- Email: [email protected]
Conformational influence on the CO stretching mode in cholesteryl alkanoates studied by Raman spectroscopy
Vibrational Spectroscopy 21 Ž1999. 27–37 www.elsevier.comrlocatervibspec
Conformational influence on the C5O stretching mode in cholesteryl alkanoates studied by Raman spectroscopy S. Bresson a , D. Bormann a
a,)
, B. Khelifa a , F. Hamza Reguig b, A. Krallafa
b
Laboratoire de Physicochimie des Interfaces et Applications, Faculte´ Jean Perrin, UniÕersite´ d’Artois, SP 18, rue Jean SouÕraz, 62307 Lens Cedex, France b (L.P.C.M.), Institut de Chimie, UniÕersite´ d’Oran Es-Senia, 31100 Es-senia, Algeria Laboratoire de Physico-Chimie et Modelisation ` Received 6 February 1999; received in revised form 26 May 1999; accepted 9 July 1999
Abstract In this work, we present Raman spectroscopy studies of the C5O 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 C5O stretching mode with regard to the alkyl chain length n. The first concerns the cholesteryl pentanoate, heptanoate, octanoate, nonanoate Ž n s 5, 7, 8, 9. and tridecanoate Ž n s 13. for which we observe an unresolved doublet. The second concerns the cholesteryl undecanoate, dodecanoate, tetradecanoate, octadecanoate and docosanoate Ž n s 11, 12, 14, 18, 22.; in these cases, we observe a resolved doublet. For n s 5, 7, 8, 9, the C5O vibrational behavior could be mainly explained by an alkyl chain length effect, whereas for n s 11, 12, 14, 18, 22, the frequency shifts of the C5O stretching mode seem to result principally from the bond angle variations induced by the unit cell packing. The C5O 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 C5O stretching mode spectral region whatever the cholesteryl alkanoate was; one broad component centered around 1739 cmy1. Only the half-width of the C5O stretching mode differs for a given cholesteryl alkanoate between the isotropic liquid phase and the mesophases. Our studies lead us to propose that the C5O stretching mode behavior is a preferential witness of structural organization. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Raman spectroscopy; C5O stretching; Cholesteryl alkanoates
1. Introduction The cholesteryl esters, in addition to the crystalline and isotropic liquid phases, can present various mesophases, in particular, smectic and cholesteric phases w1x. The arousing of the great physical interest ) Corresponding author. Tel.: q33-3-2179-1724; fax: q33321-791717; E-mail: [email protected]
for these molecules is justified by their anisotropic electro-optical properties w2–5x. The cholesteryl alkanoates are composed of a ‘‘knuckle joint’’ constituted by the carbonyl group O 3 –C 28 5O 28 which separates the rigid steroid skeleton Žrings A, B, C, D. and the isopropyl from the flexible alkyl chain ŽFig. 1.. By this simple fact, the carbonyl group will be very sensitive to the slightest modification of conformation. The double bond be-
0924-2031r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 9 . 0 0 0 4 3 - 0
28
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
noates in different phases vs. n, number of carbons in the alkyl chain.
2. Experimental
Fig. 1. The cholesteryl alkanoates molecules. The cholesteryl alkanoates molecules have a ‘‘knuckle joint’’ group: O 3 – C 28 5O 28 . It separates the steroid Žrings A, B, C, D and the isopropyl group. and the alkyl chain beginning at C 28 Ž ns1..
tween C 28 and O 28 should be a preferential witness of the cholesteryl alkanoates structural properties. Previous Raman studies in the spectral range 1550–1800 cmy1 for various cholesteryl alkanoates in the crystalline state have evidenced a undoubling of the C5O stretching mode which is interpreted as the signature of the existence of two conformations of these molecules in the crystalline state w6x. These two conformers, labeled ŽA. and ŽB., differ only by angle and bond length variations. Structural studies of the crystalline structure of cholesteryl acetate, octanoate, nonanoate, dodecanoate and tetradecanoate w7–11x by X-ray diffraction, and of cholesteryl acetate by neutron diffraction w12x, showed that an essential conformational difference between the two types of molecules results in different distortions of the rings A and B in the chair conformation between the molecules ŽA. and the molecules ŽB.. This distortion is strongly reflected by the C 2 –C 3 –O 3 –C 28 torsion angle ŽFig. 1.. This angle, for a given cholesteryl alkanoate, differs significantly between the molecules ŽA. and ŽB. and vary also from one cholesteryl alkanoate to another w8x. Moreover, the previous X-ray studies have shown that the alkyl chain length plays an important role in the crystalline structure organization of these molecules. In order to correlate the vibrational properties of the ‘‘knuckle joint’’ with the structural organization, we focused our attention on the influence of the conformational variations and the alkyl chain length on the C5O stretching mode behavior. For this aim, we performed Micro-Raman spectroscopy investigations on various cholesteryl alka-
The Micro-Raman experiments were performed with a DILOR ŽXY. 800 Raman spectrophotometer and we used an excitation wavelength of 514.5 nm. The experimental conditions and the adjustments of the spectrophotometer have been the same as for our previous work in the C5C and C5O double bond stretching region w6x except for the spectral resolution. Indeed, in order to have a high precision for the frequency values of the C5O stretching mode, the Raman spectra of the studied samples have been obtained with high dispersion. Thus, the spectral resolution was in order of 0.5 cmy1 . In this work, in order to observe the influence of the alkyl chain length, we analyze 10 cholesteryl alkanoates with n increasing from 5 to 22 ŽTable 1.. These products were supplied by the Aldrich– Sigma–Fluka society and were in a crystalline form. For the study in the polycrystalline state, the samples have been used without any peculiar treatment. A Mettler thermosystem FP900 plate coupled with the Raman spectroscopy was used to study the samples in the liquid crystalline and isotropic liquid phases.
