Langmuir 1997, 13, 6439-6446
6439
Micellar and Solvent Effects on the Geometrical Isomerism of Hydroxamic Acids and Their Anions Andrei Blasko, Clifford A. Bunton,* and Nicholas D. Gillitt Department of Chemistry, University of California, Santa Barbara, California 93106 Received June 9, 1997. In Final Form: September 9, 1997X Hydroxamic acids and their anions exhibit geometrical isomerism due to amide-like resonance and an increase in the C-N bond order which can be monitored by 1H-NMR spectroscopy. N-Phenylpropionoand dodecanohydroxamic acids, 1a and 2a, respectively, exist as E-isomers in DMSO-d6 and CD3OD but 1a is an E-Z mixture in D2O. The corresponding hydroxamate ions, 1b and 2b, also exist as E-isomers in DMSO-d6, but Z-isomers form on addition of D2O and Z-2b is dominant with χD2O g 0.9. In CD3OD E- and Z-isomers coexist, but Z-2b becomes dominant on addition of D2O. The E:Z ratio of 1b is very similar in cetyltrimethylammonium bromide (CTABr) and in D2O, but only Z-2b is detected in cationic CTABr micelles and in anionic, zwitterionic, and nonionic micelles. Formation of Z-2b in micelles and water-rich mixed solvents is favored by hydrophobic interactions between phenyl and n-alkyl groups, despite the proximity of anionoid oxygens. In micelles of sodium dodecyl sulfate the N-phenyl residue of 2b is in a more aqueous region of the micelle-water interface than in micelles of the other surfactants. Scheme 1
Introduction Hydroxamate ions are R-effect nucleophiles, i.e., their reactivity is higher than that predicted by Bronsted relations between nucleophilicity and basicity.1 They are effective deacylating and dephosphorylating agents, and reactivities of amphiphilic hydroxamate ions are increased by comicellization with inert surfactants in water.2,3 On the basis of pseudophase treatments of micellar rate effects,4,5 high local concentrations of hydroxamate ions in the interfacial micellar region are largely responsible for the rate increases.3 However, hydroxamic acids exist as E- and Z-isomers and the equilibrium composition is medium dependent.6 Charge delocalization, as in simple amides,7 generates partial double bond character and should increase on deprotonation (Scheme 1). In the present work we examined N-phenylpropionohydroxamic acid and its anion, 1a,b, R ) CH2CH3, and the corresponding N-phenyldodecanohydroxamic acid and its anion, 2a,b, R ) n-C11H23 in a variety of solvents (CD3* To whom correspondence should be addressed: tel, (805) 893 2605; fax, (805) 893 4120; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) (a) Jencks, W. P. Catalysis in Chemistry and Enzymology; McGraw-Hill: New York, 1969; Chapter 2. (b) Fina, N. J.; Edwards, J. O. Int. J. Chem. Kinet. 1973, 5, 1. (c) Harris, J. M., McManus, S. P., Eds. Nucleophilicity; Adv. Chem. Ser. 215; American Chemical Society: Washington, DC, 1987. (d) Fountain, K. R.; Dunkin, T. W.; Patel, K. D. J. Org. Chem. 1997, 62, 2738. (2) (a) Kunitake, T.; Shinkai, S.; Okahata, Y. Bull. Chem. Soc. Jpn. 1976, 49, 540. (b) Kunitake, T.; Ihara, H.; Hashiguchi, Y. J. Am. Chem. Soc. 1984, 106, 1156. (c) Anoardi, L.; de Buzzaccarini, F.; Fornasier, R.; Tonellato, U. Tetrahedron Lett. 1978, 3945. (d) Pillersdorf, A.; Katzhendler, J. J. Org. Chem. 1979, 44, 549. (3) Bunton, C. A.; Hamed, F. H.; Romsted, L. S. J. Phys. Chem. 1982, 86, 2103. (4) (a) Martinek, K.; Yatsimirski, A. K.; Levashov, A. V.; Berezin, I. V. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed., Plenum Press: New York, 1977; Vol. 2, pp 489. (b) Romsted, L. S. In ref 4a, p 509. (c) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015. (5) (a) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (c) Bunton C. A. In Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M., Kalyanasundaram, K., Eds.; Dekker: New York, 1991; Chapter 2. (6) (a) Brown D. A.; Glass, W. K.; Mageswaran, R.; Mohammed, S. A. Mag. Res. Chem. 1991, 29, 40. (b) Dietrich, A.; Powell, D. R.; EngWilmot, D. L.; Hossain, M. B.; Van der Helm, D. Acta Crystallogr. 1990, C46, 816. (7) Dahn, H.; Vantoan, V.; Pecky, P. Mag. Res. Chem. 1995, 33, 386.
