–O stretching and O–Cr–O bending modes. An adequate measure for the CrO 4 2− guest ion distortion is the site group splitting Δ ν as and Δ ν max (the difference between the highest and the lowest wavenumbered components of the stretching and bending modes). When the smaller SO 4 2− ions are replaced by the larger CrO 4 2− ions the mean frequencies of the asymmetric stretching and bending modes ( ν ¯ 3 and ν ¯ 4 ) as well as the frequencies of ν 1 of the CrO 4 2− guest ions are shifted to higher wavenumbers as compared to those in the respective neat chromates due to the larger repulsion potential at the host lattice sites (smaller values of the unit-cell volumes of the neat sulfates than those of the neat chromates). The CrO 4 2− guest ions exhibit three bands corresponding to the ν 3 modes as deduced from the site group analysis ( C 2 site symmetry in CaSO 4·2H 2O and C s site symmetry in SrSO 4, BaSO 4 and PbSO 4). However, the bending modes ν 4 and ν 2 of the CrO 4 2− guest ions in SrSO 4, BaSO 4 and PbSO 4 show an effectively higher local symmetry than the ‘rigorous’ crystallographic one (two bands for ν 4 and one band for ν 2 instead of a triplet and a doublet expected, respectively). Such different apparent site symmetries observed in various spectral regions may be attributed to the different influence of energetic and geometrical distortions of the polyatomic entities at particular site on various modes.