Polyacetylene, vibrational spectra

Polyacetylene, vibrational spectra, 42 196 Poly acids, defined, 41 117 Polyalkenamers,24 134,135 Polyamino acids, thermal, 20 374-377 catalysis by, 20 379... 

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. 

The polymerization of acetylene by using [Rh(l,5-Cod)Cl]2, where 1,5-Cod is c/j,cw-cycloocta-l,5-diene, or [Rh(NBD)Cl]2, where NBD is bicyclo[2,2,I]hepta-2,5-diene, was studied by UV-vis spectroscopy [79,80]. The growing polyacetylene chains were identified by three maxima at 500, 544, and 590 nm as a result of subtracting the spectrum of the catalyst from that of the reaction mixture. The first-order derivative of the absorption spectrum of the growing polyacetylene exhibited vibrational maxima at 480, 515, 550, and 600 nm for the cis-isomer and at 640, 670, and 710 nm for the trans-isomer. UV-vis and FTIR spectroscopies were used in the study of the structure of thin freestanding films of cis- and trans-PA obtained by using Rh(I) complexes. The absorption spectrum shows no vibrational structure, which was detected in acetylene polymerization in ethanol. The microstructure of PA is very similar to that of PA synthesized with a Luttinger catalyst in terms of sp defects in the polymer chains detected by FTIR spectroscopy. 

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 mechanism for the polymerisation of acetylene is inherently different from that of aromatic monomers such as pyrrole or thiophene. Whereas the polymerisation of pyrrole or thiophene involves a redox reaction,(77,74) the corresponding reaction of acetylene is probably initiated by acidic properties of the catalyst.(22) In the case of polyacetylene evidence has been obtained to suggest that the nature of the cations in the zeolite lattice is also important.(75) Fig. 1 shows a series of Raman spectra which illustrate the influence of various cations upon the extent of polymerisation, demonstrate the effect of elevating the acetylene pressure and indicate a role for Lewis acid sites in the reaction mechanism. Exposure of acetylene (0.1 MPa) to sodium-mordenite (NaM) at 295 K gave the spectrum displayed in Fig. 1(a). Bands at 398 and 468 cm are ascribed to lattice modes of the mordenite structure(2J), whereas the peak at ca. 1958 cm can be attributed to the Vj vibration of adsorbed monomeric acetylene bound in a side-on" manner to cation sites (16,23). Relatively small maxima at 1112 and 1502 cm are characteristic of trans-polyacetylene (5,18,24,25). Exchange of cesium for the sodium ions in mordenite was found to be beneficial for the formation of polyacetylene, as can be seen in Fig. 1 (b). In addition to the noted intensification of bands typical of rra/iy-polyacetylene at 1112 and... 

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