3. Results and discussion The Raman spectra of the different samples in the polycrystalline state are shown in Fig. 2 in the C5O Table 1 Characteristics of the studied samples Cholesteryl alkanoates
n
% of purity
Pentanoate Heptanoate Octanoate Nonanoate Undecanoate Dodecanoate Tridecanoate Tetradecanoate Octadecanoate Docosanoate
5 7 8 9 11 12 13 14 18 22
93 96 90 99 99 99 97 99 99 95
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
29
Fig. 2. Raman spectra of the C5O stretching mode for the cholesteryl pentanoate Ž n s 5., heptanoate Ž n s 7., octanoate Ž n s 8., nonanoate Ž n s 9., undecanoate Ž n s 11., dodecanoate Ž n s 12., tridecanoate Ž n s 13., tetradecanoate Ž n s 14., octadecanoate Ž n s 18. and docosanoate Ž n s 22. in the polycrystalline phase. The straight lines correspond to the fitting curves.
stretching spectral range. Considering this mode, we can distinguish two groups of cholesteryl alkanoates. The first one is composed of the cholesteryl unde-
canoate Ž n s 11., cholesteryl dodecanoate Ž n s 12., cholesteryl tridecanoate, cholesteryl tetradecanoate Ž n s 14., cholesteryl octadecanoate Ž n s 18. and
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
30
cholesteryl docosanoate Ž n s 22.. For these cholesteryl alkanoates, we observed a well-resolved doublet for the C5O stretching mode. The pointed and the fitted frequency values are the same and are given in the Table 2; these frequencies include a possible error of "0.5 cmy1 . Moreover, we observe that the undoubled C5O stretching mode frequencies, in the case of the cholesteryl undecanoate and the cholesteryl dodecanoate, respectively Ž n s 11, 12. are lowered by about 10 cmy1 in comparison with the other samples, n s 14, 18, 22 ŽTable 2.. The second group is composed of the cholesteryl pentanoate Ž n s 5., cholesteryl heptanoate Ž n s 7., cholesteryl octanoate Ž n s 8., cholesteryl nonanoate Ž n s 9. and cholesteryl tridecanoate Ž n s 13.. In contrast with n s 11, 12, 14, 18, 22, we observed only a broad peak for n s 5, 7, 8, 9, 13. In the case of an unresolved broad peak, we try to fit our curves with one or more components. As previously explained w6x, these peaks can only be successfully fitted with two components. This result is explained by the fact that whatever is the studied cholesteryl alkanoate, two types of molecules ŽA. and ŽB. coexist in the crystalline phase. The fitted frequency values are labeled U in Table 2. In this table, these frequencies include a possible error of "0.5 cmy1 . For each sample, we observe a bandwidth of about 8–10 cmy1 and we did not observe any variation
which could be correlated with the chain length. Moreover, we have chosen not to analyze the intensities of the peaks in Fig. 2 because the C5O stretching mode is polarized. The polarized Raman experiments will be performed in the immediate future. It is well-known that the supported mass by a carbon atom of a carbonyl group is a factor of the C5O stretching mode frequency shift w13x. For example, the fact to replace a carbon atom attached to a carbonyl group with a hydrogen diminishes the vibration frequency from 17 cmy1 ; nevertheless, this effect is limited w13x. To study the correlation between the O 3 –C 28 5O 28 ‘‘knuckle joint’’ bond angle variations and the C5O stretching mode behavior, we need some X-ray results. X-ray studies of cholesteryl alkanoates in the crystalline state lead Craven et al. to distinguish three types of crystalline structures with regard to the relative steroid skeleton position between the molecules ŽA. and ŽB.: the monolayer-type I, monolayer-type II and bilayer-type structures w8–11x. The reported structures for the various cholesteryl alkanoates are presented in Table 3. Craven et al. showed that the cholesteryl octanoate marks the separation between monolayer-type I and monolayer-type II structures: the cholesteryl octanoate is the only cholesteryl ester in which the steroid skeleton of the molecules ŽA. and ŽB. has the same conformation.
Table 2 Frequency values of the C5O stretching mode for the studied cholesteryl alkanoates in the polycrystalline and isotropic liquid phases. In this table, the frequency values are considered to include a possible error of "0.5 cmy1 when the cholesteryl alkanoates are in the polycrystalline phase and "0.2 cmy1 in the isotropic liquid phase Žfitted frequency values are labeled U in this table. Cholesteryl alkanoates
n
C5O stretching frequencies n Žcmy1 . Polycrystalline
n First component Pentanoate Heptanoate Octanoate Nonanoate Undecanoate Dodecanoate Tridecanoate Tetradecanoate Octadecanoate Docosanoate
5 7 8 9 11 12 13 14 18 22
U
1732.0 1735.5U 1736.5U 1737.0U 1727.5 1727.0 1738.0U 1738.5 1737.0 1738.0
Isotropic liquid
n Second component U
1735.0 1740.5U 1741.0U 1742.0U 1736.0 1735.5 1741.5U 1744.5 1743.5 1744.5
Dn
n
3.0 5.0 4.5 5.0 8.5 8.5 3.5 6.0 6.5 6.5
1738.6 1738.6 1738.8 1738.8 1738.9 1738.9 1739.0 1739.1 1739.2 1739.3
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
31
Table 3 Classification of the cholesteryl alkanoates with regard to the crystalline packaging after Barnard and Lydon w14x and Craven et al. w8–11x Groups of cholesteryl alkanoates according to the crystalline structures Barnard and Lydon w14x First group Second group Third group
Craven et al. w8–11x n s 6, 7, 8 and 9 n s 10 and 12 n s 14, 16 and 18
Monolayer-type II structure Monolayer-type I structure Bilayer-type structure
These authors’ conclusions are directly determined by the crystalline cell parameters Žsee Table 4. and the significant C 2 –C 3 –O 3 –C 28 torsion angle variations. The observed C 2 –C 3 –O 3 –C 28 angle for the studied cholesteryl alkanoates varies between 798 and 1608: acetate ŽA., 1518; acetate ŽB., 1608; octanoate ŽA., 1218; octanoate ŽB., 1218; nonanoate ŽA., 828; nonanoate ŽB., 1378; dodecanoate ŽA., 798; dodecanoate ŽB., 1278; tetradecanoate ŽA., 878; tetradecanoate ŽB., 1028 w9x. The cholesteryl octanoate steroid skeleton has the same conformation for the molecules ŽA. and ŽB., and so the C 2 –C 3 –O 3 –C 28 torsion angle between the octanoate ŽA. and octanoate ŽB. is the same. Other X-ray results are reported by Barnard and Lydon for cholesteryl alkanoates in the crystalline state w14x. Between these two X-ray experiments ŽBarnard and Lydon and Craven et al.., we can notice different cell parameters. This difference comes from the fact that in some cases, Craven et al. w8–13x doubled the number of molecules with regard to Barnard and Lydon w14x. From the various unit cell parameters of these molecules Žsee Tables 3 and 4., these authors propose another classification of the cholesteryl alkanoates according to the packing in the crystal. The peculiar role of the cholesteryl octanoate is not observed. Barnard and Lydon w14x have noticed that for n s 10, 12, 14, 16, 18, the molecules lie along an axis, whereas for n s 6, 7, 8, 9, the molecules lie parallel to the side of the unit cell. The difference between n s 10, 12 and n s 14, 16, 18 comes from the largest unit cell dimension. For n s 10, 12, this dimension is comparable with the length of the molecule whereas for n s 14, 16, 18, the largest unit cell dimension is greater than the length of the molecule. Whatever classification we consider, the transitions between the different types of packing coin-
n s 6, 7 and 8 n s 9, 10, 11 and 12 n G 14
