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J . Phys. Chem. 1992, 96, 2382-2385
Reverse Micelles of Triton X-100 in Cyclohexane. Effects of Temperature, Water Content, and Salinity on the Aggregation Behavior D.-M. Zbu,' K . 4 . Feng, and Z. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 (Received: September 30, 1991)
In reverse micelles of Triton X-100 in cyclohexane, the effects of water content, temperature, and the presence of CaC12 on aggregation number, surfactant monomer concentration, and size of aggregates were investigated by controlled partial pressure-vapor pressure osmometry and quasi-elastic light scattering. The monomer concentration is reduced by the addition of water but is increased by raising the temperature. At 30 "C, the aggregation number reaches a maximum of n = 40.4 at a water content of R = 1.0. The hydrodynamic diameter of dry reverse micelles at 25 "C is constant (Dh = 21.3 nm) in the concentration range investigated. Wet reverse micelles are larger than the dry ones, and the salt-containing aggregates are in between in size. The size of all aggregates decreases with increasing temperature. The aggregates are found not to be spherical; thus, Dh is only an apparent size parameter. A model is proposed for the description of the observed aggregation behavior which is based on the penetration of cyclohexane into the polar core of the TX-100 reverse micelle.
when necessary. The application of controlled partial pressurevapor pressure osmometry, CPP-VPO (instead of conventional VPO), is necessary whenever a reverse micellar solution studied is hygroscopic or has a nonnegligible water vapor pressure (depending on the nature of the surfactant and solvent and the amount of water present).8 In such cases, to recover the true osmometric signal, the partial pressure of water in the osmometer chamber (pW,,)must be matched to the water vapor pressure (pW) of the sample. The matching condition is found by a systematic search and is achieved by placing an aqueous H2S04solution of the proper concentration into the chamber of the osmometer, in addition to the reservoir of pure solvent (cyclohexane). In the present study, this was necessary only for solutions with R p [H20]/[TX-100] = 2.5 and 4.0. The matching conditions were achieved with 25% CH~C(CH,)~CHZC(CH~)ZC~H~(OCH~CH~-)~.~~H (w/w) and 15% (w/w) aqueous HzS04, respectively. The meain cyclohexane. In a previous study of the same system,6 by the surements were carried out at 30 and 37 "C. use of methyl orange as an absorption probe, we found that Light Scattering Mewwments. Quasi-elastic light scattering cyclohexane penetrates the polar interior of the aggregates which, measurements were performed on a Brookhaven Model BI-200SM however, is partially replaced by water molecules upon increasing instrument with an argon ion laser light source (514.5 nm), in the R value (molar ratio of H 2 0 to surfactant) of the solution. the temperature range 20-45 OC. Since the angle of detection Now, we report the effects of temperature, water content, and (in the range 30°-150°) had no effect on the sizes computed, the the presence of CaC1, on the mean aggregation number n, size, majority of the measurements were done at 120'. The translaand shape of the aggregates, as well as on the monomer contional diffusion coefficients were obtained from the correlation centration C, of the surfactant in the bulk solution. The confunction, and the hydrodynamic diameter (4)of the aggregates clusions presented are based on the results of controlled partial was calculated through the Stokes-Einstein equation. pressure-vapor pressure osmometric (CPP-VPO)7 and quasielastic light scattering measurements. Results and Discussion The range of compositions of the ternary system TX-100/ Experimental Section cyclohexane/water investigated is indicated by the area of ABC Materials. Triton X-100 (or TX-100; a Rohm and Haas in Figure 1. Within this range, clear, homogeneous solutions exist. product) was purchased from Eastman Kodak and cyclohexane At 20 OC,the dashed line AB is a phase boundary at which phase (>99.5%) from Fluka. Their water content was found through separation occurs. The BC line is not a phase boundary; it simply Karl Fischer titration (Aquastar V1B Titrator) as 0.28% (w/w) indicates our choice for the upper limit of surfactant concentrations and 0.014% (w/w), respectively. Anhydrous CaCl, was obtained used. from Matheson Coleman & Bell and hexamethylbenzene (>99%) 1. Monomer Concentration and Mem Aggregation Number. from Aldrich. Water was doubly deionized and distilled. VPO and CPP-VPO measurements were used to determine the Vapor Pressure Osmometry (VPO).A computerized Osmomat mean aggregation number n of the reverse micellar particles and 070-SA (UIC, Inc.) was