Polyacetylene vibrational structure

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. 

MBPT(2) has also been applied to calculate vibrational frequencies of polymers. With the translational symmetry, one can only calculate the vibrational modes with the reciprocal vector k = 0. These modes are of particular importance since they give rise to infrared and Raman spectra [67]. We applied MBPT(2) to polymethineimine and calculated its equilibrium structure, band gap, and vibrational frequencies with basis sets STO-3G, 6-31G and 6-31G [68]. Both basis set and electron correlation have a strong influence on its vibrational frequencies as well as its optimized geometry and band gap. With respect to in-phase (k=0) nuclear displacements, Hirata and Iwata very recently calculated the MBPT(2) vibrational frequencies of polyacetylene for basis sets STO-3G and 6-31G with analytical gradients [69], They showed that MBPT(2) greatly improves the HF vibrational frequencies for polyacetylene. 

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. 

Within the trans structure there is one acetylene per repeat unit and the factor group is isomorphous to the D2h print group. However, the situation for cis-polyacetylene is quite different. In this case cis-polyacetylene has two monomers per repeat unit with a factor group isomorphous to C2h symmetry. Hence the modes of vibration are different and are described by ... 

Since polyacetylene is insoluble in all solvents tested, identification of the isomers has been made by vibrational spectroscopic studies on thin films or by NMR spectra on the solid polymers (82). The structural aspects of polymers of acetylene or acetylene derivatives has recently been discussed by Simionescu et al. (83). The mechanism of stereoregulation in the stereospecific polymerization of acetylene or substituted acetylenes is still unclear. 

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. 

Many problems of disordered organic polymers have been treated [6-13] including the case of structurally disordered Polyacetylene for which both vibrational [45] and electronic states [46] have been calculated. 

The electronic structure of polyacetylene is very sensitive to the presence of conformational defects on the chains, and there are significant differences between the properties of polyacetylene prepared as unoriented and as stretch-oriented films. Stretch-oriented films show properties similar to those of good-quality Shirakawa polyacetylene, with the peak in the interband n-n optical absorption at around 1.9 eV [41-43]. In contrast, Ae band-gap in the unoriented material is raised, with the peak in the interband -absorption at about 2.3 eV [44-46], as seen in the optical absorption spectra shown in figure 4. There are concomitant increases in the frequencies of the Raman-active vibrational modes [44]. 

We have shown in section 5.3 that the sub-gap optical absorption of the depletion layo formed in the polyacetylene Schottky diodes shows the removal of the mid-gap soliton states associate with the extrinsic charges present in the undepleted material. The MIS structures operate in both charge accumulation and depletion, and diere is particular int st in the charge accumulation layer in that the charges injected are present without any associated dopant We have therefore carried out an extensive sales of expoiments on the electro-opticd properties of these MIS structures, covering both the electronic excitations of the solitons at mid-gap and also the IR and Raman vibrational excitations of the soliton. All the MIS structures which have been electrically characterised as described in section 6.2 have, been constructed so that they are semitransparent, either over the IR (silicon substrate) or over the IR and visible (polymer insulators, glass substrates). 

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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