CONTRIBUTION OF THE INTERMOLECULAR COUPLING AND LIBROTORSIONAL MOBILITY IN THE METHYLENE STRETCHING MODES IN THE INFRARED-SPECTRA OF ACYL CHAINS

Methylene stretching modes are often used to probe the order of polymethylene chains in condensed material including lipids and polymers. The shifts of the bands related to these vibrations are generally interpreted in terms of conformational order. In this paper, we show that considerable shifts of the methylene stretching bands (for both hydrogenated and deuterated species) can be induced without variation of the conformational order. First, the results indicate that isotopic dilution affects the frequencies of the C-H stretching bands while the C-D stretching bands are less sensitive. This effect is more pronounced in systems for which interchain interactions are important such as lipids in the ordered gel phase. The origin of these shifts is attributed to a change in Fermi resonance caused by the variation in intermolecular coupling. Second, librotorsional motions also affect the methylene stretching modes. Shifts of about 2.5 cm(-1) are observed for the C-H stretching modes when hexadecane trapped into urea clathrate is warmed from -150 to 25 degrees C. Using deuterated hexadecane, a comparable change is observed for the antisymmetric C-D stretching mode whereas the frequency of the symmetric mode is temperature insensitive. The coupling between the stretching modes and the librotorsional modes of the carbon skeleton is proposed to be at the origin of these shifts. Bandwidths of the methylene stretchings appear to be essentially sensitive to intramolecular factors. Finally, as a side aspect, it is found that the method for conformational analysis based on the CH2 wagging bands is not sensitive to intermolecular coupling, confirming the reliability of this method. In conclusion, a warning about the interpretation of shifts of the methylene stretching bands exclusively in terms of conformational order is stated.