S0743-7463(97)00602-1 CCC: $14.00
OD, DMSO-d6, and mixtures with D2O) and micellized surfactants (Chart 1). Chart 1
CTABr
cetyltrimethylammonium bromide (n-C16H33NMe3Br)
CTBABr cetyltributylammonium bromide (n-C16H33NBu3Br) SB3-14
(tetradecyldimethylammonio)propanesulfonate (n-C14H29N+Me2(CH2)3SO3-)
SDS
sodium dodecyl sulfate (n-C12H25OSO3Na)
C12E10
10-dodecyl ether (n-C12H25 (OCH2CH2)9OCH2CH2OH)
The E-Z equilibrium of hydroxamic acids has been monitored by NMR spectroscopy, and Z-isomers are favored by an increase in solvent polarity and water content.6a If hydroxamate ions behave similarly, solvent or micellar effects on nucleophilicity may be influenced by changes in the E:Z ratio because the isomers may have different reactivities. Interfacial regions of aqueous ionic surfactants are considered to have polarities similar to those of medium chain length alcohols, based largely on the use of spec© 1997 American Chemical Society
6440 Langmuir, Vol. 13, No. 24, 1997
trophotometric probes,8 and the solvatochromic scale of solvent properties has been extended to micelles.9 However, we do not know whether organic solvents and micelles will have similar effects on the E-Z equilibria of hydroxamic acids and their anions. Experimental Section Materials. The hydroxamic acids were prepared from the acid chlorides and N-phenylhydroxylamine following Exner’s general procedure.10 N-Phenylpropionohydroxamic acid, 1a, was prepared from the acid chloride (0.1 mol) and N-phenylhydroxylamine (0.2 mol) in dry Et2O (300 mL).3 After reaction the precipitate of the hydroxylamine hydrochloride was removed and the crude hydroxamic acid was chromatographed on SiO2 gel with CH2Cl2 and 1a was eluted with Et2O. This material had previously been isolated as an oil,3 but we obtained a light brown solid, mp 45-46 °C. N-Phenyldodecanohydroxamic acid, 2a, was prepared by adding dropwise the acid chloride (0.02 mol) to N-phenylhydroxylamine (0.04 mol) and Et3N (0.02 mol) in dry Et2O (60 mL). The precipitate was removed and washed, Et2O, and 2a was crystallized by cooling the combined solutions. Recrystallization from Et2O gave white crystals, mp 78-79 °C. Surfactants were materials used earlier,3,11 except for C12E10 (Sigma), which was used as received. Deuterated solvents were 99.9% D2O and DMSO-d6 (Aldrich) and 99.8% CD3OD (Cambridge Isotope Lab.). NMR Spectroscopy. The 1H-NMR spectra were monitored at 400 or 500 MHz on Varian Unity (Inova) instruments. The temperature was 21.5 °C, except as noted, and the reference was TSP (3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt) except in DMSO-d6 where we used TMS (tetramethylsilane). We used 0.01 M hydroxamic acid, which was deprotonated by 40 wt % KOD-D2O, equivalent to 0.02 M in the final solution, except in SDS where we used NaOD, equivalent to 0.1 M in the final solution, to ensure complete deprotonation,4,5 and [surfactant] ) 0.05 M. In experiments in mixed solvents dilution was with D2O-KOD or CD3OD-KOD to ensure constancy of [OD-]. The correlation spectroscopy (COSY) spectra were carried out by the GLIDE procedure. The nuclear Overhauser effect (NOE) experiments12 were made at 400 MHz with 0.01 M 2b and 0.02 M KOD in CD3OD or CTABr-D2O and 0.01 M 2b and 0.1 M NaOD in SDS-D2O. Structures of the Hydroxamic Acids. The short chain acid, 1a, is soluble in the organic solvents and in D2O. We saw the following signals of E-1a in DMSO-d6, δ, ppm referred to TMS: OH, 10.49 (s, 1H); aromatic; o, 7.63 (d, 2H); m, 7.36 (t, 2H); p, 7.13 (t, 1H); aliphatic; CH2, 2.58 (q, 2H); CH3, 1.05 (t, 3H). There were no signals of Z-1a. The spectra of 1a and 2a are similar in DMSO-d6, and for E-2a were as follows: OH, 10.50 (s, 1H); aromatic; o, 7.61 (d, 2H); m, 7.36 (t, 2H); p, 7.14 (t, 1H); aliphatic; CH2(1), ≈2.5 (overlapped by DMSO); CH2(2), 1.53 (q, 2H); CH2(3-10), 1.25 (b, 16H); CH3, 0.68 (t, 3H). The 1H-NMR spectra of 1a and 2a are also similar in CD3OD and for 1a were as follows: aromatic; o, 7.57 (d, 2H); m, 7.38 (t, 2H); p, 7.21 (b, 1H); aliphatic; CH2, 2.65 (b, 2H); CH3, 1.16 (t, 3H), and for 2a: aromatic; o, 7.56 (b,2H); m, 7.38 (b, 2H); p, 7.21 (b, 1H); aliphatic; CH2(1), 2.64 (b, 2H); CH2(2), 1.67 (b, 2H); CH2(3-10), 1.30 (b, 16H); CH3, 0.90 (t, 3H). (8) (a) Zachariasse, K. A.; Phuc, N. V.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2672. (b) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. J. Phys. Chem. 1982, 86, 3198. (c) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; 2nd ed.; VCH: New York, 1988. (d) Drummond, C. J.; Grieser, F.; Albers, S. Colloids Surf. 1991, 54, 197. (9) (a) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708. (b) Abraham, M. H.; Chadha, H. C; Dixon, J. P.; Rafols, C.; Treiner, C. J. Chem. Soc., Perkin Trans. 2 1995, 887. (c) Abraham, M. H.; Chadha, H. S.; Dixon, J. P.; Rafols, C. J. Chem. Soc., Perkin Trans. 2 1997, 19. (10) (a) Exner, O.; Simon, W. Collect. Czech. Chem. Commun. 1965, 30, 4078. (b) Exner, O. Angew. Chem., Int. Ed. Engl. 1974, 13, 376. (11) (a) Blasko, A.; Bunton, C. A.; Wright, S. J. Phys. Chem. 1993, 97, 5435. (b) Blasko, A.; Bunton, C. A.; Foroudian, H. J. J. Colloid Interface Sci. 1995, 175, 122. (c) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497. (12) Noggle, J. H.; Schirmer, R. E. The Nuclear Overhauser Effect: Chemical Applications; Academic Press: New York, 1971.
Blasko et al.