cides always with a variation of the number of molecules per unit cell in which there exists always two unequivalent molecules labeled ŽA. and ŽB. Žsee Table 4.. If we focus our attention on the cholesteryl alkanoates with n s 5, 7, 8, 9, the C5O stretching mode behavior seems to be the same for all these cholesteryl alkanoates; we observe only non-resolved doublet. We do not observe a difference between the Raman spectra of n s 8 and n s 9. However, for n s 8, the C 2 –C 3 –O 3 –C 28 torsion angle does not vary between the molecules ŽA. and ŽB., whereas for n s 9, the variation is significant w8x. This implies that the C 2 –C 3 –O 3 –C 28 torsion, which reflect the O 28 –C 28 –O 3 angle variation, seems not to influence strongly the behavior of the C5O stretching mode. For n s 5, 7, 8, 9, the packing does not induce important bond angle effects. So, we can consider that the bond angle effects seem not to be preponderant for explaining the C5O stretching mode behavior. Considering mass effects, we know that the vibrational frequency is inversely proportional to the square root of the masses for a diatomic oscillator w13x. In order to take the alkyl chain length into account, we have chosen to study the frequency values of the molecules ŽA. and ŽB. for n s 5, 7, 8, 9 according to the inverse of the square root reduced molar mass of the alkyl chain Žsee Fig. 3.. On this figure, we observe a linear behavior for the molecules ŽA. and ŽB. frequency values. This means that for n s 5, 7, 8, 9, the C5O stretching mode behavior can be interpreted as an indirect mass effect induced by the influence of the alkyl chain length on the crystal packing. For n s 11, 12, 14, 18, 22, the doublet of the C5O stretching mode is well-resolved. We can establish a correlation between this behavior and the
32
Cholesteryl alkanoates
n
Space group
a w14x ˚. ŽA
b w14x ˚. ŽA
c w14x ˚. ŽA
b w14x Ž8.
Cell w14x
a w8–11x ˚. ŽA
b w8–11x ˚. ŽA
c w8–11x ˚. ŽA
b w8–11x Ž8.
Cell w8–11x
Pentanoate Heptanoate Octanoate Nonaoate Dodecanoate Tetradecanoate Octadecanoate
5 7 8 9 12 14 18
Orthorhombic P2 1 2 1 2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1
21.45 14.02 13.95 14.44 31.80 50.30 57.50
21.50 9.23 9.20 9.33 8.92 7.50 7.55
6.40 12.54 12.67 12.81 12.92 10.18 10.20
– 92.0 94.0 95.5 93.0 92.5 96.0
4 2 2 2 4 4 4
– – 14.12 13.96 32.02 101.43 –
– – 9.20 9.18 9.01 7.59 –
– – 12.80 27.24 12.99 10.26 –
– – 93.8 91.9 91.4 94.4 –
– – 2 4 4 8 –
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
Table 4 Unit cell parameters and space group of the studied cholesteryl alkanoates in the crystalline state; after Refs. w8–11,14x. ‘‘Cell’’ represents the number of molecules in the unit cell
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
Fig. 3. Evolution of the two frequency values for the C5O stretching mode for the cholesteryl pentanoate, heptanoate, octanoate and nonanoate in the polycrystalline state vs. the square root of the reduced molar mass of the alkyl chain.
variation of the number of molecules per unit cell in the crystalline state. When the unit cell contains two molecules, the C5O stretching mode results in a broad peak Žn s 5, 7, 8, 9., whereas for four or eight molecules per unit cell, this mode is split into two well-resolved components Ž n s 11, 12, 14, 18, 22.. So, it is reasonable to suppose that there exists a strong packing variation between n s 5, 7, 8, 9 and n s 11, 12, 14, 18, 22 and that the organization of the molecules in the unit cell can be responsible for the frequency difference between the two C5O stretching mode components. The more this difference is large, the more the constraints exerted on the double bond C5O should be significant. For n s 11, 12, the difference in frequency between the two components is the most important Ž8.5 cmy1 , see Table 2. and we observe a frequency drop of 10 cmy1 in comparison with n s 14, 18, 22. All these factors reveal important constraints on the double bond C5O. This assumption is in agreement with the previous X-ray classifications. The more the conformational constraints exerted on the molecules increase, the more the bond angle effects are large. The vibra-
33
tional behavior of the double bond C5O in the crystalline phase can be interpreted as the signature of these constraints which are reflected by bond angle variations. For n s 11, 12, 14, 18, 22, the packing seems to induce some important bond angle variations. In the case of n s 13, no X-ray data is available and it is known that it presents a structural instability in the crystalline phase w15,16x. The interpretation of our data is not comparable with other structural studies but with regards to our results, we can propose that the constraints variations on the ‘‘knuckle joint’’ are not very significant between molecules ŽA. and ŽB.. Our hypothesis is that the ‘‘knuckle joint’’ conformation variations are a preferential witness of the constraints exerted on the molecules. Two types of constraints can be taken into account: the intramolecular constraints correlated with the alkyl chain length variations and the intermolecular constraints depending on the interaction between the molecules and resulting in conformational variations. In order to verify that the bond angle effects variations and consequently, the existence of two C5O stretching modes result from the constraints exerted on the molecules, we have studied our samples in the isotropic liquid phase. Indeed, the bond angle effects are due to the packing in the crystalline state; in the first approximation, in the isotropic liquid phase, the molecules should not be subjected to the conformational constraints induced by the packing in the unit cell. So, in the isotropic liquid state, we can raise the constraints on the knuckle joint angles. The transition temperatures of cholesteryl alkanoates have been well-established w15,17–21x. For some cholesteryl alkanoates, the phase transitions are monotropic: we do not observe the same phases on heating than on cooling. For these samples, the mesophases exist only on cooling from the isotropic liquid and do not appear on heating from the solid phase. Moreover, for n ) 8, the cholesteryl alkanoates possess two mesophases: the smectic and cholesteric phases, whereas for n - 9, only the cholesteric phase exists w15,17–21x. The Raman spectra of the various samples in the isotropic liquid phase are shown in Fig. 4. We remark that the behavior of the C5O stretching
34
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
Fig. 4. Raman spectra of the C5O stretching mode for the cholesteryl pentanoate Ž n s 5., heptanoate Ž n s 7., octanoate Ž n s 8., nonanoate Ž n s 9., undecanoate Ž n s 11., dodecanoate Ž n s 12., tridecanoate Ž n s 13., tetradecanoate Ž n s 14., octadecanoate Ž n s 18. and docosanoate Ž n s 22. in the isotropic liquid phase.
mode in the isotropic liquid phase is, as expected, well different from the polycrystalline phase. Whereas in the polycrystalline phase, we have no-
ticed the existence of a splitting Žresolved or not. of the C5O stretching mode corresponding to two types of molecules ŽŽA. and ŽB.., in the isotropic liquid
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
phase, we observe only one component for the C5O stretching mode for each cholesteryl alkanoate near 1739 cmy1 . The pointed frequency value of this mode for each studied sample is given in Table 2,
35
with a possible error of "0.2 cmy1 . The frequency variation of this peak is very weak considering the chain length variations. Thus, we note that the C5O stretching mode is almost insensitive to the alkyl
Fig. 5. Raman spectra of the C5O stretching mode for the cholesteryl dodecanoate Ž n s 12. in the polycrystalline, smectic, cholesteric and isotropic liquid phases.