used for the determination of the mean the monomer surfactant concentration C,,,in the bulk solution. aggregation number n of the micelles, in the CPP-VPO mode,7 The expression of the osmometric signal AB (in arbitrary units) is ( 1 ) R. A. Welch Postdoctoral Fellow. On leave of absence from Qingdao AB KCT (1) Institute of Chemical Technology, Qingdao, People's Republic of China. (2) Fendler, J. H. Membrane Mimetic Chemisfry; Wiley-Interscience: where CT stands for the total concentration of solute particles and New York, 1982. K is an instrumental constant. K was determined by using the (3) Kandori, K.; Kon-no, K.; Kitahara, A. J . Colloid Inferface Sci. 1988, nonassociating calibrating material hexamethylbenzene in cy122, 78. clohexane solution and was found to be K = 3.06 X lo4 unit X (4) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. mol-' kg at 30 OC and 3.45 X lo4 unit X mol-' kg at 37 O C . Introduction The core of reverse micelles and water-in-oil microemulsions represent compartmentalized, polar microenvironments that can be utilized for chemical reactions* and the preparation of solid colloidal parti~les.~Especially aggregates of nonionic surfactants have proven useful for the latter.4s5 For understanding the dynamics of, and the geometric limitations on, particle formation in reverse micelles, the equilibrium characterization of the host aggregates is necessary. In the present paper our focus is on reverse micelles of the nonionic surfactant Triton X- 100 (polyoxyethylene tert-octylphenyl ether), with the formula of
(5) Zhu, D.-M.; Schelly, Z. A. Manuscript in preparation. (6) Zhu, D.-M.; Schelly, Z. A. Langmuir 1992, 8, 48. (7) Ueda, M.; Schelly, Z. A . J. Colloid Interface Sci. 1988, 124, 673.
0022-365419212096-2382$03.00/0
(8) Ueda, M.; Schelly, Z. A. Lungmuir 1988, 4, 653.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2383
Reverse Micelles of Triton X-100 in Cyclohexane
-
TX 100
f
"J 1020304050607080gY)
Cyclohexane
F i i 1. Definition of range (area of ABC) of compositions investigated. At 20 OC, the dashed line AB is a phase boundary at which phase separation occurs.
TABLE I: Monomer Concentration C, (mol kg-') and Aggregation -- Number I) as a Function of R Value a 4 Temperature 30
R
cnl
0 1.o 2.5 4.0
0.052 0.047 0.022
0.0069
37
OC
OC
n
cm
n
18.6 40.4 30.5 23.8
0.063 0.051 0.029 0.014
15.8 25.8 28.1 23.7
Because of the equilibrium between surfactant monomers and aggregates, eq 1 can be written as AB = K[C, + (C - C,)/n] (2) where C is the analytical concentration of the surfactant. Our previous7-10 and present experience has shown that water molecularly dispersed in the bulk solvent does not contribute to the osmometric signal. Hence, the total solute concentration C, is just the sum of the concentrations of surfactant monomer C, and micellar aggregates, (C - C,)/n. Equation 2 can be rearranged to AB = KC,( 1 - 1/n) + K C / n (3) which yields a linear plot of AB vs C, if C, and n are constant. Indeed, in the concentration range of surfactant investigated this is the case (Figure 2). The corresponding monomer concentrations and aggregation numbers as a function of the water content (R value) of the solution and the temperature are listed in Table I. Not surprisingly, we find that at a given temperature C, decreases monotonically with increasing R because of the enhanced hydration of the polyoxyethylene chains,"J2 which promotes aggregation. The effect of temperature is just the opposite. Increasing temperature weakens molecular interactions which results in shifting the monomer-micelle equilibrium toward the monomer. The observed general decrease of the aggregation number n with increasing temperature is in accord with this picture. The effect of the water content on aggregation number, however, is more complex: At constant temperature n goes through a maximum with increasing R. At 30 OC, the maximum (n = 40.4) is at R = 1.0 and at 37 OC (n = 28.1) at R = 2.5. Based on our previous findings6 already mentioned in the Introduction, this behavior can be rationalized by the following model of the aggregation process. (1) In "dry" TX-100/cyclohexane solution with no added water present, there is only one H 2 0 molecule per 10 TX-100 molecules (R = 0.1). The surfactant molecules (including their polyoxyethylene chains) in the monomer state are solvated by cyclohexane. Upon aggregation, some of the solvent (9) Herrmann. U.; Schelly, 2.A. J . Am. Chem. Soc. 1979, 101, 2665. (10) Tamura, K.; Schelly, Z. A. J . Am. Chem. SOC.1981, 103, 1013. (1 1 ) Ravey, J. C.; Buzier, M. In Macr+ and Microemulsions. Theory and Applications; Shah, D. O., Ed.;ACS: Washington, DC, 1985, p 253. (12) Boyle, M. H.; McDonald, M. P.; Rossi, P.; Wood, R. M. In Microemulsions; Robb, I. D., Ed.;Plenum: New York, 1982; p 103.