Results NMR Spectroscopy and Signal Assignments. The hydroxamic acids, 1a and 2a, are E-isomers in DMSO-d6 and CD3OD (Experimental Section), but 1a is soluble in D2O where E- and Z-isomers coexist.6a The aromatic signals are overlapped at δ ) 7.45 ( 0.15 ppm, and the spread is much less than that seen in organic solvents. The aliphatic signals are broad and are: for Z-1a; CH2, 2.23; CH3, 1.06 and for E-1a; CH2, 2.73; CH3, 1.17, and χZ ) 0.43 and 0.40 based on the CH2 and CH3 signals, respectively. We expected the hydroxamate ion to exhibit E-Z isomerism and 1b could be examined in DMSO-d6, CD3OD, and D2O, but 2b is sparingly soluble in D2O and was examined in the organic solvents, aqueous organic mixtures, and aqueous surfactants only, but 1b is bound only to cationic micelles of CTABr. Signals of the anions in the aliphatic region were generally sharp, except in surfactant solutions and with 2b where there is a broad signal of hydrogens in the middle of the chain. Signals were broader with the acids, probably because of lower barriers to isomerization. The sharp signals had the expected multiplicity and coupling constants were J ) 7-8 Hz and 7.2-7.7 Hz for the aromatic and aliphatic regions, respectively, under all conditions in which they could be determined. We used COSY spectra to confirm the coexistence of Eand Z-isomers of 1b and 2b. With 1b there were cross peaks of CH2 and CH3 for both isomers in CD3OD-KOD. There were well-defined cross peaks of the ortho and meta and meta and para hydrogens of E-1b but signals of Z-1b were partially overlapped. However, we observed a clear signal of the meta hydrogen with cross peaks with the ortho hydrogen at 7.37 ppm and the para hydrogen at 7.32 ppm, although we could not establish the multiplicity of these signals. We saw similar cross peaks for 2b in the aromatic region. In the aliphatic region we could assign signals of CH2(3) at 1.42 and 1.18 ppm for E- and Z-2b, respectively, and CH2(10) at 1.31 ppm for both isomers, even though in the normal spectra these signals could not be separated from the others of the chain. Chemical shifts in CD3OD are given in Tables 1 and 2 for 1b and 2b, respectively, and spectra are shown in Figures S1-4. Relative amounts of E- and Z-isomers were estimated from areas of the CH2 and CH3 signals of 1a,b and of the first two CH2 signals of 2a,b. Provided that we could identify at least one signal of an aromatic hydrogen, we also estimated isomeric composition from the aromatic signals. However, estimates sometimes depended on comparisons of areas of small and large signals which decreased their reliability. In some experiments we could determine a chemical shift but not the area of a signal because of its proximity to a larger signal, e.g., of solvent or surfactant. Medium Effects on the Isomerism of the Hydroxamate Ions. Addition of D2O to DMSO-d6-KOD promotes conversion of E- into Z-1b, 2b (Tables 3 and 4 respectively) as expected based on earlier work.6a However, signals in the aromatic region tend to converge as χD2O increases and only approximate chemical shifts can be determined. Examples of 1H-NMR spectra in this solvent are shown in Figures S5-8. There are significant differences in the behaviors of 1b and 2b on addition of D2O. With DMSO-d6-D2O we see a monotonic increase in Z-1b to χZ ) 0.31 in D2O but with 2b there is a sharp increase in Z-2b up to the solubility limit and it appears that, but for its solubility, 2b would be wholly the Z-isomer in D2O (Figure 1). The amount of Z-1b also increases on addition of D2O to CD3OD-KOD (Table 1) and goes through a mild
Geometrical Isomerism of Hydroxamic Acids
Langmuir, Vol. 13, No. 24, 1997 6441
Table 1. Mole Fraction, χZ, of Z-1b and 1H Chemical Shifts in ppm of N-Phenylpropionohydroxamate Ion in CD3OD-D2Oa D2O vol. % χD2O
0.3 0.004
9.1 0.183
21.1 0.374
31.9 0.513
40 0.599
62.5 0.789
75.0 0.871
88.2 0.944
100 1.0
χZ CH2 χZ CH3 χZ Phenylb
32 35 40
38 39 41
43 42 43
43 43 37c
43 41 40c
38 39 N/C
34 37 N/C
33 34 N/C
31 30 N/C
δ CH2, Z δ CH2, E δ CH3, Z δ CH3, E δ o-Ph, Z δ o-Ph, E δ m-Ph, Z δ m-Ph, E δ p-Ph, Z δ p-Ph, E
r 2.77 1.05 1.17 7.35A 7.70 7.42 7.31 7.35A 7.12
r 2.76 1.05 1.17 7.35A 7.63 7.44 7.34 7.35A 7.16
r 2.73 1.04 1.18 7.35A 7.55 7.46 7.35A 7.35A 7.21
2.06 2.71 1.04 1.17 7.38B 7.49 7.47 7.38B 7.38B 7.25
f 2.70 1.03 1.17 v v 7.42C V V 7.28
f 2.67 1.02 1.17 v v 7.40C V V V
f 2.66 1.01 1.16 v v 7.41C V V V
2.07 2.65 1.01 1.17 v v 7.41C V V V
2.08 2.65 1.01 1.17 v v 7.41C V V V
a At 21.5 °C, δ relative to TSP and 0.02 M KOC. N/C indicates that the signal was partially obscured or overlapped by other aromatic signals. Chemical shift ranges are: A (0.05 ppm, B (0.04 ppm, and C (0.1 ppm. b Based on o- and p-E signals except where noted. c Based on p-E signal only.
Table 2. Mole Fraction, χZ, of Z-2b and 1H Chemical Shifts in ppm of N-Phenyldodecanohydroxamate Ion in CD3OD-D2Oa
b
D2O vol. % χD2O
0.3 0.004
4.8 0.101
9.1 0.184
13.0 0.252
16.7 0.310
20.0 0.360
23.1 0.403
28.6 0.473
40.0 0.599
χZ CH2(1) χZ CH2(2) χZ phenylb
29 31 30
32 33 33
35 36 37
37 39 39
40 41 42
43 44 44
45 46 46
49 50 49
54 54 57c
δ CH2(1), Z δ CH2(1), E δ CH2(2), Z δ CH2(2), E δ CH2(3-10), Z δ CH2(3-10), E δ CH3, Z δ CH3, E δ o-Ph, Z δ o-Ph, E δ m-Ph, Z δ m-Ph, E δ p-Ph, Z δ p-Ph, E
r 2.75 1.55 r r r r r 7.38B 7.70 7.38B 7.30 7.38B 7.11
r 2.75 1.55 r r r r r 7.38B 7.67 7.38B 7.32 7.38B 7.13
r 2.74 1.55 r r r r r r 7.64 7.43 7.33 r 7.14
2.05 2.74 r 1.69 r r 0.90 0.90 r 7.61 7.43 7.34 r 7.16
f 2.73 r f 1.3A 1.3A f f r 7.59 7.44 r r 7.17
f 2.73 1.54 f f f f f 7.35C 7.57 7.45 r 7.35C 7.18
f 2.72 f 1.68 f f 0.89 0.89 f 7.55 7.45 7.35C f 7.20
f 2.71 f 1.68 f f 0.89 0.89 f 7.53 7.45 f f 7.21
2.04 2.69 1.53 1.67 f f 0.89 0.89 f 7.40D 7.40D 7.40D 7.40D 7.25
a At 21.5 °C, δ relative to TSP and 0.02 M KOD. Chemical shift ranges are: A (0.2 ppm, B (0.08 ppm, C (0.05 ppm, and D (0.1 ppm. Based on o- and p-E signals except where noted. c Based on p-E signal only.
