Molecular flexibility of polymethylene molecules: a Raman spectroscopic study

The Raman spectrum in the CH stretching region of molecules containing long alkyl residues shows characteristic features which provide direct information on their molecular flexibility, as probed by the C-H stretching oscillator which vibrates on the picosecond time scale. From their Raman spectra, librational motions of the (CH2 )n units can be studied for molecules in the solid and liquid phases. In the latter case, segmental motions must be considered. In this paper, the spectra of hydrocarbons in the solid, liquid and as clathrates in urea and perhydrotriphenylene are studied in terms of their overall mobility. Using molecular dynamical calculations we show the existence of a selective coupling between CH stretchings and skeletal torsions, which is modulated by the collective mobility of the carbon skeleton. In particular, we account for the frequency dependence of the antisymmetic CH stretching mode using a model which allows for selective coupling between this high frequency mode and the low frequency torsional oscillations about the C-C bonds in the chain.