9592
J. Phys. Chem. 1996, 100, 9592-9597
Dynamic Structure and Chirality Effects on 1H and 13C NMR Chemical Shifts for Aerosol OT in Reversed Micelles Assisted by NOESY, ROESY, and 13CT1 Studies Akihiro Yoshino,† Hirofumi Okabayashi,*,† Tadayoshi Yoshida,† and Katsuhiko Kushida‡ Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan, and Varian Japan Ltd., Sumitomo Shibaura Building, 4-16-36 Sibaura, Minato-ku, Tokyo 108, Japan ReceiVed: October 6, 1995; In Final Form: March 26, 1996X
The 13C NMR spin-lattice relaxation time (T1) and 13C{1H} NOE of the individual carbon atoms of each 2-ethylhexyl chain in an aerosol OT (AOT) molecule have been investigated in the reversed micellar state, and the segmental mobility of each 2-ethylhexyl chain has been discussed independently in connection with the chirality effect on the 13C chemical shifts. Measurements of the NOESY and ROESY spectra of an AOT molecule and cross-peak assignments have been made. In particular, for the ROESY spectrum, the crosspeaks arising from the dipole-dipole interaction have been used to discuss successfully the spatial proximity of the two 2-ethylhexyl chains. It has also been found that the carbon atom having the larger 1/TDD 1 values provides larger secondary splitting of the 13C chemical shift. Furthermore, the dipole-dipole interaction between the H3 or H3′ protons and other specific protons has been found to be strongly dependent on the diastereoisomers. Selective NOE measurements were also made, and the proximity of intermolecular or intramolecular two 2-ethylhexyl chains has been discussed.
Introduction Sodium 1,2-bis(2-ethylhexyl) sulfosuccinate (aerosol OT, AOT) is a well-known representative of 2-chain type surfactants, and its solution behavior continues to attract interest. 1H, 2H, and 13C NMR spectra have played an important role in the study of the conformation and dynamics of AOT in its various aggregated structures.1-13 It has been proposed that in organic solvents AOT forms reversed micelles,1,2 which have the ability to solubilize large amounts of water in the polar core.3-7 In particular, for 1H and 13C NMR studies, discussion on the molecular conformation8,9 and dynamics8,10-13 has previously been focused on the segments in the vicinity of the polar head and the ester groups. Until now, the separate dynamic structures of the two 2-ethylhexyl chains have not been elucidated because assignment of the 1H and 13C NMR signals arising from the two chains has remained incomplete. Recently, however, complete assignment of the 1H and 13C NMR signals of the two chains has been made,14 making it possible to study the dynamic structure of each chain independently. Cross-relaxation rates measured with two-dimensional nuclear Overhauser effect spectroscopy (NOESY) have become one of the most powerful tools for the study of molecular structure and dynamics of small molecules15,16 in which magnetic dipolar relaxation of the nuclei is in the extreme narrowing limit of the resonance line. However, in the case of macromolecules or medium-sized molecules, when the molecular correlation time (τc) approaches the inverse of the Larmor frequency (ω) of the observed protons, NOE effects are often close to zero.17 The rotating-frame Overhauser enhancement spectroscopy (ROESY) is particularly suitable for molecules having a motional correlation time near the condition ωτc ) 1. That is, crossrelaxation processes in the rotating frame are sensitive to lowfrequency motions occurring in the form of a slow conformational change.18-20 Combined use of ROESY and NOESY yields enhanced information on molecular structure and dynamics. For * To whom correspondence should be addressed. † Nagoya Institute of Technology. ‡ Varian Janpan Ltd. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)02977-7 CCC: $12.00
