Theoretical analysis of the resonance Raman spectra of diacetylene single crystals

Materny, A.; Kiefer, W.
July 1992
Journal of Chemical Physics;7/15/1992, Vol. 97 Issue 2, p841
Academic Journal
Resonance Raman spectra of polymer chains in the single crystals of the diacetylenes FBS (2,4-hexadiynylene-di-p-fluorobenzene sulfonate), TS/FBS [6-(p-toluenesulfonyloxy)-2,4-p-fluorobenzene sulfonate] and TS6 (2,4-hexadiynylene-di-p-toluene sulfonate) are described theoretically by means of a Franck–Condon model, which considers a chain length dependence. The electronic transition energies and matrix elements are calculated by means of a linear-combination-of-atomic-orbitals–molecular-orbital method calculation in the Hückel approach. We are able to describe the Raman excitation profiles as well as the Raman band profiles for two Raman active modes, i.e., the C+C-, and the C 3/4 C-stretching vibrations of the polymer chains. The model also contains a description of a side group vibrational mode which is enhanced by Fermi resonance with the C=C-stretching vibration. Observed Raman excitation profiles can be well simulated by these calculations. An evaluation of the parameters shows a strong influence of defects, which is described by including inhomogeneous broadening in the theoretical model. An interaction between ensemble and intramolecular properties due to defects can be shown. Simulation of the Raman band profiles yields information on the influence of chain length distribution. Changes of position and Raman band profile as well as of the strength of the Fermi resonance can be explained by the enhancement of modes belonging to polymer chains having different chain lengths. A considerable influence of the substitutents results in remarkable changes of several parameters on which the model calculation is based.


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