Theoretical and Experimental Evaluation of Low-frequency Spectra of Nitric Acid Hydrates

The low-frequency region (below 175 cm−1) of the spectra of solid samples can provide valuable information on vibrational modes related to large amplitude motions, intermolecular vibrations or vibrations associated to weak linkages. Whereas far-infrared spectroscopy exhibits rather broad bands for solid hydrates, among other drawbacks of this technique, Raman spectroscopy produces narrow signals in this spectral region. On the other hand, the prediction of spectra from first principles is currently at a higher level of development for infrared absorption than it is for Raman scattering. This contribution presents the application of two techniques to the study of samples of atmospheric relevance, namely the hydrates of nitric acid.