0
0.2
0.4
0.6
c [mol
kg-11
0.8
Figure 2. VPO signals AB as a function of surfactant concentration C, R value, and temperature. At 30 OC: R = 0 (0),R = 1.0 (A),R = 2.5 (e),R = 4.0 (0). At 37 OC: R = 0 ( O ) , R = 1.0 (A),R = 2.5 (0). R = 4.0 (m).
molecules are squeezed out from the reverse micellar core, which increases the entropy of the system. (2) Added water enhances surfactant association by bridging adjacent polyoxyethylene chains and increases the polarity of the core. During this process additional solvent molecules are squeezed out, the free energy of the system decreases, and the aggregation number increases, eventually to a maximum. (3) With further addition of water, there is a diminishing number of cyclohexane molecules to be displaced from the core; hence, the contribution of their release to the entropy increment decreases. As a compensation, the aggregation number is reduced. A semiquantitative account of this description can be made by considering the equilibrium between surfactant monomer and aggregate, which can be expressed by the equation fH20
+ (TX-10@bH20jS)*
free H20 in bulk solvent
free TX-100 wiih associated H 2 0 and S molecules
lIn(TX-l00*RH2O*kS), + TX-100 with associated H@ and S molecules in the reverse micelle
qS
(4)
released solvent mdecules
where S denotes solvent molecule, R is the molar ratio of water to surfactant in the solution, and q ( = j - k) is the number of solvent molecules released per TX-100 molecule during the aggregation process. Since the solubility of water in cyclohexane is very low (1.48 X mol kg-' at 25 OC and 5.3 X mol kg-I at 35 "C as determined by Karl-Fischer titration), to a good approximation, all water may be assumed to be associated with the surfactant. Hence, the analytical R value of the solution is used as the coefficient of H 2 0 in the reverse micelle. In this approximation, R = f + b, where f is the number of free and b is the number of bound water molecules per surfactant monomer. The upper limit off is actually less than that corresponding to the solubility of water in the pure solvent, because of the additional binding equilibrium involving water and surfactant monomer. Addition of water increases R, and its effect on q and n depends on the original values of R and j . Up to an R value of around 1, water molecules are preferentially associated with the terminal hydroxyl group of the TX-100. Thus, in this range (R = 0.1 to R FJ l), the amount of water should have little effect o n the numerical value of j since the number of water molecules is too small for competing for the oxyethylene sites with the cyclohexane. However, within the same range, added water molecules establish further bridges between TX- 100 chains inside the reverse micelle, and thus they do reduce k by squeezing out solvent molecules. Hence, q increases. In the range above R = 1, Le., when the water content becomes sufficiently high for H 2 0 molecules to occupy also some of the oxyethylene sites in addition to the terminal hydroxyl group of the surfactant monomer, the original j is small.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
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TABLE 11: Dbof Dry TX-lOO/Cyclobexane Reverse Micelle as a Function of the Surfactant Concentration C (mol kg-') at 25 O C
Dh, nm 21.3 21.2 21.6 22.2 21.4 21.1 (Dh) = 21.3
C 0.0256 0.302 0.41 1 0.618 0.824 1.03
T. OC 0 1 .o
2.5 4.2 5.5
TABLE IV: Apparent Dh (nm) of Aggregates in TX-lOO/Cyclobexane (C = 0.824 mol kg-') as a Function of R Value at 25 OC
R
Dh 21.4 49.6 61.0
R 1.2 1.3 1.4
Dh 58.4 60.0 60.0
R 1.5 2.5
TABLE V Apparent Db (nm) of Saline (0.25 M Aqueous CaCI,) Aggregates in TX-lW/Cyclohexane (C = 0.824 mol kg-I) as a Function of R Value and Temwrature
R
TABLE 111: Apparent Db(nm) of Aggregates in TX-100/Cyclohexane (C = 0.824 mol kg-I) as a Function of R Value and Temperature T, "C R 20 25 30 35 40 45 0 31.1 21.4 18.3 16.0 14.6 13.3 1.o 61.0 38.3 27.0 20.8 18.6 2.5 51.2 37.5 29.4 24.9 21.6 4.2 44.5 35.5 28.6 24.5 21.6 5.5 46.5 35.6 29.3 24.8
0 0.8 1.0
Zhu et al.
