Theory of reorientational motional broadening of vibrational spectral bands in solid state

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Abstract
A theory of reorientational motional broadening of vibrational bands in solid state was developed by using the site model. The equation describing the correlation function of the transition moment and the Raman tensor was derived in a matrix form. In the case of an equivalent site model, the obtained correlation function indicated that the band profile was represented by the overlap of Lorentzian functions located at the same in peak position but different in half-width. The theory was applied to the Raman bands of n-C20H42 in the urea inclusion adduct, and used to interpret semiquantitatively the observed reorientational motional broadening of the bands. It was found that the amount of broadening of the infrared and Raman bands induced by molecular motion depended systematically on their symmetry species. This was interpreted theoretically by a group theoretical consideration of the reorientational relaxation.