J. Phys. Chem. 1988, 92, 5639-5642 Bifurcated H-bonding structures are often predicted by extended basis set a b initio calculation^,'^ even in cases where small basis sets calculations predict linear H bonds.lb Experimental studies of the rotation of ammonium ion in aqueous solution are in accord with bifurcated H bonds between water and ammonium iomZ0 Table V displays the total charge densities on the water molecules in each hydration cluster. The accumulation of positive charge on the water molecules increases less with the addition of each new water molecule, as would be expected. The prediction of some charge transfer seems in accord with a recent orbital analysis of t4e water dimerz1 and earlier suggestions by Klemperer?2 It is also invoked as an explanation of the failure of small basis set a b initio calculations to predict bifurcated structures.lb (1 9) See, for example: (a) Kistenmacher, H.; Popkie, H.; Clementi, E. J. Chem. Phys. 1973,58,5627. (b) Kollman, P. J . Am. Chem. SOC.1977,99, 4875. (20) Perrin, C. L.; Gipe, R. K. J . Am. Chem. Soc. 1986,108,1088; Science (Washington, D.C.) 1987, 238, 1393. (21) Reed. A. E.: Weinhold. F. J. Chem. Phvs. 1983. 78. 4066. (22j Harris, S.J:; Janda, K.'C.; Novick, S.6.;Klembrer, W. J . Chem. Phys. 1975, 63, 5285.
5639
Conclusion The results of the present AM1 calculations seem in good agreement with the experimental results of Mautner. They are also consistent with the results of large basis set calculations on smaller systems. From the present study, it would seem that the AM1 molecular orbital method might be an extremely effective and economical means of modeling systems containing hydrogen bonds. The present calculations also suggest that hydration of protonated diamines occurs primarily at the protonated nitrogen (at least when no more than four waters are considered). The initial interactions involve bifurcated H bonds between each water and two protons. After the third water is added, some solvent structuring begins to take place. The ammonium hydrogens eventually each have hydrogen bonds to two different waters, as well.
Acknowledgment. This work was supported, in part, by a PSC-BHE grant from the Research Foundation of the City University of New York. We thank Dr. Michael Mautner for several fruitful conversations. Registry No. I, 1 15960-32-2; 11, 115942-66-0; 111, 1 15942-67- 1.
Intramolecular Hydrogen Bonding In a Monoglyceride Lipid Studied by Fourier Transform Infrared Spectroscopy A. Holmgren,* G. Lindblom, and L. B.-A Johansson Department of Physical Chemistry, University of Umei. S-901 87 Umei, Sweden (Received: December 3, 1987; In Final Form: April 5, 1988)
The infrared spectra of the carbonyl and hydroxyl stretching modes of 1-monooctanoinwere examined. The C=O stretching mode of 1-monooctanoin shows two absorption bands in solvents and in a lamellar liquid crystalline phase formed with water. It is concluded that this splitting is due to intramolecular interactions. The IR spectra of the C=O and 0-H stretching modes of 1-monooctanoinin chloroform and acetonitrile and at various temperatures have been examined. Taken together, these data strongly suggest that the intramolecular interaction is a hydrogen bonding between the sn-3 hydroxyl and the carbonyl group of 1-monmtanoin. The similarities in the IR spectra of 1-monmtanoin and I-monoolein in the 1750-1700-cm-' range support the same explanation for the behavior of the C=O stretch in the latter lipid molecule.
