Assignment of the CH stretching vibrational frequencies in the Raman spectra of lipids

A Raman spectroscopic study of several substituted lipids lacking a methyl end group has been carried out in order to assign the vibrational CH stretching modes, all of which arise from the methylene group vibrations. Polarization measurements on the melt and cubic liquid crystalline phase show that the peak in the region of 2885±5 cm −1 is rather weakly polarized in the liquid and liquid crystalline phases, whereas the peak at 2850 cm −1 is rather strongly polarized. Thus it is concluded that the 2850 cm −1 peak arises mainly from the methylene symmetric stretching modes and the 2885±5 cm −1 peak arises mainly from the methylene antisymmetric stretching modes. The band at about 2930 cm −1 is assigned to a symmetric methylene stretching vibration. The implications in the interpretation of chain mobility and chain conformation are discussed.