Table 3. Mole Fraction, χZ, of Z-1b and 1H Chemical Shifts in ppm of N-Phenylpropionohydroxamate Ion in DMSO-d6-D2Oa D2O vol. % χD2O
0.3 0.012
χZ CH2 χZ CH3 χZ phenylc δ CH2, Z δ CH2, E δ CH3, Z δ CH3, E δ o-Ph, Z δ o-Ph, E δ m-Ph, Z δ m-Ph, E δ p-Ph, Z δ p-Ph, E
2.50 0.94 7.71 7.24 7.01
9.9 0.301
27.5 0.598
37.5 0.701
50.8b 0.801
69.9 0.901
100 1.0
0.07 N/C 0.10
0.15 0.19 0.18
N/C 0.17 0.18
N/C 0.24 0.24d
N/C 0.27 N/C
0.31 0.30 N/C
1.95 2.65 0.92 0.98 7.25A 8.00 7.25A 7.17 7.25A 6.89
1.95 2.62 0.95 1.03 7.28B 7.75 7.40 7.26 7.28B 7.04
1.99 2.64 0.98 1.07 7.34C 7.68 7.45 7.33 7.34C 7.12
2.01 2.62 0.99 1.10 7.41D 7.50 7.41D 7.35B 7.41D 7.21
2.04 2.63 0.99 1.12 v v 7.38E V V V
2.08 2.65 1.01 1.17 v v 7.41D V V V
a
At 21.5 °C, δ relative to TMS in DMSO and TSP in aqueous media and 0.02 M KOD. N/C indicates that the signal was partially obscured or overlapped by DMSO or other aromatic and aliphatic signals. Chemical shift ranges are A (0.15 ppm, B (0.05 ppm, C (0.04 ppm, D (0.1 ppm, and E (0.13 ppm. b Similar values of χZ were obtained in 50.0 vol % D2O. c Based on o- and p-E signals except where noted. d Based on p-E signal only.
maximum. However, the amount of Z-2b increases monotonically up to the solubility limit, as in DMSO-d6D2O (Figure 1 and Table 2). The plot of the relative amount of Z-2b against χD2O is approximately linear and extrapolation supports the evidence in DMSO-d6-D2O indicating
that in D2O 2b should be largely the Z-isomer. Examples of 1H-NMR spectra for this solvent are shown in Figures S9-13. Several factors favor E to Z conversion in polar, hydroxylic, solvents (Scheme 1). Unfavorable interactions
6442 Langmuir, Vol. 13, No. 24, 1997
Blasko et al.
Table 4. Mole Fraction, χZ, of Z-2b and 1H Chemical Shifts in ppm of N-Phenyldodecanohydroxamate Ion in DMSO-d6-D2Oa D2O vol. % χD2O
0.3 0.012
χZ CH2(1) χZ CH2(2) χZ phenylb δ CH2(1), Z δ CH2(1), E δ CH2(2), Z δ CH2(2), E δ CH2(3-10), Z δ CH2(3-10), E δ CH3, Z δ CH3, E δ o-Ph, Z δ o-Ph, E δ m-Ph, Z δ m-Ph, E δ p-Ph, Z δ p-Ph, E
c 1.55 1.25 r 7.62 7.34 7.12
9.9 0.301
27.5 0.598
65.2 0.880
7 N/C 11
N/C 25 24
69 69 N/C
1.93 2.62 d 1.49 1.26A 1.26A 0.86 r 7.20E 7.99 7.34 7.16 7.2E 6.88
1.94 c 1.44 1.53 1.2B 1.2B 0.86 f 7.27A 7.72 7.39 7.26 7.27A 7.04
2.00 2.61 1.51 1.62 1.07C 1.32D 0.70 f v v 7.33E V V V
0.86
a
At 21.5 °C, δ relative to TMS in DMSO and TSP in aqueous media and 0.02 M KOD. N/C indicates that the signal partially obscured or overlapped by DMSO or other aliphatic and aromatic signals. Chemical shift ranges are A (0.06 ppm, B (0.2 ppm, C (0.13 ppm, D (0.12 ppm, and E (0.1 ppm. b Based on o- and p-E signals. c Overlapped by DMSO. d As lower shift shoulder to signal of E-isomer.
Figure 1. Effect of χD2O on χZ-Isomer of N-phenylpropiono- (1b) and N-phenyldodecanohydroxamate (2b) ions in aqueous organic solvents: (4) 1b in CD3OD; (2) 1b in DMSO-d6; (b) 2b in CD3OD and (O) 2b in DMSO-d6 at 21.5 °C. The solid lines are guides for the eye.
between anionoid oxygens in Z-isomers decrease with an increase in dielectric constant and hydrogen-bonding donation by the solvent.6a In the more aqueous media there will be a hydrophobic interaction between the phenyl and the n-alkyl groups of Z-2b,13 but it will be relatively unimportant in Z-1b. Both water and methanol are hydrogen bond donors and they have similar effects on the E:Z ratio of 1b (Figure 1 and Table 1), and it is the hydrophobic interaction that gives the large amounts of Z-2b in the wetter solvents (Figure 1 and Tables 2 and 4). (13) (a) Tanford, C. The Hydrophobic Effect; 2nd ed.; Wiley: New York, 1980. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982.
Figure 2. 400 MHz 1H-NMR spectrum of 2b (0.01 M) in the aliphatic region in DMSO-d6 ([KOD] ) 0.02 M) with χD2O ) 0.88 at 21.5 °C.
