Elsevier

Chemical Physics

Volume 171, Issues 1–2, 1 April 1993, Pages 133-143
Chemical Physics

Dependence of the vibrational frequencies and intensities on the configuration of polyacrylonitrile: an ab initio study on model oligomers

https://doi.org/10.1016/0301-0104(93)85137-WGet rights and content

Abstract

Using the oligomer approach, RHF calculations are carried out to investigate the influence of configurational effects on the IR spectra of polyacrylonitrile (PAN). In a first step, test calculations are performed on propionitrile using the STO-3G, 3-21G, 6-31G** and 6-31+G* bases, in order to assess the reliability of calculated vibrational frequencies and IR intensities. Then, isotactic and syndiotactic acrylonitrile oligomers (up to hexamers) are investigated at the RHF/3-21G level, in the all-trans conformation. In line with previous experimental work, we find that the frequencies are not significantly affected by the chain configuration, therefore we focus on the intensities. The pattern in the 1400-1200 cm−1 region, corresponding to deformations of the methylene and methine groups, appear as a source of information on the configuration of the chain. However, it is in fair agreement with the experimental spectra only for the syndiotactic case.

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      With respect to the –CN group, theoretical studies of the valence electronic levels of acrylonitrile oligomers are limited to relatively short polymeric chains. Model isotactic and syndiotactic chains with 2,4 and 6 acrylonitrile units were investigated, particularly emphasizing the dependence of πzCN levels on increasing unit count and polymer tacticity [39,53]. The pz orbitals contributing to those levels are parallel to the carbon backbone of the chain and the pz orbitals of two neighboring –CN groups interact strongly in the case of isotactic oligomers, leading to a large energy level splitting between the two πzCN levels [39].

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