CONCERNING THE ASSIGNMENT OF METHYLENE STRETCHING VIBRATIONS IN HYDROCARBONS

Author Institution: Polytechnic Institute of New YorkDespite extensive experimental and theoretical work, uncertainties about the approximate form of normal modes of vibrations of hydrocarbon chains in the CH stretching region persist. The experimental observations are complicated by the possibility of Fermi resonance with the overtone of $CH_{2}$ deformation vibrations. Recent work on the melting of hydrocarbon chains in phospholipids has led to a reexamination of these assignments. While force field calculations appear to show that there is only a very small amount of coupling between C-H stretching vibrations on adjacent carbons, the experimental results, when one includes both infrared and Raman spectra and when one further examines both all trans and trans-gauche segments for the same molecule, lead to a different conclusion. This is particularly true for the asymmetric $CH_{2}$ stretches, which are seen to extend from 2875 to $2930 cm^{-1}$. The conclusions will be substantiated through a series of infrared and Raman difference spectra