INSIGHT INTO POLYACRYLONITRILE MOLECULAR-STRUCTURE FROM CALCULATED VIBRATIONAL FREQUENCIES AND INFRARED INTENSITIES OF MODEL OLIGOMERS

RHF/3-21G vibrational frequencies and intensities are calculated for acrylonitrile dimers (S-2-methylglutaronitrile) and pentamers differing by their conformation and/or configuration to comprehend the influence of these two structural characteristics on the IR spectra of polyacrylonitrile chains. The isotactic pentamer tends to adopt a (TG)3 helix while for the syndiotactic configuration, two conformations of similar total energy are obtained. On the basis of these structures, it is found that the calculated IR spectra follow the reported experimental trends. Of particular interest is the theoretical verification of the experimental observation made by Minagawa et al. that the intensity ratio between bands at 1230 and 1250 cm-1 depends on the stereochemical structure of polyacrylonitrile and provides a measure of their isotacticity percentage.