more detailed discussion of the dynamic structure of each AOT chain, application of the NOESY and ROESY methods to the AOT micellar system is highly desirable. In the present study, the 13C spin-lattice relaxation times (T1) and normal NOE of the individual carbon atoms of each 2-ethylhexyl chain were measured, and the segmental mobility of each chain is discussed in connection with the diastereoisomeric effect on the 13C NMR chemical shifts. Furthermore, the NOESY and ROESY spectra and selective NOE were also measured, and the dynamic structures of each 2-ethylhexyl chain are discussed in terms of the spatial proximity of the protonproton or carbon-proton internuclear distance. Experimental Section Materials. Sodium 1,2-bis-(2-ethylhexyl) sulfosuccinate (aerosol OT, AOT) (pure grade) was purchased from Tokyo Kasei Co. Ltd. A sample of AOT-C6D6-D2O (wt % ratio equal to 2:2:1) was used in the present experiment. The minimum isotopic purity of benzene-d6 (Aldrich Japan Co., Tokyo) was 99.6 atom % D. The isotopic purity of heavy water (Wako Pure Chemical Industry Co., Tokyo) was 99.75%. 13CT , 1H NOESY, and ROESY Measurements. 1D and 1 2D NMR spectra were recorded on a Varian UNITY-400 spectrometer, operating at 400 and 100.6 MHz for protons and carbon-13 atoms, respectively, at 25 °C. The 13C chemical shifts (δ, ppm) were measured relative to tetramethylsilane (TMS) as an external reference, and their precision was within (0.003 ppm. The 13C NMR spectral width was 6646.7 Hz with an aquisition time of 1.310 s. The 13C spin-lattice relaxation times (T1) were measured by the inversion recovery Fourier transform method (π-τ-π/2-delay sequence) with a spectral width of 6646.7 Hz and an acquisition time of 1.310 s and delay of 20 s. The pulse repetition time (tpr) was chosen to satisfy the relation tpr > 5T1. The NOESY experiments were performed using the standard sequence,20 and 64 or 16 transients were accumulated for the 512 or 128 values of evolution period. The 1H spectral width was 2177.2 or 1967.0 Hz, with an acquisition time of 0.235 or 0.130 s and delay of 2 or 3 s. The ROESY experiments were also performed using the standard sequence,21 © 1996 American Chemical Society
Dynamic Structure of AOT in Reversed Micelles
J. Phys. Chem., Vol. 100, No. 23, 1996 9593
and 8 transients were accumulated for the 128 values of evolution period under continuous wave spinlock. The 1H spectral width was 2118.9 Hz with an aquisition time of 0.242 s and delay of 2 s. NOE Measurements. Normal NOE measurements were achieved by gated decoupling methods (pulse sequence “S2PUL”) with Waltz noise decoupling,17 which was available on the UNITY-400. The 13C NMR spectra with NOE were measured by decoupling during the whole period (termed YYY). The spectra without NOE were measured by decoupling only during the acquisition period (termed NNY). The two types of spectra were integrated, and then the difference between them was divided by the NNY integrations of each resonance line. Thus, the NOE value (%) was expressed by the following equation.
IYYY - INNY 100 η(%) ) × INNY 1.987
(1)
When weak selective (low-power continuous wave) decoupling is operated only during the contact period, 13C NMR spectra provide the “selective” NOE information derived from the irradiated proton. Selective NOE measurements were made by the modified pulse sequence “Bilevel”, which was equipped with two decoupled power levels in each acquisition status, modified from “S2PUL”. In status A and B, the decoupling power was set to low power (weak selective), and then, in status C, the decoupling power was set to high power. Thus, the NOE values (%) were calculated from the observed intensities using eq 1. Numbering Scheme of an AOT Molecule. The numbering scheme of carbon atoms for an AOT molecule, as adopted by Ueno et al.,1 is used and that of the protons is also represented by the number of the proton-attached carbon.
