Polyacetylenes infrared spectra

Thermogravimetric curves for the polyacetylene, there are two exothermic peaks at 145 and 325°C [16]. The first of these corresponds to an irreversible cis-trans isomerization. Migration of hydrogen occurs at 325°C, open chain and cross-linking without the formation of polyacetylene volatile products. The color of the polymer becomes brown. A large number of defects appears. In the infrared spectrum there are absorption bands characteristic of the CH, CH3, -C=C- and -C2H5- groups [16]. 

Fig. 4. Infrared spectrum in polarized light of a highly oriented film of trans-polyacetylene (Assoreni) electric vector perpenthcular (dotted) and parallel (fuU line) to the stretching direction...

Figure 1.69. Infrared spectrum of unoriented polyacetylene sample. (Reprinted with permission liom ref. 172)...

The infrared spectrum of polyacetylene exhibits three peaks at 3013, 1292, and 1015 cm < > (see Table 9.9). The peak at 3013 cm is assigned to the C-H stretching vibration. Shirakawa and Ikeda assigned the mode at 1292 cm to the trans-C-H in-plane deformation and the band at 1015 cm to the out-of-plane C-H deformation. The results for these two bands are in good agreement with this interpretation. 

The infrared spectrum of doped and photoexcited polyacetylene can be nicely interpreted with ECC theory. 

Compared to the results of photoelectron spectroscopy, which are very sensitive to changes in charge distribution and electronic structure, we believe that the examination of the vibrational properties of such complexes offers a more direct probe to the actual chemical structure at the interface. In recent works [118, 120], we have described the evolution of the vibrational spectrum calculated for a polyene molecule, octatetraene, upon bonding of two A1 atoms, in order to model the Al/polyacetylene interface formation. These theoretical results indicate that important changes can be expected in the experimental infrared spectrum as a consequence of (i) the formation of Al-C covalent bonds and (ii) strong modifications in the bond pattern along the chain. 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

Nevertheless, even for polyacetylene, the electronic structure is not that of a simple metal in which the bond-alternation and the tc-tc gap have gone to zero there are infrared active vibrational modes (IRAV) and a pseudo-gap. This is indicated by the spectra in Figure 2 which demonstrate the remarkable similarity between the doping-induced absorption found with heavily doped trans-(CH)x, and the photoinduced absorption spectrum observed in the pristine semiconductor containing a very few photoexcitations. Not only are the same IRAV mode spectral features observed, they have almost identical frequencies. 

Figure 1.61. Photo-induced absorption spectrum of trans-polyacetylene at 210 K showing in addition the infrared soliton modes. (Reprinted with permission from ref 80)...

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