36
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
chain length when the cholesteryl alkanoates are in the isotropic liquid phase. Whereas, the cholesteryl alkanoates have different attitudes in the polycrystalline phase, these molecules seem to have the same behavior in the isotropic liquid phase. Thus, these spectra confirm that in the polycrystalline phase, the C5O stretching mode behavior is a good tool for studying conformational variations induced by the packing in the unit cell. Knowing that the cholesteryl alkanoates are used essentially in smectic or cholesteric phases, we have obtained the Raman spectra of all studied cholesteryl alkanoates in mesophases. Whatever were the sample and the mesophase, we found the same frequency value as that in the isotropic liquid phase for the C5O stretching mode. No difference was seen between the smectic, cholesteric and isotropic liquid phases for the C5O stretching mode, except for a variation of the half-width of the peak. We present, as an example, the Raman spectra of the cholesteryl dodecanoate Ž n s 12. in the polycrystalline, smectic, cholesteric and isotropic liquid phases in Fig. 5. Between the mode in the mesophases and the mode in the isotropic liquid phase, we found for n s 12 a variation of the half-width of q4 cmy1 . These variations are observed at temperatures close to the transition temperatures and so we can neglect the temperature effect. Thus, we well verify by a Raman study that the peak of the C5O stretching mode is larger in the isotropic liquid phase than in the smectic or cholesteric phase as the disorder increases.
4. Conclusion The comparative study of various cholesteryl alkanoates in different phases, by Raman spectroscopy, has permitted us to better understand the vibrational behavior of the C5O stretching mode. In the polycrystalline phase, the influence of the alkyl chain length on the C5O stretching mode is very significant. We can distinguish two behaviors for the C5O stretching mode. For n s 5, 7, 9, 13, we observe a broad peak, and for n s 11, 12, 14, 18, 22, a well-resolved doublet. For n s 5, 7, 8, 9, the C5O vibrational behavior could be explained mainly by an alkyl chain length effect. For n s 11, 12, 14, 18, 22, the crystal packing influences more directly the C5O
stretching mode: the frequency shifts of this mode seem to be principally due to the bond angle effects. Thus, the C5O stretching mode behavior is the witness of the two types of constraints exerted on the molecules: intramolecular constraints resulting from the alkyl chain length and intermolecular constraints depending on the crystal packing and leading to conformational variations. The study of the various cholesteryl alkanoates in the isotropic liquid phase shows that all the samples have a similar Raman spectrum; one mode near 1739 cmy1 . Except for the half-width of the mode, no difference was observable by Raman spectroscopy for the C5O stretching mode behavior between the isotropic liquid phase and the cholesteric or smectic phases whatever cholesteryl alkanoate we consider. In this work, our results permitted us to propose a suitable tool to characterize the phases and the conformational variations of the cholesteryl alkanoates: the Raman spectroscopy of the C5O stretching mode.
Acknowledgements The Laboratoire L.P.C.I.A. participates in the ‘‘Centre d’Etudes et de Recherches Lasers et Applications ŽCERLA.’’ supported by the ‘‘Ministere ` de l’Education Nationale, de la Recherche et de la Technologie’’, the ‘‘Region Nord-Pas-de-Calais’’ and the ‘‘Fonds Europeen de Developpement ´ Economique des Regions’’.
References w1x P.G. de Gennes, The Physics of Liquid Crystals, Clarendon Press, Oxford, 1974. w2x I.-C. Khoo, S.-T. Wu, Optics and Nonlinear Optics of Liquids Crystals, World Scientifics Publishing, 1993. w3x J.-C. Lee, S.D. Jacobs, J. Appl. Phys. 68 Ž12. Ž1990. 6523. w4x Z. Sekkat et al., Optics Communications 111 Ž1994. 324. w5x I.P. Il’chishin, A.Y. Vakhnin, Liq. Cryst. 265 Ž1995. 687. w6x S. Bresson, D. Bormann, B. Khelifa, Phys. Rev. E 55 Ž1997. 7429. w7x P. Sawzik, B.M. Craven, Acta Crystallogr. B 35 Ž1979. 895. w8x B.M. Craven, N.G. Guerina, Chemistry and Physics of Lipids 24 Ž1979. 157. w9x N.G. Guerina, B.M. Craven, J. Chem. Soc., Perkin Trans. 2 2 Ž1976. 1414. w10x P. Sawzik, B.M. Craven, Acta Crystallogr. B 35 Ž1979. 789.
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37 w11x B.M. Craven, G.T. Detitta, J. Chem. Soc., Perkin Trans. 2 Ž1976. 814. w12x H.-P. Weber, B.M. Craven, P. Sawzik, R.K. McMullan, Acta Crystallogr. B 47 Ž1991. 116. w13x N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, 3rd edn., Academic Press, New York, 1990. w14x J.A.W. Barnard, J.E. Lydon, Molecular Crystals and Liquid Crystals 26 Ž1974. 285. w15x E.M. Barrall, G.J. Davis, R.S. Porter, Molecular Crystals and Liquid Crystals 10 Ž1970. 1.
37
w16x E.M. Barrall, J.F. Johnson, R.S. Porter, The Homologous Series of Aliphatics Esters of Cholesterol. Thermodynamic Properties, Academic Press, New York, 1969. w17x E.M. Barrall, J.F. Johnson, R.S. Porter, The Journal of Physical Chemistry 70 Ž1966. 385. w18x E.M. Barrall, J.F. Johnson, R.S. Porter, The Journal of Physical Chemistry 71 Ž1967. 1224. w19x F.P. Price, J.H. Wendorff, The Journal of Physical Chemistry 75 Ž1971. 2839. w20x G.W. Gray, J. Chem. Soc. 3 Ž1956. 3733. w21x R.D. Ennulat, Mol. Cryst. Liq. Cryst. 8 Ž1969. 247.
