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NMR Investigation on the Various Aggregates Formed by a Gemini Chiral Surfactant† Luciana Luchetti* Dipartimento di Scienze e Tecnologie Chimiche, Universita` degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy
Giovanna Mancini* Centro CNR di Studio sui Meccanismi di Reazione c / o Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, Box 34sRoma 62, P.le Aldo Moro 5, 00185 Roma, Italy Received June 4, 1999. In Final Form: November 4, 1999 In this paper we report an NMR investigation on the chiral gemini surfactant (2S,3S)-2,3-dimethoxy1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1), carried out to study the aggregation in various solvents. If the aggregation equilibrium of 1 can be described by the mass action law model, by observing the variation of chemical shift with respect to [1], we can obtain the aggregation number n. In CDCl3 the values are n ) 2 and 19 ( 3, at low and high [1], respectively, indicating that 1 is present in dimeric assemblies and as reversed micelles. In CD3OD, despite evidence that 1 is under aggregating conditions, the model does not hold; 1 should form a relatively flexible structure, because in the 13C NMR spectra 1J(13C,14N) coupling is observed (1J ) 3.4 Hz). 1 is scarcely soluble in D2O; in the 1H NMR spectrum of 1 × 10-3 M 1, the signals relative to the tails and to one of the NCH3 groups disappear, while the other head group signals are well resolved, indicating the presence of large assemblies. These large aggregates are confirmed by preliminary experiments of dynamic laser light scattering. The signals of the two NCH3 groups are very different in all the solvents investigated. This result can be interpreted in terms of an aggregate conformation in which they undergo different environments.
Introduction Chiral aggregates, formed by self-assembly of chiral surfactant molecules, are an interesting field of investigation.1 A chiral, organized aggregate can be a useful medium both to induce enantioselectivity in well-known reactions, such as the Reformatsky2 and the Diels-Alder reactions,3 and for chiral discrimination.4 A gemini surfactant is an amphiphilic molecule with two hydrophobic tails and two polar head groups, linked by a spacer group.5 The surfactant properties are very different from those of the corresponding monocationic compound and are strictly dependent on the spacer, whose nature can be very different. In a recent paper, we studied the cyclization of 2-(3-bromopropyloxy)phenoxide ion in aqueous solutions of the chiral gemini surfactant (2S,3S)2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1).6
(2), and at 25 °C, we could perform the kinetic study only up to [1] ) 8.8 × 10-4 M.7 Besides the unexpected low solubility, 1 showed very unusual 1H NMR spectra. In fact, in CDCl3, 1 shows a typical surfactant spectrum, while in the spectrum of 1 × 10-3 M 1 in D2O the signals relative to the hydrophobic tails disappear. On the contrary, the head group signals are well resolved, indicating that this aggregate zone is hydrated and has good mobility. We explained this effect with a very tight and relatively rigid aggregate structure that made possible a very efficient relaxation process. NMR is a versatile and powerful technique to investigate surfactant aggregates in solution. Parameters such as chemical shift, line width, and spin-spin coupling constant and their variations versus [surfactant] can give useful structural information, for instance on the cmc and aggregation number.8 Because in our preliminary study, 1 showed very interesting features, to study this surfactant deeper, we performed an NMR investigation on 1 in different solvents, such as CDCl3, CD3OD, and D2O. Experimental Section Materials. Preparation and purification of (2S,3S)-2,3dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1) has been described.9 Elemental analysis and
Although two methoxy groups are present in the spacer, 1 was scarcely soluble in H2O, as compared to 1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. * To whom correspondence should be addressed.
(1) Fuhrhop, J. H.; Helfrich, W. Chem. Rev. 1993, 93, 1565 and references therein. (2) Zhang, Y.; Wu, W. Tetrahedron: Asymmetry 1997, 8, 3575. (3) Diego-Castro, M. J.; Hailes, H. C. Chem. Commun. 1998, 1549.
(4) (a) Jursic, B. S. Tetrahedron Lett. 1993, 34, 963. (b) Belogi, G.; Croce, M.; Mancini, G. Langmuir 1997, 13, 2903. (c) Borocci, S.; Erba, M.; Mancini, G.; Scipioni, A. Langmuir 1998, 14, 1960. (5) (a) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (b) Fletcher, P. D. Curr. Opin. Colloid Interface Sci. 1996, 1, 101 and references therein. (c) Danino, D.; Talmon, Y.; Zana, R. J. Colloid Interface Sci. 1997, 185, 84. (6) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G. Langmuir 1999, 15, 2631. (7) The solubility limit of 2 was [2] ) 4.78 × 10-2 at 25 °C. (8) Cerichelli, G.; Mancini, G. Curr. Opin. Colloid Interface Sci. 1997, 2, 641 and references therein.
