From butadiene to polyacetylene: An ab initio study on the vibrational spectra of polyenes

We report the results of a systematic ab initio self-consistent field (SCF) study on the structure and vibrational spectra of all-trans polyenes ranging from C₄H₆ to C₁₈H₂₀. We include the limiting case of the infinite all-trans polyacetylene. Trends in the evolution of the vibrational frequencies upon increasing the size of the polyenes and the convergence of edge effects and of bulk properties are monitored. We point out the inherent limitations of any SCF-type approach for the description of the vibrational spectra of polyenes and discuss difficulties encountered in the extrapolation to infinite chain length. On the basis of a suite of calculations on the smaller members of the polyene series trans-1,3-butadiene and all-trans-1,3,5-hexatriene carried out with a high-level electron correlation method, a new scaled force field has been suggested recently by us [M. Kofranek, A. Karpfen, and H. Lischka, Int. J. Quantum Chem. Symp. 24, 721 (1990)] which reproduces most of the experimentally known trends in these oligoenes, in particular, the vibrational frequencies in the region of the carbon-carbon double and single bond stretches. The performance of this scaled force field for the prediction of the vibrational spectra of larger all-trans polyenes is investigated in detail. We also apply it to compute the harmonic phonon dispersion curves of polyacetylene and compare with available experimental data.