Density of vibrational states in trans-polyene: comparison with the infrared, Raman and neutron spectra of trans-polyacetylene

The density of states has been calculated for the in-plane and out-of-plane vibrations in the infinite trans polyene chains. Comparison of the density of states spectra with the IR and Raman spectra of trans-polyacetylenes, (CH)_x, (¹³CH)_x and (CD)_x, shows that most of the weak bands as well as the strong bands arise from the trans polyene segments in the polymer films. The IR bands newly assigned to the trans segments include the 740 cm^-1 band, which was previously assigned to remnant cis CC bonds. The IR spectra of (¹³CH)_x and (CD)_x clearly indicate that the amount of remnant cis bonds in thermally isomerized trans films is much smaller than that proposed previously. The hydrogen-amplitude-weighted density of states calculated for the infinite polyene chain also shows satisfactory agreement with a neutron inelastic scattering spectrum of trans-(CH)_x.