Polyacetylenes Raman spectra

D. carota ssp. commutatus) were also investigated regarding the accumulation of polyacetylenes. Raman spectra prove that mainly falcarindiol is present in the individual wild species. Recently, differences in the Raman spectra of two wild carrot species have been described in more detail D. carota ssp. gummifer, D. carota ssp. maximus) [17]. Furthermore, Raman mapping clearly presented the different distribution of polyacetylenes in wild and orange carrots. It could be shown that the whole phloem tissue was rich in polyacetylenes, but the maxima were also observed near the pericyclic parenchyma. An analogous distribution of polyacetylenes was found in roots of other carrot wild species. 

A chemical reaction occurs above 1.5 GPa The sample turns black, new peaks develop in the Raman spectrum, and the absorption edge moves below 11,000cm. The recovered material has an optical band gap of 1.39eV, smaller than the band gap of polyacetylene. From the analysis of the Raman spectrum, it is seen that the C=C stretching mode completely disappears in the reaction product, while the C=N stretching band is present but at a different frequency than in cyanocetylene. In addition, the Raman bands of polyacetylene are observed with their characteristic frequency dependence on the wavelength... 

Figure 6.8 Light-optical microscopic image of chamomile inflorescence (a), FT-Raman spectrum obtained from the receptacle (b) Raman mapping showing the distribution of polyacetylenes (c) and carotenoids (d). The relative concentration of polyacetylenes...

Fig. 11-13 Resonance Raman spectrum of cis-polyacetylene. After Reference [166], reproduced with permission.

Two different classes of feature are observed in the difference spectrum of accumulation layers in Durham polyacetylene shown in figure 40. The main class of feature is negative and arises from the bleaching of the resonance-enhanced Ag modes that dominate the normal Raman spectrum of trans- polyacetylene. The two main modes are near 1100 cm-i and 1500 cm-i with the exact frequency a function of the excitation energy. The bleaching of these two main modes dominates the difference spectrum. The dispersion of the... 

Figure 40. Difference spectra due to charge accumulation layer in a polyacetylene MISFET structure, from [14]. The solid line is the difference signal the broken line is the normal Raman spectrum. The excitation wavelength is 584 run.

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. ... 

There have been many studies of Raman spectra of polyacetylene [173-175] in order to determine both conjugation lengths and doping and also as a basic fingerprint technique in order to measure differences between cis- and rmn -polyacetylene. Figure 1.72 shows a typical spectrum for polyacetylene. There are two strong Raman bands for the trans polymer and three for the cis polymer. 

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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