Ab Initio Study of Vibrational Anharmonic Coupling Effects in Oligo( para -phenylenes)
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Ab Initio Study of Vibrational Anharmonic Coupling Effects in Oligo(para-phenylenes) G. Heimel,a) D. Somitsch,b) P. Knoll,b) and E. Zojera) a) Institute of Solid State Physics, Graz University of Technology, Graz, Austria b) Institute of Experimental Physics, University of Graz, Graz, Austria ABSTRACT In this study we present a theoretical approach to simulate vibrational anharmonic coupling effects seen in the Raman spectra of oligo(para-phenylenes). Quantum chemical ab inito methods are applied to determine anharmonic force constants and energy corrections on the harmonic vibrational frequencies of the isolated molecules. Semiempirical methods are applied to compute Raman intensities of fundamentals and combination bands. This methodology is then used to characterize a previously unassigned Fermi resonance around 1600 cm-1. The evolution of this quantum mechanical resonance with oligomer length and planarity is compared to experimental data. INTRODUCTION The vibrational spectra of poly(para-phenylene) (PPP), its oligomers and its planarized derivatives have been subject to numerous theoretical [1,2] and experimental [3] investigations. Particular interest was taken in the Raman and resonant Raman spectra of pristine [4,5] and doped [6,7,8] PPP, since they yield information about the mechanism of electron-phonon coupling in the material. There are three predominant features in the Raman spectrum of the polymer, at 1220 cm-1, at 1280 cm-1 and around 1600 cm-1. They have been assigned to C-H in plane bend, C-C inter-ring stretch and aromatic C-C on ring stretch modes, respectively. The 1600 cm-1 Ag mode is of major importance for electron-phonon coupling in PPP. [1,4] As we will show in the present work, this mode also yields important information about the structural conformation of phenylene based molecules. If we take a closer look at the 1600 cm-1 band, we note that, while there is only one single peak in the polymer [3,5,6,7], a clear double peak structure can be resolved in all investigated oligomers that persists in solution, melt and gas phase. Ohtsuka et al. [9] mention that it might be best to regard the two bands in oligo(para-phenylenes) (OPP) as a Fermi resonance between a strong fundamental at 1600 cm-1 and some unidentified combination/overtone band. METHODOLOGY Experimental Benzene, biphenyl (2P) and p-quaterphenyl (P4P) have been purchased from Fluka, pterphenyl (P3P) from Aldrich and p-sexiphenyl (P6P) from Tokyo Kasei Kogyo Co. LTD. All chemicals were of >99% purity and used without further processing. Infrared (IR) absorption spectra of 2P, P4P and P6P have been recorded in KBr pellets with a Perkin Elmer Spectrum One FT-IR spectrometer at a resolution of 0.5 cm-1. Raman spectra were recorded for polycrystalline powder (or in the case of 2P and P3P also in CH2Cl2 solution) with a Dilor OMARS 89 triple-spectrometer, using the 647 nm line of a Krypton Ion Laser (Coherent Innova BB3.10.1
90) at ∼100 mW in a standard backscattering configuration. Incident Laser light was collimated to a ∼2×0.05 mm l
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