Under most conditions chemical shifts of ω-CH3 and CH2(3-10) are very similar for E- and Z-2b, indicating that the alkyl group is largely extended, but the situation changes with high D2O in DMSO where chemical shifts differ in the E- and Z-isomers. In all conditions chemical shifts in the aliphatic region are lower for the Z- than for the E-isomer,6a due to shielding by the phenyl group. In DMSO-d6 with χD2O ) 0.9 the hydrophobic effect causes the alkyl group to take up cyclic rather than extended conformations, which brings ω-CH3 and the rest of the chain close to the hydroxamate residue. We therefore see separate signals, not only for the first two CH2 groups but also for ω-CH3 and the rest of the chain (Figure 2 and Table 4). Micellar Effects. The short chain hydroxamate ion, 1b, should bind to CTABr in D2O-KOD.4,5 The aliphatic signals are not obscured by those of CTABr (Figure S14) and χZ ) 0.45, which is slightly higher than χZ ) 0.34 and 0.31 in CD3OD and D2O, respectively (Table 1), and NMR signals are similar. The aromatic signals are heavily overlapped, as in D2O-KOD (Table 1). It appears that 1b is located, on the average, in a water rich region at the micellar surface where it has considerable freedom of movement. We did not examine the NMR spectrum in anionic or nonionic micelles where 1b would remain largely in the water.4,5 The Z-isomer of 2b is dominant in comicelles with all the inert surfactants used, regardless of their charge (Figure 3 and Table 5). Except for CTBABr as surfactant we saw no signals of CH2(1) of E-2b which should be at δ ) 2.6-2.7 ppm (Tables 2 and 4) and would not be obscured by those of the surfactants, and except in CTBABr and C12E10, we saw no well-separated aromatic signals of E-2b (Figure 4). In general the aromatic signals of Z-2b have chemical shifts similar to those of Z-2b in hydroxylic solvents (Figure 5). The formation of E-2b, χE ) 0.17 calculated from integration of E- and Z-CH2(1), in micelles of CTBABr (Table 5 and Figure 6) deserves comment. We could not see signals of E-CH2(2) because they are obscured by those of CTBABr itself (Figure 6), but the additional signals of E-CH2(1) and those in the aromatic region are evident, although here there is some overlap of the E- and Z-signals. Signals are not sharp, but signals of the small amount of Et2O in the solution are sharp, as in other surfactants. In all the surfactant solutions we use TSP, δ ) 0 ppm, as the internal reference, although it is anionic and may bind to the cationic micelles. There may be errors in our chemical
Geometrical Isomerism of Hydroxamic Acids Table 5.
1H
Langmuir, Vol. 13, No. 24, 1997 6443
Chemical Shifts in ppm of Z-2b in D2O-KOD in Surfactants and Saltsa SB3-14
CTABr δ CH2(1) δ CH2(2) δ o-Ph δ m-Ph δ p-Ph
2.00 (2.01) 1.55 (1.54) 7.36A (7.35A) 7.36A (7.35A) 7.23 (7.22)
CTBABrb 2.02 c 7.37A 7.37A 7.27
no salt 2.05 (2.06) 1.58 (1.58) 7.41C (7.40A) 7.41C (7.40A) 7.30 (7.27)
SDS
0.2 M NaClO4 2.06 1.58 7.36C 7.46 7.36C
C12E10
no salt
0.5 M TMAN
2.03 (2.13) c c 7.36A (7.41A) 7.36A (7.41A) 7.23 (7.29)
2.09 (2.08) 1.58 (1.56) 7.40 (7.39) 7.50 (7.47) 7.40B (7.37)
2.08 1.58 7.41 7.49 7.37
a At 25.0 °C, δ relative to TSP with [2b] ) 0.01 M, [surf]/[2b] ) 5 and 0.02 M KOD, except for experiments with SDS where 0.1 M NaOD was used. Values in parentheses are those at 50 °C. Chemical shift ranges are A (0.05 ppm, B (0.03 ppm, and C (0.06 ppm. b CTBABr also shows some E-isomer signals which are: aliphatic; CH2(1), 2.67 ppm; CH2(2) obscured by surfactant and aromatic signals at 7.65 ppm, probably o-H and 7.17 ppm, probably p-H, with the m-H probably obscured by the Z signals. c Signal obscured by surfactant signals.
Figure 5. 400 MHz 1H-NMR spectra of 2b (0.01 M) in CD3OD (0.02 M KOD) at 21.5 °C.
Figure 3. 500 MHz 1H-NMR spectra of 2b (0.01 M) comicellized with (a) SDS (0.1 M NaOD) and (b) CTABr (0.02 M KOD) at 50 °C and [surf]/[2b] ) 5.
Figure 6. 400 MHz 1H-NMR spectra of 2b (0.01 M) comicellized with CTBABr (0.02 M KOD) at 25.0 °C and [CTBABr]/[2b] ) 5.
Figure 4. 500 MHz 1H-NMR spectra of 2b (0.01 M) comicellized with C12E10 (0.02 M KOD) at 50 °C and [C12E10]/[2b] ) 5.
shifts due to binding of TSP, but they should be small, because the observed chemical shift of the methylene protons of Et2O is always at ca. 2.50 ppm.