Introduction In recent years FT-IR spectroscopy has been shown to be a useful technique for studies of physicochemical properties of surfactants in solution,14 model membranes,%* and the lipids in biological membra ne^.^^'^ In particular we are interested in using FT-IR for studies of linear dichroism of macroscopically aligned (1) Umemura, J.; Cameron, D. G.; Mantsch, H. H. J. Chem. Phys. 1980, 84, 2212. (2) Umemura, J.; Mantsch, H. H.; Cameron, D. G. J. Colloid Interface Sci. 1981, 83, 558. (3) Kawai, T.; Umemura, J.; Takenaka, T. Colloid Polym. Sci. 1984,262,
61. (4) Holmgren, A.; Fontell, K.; Lindblom, G. Acta Chem. Scand. 1986,40, 299. (5) Holmgren, A,; Johanuon, L. B.-A.; Lindblom, G. J. Phys. Chem. 1987, 91, 5298. (6) Mantsch, H. H.; Cameron, D. G.; Umemura, J.; Casal, H. L. J. Mol. Struct. 1980, 60, 263. (7) Mantsch, H. H.; Martin, A.; Cameron, D. G. Biochemistry 1981,20, 3138. (8) Mendelsohn, R.; Dluhy, R. A.; Crawford, T.; Mantsch, H. H. Biochemistry 1984, 23, 1498. (9) Mendelsohn, R.; Anderle, G.; Jaworsky, M.; Mantsch, H. H.; Dluhy, R. A. Biochim. Biophys. Acta 1984, 775,215. (IO) C a d , H. L.; Cameron, D. G.; Jarell, H. C.; Smith, I. C. P.; Mantsch,, H. H. Chem. Phys. Lipids 1982, 30, 17.
0022-3654/88/2092-5639$01 .50/0
lamellar liquid crystalline phases, in order to get information about the system (e.g., hydrogen bonding in membrane surfaces) that is not easily obtained by other spectroscopical methods. During the development of the FT-IR linear dichroism spectroscopy,s we found that lamellar phases of monoglycerides (monooctanoin and monoolein) gave rise to two absorption bands in the region of the carbonyl stretching vibrations. Previously it has been that the carbonyl stretching region (1700-1750 cm-') for dipalmitoylphosphatidylcholine (DPPC) is similarly comprised of at least two absorption bands having their maxima at 1721 and 1739 cm-I. These two bands were assigned to the carbonyl groups of the two different acyl chains of the lecithin molecule. It was suggested that the carbonyl group of the acyl chain sn-2 is located more closely to the polar head group region while the carbonyl group of the sn-1 acyl chain has a more hydrophobic environment. Lysophosphatidylcholine, having only one acyl chain, on the other hand, showed only one band in this spectral region. Therefore, the phosphatidylcholine investigations strongly indicate that the lamellar phase structure is not causing the two bands observed (11) Mushayakarara, E.; Levin, I. W. J . Phys. Chem. 1982, 86, 2324. (12) Levin, I. W.; Mushayakarara, E.; Biffman, R. J . Raman Spectrosc. 1982, 13, 23. (13) OLeary, T. J.; Levin, I. W. J. Phys. Chem. 1984, 88, 1790.
0 1988 American Chemical Society
5640 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
Holmgren et al.
DI Y
c m
n L
Y
0
rn c n
v)
n
L
a
0
ul
n
a
1800
1750 1700 W avenumber s / cm-'
Figure 1. Infrared spectra of the carbonyl stretching mode region of monooctanoin in CCI,,isotropic solution (a) 0.02 M and H20, lamellar liquid crystalline phase (b). For (c) and (d) the concentrations in CCI, were 0.01 and 0.005 M, respectively. The peak height of trace b is scaled.
at 1724 and 1738 cm-' for the monoglycerides, having just one acyl chain. The aim of the present study is to try to get an understanding of the appearance of these two absorption bands in the carbonyl region of the IR spectrum for monooctanoin.
