Vibrational spectra ofcis−(CH)xandcis−(CD)x: A theoretical study

Theoretical vibration spectra of $\mathrm{cis}-{\left(\mathrm{CH}\right)}_x$ and $\mathrm{cis}-{\left(\mathrm{CD}\right)}_x$ are presented, based on a single-periodic-chain model and a harmonic potential with parameters transferred from hexatrienes. The out-of-plane degrees of freedom and the infrared absorption intensities were included in the calculations. Good overall agreement with the experiment is found, and hence the electron-phonon coupling in cis-polyacetylene is relatively weak, as in $\mathrm{poly}(p-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e})$—as compared to trans-polyacetylene, where the frequencies of the principal Raman lines are significantly lowered and where the supposedly strongest infrared absorption band is missing. The difference originates from the ground-state degeneracy of trans-polyacetylene, which is absent in the other two polymers.