Chemical shifts of the hydroxamate ions in the aliphatic region are not very sensitive to solvent or micellar effects (Tables 1-5), but, as noted, chemical shifts of the E-isomers in the aromatic region are solvent sensitive. The preference for Z-2b in micelles allows the phenyl and alkyl groups to interact with the more apolar regions of micelles. This interaction will be somewhat less important in micelles of CTBABr where relatively hydrophobic butyl groups are at the cationic center. There is no Coulombic interaction of 2b with nonionic micelles of C12E10, and if the phenyl and alkyl groups of Z-2b interact with the apolar core, the anionoid oxygens will be in the hydrated palisade layer,14 but interactions with E-2b will be less favorable. However, there were minor
6444 Langmuir, Vol. 13, No. 24, 1997
signals (χE < 0.1) which appeared to be of E-2b (Figure 4). Their chemical shifts are very different from those of E-isomers in aqueous media (Tables 2 and 4) and are probably due to material in the palisade layer rather than residual 2b in the bulk solvent. Signals of 2b in comicelles with CTABr, CTBABr, SB314, and C12E10 in the aliphatic region are broad and are not split, and aromatic signals are not well separated, as shown for CTABr in Figure 3b. Here there is one large and one small signal with relative areas 4:1, and the smaller signal, with the lower chemical shift, is assigned to the para-H. Signals in the aliphatic and aromatic regions are relatively sharp in SDS (Figure 3a), but less so than those obtained in DMSO-d6 or CD3OD (Figure 5). There are two signals in the aromatic region, with relative areas 2:3. That at the higher chemical shift is a well-defined triplet (2H), and we assign it to meta-H. The larger signal appears to be the result of an overlapping doublet (2H), due to ortho-H, and a triplet (1H) of para-H, at lower chemical shift. The spectra at 50 °C are significantly sharper than those at 25 °C, but forms are similar. Complete spectra for all surfactants at 25 and 50 °C are given in Figures S15-23. The sharpening of signals with an increase in temperature is more evident with SDS than with the other surfactants, and with SDS there is a change in the usual sequence of chemical shifts for the E-isomer in the aromatic, but not in the aliphatic, regions (Tables 1-4 and Figure 5). We show in these tables chemical shifts in the aromatic region that are imprecise but are within the specified range. In some spectra there is a sharp signal of the methylene protons of Et2O at δ ≈ 2.5 ppm, with the predicted multiplicity, showing that the broad signals of the hydroxamate ions are not due to instrumental limitations. The differences between the 1H-NMR spectrum of Z-2b in SDS and the other surfactants (Figures 3 and S15-23) indicate that there are differences in location, at least of the phenyl groups. The phenyl group has more mobility in SDS than in other surfactants due to several factors: (i) the mobility of the phenyl group in CTABr, CTBABr, and SB3-14 will be limited by interaction with the cationic centers of the head groups15 and by interaction with the apolar interior of C12E10 micelles; (ii) electrostatic interactions between the anionic residues of Z-2b and SDS will favor location of the hydroxamate residue away from the apolar region of the micelle and in a more “water-rich” part of the interfacial region as compared with the location in micelles of CTABr, CTBABr, or SB3-14. Salt-Surfactant Interactions. Quaternary ammonium ions bind to anionic micelles,4bc,16 and there is kinetic and physical evidence for strong interactions between ClO4- and other low charge-density anions and micelles of SB3-14.11b,17 Addition of Me4NNO3 (TMAN) to SDS at 25 °C has little effect on chemical shifts of 2b in the aliphatic region (Table 5), and signals in the aromatic region (Figure 7) are similar to these in SDS at 50 °C (Figure 3a). Addition of NaClO4 to SB3-14 has little effect upon the chemical shifts of signals of 2b in the aliphatic (14) (a) Elworthy, P. H.; Florence, A. T.; Macfarlane, C. B. Solubilization by Surface-Active Agents; Barnes and Noble: New York, 1968; Chapter 1. (b) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425 and references cited therein. (15) (a) Foreman, M. I. In Nuclear Magnetic Resonance; Harris, R. K., Ed.; Specialist Periodical Reports, Royal Society of Chemistry: London, 1972; p 295. (b) Bunton, C. A.; Cowell, C. P. J. Colloid Interface Sci. 1988, 122, 154. (c) Bachofer, S. J.; Simonis, U.; Nowicki, T. A. J. Phys. Chem. 1991, 95, 480. (d) Kreke, P. J.; Magid, L. J.; Gel, J. C. Langmuir 1996, 12, 699. (16) Romsted, L. S.; Yoon, C.-O. J. Am. Chem. Soc. 1993, 115, 989. (17) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 854. (b) Savelli, G. Personal communication.
Blasko et al.
Figure 7. Effect of added electrolyte on the 400 MHz 1H-NMR spectra of Z-2b (0.01 M) in the aromatic region in D2O at 25.0 °C: (a) SB3-14 in 0.2 M NaClO4 (0.02 M KOD); (b) SDS in 0.5 M Me4NNO3 (0.1 M NaOD).
region (Table 5) but it changes those in the aromatic region, which become similar to those in SDS (Figure 7). Addition of NaClO4 could decrease the extent of deprotonation of 2a in micelles of SB3-14 by excluding OH- from the micellar pseudophase,5,18 but we obtain similar NMR spectra in 0.02 and 0.05 M KOD. Comparison of the spectra in Figures 3a and 7b shows the effect of temperature in SDS in the absence of salt. Observation of NOE. Nuclear Overhauser effects (NOEs) provide evidence on the proximity of groups.12 Conditions are given in the Experimental Section. We did not see an NOE with an E-Z mixture of 2b in CD3OD probably because isomers are interconverting on the time scale of the NOE experiment. There were clear signals in 0.05 M CTABr, 0.02 M KOD, and 0.01 M 2b in D2O at 25 °C, where only the Z-isomer is present. Irradiation of CH2(1) decreased the intensity of the aromatic signals and irradiation of the aromatic region decreased the intensities of signals of CH2(1) and CH2(2). The signal of CH2(2) was too close to signals of CTABr for it to be irradiated. The evidence for an NOE in 0.05 M SDS, 0.1 M NaOD, and 0.01 M 2b in D2O at 25 °C was uncertain and any NOE was minor. We did not use SB3-14 and C12E10 because some of their methylene signals are too close to those of Z-2b. Discussion Locations of Hydroxamate Ions in Micelles. The short chain hydroxamate ion, 1b, is an E-Z mixture in both D2O and CTABr, and chemical shifts are similar in the two media (Table 1 and Figure S14). It appears to be located in a water-rich region of the interface where it has considerable mobility. The behavior of the long chain 2b is very different. Although 2b is insoluble in D2O, the results in mixed solvents (Figure 1 and Tables 2 and 4) indicate that the Z-isomer would be dominant in D2O. In mixed and organic solvents the alkyl group is probably (18) Bunton, C. A.; Mhala, M..; Moffatt, J. R. J. Org. Chem. 1987, 52, 3832.