Experimental Section 1-Monooctanoin was purchased from Syntestjanst, Lund, Sweden, and used without further purification. Water for the preparation of the lamellar phase was distilled in an all-quartz apparatus. The solvents carbon tetrachloride, chloroform, and acetonitrile were of pro analysi quality. Solutions were prepared by weighing, and the concentration of monooctanion was kept below 0.01 M to eliminate the effect of dimerization. The reference cell contained the pure solvent or solvent mixture used for the solutions. For the study of the temperature dependence of the absorption bands, a temperature control unit and a variable temperature cell from Beckman Instruments were used. Spectra were recorded by using a Bruker IFS 113 V spectrometer equipped with a DTGS detector. One thousand scans were combined, and the resultant interferogram was Fourier transformed to obtain a resolution of 1 cm-I over the spectral range. Results and Discussion Comparisons between Spectra from an Isotropic Solution and a Lamellar Phase. To investigate whether the two absorption bands were dependent on the structure of the lamellar liquid crystalline phase, spectra were recorded for monooctanoin at low concentration (-0.01 M) in CCl, and for a lamellar phase sample at the composition 70% monooctanoin and 30% HzO. Figure 1 shows that the spectral features of monooctanoin are very similar in an isotropic solution and in an anisotropic lamellar liquid crystal. The solvent CC14, however, induces a shift in the peak frequencies from 1738 and 1724 cm-' (lamellar phase) to 1744 and 1727 cm-I, respectively. Note that the high-frequency peaks are still very close, indicating a low-permittivity hydrocarbon environment for the carbonyl located in the bilayers of the monooctanoin-water system. Thus, from the experimental findings of Figure 1 it can be concluded that the double band profile is not induced by the aggregation of monooctanoin molecules in the lamellar phase but is caused by the physicochemical properties of the monooctanoin molecules themselves. A possible explanation would then be that the carbonyl group is subjected to two different environments. However, the molecular ordering in the bilayer of the carbonyl group has to be about the same, since the dichroic ratio over the C=O stretching band (1700-1750 cm-') is constant. It can therefore be assumed that the best candidate that can give rise to the observed behavior is the formation of hydrogen bonds between the OH- and CO-groups of the monooctanoin molecule. These hydrogen bonds can be either inter- or intramolecular. When they are formed, the frequency of the stretching vibration for the electron donating group (e.g., C=O) decreases as well as the OH stretching frequency. Intra- and Intermolecular Interactions. It is well documented that absorption bands caused by hydrogen bonding are broader
3700
3500
3300
3100
Wavenurnbers/cm-1
Figure 2. Infrared spectra of propanediol (a), monooctanoin (b), and ethylene glycol (c) in the region of the hydroxyl stretching mode. For traces a, b, and c the concentrations in CCI4were 0.02,0.01, and 0.01 M, respectively.
and more intense than the bands of nonbonded g r o ~ p s . ' ~ JOf ~ course, the same groups that form intermolecular H-bonds may form intramolecular hydrogen bonds if the spatial configuration is favorable. For intramolecular hydrogen bonds this very often means that the H-bond is bent rather than linear. The bent configuration may be the reason why the intensity and bandwidth enhancement are usually smaller than those for intermolecular H-bonds. l 4 Change in Monooctanoin Concentration. To descriminate between intra- and intermolecular interactions, we investigate how the spectral behavior varies with the concentration of monooctanoin in CCl,. It is well-known that the effects of concentration on the intraor intermolecular interactions are significantly differer~t.'~J~ Since the intramolecular interaction is an internal effect, it persists even at the lowest concentrations and therefore retains its spectral behavior over the whole region of concentration. This is in contrast to the absorption bands resulting from the intermolecular interaction, which disappear at low concentrations. The infrared spectra in Figure 1, therefore, strongly indicate that the observed band profile originates from intramolecular interactions. Even at as low concentrations as 0.0015 M, the dominant spectral feature is retained. Figure 2 shows the spectral range for the hydroxyl vibrations viz., 3700-3300 cm-'. The absorption band profile for this range is rather complex, indicating many different conformations of the hydroxyl moieties. At 25 OC, the peak values of the most dominant spectral features in this region are found at 3622, 3601, and 3518 cm-'. Studies of intra- and intermolecular hydrogen bonds in 1 , 2 - d i o l ~ ' ~provide ,'~ information about the origin of the bands at 3622 and 3601 cm-I. The peak frequencies of the OH-band of ethylene glycol at 3644 and 3612 cm-l are assigned to free and intramolecularly hydrogen bonded OH-groups, respectively. A reexamination of this compound (0.01 M in CC14) yielded the values of 3644 and 361 1 cm-I in excellent agreement with the previous investigation. The lower frequency band is broader, as expected from an OH-group forming an internal hydrogen bond. It should be mentioned that the band due to intermolecular bonded OH, at still lower frequency, does not appear in our ethylene glycol spectrum. On the bases of these findings we assign the 3622 and 3601 cm-' peaks of monooctanoin to originate from the free OH-groups and the hydroxyl groups that form a H-bond to the oxygen atom of the free OH-groups. The absorption feature at 3518 cm-' has (14) Pimentell, G . C.; McClellan, A. L. The Hydrogen Bond; W. H . Reeman: London, 1960; Chapter 5 . (15) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding, Marcel Dekker: New York, 1974; Chapter 5 . (16) Kuhn, L. P. J . Am. Chem. SOC.1952, 74, 2492. (17) Fishman, E.; Chen, F. L. Spectrochim. Acta, Part A 1%9,25A, 1231.