Results and Discussion 13C NMR T Values, 13C{1H} NOE, and Secondary 1 Splitting. Ueno et al.1b have already reported the 13C NMR spin-lattice relaxation studies for the dynamic behavior of AOT in normal and reversed micelles. The segmental mobility of each chain of the two 2-ethylhexyl chains of an AOT molecule has not yet been discussed, since the 13C signals for the carbon atoms other than those for C3 and C3′, in those arising from the corresponding carbon atoms (Ci and Ci′) of the two chains, overlapped each other. In our previous paper,14 we used two-dimensional pulse techniques to successfully assign all the 13C signals arising from the R- and β-chains, with the exception of those for the C3 and C3′ carbon atoms, to the individual carbon atoms of each chain. Furthermore, we found that the chirality effect causes secondary splitting (∆Si or ∆Si′) of the 13C NMR signals of an AOT molecule and the effect of the chiral centers (C4 and C4′) is concentrated on the R-chain and reaches the C6′ carbon. In this study, we have measured the 13C spin-lattice relaxation times (T1) and the 13C{1H} NOE for the individual carbon atoms of each chain in order to elucidate the relationship between the chirality effect and the segmental mobility. The
observed 13C T1 values of an alkyl chain are usually dominated by the dipole-dipole interaction between the 13C atoms and the protons bonded directly to the carbon atom. However, when the mobility of a molecular segment is extremely restricted, the dipole-dipole interactions between the 13C atom and the nonbonded protons also contribute to the observed T1 values. Therefore, the 13C spin-lattice relaxation times caused by the dipoledipole interactions become an indicator of segmental mobility. When the carbon atom interacts with plural protons, the 13C spin-lattice dipole-dipole interaction terms (TDD 1 ) of the relaxation times can be written in the following form:22 -6 2 2 2 1/T DD 1 ) p γCγH ∑ rCHiτC
(2)
i
where γH and γC are the gyromagnetic ratios of the 1H and 13C atoms, respectively, τC is the rotational correlation time of a carbon atom, and rCHi is the distance between the 1Hi proton term can be separated from the and 13C carbon. The TDD 1 observed spin-lattice relaxation times (Tobs 1 ) by the following equation:17 DD η/η0 ) Tobs 1 /T1
(3)
where η0 and η are the maximum and observed NOE values, respectively. The 1/TDD 1 values of each carbon atom are listed in Table 1, together with the observed T1 and normal NOE values. When we compare the 1/TDD 1 values between the R- and β-chains, we found that there exists a marked difference in the segmental mobility between the R- and β-chains. For an AOT molecule, the segments in the neighborhood of the polar head group are more restricted in internal motion than are the other segments of the two chains. In particular, the internal motion of the C3C4 segment is more restricted than that of the C3′-C4′ segment. Furthermore, it should be noted that the C4′-C5′ segment of the R-chain is extremely restricted in motion compared with the C4-C5 segment of the β-chain. The internal motions of the C9 and C9′ carbons, which are directly bonded to the C4 and C4′ carbons, respectively, are also restricted, and the C9 atom is more restricted in motion than is the C9′ atom. Thus, the mobility of the whole R-chain is more restricted than that of the whole β-chain. We may now discuss the secondary splitting induced by the chirality effect with reference to the segmental mobility.14 Table 1 shows the secondary splitting on the 13C NMR signals of individual carbon atoms for AOT in reversed micelles. It is clear that the chiral center effect of the R-chain extends as far as the C6′ carbon, while that of the β-chain does not even extend as far as the C5 carbon. It is easily seen from comparison of the carbon-position dependence of the secondary splitting with value for the corresponding carbon atoms that of the 1/TDD 1 between the R- and β-chains that the carbon atom with the larger value provides larger ∆S values. Therefore, we may 1/TDD 1 assume that the restricted segmental mobility enhances the effect of the chiral center. In particular, we may emphasize that the great rigidity of the R-chain compared with the β-chain enhances the effect of the chiral centers on the R-chain and causes the secondary splitting to be concentrated on the R-chain. NOESY and ROESY Spectra and Dynamic Structure. Figures 1 and 2 show the expanded parts of the NOESY and ROESY spectra for AOT in the reversed micellar state, respectively. The assignment of the 2-ethylhexyl protons and the polar segment protons in the 1H NMR spectra, shown on the vertical or horizontal axis, which has been made by the selective the 1H-decoupling method,14 is useful for assigning the cross-peaks in the NOESY and ROESY spectra.
9594 J. Phys. Chem., Vol. 100, No. 23, 1996
Yoshino et al.