Geometrical Isomerism of Hydroxamic Acids
extended, but in DMSO-d6-D2O with χD2O ) 0.9 hydrophobic interactions may induce coiling in the chain. Although we saw no evidence for E-2b in micelles of CTABr, some is present (χE ) 0.17) in micelles of CTBABr (Figure 6) indicating that the hydroxamate moiety is in a less hydrophilic region of the micellar surface than in the other surfactant micelles. This difference is understandable because the three n-butyl residues should decrease the amount of water adjacent to the cationic head groups. Interfacial regions of aqueous micelles and similar association colloids are somewhat less polar than water,8,9 and effects on the rate constants of reactions in these regions are often interpreted on the assumption that polarities are similar to those of medium chain length alcohols.5 Micellar effects on the E-Z equilibrium do not correlate with the polarity of the interfacial region, because an aprotic solvent such as DMSO strongly favors the E-isomer and it is slightly preferred to the Z-isomer in CD3OD, but except for CTBABr there is little or no E-isomer in micelles, regardless of their charge (Figure 3). The hydroxamate ion 2b is only slightly soluble in water, so incomplete micellar incorporation is not a factor, and it appears that the preference for the Z-isomer is governed by geometrical constraints. Location of the hydroxamate residue of Z-2b in comicelles depends upon their charge, and differences in location in SDS, as compared with other micellized surfactants, shows the limitation of the implicit assumption that the interfacial region can be treated as if it is a uniform microsolvent.4,5 The addition of NaCIO4 to micellized SB3-14 and Me4NNO3 to micellized SDS affects 1H-NMR signals in the aromatic region (Table 5 and Figure 7). There are indications that anions interact more strongly than cations with zwitterionic, sulfobetaine, micelles which should then behave more like anionic micelles. These interactions are much stronger for polarizable anions, e.g., Br-, ClO4-, or IO4-, than for very hydrophilic, high-charge density anions, e.g, OH- or F-.11b,17-19 Accordingly, aromatic signals of 2b in micelles of SB3-14 (Figure 7a) are similar to those in micelles of CTABr (Figure 3b), but on addition of NaClO4 they look much more like the signals of 2b in SDS micelles (Figures 3a and 7b). It appears therefore that ClO4- is interacting with the quaternary ammonium centers of the sulfobetaine head groups and generating an anionoid micelle in which the hydroxamate residue of 2b will be in a water-rich environment, as in SDS micelles. We explain the effects of NaClO4 on the 1H-NMR spectrum of 2b in SB3-14 in terms of generation of some negative charge in the micelle, and conversely we might expect addition of Me4NNO3 to reduce the negative charge on the SDS micelle so that it should behave more like a zwitterionic, sulfobetaine, micelle, but this simplistic approach does not fit the data. The 1H-NMR spectrum in the aromatic region changes very little on the addition of Me4NNO3 to SDS (Figure 7b). The “footprint” of the signal increases slightly on addition of Me4NNO3, possibly due to growth of the micelle, but the spectrum at 25 °C is very similar to that in SDS at 50 °C (Figure 3a) without added salt. Therefore it appears that the location of 2b in the interfacial region is not significantly affected by addition of Me4NNO3, perhaps because the relatively hydrophobic quaternary ammonium ion does not pair with the hydrophilic sulfate residue, but interacts largely with the beginning of the chain. (19) (a) Baptista, M. S.; Cuccovia, I.; Chaimovich, H.; Politi, M. J.; Reed, W. F. J. Phys. Chem. 1992, 96, 6442. (b) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234.
Langmuir, Vol. 13, No. 24, 1997 6445
Added electrolytes decrease rates of micellar-mediated bimolecular reactions of counterions with micellar-bound substrates. These inhibitions are fitted quantitatively by treatments that describe interionic competition for micelles.4bc,5,20 However, based on our results with micelles of SB3-14 and NaClO4 it appears that added electrolytes may affect locations of reactants in the interfacial region of the micelle, and in these special cases the simple ionexchange models of salt effects will be inadequate. There are many examples of rate enhancements of bimolecular reactions by association colloids, e.g., cationic micelles accelerate bimolecular reactions of anionic bases and nucleophiles.4,5 Typically second-order rate constants calculated in terms of molarity in the micellar pseudophase are similar to those in water showing that the rate enhancements are governed largely by concentration of reactants in the micellar pseudophase. This generalization also applies to reactions mediated by vesicles,21 oilin-water micremulsions,22 and nonmicellar amphiphilic clusters.23,24 The question of uniform composition of the interfacial region and its relation to reactivity in the micellar pseudophase has been discussed extensively, especially as regards bimolecular reactions involving small ionic reagents,4,5,20,26 but our NMR data indicate that location of the nucleophilic hydroxamate residue depends both on the hydrophobicity of the alkyl group and on the micellar charge. For example, the short chain hydroxamate ion, 1b, bound to micelles of CTABr (Figure S14) has an E:Z ratio very similar to that in D2O (Table 1), indicating that it is in a water-rich region. But the long chain hydroxamate ion, 2b, comicellized with CTABr is, within experimental error, exclusively the Z-isomer (Figure 3b), and its structure and location in the micelle appear to be different from those of 1b. Location of the hydroxamate residue of 2b in comicelles depends to some extent on their charge, based on differences in the forms of the aromatic signals in SDS comicelles and those of the other surfactants (Figure 3). It appears that in SDS the hydroxamate residue is further away from the micellar core and in a more hydrophilic region than in comicelles with CTABr, CTBABr, SB3-14, or C12E10. These differences in location are understandable in view of unfavorable electrostatic interactions between the hydroxamate and sulfate residues. In addition, in CTABr and especially in CTBABr, the cationic nitrogen is shielded from water and hydrophilic anions by methyl groups but there is no such shielding in SDS. We note that in interactions with ions, zwitterionic, sulfobetaine, micelles behave more like cationic than anionic micelles.17-19 Estimates of the polarities of micellar interfacial regions, as being similar to, but lower than that of water,4,5,8,9 fit the effect of CTABr on the isomeric composition of the (20) (a) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (b) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538. (c) Rodenas, E.; Ortega, F. J. Phys. Chem. 1987, 91, 837. (d) Al-Lohedan, H. A. J. Chem. Soc., Perkin Trans. 2 1995, 1707. (21) Kawamuro, M. K.; Chaimovich, H.; Abuin, E. B.; Lissi, E. A.; Cuccovia, I. M. J. Phys. Chem. 1991, 95, 1458. (22) (a) Mackay, R. A. Adv. Colloid Interface Sci. 1981, 15, 131. (b) Mackay, R. A. J. Phys. Chem. 1982, 86, 4756. (c) Athanassakis, V.; Bunton, C. A.; de Buzzaccarini, F. J. Phys. Chem. 1982, 86, 5002. (23) Bunton, C. A.; Hong, Y. S.; Romsted, L. S.; Quan, C. J. Am. Chem. Soc. 1981, 103, 5788. (24) These generalizations do not apply to reactions of anionic electrophiles.11,25 (25) (a) Bacaloglu, R.; Blasko, A.; Bunton, C. A.; Foroudian, H. J. J. Phys. Org. Chem. 1992, 5, 171. (b) Cerichelli, G.; Grande, C.; Luchetti, L.; Mancini, G. J. Org. Chem. 1991, 56, 3025. (26) Davies, D. M., Gillitt, N. D., Paradis, P. M. J. Chem. Soc., Perkin Trans. 2 1996, 659.