Conformational influence on the C5O stretching mode in cholesteryl alkanoates studied by Raman spectroscopy S. Bresson a , D. Bormann a
a,)
, B. Khelifa a , F. Hamza Reguig b, A. Krallafa
b
Laboratoire de Physicochimie des Interfaces et Applications, Faculte´ Jean Perrin, UniÕersite´ d’Artois, SP 18, rue Jean SouÕraz, 62307 Lens Cedex, France b (L.P.C.M.), Institut de Chimie, UniÕersite´ d’Oran Es-Senia, 31100 Es-senia, Algeria Laboratoire de Physico-Chimie et Modelisation ` Received 6 February 1999; received in revised form 26 May 1999; accepted 9 July 1999
Abstract In this work, we present Raman spectroscopy studies of the C5O 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 C5O stretching mode with regard to the alkyl chain length n. The first concerns the cholesteryl pentanoate, heptanoate, octanoate, nonanoate Ž n s 5, 7, 8, 9. and tridecanoate Ž n s 13. for which we observe an unresolved doublet. The second concerns the cholesteryl undecanoate, dodecanoate, tetradecanoate, octadecanoate and docosanoate Ž n s 11, 12, 14, 18, 22.; in these cases, we observe a resolved doublet. For n s 5, 7, 8, 9, the C5O vibrational behavior could be mainly explained by an alkyl chain length effect, whereas for n s 11, 12, 14, 18, 22, the frequency shifts of the C5O stretching mode seem to result principally from the bond angle variations induced by the unit cell packing. The C5O 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 C5O stretching mode spectral region whatever the cholesteryl alkanoate was; one broad component centered around 1739 cmy1. Only the half-width of the C5O stretching mode differs for a given cholesteryl alkanoate between the isotropic liquid phase and the mesophases. Our studies lead us to propose that the C5O stretching mode behavior is a preferential witness of structural organization. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Raman spectroscopy; C5O stretching; Cholesteryl alkanoates
1. Introduction The cholesteryl esters, in addition to the crystalline and isotropic liquid phases, can present various mesophases, in particular, smectic and cholesteric phases w1x. The arousing of the great physical interest ) Corresponding author. Tel.: q33-3-2179-1724; fax: q33321-791717; E-mail: [email protected]
for these molecules is justified by their anisotropic electro-optical properties w2–5x. The cholesteryl alkanoates are composed of a ‘‘knuckle joint’’ constituted by the carbonyl group O 3 –C 28 5O 28 which separates the rigid steroid skeleton Žrings A, B, C, D. and the isopropyl from the flexible alkyl chain ŽFig. 1.. By this simple fact, the carbonyl group will be very sensitive to the slightest modification of conformation. The double bond be-
0924-2031r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 9 . 0 0 0 4 3 - 0
28
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
noates in different phases vs. n, number of carbons in the alkyl chain.
2. Experimental
Fig. 1. The cholesteryl alkanoates molecules. The cholesteryl alkanoates molecules have a ‘‘knuckle joint’’ group: O 3 – C 28 5O 28 . It separates the steroid Žrings A, B, C, D and the isopropyl group. and the alkyl chain beginning at C 28 Ž ns1..
tween C 28 and O 28 should be a preferential witness of the cholesteryl alkanoates structural properties. Previous Raman studies in the spectral range 1550–1800 cmy1 for various cholesteryl alkanoates in the crystalline state have evidenced a undoubling of the C5O stretching mode which is interpreted as the signature of the existence of two conformations of these molecules in the crystalline state w6x. These two conformers, labeled ŽA. and ŽB., differ only by angle and bond length variations. Structural studies of the crystalline structure of cholesteryl acetate, octanoate, nonanoate, dodecanoate and tetradecanoate w7–11x by X-ray diffraction, and of cholesteryl acetate by neutron diffraction w12x, showed that an essential conformational difference between the two types of molecules results in different distortions of the rings A and B in the chair conformation between the molecules ŽA. and the molecules ŽB.. This distortion is strongly reflected by the C 2 –C 3 –O 3 –C 28 torsion angle ŽFig. 1.. This angle, for a given cholesteryl alkanoate, differs significantly between the molecules ŽA. and ŽB. and vary also from one cholesteryl alkanoate to another w8x. Moreover, the previous X-ray studies have shown that the alkyl chain length plays an important role in the crystalline structure organization of these molecules. In order to correlate the vibrational properties of the ‘‘knuckle joint’’ with the structural organization, we focused our attention on the influence of the conformational variations and the alkyl chain length on the C5O stretching mode behavior. For this aim, we performed Micro-Raman spectroscopy investigations on various cholesteryl alka-
The Micro-Raman experiments were performed with a DILOR ŽXY. 800 Raman spectrophotometer and we used an excitation wavelength of 514.5 nm. The experimental conditions and the adjustments of the spectrophotometer have been the same as for our previous work in the C5C and C5O double bond stretching region w6x except for the spectral resolution. Indeed, in order to have a high precision for the frequency values of the C5O stretching mode, the Raman spectra of the studied samples have been obtained with high dispersion. Thus, the spectral resolution was in order of 0.5 cmy1 . In this work, in order to observe the influence of the alkyl chain length, we analyze 10 cholesteryl alkanoates with n increasing from 5 to 22 ŽTable 1.. These products were supplied by the Aldrich– Sigma–Fluka society and were in a crystalline form. For the study in the polycrystalline state, the samples have been used without any peculiar treatment. A Mettler thermosystem FP900 plate coupled with the Raman spectroscopy was used to study the samples in the liquid crystalline and isotropic liquid phases.
3. Results and discussion The Raman spectra of the different samples in the polycrystalline state are shown in Fig. 2 in the C5O Table 1 Characteristics of the studied samples Cholesteryl alkanoates
n
% of purity
Pentanoate Heptanoate Octanoate Nonanoate Undecanoate Dodecanoate Tridecanoate Tetradecanoate Octadecanoate Docosanoate
5 7 8 9 11 12 13 14 18 22
93 96 90 99 99 99 97 99 99 95
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
29
Fig. 2. Raman spectra of the C5O stretching mode for the cholesteryl pentanoate Ž n s 5., heptanoate Ž n s 7., octanoate Ž n s 8., nonanoate Ž n s 9., undecanoate Ž n s 11., dodecanoate Ž n s 12., tridecanoate Ž n s 13., tetradecanoate Ž n s 14., octadecanoate Ž n s 18. and docosanoate Ž n s 22. in the polycrystalline phase. The straight lines correspond to the fitting curves.