TABLE 1: 13C Chemical Shifts (δ), Secondary Splitting (∆S), 13C Spin-Lattice Relaxation Time (T1, s),a and Normal NOE Value (NOR, η%) of r- and β-Chains of AOT in the Reversed Micellar State R δ (ppm) ∆S (ppm) T1 (s) 1/TDD 1 NORc
C3′
C4′
67.83 0.05 0.293 0.288 (0.010) (0.011) 1.879 1.912 65 73
39.06 39.09 0.03 0.459 (0.004) 1.173 58
67.78
β δ (ppm) ∆S (ppm) T1 (s) 1/TDD 1 NORc a
C5′
C3 68.94
68.99
0.223 (0.006) 2.160 58
30.59
30.62 0.02 0.409 0.416 (0.002) (0.004) 1.599 1.572 79 78
C4
0.05
39.03
C6′
39.12
29.41
29.42 0.01 0.614 0.610 (0.004) (0.007) 1.171 1.178 100
C5
C6
30.55
29.35
C7 23.44
C8′
C9′
23.51
14.45
23.92
1.026 (0.016) 0.714 57
2.155 (0.022) 0.378 86
0.388 (0.002) 1.889 76
C8 14.34
0.09 0.221 (0.004) 2.179 55
0.445 (0.004) 1.210 61
0.412 (0.006) 1.307
0.816 (0.038) 0.801 72
0.718 (0.002) 1.001 71
1.214 (0.007) 0.604 100
C10′ b
C7′
2.354 (0.024) 0.346 95
11.02
11.08 0.06 1.686 1.708 (0.012) (0.025) 0.471 0.465 97 83 C10b
C9 23.85 0.07 0.348 (0.028) 2.106 76
11.19
11.29 0.10
1.506 (0.018) 0.527 100
1.708 (0.028) 0.465 100
Errors are listed in parentheses. b Corrected assignment from ref 14. c Normal NOE value (η%, precision: 2%).
Figure 1. 1H-1H internuclear correlation between the two 2-ethylhexyl groups (A) and between the 2-ethylhexyl group and the polar segment (B)in the NOESY spectrum of AOT-C6D6-D2O. Positive and negative contour levels are plotted without distinction. The number of the crosspeak assignment implies the numbering of the two protons having the internuclear correlation.
Three types of cross-peaks are observed in the NOESY and ROESY spectra: (1) genuine NOE (or ROE) peaks caused by incoherent transfer of magnetization between dipole-dipole
Figure 2. 1H-1H internuclear correlation between two 2-ethylhexyl groups (A) and between the 2-ethylhexyl group and the polar segment (B) in the ROESY spectrum of AOT-C6D6-D2O. Positive and negative contour levels are plotted without distinction. The number of the crosspeak assignment implies the numbering of the two protons having the internuclear correlation.
interacted protons; (2) J-peaks arising from coherent transfer mediated by spin-spin coupling (J-coupling); (3) the so-called J-relayed peaks arising from the combined process of the true
Dynamic Structure of AOT in Reversed Micelles NOE and spin-spin coupling.18-20,23,24 Detailed analysis of the 1H NMR spectrum of AOT in reversed micelles, to provide coupling constant data, has been reported.14 We can therefore use the knowledge of the 1H-1H coupling constants for the AOT-C6D6-D2O system for identification of the three types of the cross-peaks. Cross-Peak Assignment. In the NOESY spectrum (Figure 1A), for the R-chain, the 4′-5′, 7′-8′, and 9′-10′ cross-peaks are obviously mediated by the spin-spin coupling between the two corresponding protons. For the β-chain, 4-5 and 9-10 cross-peaks, caused by J-coupling, are observed. The 4-9 and 7-8 cross-peaks have very low contour levels, although the two corresponding protons should have three-bond coupling. The 4′-9, 4′-10, 5-9′, 5-10′, and 6′-9 cross-peaks are assigned to the genuine NOE peaks, since the corresponding coupling constants are eventually zero.14 The 1H-1H internuclear correlations between the 2-ethylhexyl protons and the polar segment protons, observed in the NOESY experiment, are shown in