Stereostructural and Vibrational Analyses of cis-Polyacetylene Based on Density Functional Calculations of Oligoenes

The total energies and optimized structures of all-trans-transoid (Tt), all-cis-transoid (Ct), all-trans-cisoid (Tc), and all-cis-cisoid (Cc) oligoenes with various chain lengths were calculated using the Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with the 6-31G* basis set. The optimized structures of the Tt and Ct oligoenes were found to be planer, whereas the Tc and Cc oligoenes have helical structures in which the conformations about the C-C bonds deviate from planar s-cis. The total energies of the Tt, Ct, Tc, and Cc oligoenes with the same chain length increase in this order. The vibrational force fields of the Ct and Tc oligoenes were calculated at the B3LYP/6-31G* level. The structures and force fields of the Ct and Tc polyacetylene were estimated by analyzing the chain-length dependence of the structural parameters and force constants of the oligoenes. The normal frequencies and the inelastic neutron scattering spectra were calculated for Ct and Tc polyacetylene on the basis of the thus-obtained structures and force fields. A comparison between the calculated and observed results clearly indicates that as-polymerized cis-rich polyacetylene consists of planar Ct chains. The comparison also suggests that the observed Raman bands of cis-rich polyacetylene films arise from Ct segments with intermediate conjugation lengths.