10.1021/la990713z CCC: $19.00 © 2000 American Chemical Society Published on Web 01/04/2000
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Figure 1. 1H NMR spectra of 1 in CDCl3 (a) and CD3OD (b) at 25 °C. [1] ) 0.104 M (a) and 0.147 M (b). In spectrum a the signal at δ ) 2.554 ppm is H2O. solubility tests reveal the presence of crystallization water that could not be removed after several days of drying at 50 °C under vacuum. Anal. Calcd for C42H90Br2N2O2‚1/2H2O: C, 60.67; H, 10.97; N, 3.37. Found: C, 59.99; H, 10.90; N, 3.38. NMR. The experiment temperature was T ) 25.0 °C. NMR measurements were carried out on a Brucker AC300P instrument operating at 300.13 and 75.468 MHz for 1H and 13C, respectively. The peaks were referenced with respect to TMS (δ ) 0.000 ppm), used as internal standard, in CDCl3 and CD3OD and to DOH (δ ) 4.750 ppm) in D2O.
Results and Discussion 1H
NMR spectra of 0.104 M 1 in CDCl3 and 0.147 The M 1 in CD3OD are reported in Figure 1. The NCH2CHO spacer signal should be formed by four doublets of doublets, typical of an AMX system, because the NCH2 protons are diastereotopic. In CDCl3 these signals are not well resolved, because one of the NCH2 signals overlaps the CHO signal while in CD3OD they are better separated. In CD3OD we can individually assign these signals to the two protons by observing the shielded signal at δ ) 3.88 ppm. In fact, we can measure the coupling constants 2J(1H,1H) and 3J(1H,1H) (13 and 4.5 Hz, respectively), and this 3J(1H,1H) value is in agreement with a dihedral angle Φ between the C-H bonds of about 60°, typical of syn-protons. This value depends on the fact that the rotation around the single C-C bond is restricted in order to minimize the steric repulsion between the substituents. Another interesting feature is that the two NCH3 signals are not equivalent, even though these groups are four bonds removed from the chiral center. In this paper, we (9) Cerichelli, G.; Luchetti, L.; Mancini, G. Tetrahedron 1996, 52, 2465.
denote the shielded signal and the deshielded signal as NCH3′ and NCH3′′, respectively. In different solvents, the prime could be attribute to different NCH3 groups because the relative chemical shift could vary, depending on the solvent. The difference between the two NCH3 signals (∆δ) is higher in CD3OD than in CDCl3 (∆δ ) 0.20 and 0.05 ppm, respectively) and is also observed in the 13C NMR spectra, where ∆δ ) 5.90 and 5.80 ppm, in CD3OD and in CDCl3, respectively. An unusual effect is shown in the 13C NMR spectrum in CD3OD, which is partly reported in Figure 2. The signals have been assigned on the basis of 1H-13C COSY. The NCH2 signals of the spacer (δ ) 70.98 ppm) and NCH3′ (δ ) 57.24 ppm) are triplets, while the signals for NCH3′′ (δ ) 51.44 ppm) and 1-CH2 of the hydrophobic tail (δ ) 65.23 ppm) are quite large, compared with that for OCH3 (δ ) 57.90 ppm), as shown by their line widths at halfheight (lw ) 7.5, 5.3, and 1.4 Hz, respectively). This effect is due to a 1J(13C,14N) coupling. Generally, this coupling is not resolvable in the 13C NMR spectra due to the small magnitude and quadrupolar broadening. In our case, however, the electric field gradients around 14N must be quite small compared to those of most ammonium surfactants, wherein 14N electric quadrupole-induced relaxation is fast enough to average 1J(13C,14N) coupling, and the effect is detectable, with 1J(13C,14N) ) 3.4 Hz for both NCH2 and NCH3′ signals. Because this coupling is not observed in the CDCl3 spectra, this absence can be explained either with an interaction of the 14N nucleus with the solvent or with an aggregate structure that restricts the internal rotations. This effect is similar to that observed in the 13C NMR spectra of phosphatidyl-
Gemini Chiral Surfactant
Figure 2.
13C
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NMR spectrum of the head group signals of 1 in CD3OD at 25 °C. [1] ) 0.0736 M. The CH signal is not reported.
Figure 3. Variation of δobs versus 1/[1] in the 1H NMR spectra of the NCH3′ signal in CDCl3 (b) and CD3OD (9) at 25 °C.
