L. A. Gribov

Vibrational spectroscopy of polypyrrole, theoretical study

Theoretical vibrational spectra of polypyrrole are presented, based on a single‐periodic‐chain model and harmonic potential in‐plane and out‐of‐plane parameters transferred from the pyrrole molecule. The infrared‐absorption intensities were included in the calculations for the first time in polypyrrole. Experimental spectra are analyzed and compared with theoretical ones. Some modes in the experimental spectra indicate the presence of nonplanar conformation. Also, Raman frequencies were compared with experimental ones. Significant effects of electron–phonon coupling are absent.

IR spectra and structure of poly(p-phenylene) - a theoretical study

Abstract The frequencies and the intensities of IR absorption bands are calculated for poly(p-phenylene) (PPP) transferring the parameters from toluene, and they are found to correspond well to the experimental ones. Hence significant effects of electron delocalization and electron-phonon coupling are absent in (the IR spectrum of) PPP, in contrast to polyacetylene; the electronic structure of PPP is similar to that of the monomer. Symmetry considerations are utilized to discuss effects of possible nonplanarity of PPP and of orientation.

The solution of the inverse spectroscopic problem for the IR spectra of trans- and cis-hexatrienes

The “generalized inverse spectral problem” for trans- and cis-hexa-1,3,5-trienes and their 2,3,4,5-tetradeutero analogues is solved in the framework of valence-optical theory and the force and electrooptical fields are obtained. The basic factors influencing the positions and intensities of the absorption bands are determined. The agreement between experimental and calculated spectra justifies the chosen models which are also qualitatively confirmed by quantum chemical CNDO calculations.

Calculation and interpretation of infrared spectra of poly(phenylene vinylene): Analysis of the character of chemical bonds and conformational states

Abstract Spectral distribution curves of the absorption coefficients of vibrational spectra of two poly(phenylene vinylene) conformations are calculated. Vibration dynamics are analyzed and absorption bands sensitive to the polymerization are revealed. Histograms of electronic states for isolated polymeric chains are calculated with the help of the Huckel extended method. The values of force constants and of electro-optical parameters found previously are shown to be convenient for reasonably accurate description of the IR spectra of the polymer; these show that at polymerization the electronic structure of isolated structural elements stays constant. A feeble influence of the intermolecular interactions on the spectra of basic chains is discovered, that makes it possible to use an approximation to examine these chains independently of their environment. The histograms of the electronic states show that these polymers can have semiconductor features.

Infrared Absorption Puzzles in Polyacetylene and Poly(Pyrrole)

Abstract Theoretical infrared (IR) absorption spectra have been computed for various models of trans- and cis-polyace-tylene, poly(p-phenylene) and poly(pyrrole). Surprisingly, some supposedly strong bands are missing in the observed IR absorption spectra.

Infrared spectroscopy and stereochemical structure of poly(P-phenylene vinylene)

Abstract Theoretical infrared (ir) spectra of trans- and cis- forms of poly(p-phenylene vinylene) (PPV) are presented, based on a single-period-chain model and harmonic potential with parameters transferred from p-divinyl benzen molecule. The out-of-plane degrees of freedom and the infrared intensities were included in the calculations. The calculations of polarized infrared spectra of cis- and trans-PPV do not show practically any difference. However, as the calculations evince the absence of the in-plane C-H bending infrared absorption band of trans-vinylene portion of experimental infrared spectrum of PPV, which is a consequence of a delocalized π-electron system (similarly to the case of trans-polyacetylene but not cis-polyacetylene), it seems that stereochemical structure of PPV is of the trans-form. Additional X-ray analysis is still necessary to support our conclusion.

Vibrational spectroscopy and stereochemical structure of poly (p-phenylene) and cis-polyacetylene

Abstract Theoretical vibrational spectra of pristine poly(p-phenylene) (PPP) and cis -polyacetylene (PA) are presented, based on a single-periodic-chain model and harmonic potential with parameters transferred from toluene and cis -hexatriene, respectively. The results for a planar PPP model are in good agreement with the observed infrared (ir) and Raman spectra. The only exception is the weak additional low-temperature Raman feature at ∼ 1250 cm−1, which could be explained by alternate phenyl rings in PPP weakly tilted out of plane at low temperature, as is the case in shorter oligomers. For cis-PA , the results for a planar model are also in good agreement with the observed spectra of both Shirakawa cis -PA and also Baker and Bates PA single crystals, in contrast to the calculated ir spectrum for 31 helical PA. Some exceptions are the weak additional Raman feature at ∼ 750 cm−1, and the weak ir feature at ∼ 1250 cm−1, which could be explained by weak nonplanarity of cis-PA, as is the case in shorter oligomers. However, this cannot explain the unusually strongly activated ir absorption C-C-C deformation band at ∼ 450 cm−1.

Vibrational Cluster Calculations of Zig-Zag Carbon Single-Wall Nanotubes

The results of theoretical cluster calculations of positions of the vibrational IR and Raman active modes of zig-zag carbon single-wall nanotubes of small diameter are presented. These results might be significant for fast diagnostics of the structure of synthesized carbon single-wall nanotubes.

Vibrational spectroscopy and comparative study of trans-polyacetylene, poly(p-phenylene) and poly(p-phenylene vinylene)

Abstract Theoretical vibrational spectra of trans -polyacetylene (PA), poly(p-phenylene) (PPP) and poly(p-phenylene vinylene) (PPV) are presented, based on a single-periodic-chain model and a harmonic potential with parameters transferred from trans -hexatriene, toluene, and p-divinyl benzene, respectively. The out-of-plane degrees of freedom and the infrared absorption intensities were included in the calculations. The calculations evince the absence of the in-plane C-H bending infrared absorption band of the trans -vinylene portion of trans -PA and PPV, which is a consequence of a delocalized π-electron system. The electron-phonon coupling in PPP and PPV is relatively weak - in contrast to trans -PA, where the frequencies of the principal Raman lines are significantly lowered. As a consequence, the PPV is an example of pristine polymer with well delocalized π-electron system, but without significant electron-phonon coupling.