stretching spectral range. Considering this mode, we can distinguish two groups of cholesteryl alkanoates. The first one is composed of the cholesteryl unde-
canoate Ž n s 11., cholesteryl dodecanoate Ž n s 12., cholesteryl tridecanoate, cholesteryl tetradecanoate Ž n s 14., cholesteryl octadecanoate Ž n s 18. and
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
30
cholesteryl docosanoate Ž n s 22.. For these cholesteryl alkanoates, we observed a well-resolved doublet for the C5O stretching mode. The pointed and the fitted frequency values are the same and are given in the Table 2; these frequencies include a possible error of "0.5 cmy1 . Moreover, we observe that the undoubled C5O stretching mode frequencies, in the case of the cholesteryl undecanoate and the cholesteryl dodecanoate, respectively Ž n s 11, 12. are lowered by about 10 cmy1 in comparison with the other samples, n s 14, 18, 22 ŽTable 2.. The second group is composed of the cholesteryl pentanoate Ž n s 5., cholesteryl heptanoate Ž n s 7., cholesteryl octanoate Ž n s 8., cholesteryl nonanoate Ž n s 9. and cholesteryl tridecanoate Ž n s 13.. In contrast with n s 11, 12, 14, 18, 22, we observed only a broad peak for n s 5, 7, 8, 9, 13. In the case of an unresolved broad peak, we try to fit our curves with one or more components. As previously explained w6x, these peaks can only be successfully fitted with two components. This result is explained by the fact that whatever is the studied cholesteryl alkanoate, two types of molecules ŽA. and ŽB. coexist in the crystalline phase. The fitted frequency values are labeled U in Table 2. In this table, these frequencies include a possible error of "0.5 cmy1 . For each sample, we observe a bandwidth of about 8–10 cmy1 and we did not observe any variation
which could be correlated with the chain length. Moreover, we have chosen not to analyze the intensities of the peaks in Fig. 2 because the C5O stretching mode is polarized. The polarized Raman experiments will be performed in the immediate future. It is well-known that the supported mass by a carbon atom of a carbonyl group is a factor of the C5O stretching mode frequency shift w13x. For example, the fact to replace a carbon atom attached to a carbonyl group with a hydrogen diminishes the vibration frequency from 17 cmy1 ; nevertheless, this effect is limited w13x. To study the correlation between the O 3 –C 28 5O 28 ‘‘knuckle joint’’ bond angle variations and the C5O stretching mode behavior, we need some X-ray results. X-ray studies of cholesteryl alkanoates in the crystalline state lead Craven et al. to distinguish three types of crystalline structures with regard to the relative steroid skeleton position between the molecules ŽA. and ŽB.: the monolayer-type I, monolayer-type II and bilayer-type structures w8–11x. The reported structures for the various cholesteryl alkanoates are presented in Table 3. Craven et al. showed that the cholesteryl octanoate marks the separation between monolayer-type I and monolayer-type II structures: the cholesteryl octanoate is the only cholesteryl ester in which the steroid skeleton of the molecules ŽA. and ŽB. has the same conformation.
Table 2 Frequency values of the C5O stretching mode for the studied cholesteryl alkanoates in the polycrystalline and isotropic liquid phases. In this table, the frequency values are considered to include a possible error of "0.5 cmy1 when the cholesteryl alkanoates are in the polycrystalline phase and "0.2 cmy1 in the isotropic liquid phase Žfitted frequency values are labeled U in this table. Cholesteryl alkanoates
n
C5O stretching frequencies n Žcmy1 . Polycrystalline
n First component Pentanoate Heptanoate Octanoate Nonanoate Undecanoate Dodecanoate Tridecanoate Tetradecanoate Octadecanoate Docosanoate
5 7 8 9 11 12 13 14 18 22
U
1732.0 1735.5U 1736.5U 1737.0U 1727.5 1727.0 1738.0U 1738.5 1737.0 1738.0
Isotropic liquid
n Second component U
1735.0 1740.5U 1741.0U 1742.0U 1736.0 1735.5 1741.5U 1744.5 1743.5 1744.5
Dn
n
3.0 5.0 4.5 5.0 8.5 8.5 3.5 6.0 6.5 6.5
1738.6 1738.6 1738.8 1738.8 1738.9 1738.9 1739.0 1739.1 1739.2 1739.3
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
31
Table 3 Classification of the cholesteryl alkanoates with regard to the crystalline packaging after Barnard and Lydon w14x and Craven et al. w8–11x Groups of cholesteryl alkanoates according to the crystalline structures Barnard and Lydon w14x First group Second group Third group
Craven et al. w8–11x n s 6, 7, 8 and 9 n s 10 and 12 n s 14, 16 and 18
Monolayer-type II structure Monolayer-type I structure Bilayer-type structure
These authors’ conclusions are directly determined by the crystalline cell parameters Žsee Table 4. and the significant C 2 –C 3 –O 3 –C 28 torsion angle variations. The observed C 2 –C 3 –O 3 –C 28 angle for the studied cholesteryl alkanoates varies between 798 and 1608: acetate ŽA., 1518; acetate ŽB., 1608; octanoate ŽA., 1218; octanoate ŽB., 1218; nonanoate ŽA., 828; nonanoate ŽB., 1378; dodecanoate ŽA., 798; dodecanoate ŽB., 1278; tetradecanoate ŽA., 878; tetradecanoate ŽB., 1028 w9x. The cholesteryl octanoate steroid skeleton has the same conformation for the molecules ŽA. and ŽB., and so the C 2 –C 3 –O 3 –C 28 torsion angle between the octanoate ŽA. and octanoate ŽB. is the same. Other X-ray results are reported by Barnard and Lydon for cholesteryl alkanoates in the crystalline state w14x. Between these two X-ray experiments ŽBarnard and Lydon and Craven et al.., we can notice different cell parameters. This difference comes from the fact that in some cases, Craven et al. w8–13x doubled the number of molecules with regard to Barnard and Lydon w14x. From the various unit cell parameters of these molecules Žsee Tables 3 and 4., these authors propose another classification of the cholesteryl alkanoates according to the packing in the crystal. The peculiar role of the cholesteryl octanoate is not observed. Barnard and Lydon w14x have noticed that for n s 10, 12, 14, 16, 18, the molecules lie along an axis, whereas for n s 6, 7, 8, 9, the molecules lie parallel to the side of the unit cell. The difference between n s 10, 12 and n s 14, 16, 18 comes from the largest unit cell dimension. For n s 10, 12, this dimension is comparable with the length of the molecule whereas for n s 14, 16, 18, the largest unit cell dimension is greater than the length of the molecule. Whatever classification we consider, the transitions between the different types of packing coin-
n s 6, 7 and 8 n s 9, 10, 11 and 12 n G 14
cides always with a variation of the number of molecules per unit cell in which there exists always two unequivalent molecules labeled ŽA. and ŽB. Žsee Table 4.. If we focus our attention on the cholesteryl alkanoates with n s 5, 7, 8, 9, the C5O stretching mode behavior seems to be the same for all these cholesteryl alkanoates; we observe only non-resolved doublet. We do not observe a difference between the Raman spectra of n s 8 and n s 9. However, for n s 8, the C 2 –C 3 –O 3 –C 28 torsion angle does not vary between the molecules ŽA. and ŽB., whereas for n s 9, the variation is significant w8x. This implies that the C 2 –C 3 –O 3 –C 28 torsion, which reflect the O 28 –C 28 –O 3 angle variation, seems not to influence strongly the behavior of the C5O stretching mode. For n s 5, 7, 8, 9, the packing does not induce important bond angle effects. So, we can consider that the bond angle effects seem not to be preponderant for explaining the C5O stretching mode behavior. Considering mass effects, we know that the vibrational frequency is inversely proportional to the square root of the masses for a diatomic oscillator w13x. In order to take the alkyl chain length into account, we have chosen to study the frequency values of the molecules ŽA. and ŽB. for n s 5, 7, 8, 9 according to the inverse of the square root reduced molar mass of the alkyl chain Žsee Fig. 3.. On this figure, we observe a linear behavior for the molecules ŽA. and ŽB. frequency values. This means that for n s 5, 7, 8, 9, the C5O stretching mode behavior can be interpreted as an indirect mass effect induced by the influence of the alkyl chain length on the crystal packing. For n s 11, 12, 14, 18, 22, the doublet of the C5O stretching mode is well-resolved. We can establish a correlation between this behavior and the
32
Cholesteryl alkanoates
n
Space group
a w14x ˚. ŽA
b w14x ˚. ŽA
c w14x ˚. ŽA
b w14x Ž8.