choline (3),10 where the coupling is observed in CD3OD but not in CDCl3 and D2O, where 3 is known to exist as reversed micelles or as liposomes, respectively.11 We studied the effect of the concentration of 1 on 1H spectra, in CDCl3, and on 1H and 13C NMR chemical shifts, in CD3OD. The data are reported as Supporting Information (Tables S1-S3). In both solvents, there is a break in the plot of chemical shift versus 1/[1] at [1] ) 0.04 and 0.03 in CDCl3 and in CD3OD, respectively, as reported for the NCH3′ signal in Figure 3; these breaks should correspond to the cmc of an aggregation process. Reversed micelles are easily formed in CDCl3 in the presence of water; in our case, we did not have to add water because the crystallization water was sufficient for the process (see Experimental Section). On the contrary, the break in the CD3OD plots is a significant result because CD3OD is generally considered a nonmicellizing solvent. This result confirms the unusual behavior of gemini surfactants. These surfactants are widely studied in water but, to the best of our knowledge, there are few reports of aggregation in organic solvents.12 The aggregation equilibrium of 1 can be described by the mass action law model in eq 1, where n is the aggregation number, 1a and 1b are the monomer and the associated form of surfactant 1, and K is the equilibrium constant. The observed chemical shift (δobs) at the analytical concentration [1] is given by eq 2, where x1a and x1b are the molar fractions and δ1a and δ1b are the chemical shifts of 1a and 1b. δ1a and δ1b can be extrapolated from (10) (a) Murari, R.; Baumann, W. J. J. Am. Chem. Soc. 1981, 103, 1238. (b) Murari, R.; Abd El-Rahman, M. M. A.; Wedmid, Y.; Parthasarathy, S.; Baumann, W. J. J. Org. Chem. 1982, 47, 2158. (11) Birdsall, N. J. M.; Feeney, J.; Lee, A. G.; Levine, Y. K.; Metcalfe, J. C. J. Chem. Soc., Perkin Trans. 2 1972, 1441.
Figure 4. Plot of eq 3 of the 1-CH2 signal in CDCl3 at 25 °C.
plots of δobs versus [1] and δobs versus 1/[1], respectively.14 Eqs 1 and 2 can be arranged to give eq 3.
If the mass action law model holds, the plot of log {[1](δ1b - δobs)} versus log{[1](δobs - δ1a)} should give a straight line, the slope of which yields the value of the aggregation number n. When we plotted eq 3 for the 1H NMR signal of the CDCl3 spectra, as reported in Figure 4 for the 1-CH2 signal, we obtained two straight lines with an intersection that corresponds to [1] ) cmc, that (12) In a recent paper, Oda et al. pointed out that gemini surfactants with chiral counterions formed gel structures in nonanhydrous chlorinated solvents and in water. They also reported that addition of alcohol disrupted these gels, but they did not investigate if an aggregate structure was present.13 (13) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. Engl. 1998, 71, 2689. (14) Drakenberg, T.; Lindman, B. J. Colloid Interface Sci. 1973, 44, 184.
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Figure 5.
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NMR spectrum of 1 in D2O at 25 °C. [1] ) 1 × 10-3 M.
is, at the same concentration where we noted the break in the plot of δobs versus 1/[1]. This pattern is due to two different equilibria. At low [1], the variation of the NMR signals gives a line with n ) 2, corresponding to a dimerization process. It is important to point out that we have the same slope for every well-resolved signal, with the exception of the OCH3 and NCH3′′ signals. For these signals, there is not a continuous decreasing of δobs versus [1] but δobs goes through a maximum, and the difference ∆δ′ ) δ1b - δ1a is relatively small compared to the experimental errors. This is probably due to the fact that OCH3 and NCH3′′ are the most hydrophilic groups of the molecule. These groups are always in contact with the water molecules, and their environment varies only slightly with [1]. At high [1], 1 is present under aggregate conditions as reversed micelles, with n ) 19 ( 3. This value is in agreement with those generally observed in nonpolar solvent, even if exceptions to this behavior are reported in the literature.15 The good linearity obtained and the fact that n is in good agreement for different signals mean that at high [1] there is a single type of aggregate. From eq 3, K should also be calculated but with a high degree of uncertainty. It is interesting to observe the variation of the chemical shift of the H2O (δwater). At low [1], δwater ) 1.569 ppm, very close to the value of dissolved, monomeric water in CDCl3 (δwater ∼ 1.55 ppm), and then gradually increases to δwater ) 2.601 ppm at [1] ) 0.15. On the contrary, in the plots of the 1H and 13C NMR spectra in CD3OD, we did not obtain straight lines, for any of the signals investigated. This could be ascribed to smaller differences between δ1a and δ1b than those observed in CDCl3, which corresponds to larger errors in the calculations, or to the fact that the mass action law model does not hold, for instance for the presence of different kinds of aggregates, with different aggregation numbers. For this purpose, we made a conductivity study of 1 in CH3OH at various concentrations. From the data, we (15) Nagarajan, R.; Wang, C.-C. J. Colloid Interface Sci. 1996, 178, 471 and references therein.
could extrapolate a cmc value (0.02 M) similar to that observed by NMR, confirming that the small difference between δ1a and δ1b is responsible for the nonlinearity observed. It is not simple to study 1 under aqueous aggregating conditions because 1 is scarcely soluble in water. We were not able to measure the cmc of 1 in D2O, neither by NMR nor by conductivity, but it is certainly
