Self-consistent-field calculations of the vibrational spectrum of undoped and doped transpolyacetylene

Semiempirical self-consistent-field calculations are carried out to compute the force constants for various cluster models of periodic transpolyacetylene and of neutral and charged soliton and polaron defects. The phonon spectrum of undoped polyacetylene is analyzed and compared with the vibrational spectrum of various polyene chains in which soliton or polaron defects have been introduced in a random way. The results support the interpretation that the 1100 cm−1 Raman band of undoped polyacetylene should be ascribed to a C—C single bond stretching motion weakly coupled to a C—H in-plane bending vibration. The 1292 cm−1 infrared peak of pure transpolyacetylene is ascribed to the C—H in-plane deformation, the strong adsorption at 1015 cm−1 has to be assigned to an infrared-active out-of-plane C—H deformation. The new narrow band around 1300 cm−1 seen in the infrared spectrum of doped polyacetylene is ascribed to C—C bond stretching, the additional very broad band between 800 and 900 cm−1 results from in-plane C—C—H bending and C—H out-of-plane bending motions involving relaxed bond lengths of the defect sites. The interpretation is not uniquely in favor of the soliton picture, polaron defects can serve as alternative explanation.