Figure 1B. The very weak 3-4 and 3′-4′ cross-peaks can be clearly assigned to the J-peaks. For the 3′-5′ and 3-9 cross-peaks, the possibility of J-relayed peaks is present, since spin-spin coupling occurs between the two protons which are four bonds apart. The cross-peaks at the positions of 3-8, 3-10, 3-8′, 3-10′, 3′-7′, and 3′-8′ obviously come from the genuine NOE. In the NOESY spectrum for the polar segment protons, 1-3, 1-3′, 1′-3, 1′-3′, and 3-3′ cross-peaks arising from the genuine NOE were also observed, in addition to the 1-1′ crosspeak arising from spin-spin coupling (spectrum not shown). In the expanded part of the ROESY spectrum (Figure 2A) for the two 2-ethylhexyl protons, we found cross-peaks that correspond closely to the cross-peaks in the NOESY spectrum. In particular, the cross-peaks of 4-4′, 4-6′, 4′-6, 4′-7, 4-8′, 4′-8, and 7′-8 are clearly observed and are assigned to the genuine ROE peaks, indicating a 1H-1H interchain correlation between the R- and β-chains. NOE cross-peaks corresponding to these genuine ROE peaks are not observed in the NOESY spectrum (Figure 1A). The ROE correlation between the 2-ethylhexyl protons and the polar segment protons is shown in Figure 2B. A 1H-1H internuclear correlation, which was not observed in the NOESY spectrum, is also found in this region. It should be noted that weak 1-5′, 1-7′, 1-8, and 1-10 cross-peaks and very weak 1′-7, 1′-7′, 1′-8, and 1′-10 cross-peaks seemingly arise from a dipole-dipole interaction. Dynamic Structure. One may compare the results of the NOESY experiment with those of the ROESY experiment. In the NOESY spectrum, J-peaks and J-relayed peaks are predominant, and genuine NOE peaks arising from a dipole-dipole interaction are very few. However, in the ROESY spectrum, many cross-peaks arising from dipole-dipole contacts are found, in addition to J-peaks and J-relayed peaks. The genuine ROE peaks provide information on the probability of spatial proximity of the proton-proton internuclear distance. Therefore, we may discuss the dynamic structure of AOT by concentrating on the genuine ROE peaks. The dipoledipole contacts detected in the ROESY experiment are listed in Table 2. The very strong 4-4′ cross-peak indicates that there is very close spatial proximity between the H4 and H4′ protons. The existence of the 4-6′ and 4-8′ cross-peaks implies that the H4 proton contacts the H6′ and H8′ protons by dipoledipole interaction. Furthermore, the H4′ proton of the R-chain has dipole-dipole contacts with the H6, H7, and H8 protons. In particular, the shortest mean distance between the protons is between H4′ and H7. Thus, we may assume that the probability
J. Phys. Chem., Vol. 100, No. 23, 1996 9595 TABLE 2: Proton-Proton Dipolar Contacts Detecteda in the ROESY Spectrum intrachain correlation R-chain
β-chain
(1′-3′)N (1′-7′)N (3′-6′)P (3′-8′)N (3′-10′)N (5′-8′)N (7′-10′)P
(1-3)N (1-8)N (1-10)N (3-8)N (3-10)N (5-8)N
R-, β-interchain correlation (1-5′)N (1-7′)N (1-8′)N (1′-3)N (1′-7)N (1′-8)N (1′-10)N
(3-5′)N (3-7′)N (3-8′)N (3′-5)P (3′-7)N (3′-10)N
(4-4′)P (4-6′)N (4-8′)N (4′-6)N (4′-7)P (4′-8)P
(5-8′)N (5-10′)P (5′-9)N (7′-8)N (7′-10)N (8′-9)P (9′-8)P
a Only genuine ROE cross-peaks are listed. N: negative ROE, P: positive ROE.
Figure 3. Rotational isomers (I, II, and III) about the C1-C1′ single bond.
