Polymorphism of the smectogen ethyl-p-azoxybenzoate: Far-infrared and Raman study

The far-infrared spectra of the different phases of ethyl-p-azoxybenzoate have been studied in the 30–210 cm–1 range. Distinct differences are observed between the spectra of the two crystalline polymorphs that are stable above and below 102 °C, solid I and solid II, respectively. The spectrum of solid I resembles that of the isotropic phase, while that of the smectic phase is highly broadened. These results are discussed in terms of the structure, molecular conformation, and packing obtaining in the different phases. In the region of the internal modes, the same Raman frequencies are observed in both polymorphs although relative intensity changes occur for several lines. From a comparison of the Raman line shapes corresponding to the N–O and N–N bond stretching vibrations in solid II and solid I, we conclude that in the latter case the reorientations of the azoxy group occur at a rate too slow to affect the vibrational relaxation of these modes