Dh 59.0 51.2
Hence, when more water is added, the number (4)of cyclohexane molecules released from the core is small, too. The general effect of n decreasing with increasing temperature has already been mentioned in a previous paragraph. This effect, however, becomes almost negligible at high water content. (See the n values for R = 4.0 in Table I.) This behavior is in accord with the above model, as well as with the observation13 that the temperature-induced decrease of core polarity (increase of cyclohexane penetration) is diminishing as the water content is raised. 2. Size of Micellar Aggregates. (a) Effect of Surfactant Concentration. The hydrodynamic diameters Dh of dry reverse micelles determined by light scattering at various concentrations of TX-100 are listed in Table 11. In the concentration range investigated, Dh is essentially constant, with a mean value of ( D h )= 21.3 nm. This is consistent with the constant aggregation number (at fixed R value) found through vapor pressure osmometry in the same concentration range (eq 3 and Figure 2) and with the unchanging microenvironment inside the reverse micelle monitored by methyl orange as a probeU6 Dh is calculated from the Stokes-Einstein equation which is for spherical particles. The obtained (4)is, however, significantly greater than twice the linear dimension of the TX-100 chain containing 14 C-C bonds, 21 C-O bonds, a benzene ring, and an 0-H bond. Consequently, the micelle cannot be spherical, and Dhshould only be considered as an apparent size parameter. This conclusion holds also for wet reverse micelles (microemulsion droplets) discussed below. (b) Effects of Water Content and Temperature. Table I11 lists the apparent D hvalues obtained for a 0.824 m TX-100 solution at several different water contents and temperatures. Compared to the dry reverse micelles, the size of the microemulsion droplets is greater; however, their size does not increase monotonically with R. For instance, at 25 OC, 4 has a maximum at around R = 1.0, and at 30 OC, there are a local maximum and a local minimum. At 25 OC, by increasing the R value in smaller steps, we examined the neighborhood of the maximum in greater 0 detail (Table IV). The results reveal a plateau of Dh ( ~ 6 nm), extending over the range of R = 1 .O to R = 1.5. The complex (13) Zhu, D.-M.; Schelly, 2. A. Unpublished results. Using methyl orange as a probe of polarity of the core, similarly to the studies described in ref 6.