of spatial proximity between the R- and β-chains is highest at the internuclear positions of H4-H4′ and H4′-H7. The very close proximity between the R- and β-chains may be caused by the restricted state of the 1CH-1′CH2 segment. In fact, three rotational isomers (I, II, and III) about the 1CH1′CH single bond are possible (Figure 3). However, it has been 2 found that in the reversed micellar state the rotational isomer III is preferentially stabilized (PI:PII:PIII ) 0.21:0.04:0.75).14 This restricted state of the 1CH-1′CH2 segment must bring about the close proximity between the R- and β-chain. We were able to confirm such a proximity by consideration of a molecular model. Furthermore, the restricted state of the C3′-C4′-C5′ and C3-C4 segments, assumed from the correlation times of the internal motion, may promote proximity of the two chains. The presence of the genuine ROE cross-peak (4-8′) implies that the terminal CH3 group of the R-chain is very close to the H4 proton of the β-chain. Conversely, the terminal CH3 group of the β-chain is not close to the H4 proton of the R-chain, since the 4′-8 cross-peak is weak in intensity. It should be noted that there are dipole-dipole connectivities between the polar segment protons and the terminal CH3 protons. The 1-8 and 3-8 cross-peaks indicate that the terminal CH3 protons of the β-chain have spatial proximity with the H1 and H3 protons. We note that in the homonuclear NOE and ROE experiments, overlapping and splitting of 1H resonance peaks makes it difficult to discuss the mobility quantitatively. For this reason we describe the genuine 1-8′, 3-8′, and 3′-8′ ROE peaks as having very low contour levels and conclude that spatial proximity between the terminal CH3 protons of the R-chain and the polar segment protons may not be as large. Such a difference between the R-terminal CH3-polar segment proximity and the β-terminal CH3-polar segment proximity may be due to the difference in the restricted segment of each chain. That is, contact between the R-terminal CH3 protons and the polar segment protons may be hindered by the existence of the restricted C3′-C4′-C5′ segment. However, for the β-chain, even if the C3-C4 segment is extremely restricted, the high mobility of the C5-C8 segment results in contact between the β-terminal CH3 protons and polar segment protons. In our previous paper,14 in which we examined the coupling constant data for the three possible rotational isomers about the C3′-C4′ and C3-C4 single bonds, we suggested that two
9596 J. Phys. Chem., Vol. 100, No. 23, 1996
Yoshino et al. TABLE 3: NOE Values (η%, Precision: 2%) of r- and β-Chains R NOR H1 H3 H3′ H1′ H4 H4′ H9 β
Figure 4. 13C NMR spectrum of the AOT sample, measured under high-power decoupling and operating under low-power decoupling of the H3 protons.
isomers have preferential stabilization of the C3′-C4′ segment, while all three isomers have the same populations for the C3C4 segment. The results imply that the R-chain takes up a specific direction, while the β-chain has no directionality. Thus, we may assume the following overall structure of AOT in the reversed micellar state. The restricted C3′-C4′-C5′C6′ segment of the R-chain is the main contribution to the thickness of hydrophobic core, while the β-chain takes up various conformations, thereby filling the grooves among the AOT molecules. Chirality Effect in the ROESY Spectrum. The resonance signals of the H3 protons consist of superimposition of the AB part of an ABX system and the A2 part of an A2X system.9a,14 For the H3′ protons, it has been found that the very complex signals consist of superimposition of four AB parts.14 Such superimposition of the AB and A2 parts or four AB parts is caused by the diastereoisomeric effect arising from the three asymmetric carbons (C1, C4, and C4′). The diastereoisomeric effect is also found in the ROESY spectrum, as seen in Figure 2B. For example, the 3′-5 and 3′-6′ cross-peaks arising from the dipole-dipole interaction indicate that the H5 and H6′ protons have strong connectivities with the 3A protons rather than with the 3B protons. The 3′10′ cross-peak also indicates the existence of a strong contact between the 3A and H10 protons. For the 3-8, 3-10, 3-5′, and 3-7′ cross-peaks, we