6446 Langmuir, Vol. 13, No. 24, 1997
short chain hydroxamate ion, 1b, but they do not fit micellar effects on the isomeric composition of the long chain, 2b. Our evidence on micellar effects upon both the isomeric composition of 2b and its locations in the interfacial region illustrates a limitation in the pseudophase treatment of second-order rate constants in aqueous association colloids.3-5 One test of this treatment involved comparison of second-order rate constants for dephosphorylation by 1b in water and by the C14 analog of 2b in comicelles with CTABr.3 Second-order rate constants, calculated with local hydroxamate ion concentrations in the aqueous and micellar pseudophases, were similar, indicating that the rate enhancements of the overall reaction were due largely to concentration of hydroxamate ion in the small volume of the interfacial region. This general approach has been used to analyze rate enhancements by other functional micelles and comicelles.5,27 Analysis of the hydroxamate data in micellar and nonmicellar conditions3 is also clouded by the fact that in water the hydroxamate ion is a mixture of E- and Z-isomers which may have different nucleophilicities, but in comicelles a long chain hydroxamate ion exists wholly as the Z-isomer (Figure 3). This medium effect on the structure of a nucleophile complicates analysis of kinetic solvent effects for those R-effect nucleophiles which can exist as geometrical isomers.1 The observation that in comicelles with SDS the hydroxamate residue of 2b is in a more aqueous region than in comicelles with cationic, zwitterionic, and nonionic surfactants is consistent with the low overall rate of reaction of 2b with p-nitrophenyl diphenyl phosphate, as compared with rates in comicelles with other surfactants.3,28 The very hydrophobic ester is probably bound at the apolar micellar core, and in an SDS comicelle its location could be very different from that of the hydroxamate residue. Analyses of nonionic solute distributions between water and micelles based on solvatochromic scales indicate that micellar charge influences the properties that control interactions.9 In particular, the cavity term, which includes hydrophobic interactions, is smaller for SDS than for CTABr and the hydrogen-bond donor term of SDS is smaller than that of CTABr but similar to that of water. This treatment does not apply to ionic solutes, but it indicates that solute location in the interfacial region may depend on specific interactions between solute and head group which are related to micellar charge. The question of relative locations of an apolar substrate and an ionic reagent, e.g., a nucleophile, in the micellar pseudophase may be more serious with amphiphilic nucleophiles, e.g., 2b, than with small ions.4,5 As noted the hydrophobic residues of 2b, for example, probably (27) (a) Fornasier, R.; Tonellato, U. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1301. (b) Bunton, C. A.; Scrimin, P.; Tecilla, P. J. Chem. Soc., Perkin Trans. 2 1996, 419. (28) Bunton, C. A.; Foroudian, H. J.; Gillitt, N. D. Unpublished results.
Blasko et al.
govern the location of the reactive hydroxamate residue, but interactions of small ions involve head groups in the interfacial regions rather than the apolar tails. For example, ionic micelles inhibit but do not suppress, bimolecular reactions of hydrophilic co-ions,29 and based on estimated local ionic concentrations, second-order rate constants in the micellar pseudophase are generally not very sensitive to micellar charge.30 However, these comparisons were for reactions of small inorganic ions, unlike 1b and 2b, where changes in isomeric composition may affect reactivity. One quantitative treatment of micellar effects in bimolecular reactions of small inorganic ions is based on a predicted nonuniform ionic distribution in the interfacial region and estimated local second-order rate constants depend on the geometry of this region.11a,20b-d,26 Other theoretical treatments involve an assumed uniform ionic distribution4,5 as do some experimental methods,31 and these calculated second-order rate constants vary linearly with volume of this region. The dediazonization trapping method involves no assumptions regarding the geometry of the interfacial region,32 but it is not applicable to very basic nucleophiles. Our main concern in this work is the structure of hydroxamate ions and their location in micelles, but solvent effects upon the E-Z equilibria (Tables 1-4) show that one has to be cautious in interpreting solvent effects upon reactivities of R-effect nucleophiles1,33 which exhibit E-Z isomerism. Acknowledgment. Support by the US Army Research Office is gratefully acknowledged. Upgrading of the NMR spectrometers was made possible by the National Science Foundation Grant CHE-940775. Supporting Information Available: Figures S1-S13 showing 1H-NMR spectra of 1b and 2b in selected organic solvents and mixtures with D2O and Figures S14-S23 showing 1H-NMR spectra in surfactants (48 pages). Ordering information is given on any current masthead page. LA9706025 (29) (a) Chaimovich, H.; Aleixo, R. M. V.; Cuccovia, I. M.; Zanette, D.; Quina, F. H. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. 2, p 949. (b) Lelievre, J.; Gaboriaud, R. J. Chem. Soc., Faraday Trans. 1 1985, 81, 335. (30) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 7851. (b) Blasko, A.; Bunton, C. A.; Armstrong, C.; Gotham, W.; Zhen-Miu, H.; Nikles, J.; Romsted, L. S. J. Phys. Chem. 1991, 95, 6746. (c) Aonado, S.; Garcia-Rio, L.; Leis, J. R.; Rios, A. Langmuir 1997, 13, 687. (31) (a) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (b) Abuin, E. B.; Lissi, E.; Aranjo, R. S.; Aleixo, R. M. V.; Chaimovich, H.; Bianchi, N.; Miola, L.; Quina, F. J. Colloid Interface Sci. 1982, 88, 594. (c) Blasko, A.; Bunton, C. A.; Cerichelli, G.; Mckenzie, D. C. J. Phys. Chem. 1993, 97, 11324. (32) (a) Chaudhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (b) Chaudhuri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362. (33) (a) Buncel, E.; Chuaqui, C.; Wilson, H. J. Org. Chem. 1980, 45, 3621. (b) Buncel, E.; Wilson, H.; Chuaqui, C. J. Am. Chem. Soc. 1982, 104, 4896. (c) Hoz, S.; Buncel, E. Isr. J. Chem. 1984, 26, 313.
