Analysis of vibrational Raman optical activity signatures of the (TG) and (GG) conformations of isotactic polypropylene chains in terms of localized modes

In a previous study [Lamparska, E.; Liégeois, V.; Quinet, O.; Champagne, B. ChemPhysChem 2006, 7, 2366-2376], signatures associated to the helical structure of a small oligomer of a polypropylene chain were highlighted in the vibrational Raman optical activity (VROA) spectra. Nevertheless, it was difficult to pursue the analysis of longer chains. Indeed, the number of normal modes is becoming large and they are delocalized over the whole chain, increasing the complexity of their analysis. With a new tool developed to analyze the vibrational spectra [Jacob, Ch. R.; Reiher, M. J. Chem. Phys. 2009, 130, 084106], one can understand the normal modes, the VROA intensity of the bands, and the band shapes of long polymer chains by investigating the vibrational coupling matrices and the intensity coupling matrices. The VROA couplet at around 1100 cm (previously evidenced as a signature of the (TG) helical pitch) can now be thoroughly analyzed and compared to the corresponding signature in the (GG) conformer. The mode localization approach shows that for both conformations this couplet arises from a phase difference within the localized modes of both peaks, leading to the inversion of the sign of the total VROA intensity. Comparing the (TG) and (GG) conformers, the vibrational and intensity coupling matrices completely change with the modification of the structure. This leads for the (TG) conformer to a negative-positive couplet, whereas for the (GG) conformation, a characteristic positive-negative-positive pattern is found.