may assume that the H8, H10, H5′, and H7 protons have contact mainly with the 3A2 protons and that the dipole-dipole interaction of these protons with the 3A and 3B protons is weak. These observations show that the dipole-dipole contact between the H3 or H3′ protons and other protons strongly depends upon the diastereoisomers. Selective NOE and Proximity of Two Chains. To obtain further information on the steric proximity of the R- and β-chains of an AOT molecule in the reversed micellar state, we carried out 13C{1H} NOE experiments using a selective decoupling method. Figure 4 shows the 13C NMR spectrum of the AOT sample in reversed micelles, measured under high-power decoupling and operating under low-power decoupling of H3 protons only during the contact period. Comparison of the intensity of the C3 signal at 69.04 ppm with that of the latter signal at 67.80 ppm shows that the former is greater than that of the latter, indicating that the NOE observed in the 13C NMR spectrum is derived from the H3 protons. Integration of 13C NMR signals of the 5′, 5 and 9′, 9 carbons (curve not shown) provided ample evidence for the presence of NOE, although the NOE values of the 6′, 6 and 7, 7′ signals were not so large. Since these observations were obtained from irradiation of the
NOR H1 H3 H3′ H1′ H4 H4′ H9
C3′ 65 39 36 39 36 31 29 30
C4′
73 38 33 36 37 31 29 31 C3
58 41 37 37 35 26 25 25
55 38 38 39 35 25 23 23
C5′
58 17 11 14 16 24 24 28
79 15 15 16 19 30 30 30 C4 61
15 13 19 16 24 23 28
15 18 18 17 21 22 26
C6′
78 17 17 17 19 31 31 33
C7′ C8′ C9′
100 6 10 2 9 5 11 10 15 13 21 12 21 12 21
57 7 7 10 4 5 4 19
86 1 2 16 0 4 1 13
76 17 14 17 22 32 32 32
C5
C6
C7
C8
C9
72 14 12 16 21 30 29 29
71 11 5 10 13 21 21 21
100 3 7 19 0 8 4 15
95 2 8 13 0 0 0 12
76 17 14 17 22 32 32 32
C10′ 97 83 3 5 7 4 7 6 5 6 5 10 5 9 7 11 C10
100 6 6 10 3 10 9 14
100 11 1 5 5 11 11 18
H3 proton, the observed NOE derives from the steric proximity of the H3 protons to these carbon atoms. Similar selective NOE experiments were made for other protons in the neighborhood of the polar groups. To obtain NOE information on each of the secondary-split signals, the NOE value of each 13C resonance signal was calculated from the signal intensity of each line. The selective NOE values thus obtained are listed in Table 3. When each of the H1, H3, H4, and H9 protons is selectively irradiated with low power, the NOE values of the C3′ signal lie between 30 and 39%. Furthermore, under low-power irradiation of each of the H1′, H3′, and H4′ protons, NOE values of the C3 signal are within the range 23-39%. These selective NOE observations indicate access of the C3′ carbon (or C3 carbon) to the polar segment of the β-chain (or R-chain), implying that there exists an intramolecular or intermolecular proximity of polar segments between the R- and β -chains in the reversed micelles. Under low-power irradiation of the H4 (or H4′) protons, the selective NOE values of the C4′, C5′, and C6′ (or C4, C5, and C6) carbon atoms are found to be 21-33% (or 21-29%). These NOE values also indicate that access of the C4 (or C4′) methyl group to the C4′, C5′, and C6′ (or C4, C5, and C6) segments occurs between the intermolecular or intramolecular R- and β-chains. Furthermore, it should be emphasized that the selective NOE values, 13-19%, for the C7 and C8 carbons are obtained under irradiation of the H3′ protons. This observation may imply that the high mobility of the C7 and C8 segment of the β-chain allows it to approach the H3′ protons. This access may occur between intermolecular R- and β-chains. Conclusions To elucidate the relationship between the chirality effect and the segmental mobility for AOT in the reversed micellar state, we have measured the 13C spin-lattice relaxation times (T1) and the normal NOE of the individual carbon atoms of the Rand β-chains of an AOT molecule in reversed micelles. The values were separated from the observed T1 values, to 1/TDD 1 give an indication of the internal motion of each carbon, and it was possible to discuss the segmental mobility of the two chains independently. It has been found that the R-chain is more restricted than the β-chain. Indeed, there is evidence for a marked difference between the mobilities of the R- and β-chains. In particular, the C4′-C5′ segment of the R-chain is more restricted in mobility when it is compared with the corresponding segment of the β-chains. It was also found that the carbon atom
