Vibrational Spectroscopy of Heteropoly Acids

The vibrational spectra of the heteropoly acids can be conveniently regarded as composed of contributions from the polyoxometalate anion (the primary structure) and from the water of crystallisation and hydrated protons (the secondary structure). Following a brief general survey of vibrational spectra of hydrogen-bonded systems, the spectra of water and hydrated protons in crystalline solids are reviewed. The vibrational spectra of the primary structure of heteropoly acids (the Keggin anion) are described and the observed spectra of the secondary structures of highly hydrated, hexahydrated and dehydrated heteropoly acids are discussed in relation to the spectra expected for protons in different environments.