We present for the first time a comparative vibrational study of polythiophene, polypyrrole, and polyisothianaphthene. The calculation is performed at k=0 in the plane of the chains for both aromatic-like and quinoid-like structures. The Ag species which are present in the Raman spectra are more particularly investigated. The frequency changes, the atomic displacements, and the influence of the interring distance r and the interring force constant F(r2) are analyzed when the quinoid character is varied. We demonstrate that the ring linkage vibrations C–C or C=C give frequencies in the range 1450–1800 cm−1. It is found that a similar dependence of these frequencies on F(r2) and r exists for all the forms except for aromatic polyisothia naphthene. Also, analogies between the three polymers in the experimental Raman spectra are evidenced. Refined sets of force constants very close to those of the monomers are suitable for explaining the main features of these spectra and can be used to determine the degree of aromaticity and the degree of bond alternation.

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