20 31.1 64.6 49.3
25 21.4 56.1 45.5 37.3 43.6
30 18.3 36.5 34.6 29.6 31.2
35 16.0 28.1 28.6 25.4 28.7
40 14.6 22.5 23.8 22.2 24.0
45 13.3 19.3 22.5 19.1 20.5
TABLE VI: Apparent D b(nm) of Saline (0.25 M Aqueous CaCI,) Aggregates in TX-lOO/Cyclobexane (C = 0.824 mol kg-') as a Function of R Value at 25 OC
R 0 0.7 0.9
Dh 21.4 45.3 53.1
R 1.0 1.3 1.5
Dh 56.1 54.0 54.2
R 1.8 2.2
Dh 51.3 47.5
behavior may reflect the delicate balance the multitude of molecular interactions must reach, which, together with geometrical constraints, determine the apparent particle size. (c) Effects of Salinity and Temperature. The effects of salinity on apparent aggregate size were examined in microemulsions that were prepared by adding appropriate amounts of 0.25 M aqueous CaCI, solution (instead of pure water) to dry TX-100 reverse micelle. Thus, the amount of salt present in the core of the aggregates is linearly proportional to the R value of the solution. The results are summarized in Table V. Generally, at a given R value and temperature, the aggregates containing CaCl, are smaller than those without salt (cf. Table 111). The difference is the larger the greater the R value-and thus the CaClz content-of the saline systems. With increasing temperature the difference decreases, or even the rank of sizes becomes reversed. (Saline aggregates with R = 1.Oat temperature 2 3 5 O C and those with R = 2.5 at 45 "C are somewhat larger than the salt-free counterparts.) In contrast to that of the salt-free systems, Dh shows a pronounced local maximum and a local minimum at each temperature 125 OC we investigated. For a comparison with the results obtained on salt-free solutions at 25 "C (Table IV), we also examined the neighborhood of the local maximum exhibited by the corresponding saline systems, by changing R in small increments (Table
VI). In addition to the obvious difference of reduced apparent size of the aggregates, Dh of the saline systems does not exhibit a plateau region. As the results indicate, the effect of the presence of an electrolyte in the polar core of the reverse micelles is quite complex. Although the details are unclear at the present time, the general trends can be understood in terms of ion-dipole interaction or complexation between the Ca2+ion and the polyoxyethylene chain of the surfactant which affect shrinkage of the aggregates. With increasing temperature, the interactions are weakened and the extent of the reduction in size becomes less. Summary Triton X-100forms nonspherical dry (R= 0.1) and swollen reverse micelles in cyclohexane; thus, the hydrodynamic diameter Dh, as determined through quasi-elastic light scattering, should only be viewed as an apparent size parameter. For dry systems at 25 OC, Dh is independent of the surfactant concentration and has a constant value of 21.3 nm. The effects of water content R, temperature, and added salt (CaCl,) on the aggregation number n, monomer concentration, C, and apparent size Dh have been investigated. C, is reduced by the addition of water but increased by raising the temperature. Around room temperature (at 30 "C), the aggregation number goes through a maximum (n = 40.4 at R = 1.0) with increasing water content. This behavior is consistent with our previous finding of cyclohexane penetrating the polar core of the reverse micellea6 Accordingly, added water bridges the polar segments of adjacent
J. Phys. Chem. 1992,96, 2385-2390 surfactant molecules in the micelle and displaces solvent molecules from the polar core of the aggregates. Once most of the cyclohexane has been squeezed out, further addition of water reduces n. Generally, wet TX-100 reverse micelles are larger than the dry ones, and the saline aggregates are in between in size. Dh generally increases with water content, but not monotonically. However,
2385
the size of all aggregates is reduced by increasing temperature. Acknowledgment. This material is based in part upon work supported by the National Science Foundation (Grant CHE8706345), the R. A. Welch Foundation, the Texas Advanced Research Program under Grant 1766, and Alcon Laboratories, Inc. Their support is gratefully acknowledged.
Experimental Determination of the Electron Affinities of Nitrobenzene, Nitrotoluenes, Pentafluoronitrobenzene,and Isotopic Nitrobenzenes and Azulenes E. C.M. Chen, E. S. Chen, M. S. Milligan, School of Natural and Applied Science, University of Houston, Clear Lake, Houston, Texas 77058
W. E,Wentworth,* and J. R.Wiley? Department of Chemistry, University of Houston, Houston, Texas 77204 (Received: June 10, 1991) The absolute electron affinities of nitrobenzene, m-, 0-,and p-nitrotoluene, pentafluoronitrobenzene, deuterated nitrobenzene, ISN-substitutednitrobenzene, azulene, and deuterated azulene have been determined by measuring the temperature dependence of the response of the electron capture detector (ECD). These are the first ECD determinations of the absolute electron affinities of these compounds and the only values of the electron affinities of the isotopically substituted compounds. The values for the substituted compounds do not differ from the unsubstituted compounds by more than the error. The best reproducibility for a single ECD determination is *0.02 eV. The ECD electron affinities differ from the literature values by less than the errors in the determinations. The electron affinities are as follows (eV): C6H5N02= 1.0 0.02, C6D5NO2 = 1.O f 0.03, C6H5l5NO2 = 0.99 f 0.03, C ~ F S N = O ~1.5 f 0.25, C,oHg = 0.69 f 0.04, C,oDg = 0.70 f 0.03, m-C7H,NO, = 0.98 0.03, o-C7H7N02= 0.89 + 0.03, and p-C7H7N02= 0.94 0.03.