Dynamic Structure of AOT in Reversed Micelles having larger 1/TDD 1 values provides larger secondary splitting of the 13C chemical shift arising from the diastereoisomeric effect. Thus, we may emphasize that the great rigidity of the R-chain compared with the β-chain enhances the effect of the chiral centers on the chain. The NOESY and ROESY spectra of an AOT molecule in the reversed micellar state have been measured, and assignment of the cross-peaks has been made. The genuine ROE peaks detected in the ROESY experiment were used to discuss the probability of spatial proximity of the 1H-1H internuclear distance in intra- or interchains. It was assumed that the probability of spatial proximity between the R- and β-chains was highest at the internuclear positions of H4-H4′ and H4′H7. Moreover, dipole-dipole interactions between the H3 or H3′ protons and other protons were found to be strongly dependent upon the presence of the diastereoisomers. Selective NOE measurements were also made, and proximity between intermolecular or intramolecular R- and β-chains has been discussed in detail. Thus, from the differences in the chirality effects and segmental mobility and by taking into account the proximity between the two 2-ethylhexyl chains, we may assume the following overall structure of AOT in the reversed micellar state. The restricted C3′-C4′-C5′-C6′ segment of the R-chain is the main contribution to the thickness of the hydrophobic core of the reversed micelle, and the β-chain, taking up various conformations, fills the grooves among the AOT molecules. Acknowledgment. We express gratitude to Professor Charmian J. O’Connor (Department of Chemistry, The University of Auckland, New Zealand) for reading the manuscript prior to publication and making suggestions for its revision. References and Notes (1) (a) Ueno, M.; Kishimoto, H.; Kyogoku, Y. Bull. Chem. Soc. Jpn. 1976, 49, 1776. (b) Ueno, M.; Kishimoto, H.; Kyogoku, Y. J. Colloid Interface Sci. 1978, 63, 113.
J. Phys. Chem., Vol. 100, No. 23, 1996 9597 (2) Ueno, M.; Kishimoto, H.; Kyogoku, Y. Chem. Lett. 1977, 599. (3) Eicke, H.-F.; Zinsli, P. E. J. Colloid Interface Sci. 1978, 65, 131. (4) Maitra, A.; Eicke, H.-F. J. Phys. Chem. 1981, 85, 2687. (5) Martin, C. A.; Magid, L. J. J. Phys. Chem. 1981, 85, 3938. (6) Maitra, A.; Vasta, G.; Eicke, H.-F. J. Colloid Interface Sci. 1983, 93, 383. (7) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (8) Okabayashi, H.; Taga, K.; Tsukamoto, K.; Matsushita, K.; Kamo, O.; Yoshikawa, K. Colloids Surf. 1987, 24, 337. (9) (a) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1987, 83, 517. (b) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1988, 84, 343. (c) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1989, 85, 917. (10) Olsson, U.; Wong, T. C.; So¨derman, O. J. Phys. Chem. 1990, 94, 5356. (11) Yoshino, A.; Okabayashi, H.; Yoshida, T. J. Phys. Chem. 1994, 98, 7036. (12) El Seoud, O. A.; El Seoud, M. I.; Mickiewicz, J. A. J. Colloid Interface Sci. 1994, 163, 87. (13) Varshney, M.; Maitra, A. Colloids Surf. 1995, A96, 165. (14) Yoshino, A.; Sugiyama, N.; Okabayashi, H.; Taga, K.; Yoshida, T.; Kamo, O. Colloid Surf. 1992, 67, 67. (15) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. (16) Macura, S.; Ernst, R. R. Mol. Phys. 1980, 41, 95. (17) Noggle, J. H.; Shirmer, R. E. The Nuclear OVerhauser Effect; Academic Press: New York, London, 1971. (18) Bothner-By, A. A.; Stephens, R. L.; Lee, J.; Warren, C. D.; Jeanloz, R. W. J. Am. Chem. Soc. 1984, 106, 811. (19) Bax, A.; Davis, D. G. J. Magn. Reson. 1985, 63, 207. (20) Farmer, B. T., II; Macura, S.; Brown, L. R. J. Magn. Reson. 1988, 80, 1. (21) (a) States, D. J.; Haberkorn, R. A.; Ruben, D. J. J. Magn. Reson. 1982, 48, 286. (b) Kessler, H.; Griesinger, C.; Kerssebaum, R.; Wagner, K.; Ernst, R. J. Am. Chem. Soc. 1987, 109, 607. (22) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: London, 1961; Chapter 8. (23) Bax, A. J. Magn. Reson. 1988, 77, 134. (24) Neuhaus, D.; Keeler, J. J. Magn. Reson. 1986, 68, 568.
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