The Journal of Physical Chemistry, Val. 92, NO. 20, 1988 5641
Hydrogen Bonding in a Monoglyceride Lipid
I
N
ix
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2:
0) Y
c rn n L 0
m
D
4
1800
3600
3500
3400
Wave numb er s/cm
3 300
-I
1750
1700
W a v e n u m b er s / c m - I Figure 4. Infrared spectra showing the effect of CHCl, (c) and CH3CN (b) on the C=O band feature of 0.01 M monooctanoin in CC14 (a).
Figure 3. Temperature dependence for the carbonyl stretching mode (a-c) and for the OH-stretching mode (d-f) of 0.01 M monooctanoin in CC14. The temperature of the sample was 25 (a,d), 40 (b,e) and 65 'C (CA.
a shoulder at 3485 cm-'. These two absorption bands must also originate from an intramolecular effect since they remain at very low concentration of monooctanoin (0.0015 M). Probably these bands are caused by intramolecular hydrogen bonding to the carbonyl group. At 0.02 M, the intensity maximum is shifted to 3480 cm-', influenced by intermolecular hydrogen bonding, while a 0.2 M solution leads to a broad band with intensity maximum at 3425 cm-' (not shown in Figure 2). The effect of intermolecular hydrogen bonding is demonstrated by the spectrum of propanediol. At a concentration of 0.01 M the spectrum does not reveal any intermolecular H-bonding (not shown in Figure 2). However, at 0.02 M a broad band appears at 3435 cm-', which is assigned to intermolecular hydrogen bonding (Figure 2). Change in Temperature. So far, we have shown that intramolecular interactions are the origin of the 17272cm-' band in monooctanoin and that intramolecular H-bonds are present. However, it still remains to show that the C=O vibration band at 1727 cm-' is due to intramolecular hydrogen bonding. This may be conveniently studied by varying the temperature of the sample, since hydrogen bonds are very sensitive to temperature variations. A temperature rise of 10-20 OC is sufficient to cause an appreciable shift of most equilibria of hydrogen bonding in favor of nonbonded species. Although the effect of temperature is essentially the same for inter- and intramolecular H-bonds, a temperature study of monooctanoin (at low concentration) would provide evidence for a connection between the behavior of the intramolecular H-bond and the assumed H-bonded C=O stretching feature a t 1727 cm-I. Spectra in Figure 3 show the C = O absorption band of monooctanoin at a concentration of 0.01 M in CC14 and a t the different temperatures of 25, 40, and 65 OC. In addition to a small temperature-induced frequency shift, a marked decrease of the absorption intensity at 1727 cm-' occurs. This is the expected result if the band at 1727 cm-l is caused by hydrogen bonding between the carbonyl and hydroxyl moieties, since an increase in temperature would lead to the disruption of hydrogen bonds. From the band profile it is also evident that the C = O stretching band at low frequency is somewhat broader than the high-frequency band. The broadening of the low-frequency band is also an indication of hydrogen bonding. The two most apparent changes in the 3700-330O-cm-' region on heating the sample (0.01 M monooctanoin in CCI,) from 25 to 65 OC are the increase of the intensity of free hydroxyl groups (3622 cm-') relative to hydrogen-bonded groups (3601 cm-') and the decrease of the intensity of the hydrogen-bonding feature at 3518 cm-' (Figure 3). The latter band is also shifted 16 cm-' to higher frequency caused by this temperature increase. On a comparison of the behavior of monooctanoin in the two spectral ranges 3700-3300 and 1750-1700 cm-', it is apparent that the bands at 1727 and 3518 cm-' change with temperature in about the same way. This may be taken as a clear indication of the formation
2700
2600
2500
Wavenumbers/cm-'
Figure 5. Infrared spectra of the OD-stretching vibration of 0.01 M monooctanoin in pure CC14 (a) and 9:l CCl,-CHCl, (b) solvents. Spectra for the C=O stretching region, pure CCI4 (c) and 9:l CC14CHCl, (d), are included.