Cell w14x
a w8–11x ˚. ŽA
b w8–11x ˚. ŽA
c w8–11x ˚. ŽA
b w8–11x Ž8.
Cell w8–11x
Pentanoate Heptanoate Octanoate Nonaoate Dodecanoate Tetradecanoate Octadecanoate
5 7 8 9 12 14 18
Orthorhombic P2 1 2 1 2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1 Monoclinic P2 1
21.45 14.02 13.95 14.44 31.80 50.30 57.50
21.50 9.23 9.20 9.33 8.92 7.50 7.55
6.40 12.54 12.67 12.81 12.92 10.18 10.20
– 92.0 94.0 95.5 93.0 92.5 96.0
4 2 2 2 4 4 4
– – 14.12 13.96 32.02 101.43 –
– – 9.20 9.18 9.01 7.59 –
– – 12.80 27.24 12.99 10.26 –
– – 93.8 91.9 91.4 94.4 –
– – 2 4 4 8 –
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
Table 4 Unit cell parameters and space group of the studied cholesteryl alkanoates in the crystalline state; after Refs. w8–11,14x. ‘‘Cell’’ represents the number of molecules in the unit cell
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
Fig. 3. Evolution of the two frequency values for the C5O stretching mode for the cholesteryl pentanoate, heptanoate, octanoate and nonanoate in the polycrystalline state vs. the square root of the reduced molar mass of the alkyl chain.
variation of the number of molecules per unit cell in the crystalline state. When the unit cell contains two molecules, the C5O stretching mode results in a broad peak Žn s 5, 7, 8, 9., whereas for four or eight molecules per unit cell, this mode is split into two well-resolved components Ž n s 11, 12, 14, 18, 22.. So, it is reasonable to suppose that there exists a strong packing variation between n s 5, 7, 8, 9 and n s 11, 12, 14, 18, 22 and that the organization of the molecules in the unit cell can be responsible for the frequency difference between the two C5O stretching mode components. The more this difference is large, the more the constraints exerted on the double bond C5O should be significant. For n s 11, 12, the difference in frequency between the two components is the most important Ž8.5 cmy1 , see Table 2. and we observe a frequency drop of 10 cmy1 in comparison with n s 14, 18, 22. All these factors reveal important constraints on the double bond C5O. This assumption is in agreement with the previous X-ray classifications. The more the conformational constraints exerted on the molecules increase, the more the bond angle effects are large. The vibra-
33
tional behavior of the double bond C5O in the crystalline phase can be interpreted as the signature of these constraints which are reflected by bond angle variations. For n s 11, 12, 14, 18, 22, the packing seems to induce some important bond angle variations. In the case of n s 13, no X-ray data is available and it is known that it presents a structural instability in the crystalline phase w15,16x. The interpretation of our data is not comparable with other structural studies but with regards to our results, we can propose that the constraints variations on the ‘‘knuckle joint’’ are not very significant between molecules ŽA. and ŽB.. Our hypothesis is that the ‘‘knuckle joint’’ conformation variations are a preferential witness of the constraints exerted on the molecules. Two types of constraints can be taken into account: the intramolecular constraints correlated with the alkyl chain length variations and the intermolecular constraints depending on the interaction between the molecules and resulting in conformational variations. In order to verify that the bond angle effects variations and consequently, the existence of two C5O stretching modes result from the constraints exerted on the molecules, we have studied our samples in the isotropic liquid phase. Indeed, the bond angle effects are due to the packing in the crystalline state; in the first approximation, in the isotropic liquid phase, the molecules should not be subjected to the conformational constraints induced by the packing in the unit cell. So, in the isotropic liquid state, we can raise the constraints on the knuckle joint angles. The transition temperatures of cholesteryl alkanoates have been well-established w15,17–21x. For some cholesteryl alkanoates, the phase transitions are monotropic: we do not observe the same phases on heating than on cooling. For these samples, the mesophases exist only on cooling from the isotropic liquid and do not appear on heating from the solid phase. Moreover, for n ) 8, the cholesteryl alkanoates possess two mesophases: the smectic and cholesteric phases, whereas for n - 9, only the cholesteric phase exists w15,17–21x. The Raman spectra of the various samples in the isotropic liquid phase are shown in Fig. 4. We remark that the behavior of the C5O stretching
34
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
Fig. 4. Raman spectra of the C5O stretching mode for the cholesteryl pentanoate Ž n s 5., heptanoate Ž n s 7., octanoate Ž n s 8., nonanoate Ž n s 9., undecanoate Ž n s 11., dodecanoate Ž n s 12., tridecanoate Ž n s 13., tetradecanoate Ž n s 14., octadecanoate Ž n s 18. and docosanoate Ž n s 22. in the isotropic liquid phase.
mode in the isotropic liquid phase is, as expected, well different from the polycrystalline phase. Whereas in the polycrystalline phase, we have no-
ticed the existence of a splitting Žresolved or not. of the C5O stretching mode corresponding to two types of molecules ŽŽA. and ŽB.., in the isotropic liquid
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
phase, we observe only one component for the C5O stretching mode for each cholesteryl alkanoate near 1739 cmy1 . The pointed frequency value of this mode for each studied sample is given in Table 2,
35
with a possible error of "0.2 cmy1 . The frequency variation of this peak is very weak considering the chain length variations. Thus, we note that the C5O stretching mode is almost insensitive to the alkyl
Fig. 5. Raman spectra of the C5O stretching mode for the cholesteryl dodecanoate Ž n s 12. in the polycrystalline, smectic, cholesteric and isotropic liquid phases.
