Vibrational spectra in the CH stretching region and the structure of the polymethylene chain

Abstract The CH stretching regions of both the Raman and i.r. spectra of the extended polymethylene chain have broad secondary maxima. In the case of the Raman spectrum, line shapes are dependent on the environment of the chain, a fact which has been previously exploited in the study of biosystems. We have explained this phenomenon in terms of Fermi resonance interaction between the methylene symmetric CH stretching mode and appropriate binary combinations involving the methylene bending mode. It is emphasized that appropriate binary states are to be found throughout the Brillouin zone and not just at its center. It is the resulting continuum of binary states which leads to broad secondary bands. The shapes of these bands depend on the dispersion of the bending mode fundamental. For the isolated chain only parallel dispersion is involved, but in the case of the crystal perpendicular dispersion is equally important and leads to the observed dependence on crystal structure. All secondary bands have been accounted for in these terms. The ratio of the Raman intensities of the symmetric CH stretching fundamentals to the antisymmetric is found to be about 5 and is independent of environment. The relevance of these results to studies on biosystems is briefly discussed.