of an internal H-bond between one of the hydroxyl groups and the ester carbonyl group. Change in Solvent. Another way to gain information about the property of the observed intramolecular interaction is to replace CC4by solvents able to "dissolve" hydrogen bonds. Chloroform is known as a solvent that loosens hydrogen bonds by forming H-bands itself. By this choice of solvent the intramolecularly hydrogen bonded carbonyl groups would be solvated by chloroform and therefore give rise to a single absorption band. On the other hand, acetonitrile is capable of forming hydrogen bonds with the hydroxyl groups, thereby reducing the effect of hydrogen bonding to the carbonyl moiety. The influence of these two solvents on the C=O stretching mode is shown in Figure 4. As expected, only one absorption band is observed for both the solvents CHC1, and CH3CN. The half-width values are 34 and 19 cm-' for the C=O stretching mode in chloroform and acetonitrile, respectively. The larger half-width value in CHC1, as compared to CH3CN may be explained by the vibrational dephasing Thus, CHC1, would give a broader distribution of vibrational energy levels caused by a stronger perturbation of the carbonyl stretching vibration. This may be taken as evidence for a H-bond between CHCI, and the C=O group. In order to compare the effect of chloroform on C=O and OH stretching modes, OD had to be substituted for OH in monooctanoin, since CHCl, is a rather strong infrared absorber in the 3700-3500-cm-' range. However, in the region of the OD stretching mode, viz., 2700-2500 cm-', the infrared measurements are not affected by chloroform. The sample contained monooctanoin dissolved in a mixture of CCl, and CHC1, in the volume (18) Rothschild, W. G . Dynamics of Molecular Liquids; Wiley: New York, 1984;pp 53-55. (19) Oxtoby, D.W. Annu. Rev. Phys. Chem. 1981, 32, 77.
J . Phys. Chem. 1988, 92, 5642-5648
5642
ratio of 9 to 1. The spectra in Figure 5 are recorded for monooctanoin in pure CC14 and in the solvent mixture. For monooctanoin in C C 4 , we recognize the same spectral feature for the OD-stretching region (Figure 5 ) as for the OH-stretching mode (Figure 2). The major OD absorption bands are found at 2675, 2658, and 2607 cm-I. However, the frequency shifts between hydrogen-bonded (2658, 2607 cm-’) and “free” (2675 cm-I) OD-stretching vibrations are smaller than the corresponding shift for OH-stretching vibrations. A smaller frequency shift is often correlated with a weaker hydrogen bond. However, this is not reflected in the frequency distance between the two ester carbonyl groups in monooctanoin. If the solvent with 10 vol % chloroform is used, we observe a dramatic change of the spectral region in Figure 5 . The feature of the OD-stretching mode is altered from a complex spectral pattern to a single OD-peak at 2679 cm-’ with some residual hydrogen-bonding characteristics at ~ 2 6 6 and 0 2600 cm-I. The marked change of the 2600-cm-’ band is reflected in the spectral behavior of the C=O band at 1727 cm-I. At 10% CHC13 this band appears as a shoulder on the absorption band of the unbonded carbonyl group, approaching the band shape of the C=O moiety in pure CHC13, shown in Figure 4. The simultaneous alteration of these two spectral regions upon addition of CHC13 to CC14 clearly indicates that the double band feature of the ester carbonyl vibration is due to intramolecular hydrogen bonding.
