Spectroscopic probing of site symmetry in solids

Abstract

Infrared (IR), electronic absorption and Raman spectroscopy are employed to probe the site symmetry, coordination geometry and structural changes in rare-earth complexes of high and low symmetry. The results of these studies are compared with X-ray data. Although unpolarized IR spectra do not enable, a priori, determination of the site symmetry of an ion in a crystal, the spectra can be shown to be consistent with crystallographic data. In addition, IR spectra can be employed to indicate subtle changes in the distances of nuclei bonded to hydrogen, which are not available from X-ray data, as exemplified herein for PH bonds in hypophosphite complexes. The use of electronic absorption spectra to explore the coordination geometry (rather than the site symmetry) of a complexed rare-earth ion is considered. Both the IR and electronic absorption techniques are found to be useful in fingerprinting given structural types. Small perturbations on distant neighbours of a chromophore may be silent in IR and Raman spectra, but significant in electronic absorption spectra.