36
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37
chain length when the cholesteryl alkanoates are in the isotropic liquid phase. Whereas, the cholesteryl alkanoates have different attitudes in the polycrystalline phase, these molecules seem to have the same behavior in the isotropic liquid phase. Thus, these spectra confirm that in the polycrystalline phase, the C5O stretching mode behavior is a good tool for studying conformational variations induced by the packing in the unit cell. Knowing that the cholesteryl alkanoates are used essentially in smectic or cholesteric phases, we have obtained the Raman spectra of all studied cholesteryl alkanoates in mesophases. Whatever were the sample and the mesophase, we found the same frequency value as that in the isotropic liquid phase for the C5O stretching mode. No difference was seen between the smectic, cholesteric and isotropic liquid phases for the C5O stretching mode, except for a variation of the half-width of the peak. We present, as an example, the Raman spectra of the cholesteryl dodecanoate Ž n s 12. in the polycrystalline, smectic, cholesteric and isotropic liquid phases in Fig. 5. Between the mode in the mesophases and the mode in the isotropic liquid phase, we found for n s 12 a variation of the half-width of q4 cmy1 . These variations are observed at temperatures close to the transition temperatures and so we can neglect the temperature effect. Thus, we well verify by a Raman study that the peak of the C5O stretching mode is larger in the isotropic liquid phase than in the smectic or cholesteric phase as the disorder increases.
4. Conclusion The comparative study of various cholesteryl alkanoates in different phases, by Raman spectroscopy, has permitted us to better understand the vibrational behavior of the C5O stretching mode. In the polycrystalline phase, the influence of the alkyl chain length on the C5O stretching mode is very significant. We can distinguish two behaviors for the C5O stretching mode. For n s 5, 7, 9, 13, we observe a broad peak, and for n s 11, 12, 14, 18, 22, a well-resolved doublet. For n s 5, 7, 8, 9, the C5O vibrational behavior could be explained mainly by an alkyl chain length effect. For n s 11, 12, 14, 18, 22, the crystal packing influences more directly the C5O
stretching mode: the frequency shifts of this mode seem to be principally due to the bond angle effects. Thus, the C5O stretching mode behavior is the witness of the two types of constraints exerted on the molecules: intramolecular constraints resulting from the alkyl chain length and intermolecular constraints depending on the crystal packing and leading to conformational variations. The study of the various cholesteryl alkanoates in the isotropic liquid phase shows that all the samples have a similar Raman spectrum; one mode near 1739 cmy1 . Except for the half-width of the mode, no difference was observable by Raman spectroscopy for the C5O stretching mode behavior between the isotropic liquid phase and the cholesteric or smectic phases whatever cholesteryl alkanoate we consider. In this work, our results permitted us to propose a suitable tool to characterize the phases and the conformational variations of the cholesteryl alkanoates: the Raman spectroscopy of the C5O stretching mode.
Acknowledgements The Laboratoire L.P.C.I.A. participates in the ‘‘Centre d’Etudes et de Recherches Lasers et Applications ŽCERLA.’’ supported by the ‘‘Ministere ` de l’Education Nationale, de la Recherche et de la Technologie’’, the ‘‘Region Nord-Pas-de-Calais’’ and the ‘‘Fonds Europeen de Developpement ´ Economique des Regions’’.
References w1x P.G. de Gennes, The Physics of Liquid Crystals, Clarendon Press, Oxford, 1974. w2x I.-C. Khoo, S.-T. Wu, Optics and Nonlinear Optics of Liquids Crystals, World Scientifics Publishing, 1993. w3x J.-C. Lee, S.D. Jacobs, J. Appl. Phys. 68 Ž12. Ž1990. 6523. w4x Z. Sekkat et al., Optics Communications 111 Ž1994. 324. w5x I.P. Il’chishin, A.Y. Vakhnin, Liq. Cryst. 265 Ž1995. 687. w6x S. Bresson, D. Bormann, B. Khelifa, Phys. Rev. E 55 Ž1997. 7429. w7x P. Sawzik, B.M. Craven, Acta Crystallogr. B 35 Ž1979. 895. w8x B.M. Craven, N.G. Guerina, Chemistry and Physics of Lipids 24 Ž1979. 157. w9x N.G. Guerina, B.M. Craven, J. Chem. Soc., Perkin Trans. 2 2 Ž1976. 1414. w10x P. Sawzik, B.M. Craven, Acta Crystallogr. B 35 Ž1979. 789.
S. Bresson et al.r Vibrational Spectroscopy 21 (1999) 27–37 w11x B.M. Craven, G.T. Detitta, J. Chem. Soc., Perkin Trans. 2 Ž1976. 814. w12x H.-P. Weber, B.M. Craven, P. Sawzik, R.K. McMullan, Acta Crystallogr. B 47 Ž1991. 116. w13x N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, 3rd edn., Academic Press, New York, 1990. w14x J.A.W. Barnard, J.E. Lydon, Molecular Crystals and Liquid Crystals 26 Ž1974. 285. w15x E.M. Barrall, G.J. Davis, R.S. Porter, Molecular Crystals and Liquid Crystals 10 Ž1970. 1.
37
w16x E.M. Barrall, J.F. Johnson, R.S. Porter, The Homologous Series of Aliphatics Esters of Cholesterol. Thermodynamic Properties, Academic Press, New York, 1969. w17x E.M. Barrall, J.F. Johnson, R.S. Porter, The Journal of Physical Chemistry 70 Ž1966. 385. w18x E.M. Barrall, J.F. Johnson, R.S. Porter, The Journal of Physical Chemistry 71 Ž1967. 1224. w19x F.P. Price, J.H. Wendorff, The Journal of Physical Chemistry 75 Ž1971. 2839. w20x G.W. Gray, J. Chem. Soc. 3 Ž1956. 3733. w21x R.D. Ennulat, Mol. Cryst. Liq. Cryst. 8 Ž1969. 247.