*
*
*
Introduction The adiabatic electron affinity (EA) of organic molecules is an important fundamental property. Prior to 1960, there were no routine experimental methods for the determination of molecular electron affinities in the gas phase. This was emphasized by Mulliken in 1953 as follows: “Quantitative methods for determining molecular electron affinities are not very well developed, but there seem to be possibilities for the future.”’ It has taken over three decades for that prediction to come true. As recently as the mid-1980s there were fewer than a dozen values of molecular electron affinities which had passed the crucial test of being determined by more than one independent experimental procedure in the gas phase.* Presently, more than 200 molecular electron affinities have been reported, the majority of which have been determined by either the electron capture detector (ECD) method or the thermal charge transfer (TCT) methode3 However, only about seven molecules have been studied by both techniques (see Table I). In this paper, a distinction is made between organic molecules and organic radicals. Many electron affinities of organic radicals and small molecules such as O2 and SOz have been determined quite accurately by photodetachment and photoelectron ~pectroscopy.~ In addition the purpose of this paper is to present new ECD data for the subject compounds and to establish the experimental reproducibility of the ECD electron affinities determined using an internal standard. This will better establish the relationship between the ECD and TCT results. Another objective is to investigate the effect of isotopic substitution on the electron affinity. In the late 197Os, relative electron affinities of organic molecules were determined by using ICR mass spectrometry to measure the equilibrium constant for thermal charge-transfer reactions (TCT-ICR):4 AB-
+ CD
-
AB
+ CD-
‘Present address: University of Texas, Permian Basin, Odessa, TX 79762.
0022-365419212096-2385$03.00/0
In the mid-1 980s, the “high-pressure” (TCT-HPMS) pulsed electron beam technique was applied to this reaction and in addition the temperature dependence was determined so that enthalpy and entropy values were obtained. These relative values were referenced to the ECD value for benzophenone. Later, the TCT values were referenced to the electron affinity of SO2so that the TCT method and the ECD method can be independently ~ompared.~,~ The previously reported ECD values range from 0.15 f 0.05 eV for naphthalene2 to 0.88 f 0.04 eV for tetracene.’ The ECD procedure has been improved to allow measurement of higher electron affinities. We report the electron affinities for the nitrotoluenes C6F5N02,C6H5N02,C6D5NO2,and C6H5l5NO2and the azulenes CloHgand CLODs.Multiple determinations have been carried out for many compounds. The ECD results differ from the TCT results by less than the experimental error. The effect of isotopic substitution on the electron affinity is small. The best reproducibility that was realized in the multiple determinations is f0.02 eV. The ECD values are compared with recently determined half-wave reduction potentials* and photodetachment threshold^.^ The photodetachment thresholds give upper limits to the electron affinity while the half-wave reduction potential data can be used to estimate solvation energies for the anions. The photodetachment thresholds are all greater than the EAs obtained from the ECD. The combination of the ECD data with the (1) Mulliken, R. S.In Molecular Complexes; Mulliken, R. S., Person, W. B., Eds.; Wiley: New York, 1981; p 373. (Original reference: Proceedings of Int. Conference on Theoretical Physics, Kyoto and Tokyo, Sept 1953). (2) Chen, E. C. M.; Wentworth, W. E. Mol. Crysr. Liq. Crysr. 1989, 171, 271. (3) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G . J. Phys. Chem. Re$ Data 1988, 17. (4) Rains, L. J.; Moore, H. W.; McIver, R. J. J. Chem. Phys. 1978, 68, 3309. ( 5 ) Caldwell, G.;Kebarle, P. J. Chem. Phys. 1984, 80, 1. (6) Kebarle, P.; Chowdhury, S. Chem. Rev. 1987, 87, 513. (7) Lyons, L. E.; Morris, G. C.; Warren, L. J. J. Phys. Chem. 1968, 72, 3677. (8) Shalev, H.; Evans, D.H. J. Am. Chem. SOC.1989, 1 1 1 , 2667. (9) Mock, R. S.;Grimsrud, E. P. J. Am. Chem. SOC.1989, 111, 2861.
0 1992 American Chemical Society
