Structure, vibrational spectra, and IR intensities of polyenes from ab initio SCF calculations

Ab initio SCF calculations are presented which deal with the structure of polyenes, as model systems for polyacetylene (PA). We used a double zeta quality basis set, and determined the structure of clusters up to C₂₂H ₂₄ as well as the vibrational spectra of clusters up to C ₁₂H₁₄, including infrared intensities. All of these data are analyzed by comparing with experimental information available on polyacetylene and previous theoretical studies. Specifically, we identified the vibrational modes corresponding to the observed absorption bands of PA. We comment on controversial points such as the relative contributions of internal coordinates to these modes, on the magnitude of interaction force constants, and also on the effect of light polarization on the absorption of some vibrational modes. We establish that the inclusion of polarization function in the basis set does not lead to significant changes in structural or energetic properties. Convergence of the cluster properties toward the 1D polymer properties was also tested with ab initio crystal orbital calculations.