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 Becke 3-Lee-Yang-Parr (B3LYP) hybrid functional with the 6-31G basis set. T he optimized structures of the Tt and Ct oligoenes were found to be pl aner, whereas the Te 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 Te oligoenes were calculated at the B3LYP/6-31G level. The struc tures and force fields of the Ct and Te polyacetylene were estimated b y analyzing the chain-length dependence of the structural parameters a nd force constants of the oligoenes. The normal frequencies and the in elastic neutron scattering spectra were calculated for Ct and Te polya cetylene on the basis of the thus-obtained structures and force fields . A comparison between the calculated and observed results clearly ind icates that as-polymerized cis-rich polyacetylene consists of planar C t chains. The comparison also suggests that the observed Raman bands o f cis-rich polyacetylene films arise from Ct segments with intermediat e conjugation lengths.