Conclusions A consideration of the infrared spectra of the carbonyl and hydroxyl stretching mode regions of monooctanoin strongly indicates intramolecular hydrogen bonding to the C=O group having IR absorption bands at 1744 and 1727 cm-’ in carbon tetrachloride. The carbonyl stretching mode exhibits a lower vibrational frequency when the C=O group is intramolecularly hydrogen bonded than for the “free” ester group. The resemblance between the C=O frequency spectra of monooctanoin in a lamellar phase and of monooctanoin dissolved in carbon tetrachloride (monomeric solution) gives further evidence for possible intramolecular interactions of lipids in the lamellar phase. For lysopalmitoylphosphatidylcholineonly one C=O vibration band is observed at 1733 cm-I. This indicates that the sn-3 hydroxyl group of monoctanoin may be the interacting OH group giving rise to the observed split of the C=O stretching mode into the two bands. The similarities in the IR spectra for monooctanoin and monoolein in the 1750-1700-cm-’ range suggest the same explanations for the behavior of the carbonyl stretching band of the latter lipid molecule. Acknowledgment. We are indebted to the Swedish Natural Science Research Council for financial support. Registry No. 1-Monooctanoin, 502-54-5; l-monooctanoin-d2, 116052-34-7;1-monoolein, 11 1-03-5.
Ternary Complexes Containing Ethylene, Hydrogen Chloride, and Water Bengt Nelander Division of Thermochemistry, Chemical Center, University of Lund, PO Box 124, S-221 00 Lund. Sweden (Received: December 9, 1987; In Final Form: March 18, 1988)
Infrared spectra of ternary complexes containing ethylene, hydrogen chloride, and water isolated in argon matrices have been studied. The complex shifts for several isotopomers of the complex components have been used to find qualitative structures for the observed complexes. The interaction between hydrogen chloride and another molecule appears to be strongly modified by the presence of a third molecule. Fermi resonances between fundamental vibrations in different molecules have been observed.
Introduction Matrix isolation spectroscopy in rare-gas matrices has been used successfully to study numerous binary molecular complexes. At present there appears to be no clear evidence for significant matrix perturbation on either complex structures or on infrared spectra. Studies of ternary complexes are difficult, both in matrices and in the gas phase. The introduction of Fourier transform infrared spectrometers for general use in combination with the considerable database now available on binary complexes has made matrix isolation studies of such complexes more feasible. Since knowledge about the pairwise interactions involved is rarely sufficient to predict the properties of a ternary complex, studies of ternary complexes are of considerable interest. A ternary complex can serve as the simplest possible model for solvent modification of a molecular interaction or for reaction catalysis. The few matrix isolation studies of ternary complexes that have been performed have in general been byproducts of studies of binary complexes. The hydrogen halide trimers have been the subject of a more definitive study.’ Hydrogen fluoride appears to form both cyclic and open chain trimers and tetramers, but the cyclic forms are more The water trimer has recently (1) Maillard, D.; Schriver, A,; Perchard, J. P.; Girardet, C. J. Chem. Phys. 1979, 71, 505.
been investigated in our laboratory: and a preliminary account of the ternary complexes containing both water and hydrogen chloride has been given.5 In all these cases, the stable complexes were found to be cyclic, a result clearly to be expected, since it allows the complex components to make maximum use of their hydrogen-bond-forming capabilities. In a study of the water-ethylene complex,6 a ternary complex formed from one ethylene molecule and two water molecules was observed. In this case the complex appears to have an open, noncyclic structure, which can best be described as a complex between a water dimer and ethylene. The proton acceptor of the water dimer forms a hydrogen bond to the ?r-orbital of ethylene. The water dimer appears to form a significantly stronger complex with ethylene than does monomeric water, judging from the shifts of the OH fundamentals involved in the bond. The present paper extends the previous studies to ternary complexes containing ethylene, hydrogen chloride, and water. (2) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984, 88,425. ( 3 ) Andrews, L.; Bondybey, V. E.; English, J. H. J . Chem. P h p . 1984,81,
3452. (4) Engdahl, A.; Nelander, B. J . Chem. Phys. 1987, 86, 4831. (5) Maillard, D.; Perchard, C . Spectroscopie des Especes IsolCs en Matrice, Colloque International, 1985. (6) Engdahl, A.; Nelander, B. Chem. Phys. Lett. 1985, 113, 49.
0022-36S4/88/2092-5642$01.50/00 1988 American Chemical Society
