Conformational influence on the C=O stretching mode in cholesteryl alkanoates studied by Raman spectroscopy

In this work, we present Raman spectroscopy studies of the C=O stretching m ode for various cholesteryl alkanoates in the polycrystalline, cholesteric, smectic and isotropic liquid phases. For the polycrystalline phase, we can distinguish two behaviors of the C=O stretching mode with regard to the al kyl chain length n. The first concerns the cholesteryl pentanoate, heptanoa te, octanoate, nonanoate(n = 5, 7, 8, 9) and tridecanoate (n = 13) for whic h we observe an unresolved doublet. The second concerns the cholesteryl und ecanoate, dodecanoate, tetradecanoate, octadecanoate and docosanoate (n = 1 1, 12, 14, 18, 22); in these cases, we observe a resolved doublet. For n = 5, 7, 8, 9, the C=O vibrational behavior could be mainly explained by an al kyl chain length effect, whereas for n = 11, 12, 14, 18, 22, the frequency shifts of the C=O stretching mode seem to result principally from the bond angle variations induced by the unit cell packing. The C=O stretching mode behavior can be interpreted as the signature of the constraints on the mole cules. Hy raising the structural constraints, we observe, for the isotropic liquid, smectic and cholesteric phases, similar Raman spectra in the C=O s tretching mode spectral region whatever the cholesteryl alkanoate was; one broad component centered around 1739 cm(-1). Only the half-width of the C=O stretching mode differs for a given cholesteryl alkanoate between the isot ropic liquid phase and the mesophases. Our studies lead us to propose that the C=O stretching mode behavior is a preferential witness of structural or ganization. (C) 1999 Elsevier Science B.V. All rights reserved.