Conformational influence on the CO stretching mode in cholesteryl alkanoates studied by Raman spectroscopy
In this work, we present Raman spectroscopy studies of the CO stretching mode for various cholesteryl alkanoates in the polycrystalline, cholesteric, smectic and isotropic liquid phases. For the polycrystalline phase, we can distinguish two behaviors of the CO stretching mode with regard to the al...
Saved in:
Published in | Vibrational spectroscopy Vol. 21; no. 1; pp. 27 - 37 |
---|---|
Main Authors | , , , , |
Format | Journal Article |
Language | English |
Published |
Elsevier B.V
01.12.1999
|
Subjects | |
Online Access | Get full text |
Cover
Loading…
Summary: | In this work, we present Raman spectroscopy studies of the CO stretching mode for various cholesteryl alkanoates in the polycrystalline, cholesteric, smectic and isotropic liquid phases. For the polycrystalline phase, we can distinguish two behaviors of the CO stretching mode with regard to the alkyl chain length
n. The first concerns the cholesteryl pentanoate, heptanoate, octanoate, nonanoate (
n=5, 7, 8, 9) and tridecanoate (
n=13) for which we observe an unresolved doublet. The second concerns the cholesteryl undecanoate, dodecanoate, tetradecanoate, octadecanoate and docosanoate (
n=11, 12, 14, 18, 22); in these cases, we observe a resolved doublet. For
n=5, 7, 8, 9, the CO vibrational behavior could be mainly explained by an alkyl chain length effect, whereas for
n=11, 12, 14, 18, 22, the frequency shifts of the CO stretching mode seem to result principally from the bond angle variations induced by the unit cell packing. The CO stretching mode behavior can be interpreted as the signature of the constraints on the molecules. By raising the structural constraints, we observe, for the isotropic liquid, smectic and cholesteric phases, similar Raman spectra in the CO stretching mode spectral region whatever the cholesteryl alkanoate was; one broad component centered around 1739 cm
−1. Only the half-width of the CO stretching mode differs for a given cholesteryl alkanoate between the isotropic liquid phase and the mesophases. Our studies lead us to propose that the CO stretching mode behavior is a preferential witness of structural organization. |
---|---|
ISSN: | 0924-2031 1873-3697 |
DOI: | 10.1016/S0924-2031(99)00043-0 |