Raman Spectroscopy of Complex Ions in Solution

Abstract

A complete understanding of complex ions in solution depends on knowledge of (1) the identity of the complexes; (2) the structures of the species; (3) the equilibrium constants; (4) the thermodynamics of formation; and (5) the mechanisms of formation of the species. Photoelectric Raman spectroscopy is a powerful technique for obtaining some of this information. The number of Raman bands and their degrees of depolarization are often sufficient to establish both the identity and the structure of the species. In principle, infrared data are necessary for a complete vibrational analysis, but for many of the simple structures involved, an unambiguous conclusion can be drawn without infrared data. Many of the vibrational modes of metal complexes are at frequencies less than 400 cm− 1. Raman spectrophotometers provide useful information even down to 40 cm−1, but specialized equipment is needed for infrared studies in this spectral region. Furthermore, for many studies of interest to date, the complex ions exist in aqueous solution. These systems present no special difficulty for the Raman technique. On the other hand, infrared studies, especially quantitative studies, cannot easily be made on these systems, although the development of new cell-window materials and the attenuated total reflectance technique will probably make more infrared data available in the future. If the mercury 4358-A line is used for Raman excitation, investigation is restricted to colorless or nearly colorless systems. The development of sources which utilize other excitation frequencies is enlarging the number and types of systems which can be investigated.