769
Langmuir 1987, 3, 769-773
Electrochemistry of Methylviologen in the Presence of Sodium Decyl Sulfate Pablo A. Quintela and Angel E. Kaifer* Department of Chemistry, University of Miami, Coral Gables, Florida 33124 Received July 30, 1986. I n Final Form: February 26, 1987 The reductive electrochemistry of methylviologen (MV2') was surveyed in the presence of sodium decyl sulfate (SdecS) at concentration levels below and above the critical micelle concentration (cmc) of this anionic surfactant. The SdecS micelles were found to increase the equilibrium constant for the conproportionation reaction M V + + MV * MV'+as compared to the value obtained in surfactant-free solution. Absorption spectra showed that the equilibrium constant for dimerization of the cation radical is notably lowered by the micelles when their concentration is similar to that of MV". In contrast to this, the dimerization of the cation radical is remarkably enhanced by the surfactant system at low micelle concentrations, i.e., at concentrations of surfactant slightly above the cmc. The adsorption behavior of the cation radical on the electrode surface at SdecS concentrations under the cmc was also investigated.
Introduction The subject of electrochemistry in micellar solutions has received a fair amount of attention during the last decade. Most studies have addressed the effects of micellar aggregates on the redox potentials and diffusion coefficients of electroactive species with varying hydrophobic nature.I+ Changes in the reduction mechanism have also been observed in some instance^.'.^ Quite recently, polarographic measurements have been applied to obtain diffusion coefficients of micelles incorporating electroactive probes? Surfactant ferrocene derivatives have been used to demonstrate redox control over their aggregation properties.1° All these reports have ultimately been focused on surfactant concentrations above the critical micelle concentration (cmc). Far less effort has been invested in the investigation of electrochemical properties of surfactant solutions below the cmc. Indeed, micelles are not present a t this concentration range, but, as pointed out by Batina, Cosovic, and Adzic,l' the adsorption of surfactant molecules onto the electrode surface may give rise to modifications of electrochemical behavior which are yet mostly unknown. These authors have shown that the underpotential deposition of thallium and lead on a Ag(100) electrode is remarkably altered by the presence of sodium dodecyl sulfate (SDS) at premicellar 1evels.ll We have previously reported the weak adsorption of the cation radical of l,l'-dimethyL4,4'-bipyridinium ion (methylviologen, W + ) in the presence of SDS a t concentration levels below the cmc. The adsorption of the cation radical was found to be eliminated by the onset of SDS micelles.12 These considerations, along with the frequent use of MV2+ as electron-transfer mediator in different media, ~
(1)Yeh, P.;Kuwana, T. J. Electrochem. SOC.1976,123, 1334. (2)McIntire, G. L.;Blount, H. N. J. Am. Chem. SOC.1979,101,7720. ( 3 ) Ohsawa, Y.;Shimazaki, Y.; Aoyagui, S. J.Electroanal. Chem. 1980, 114,235. (4)Ohsawa, Y.; Aoyagui, S. J. Electroanal. Chem. 1982, 136, 353. (5)Ohsawa, Y.;Aoyagui, S. J. Electroanal. Chem. 1983, 245, 109. (6)Georges, J.; Desmettre, S. Electrochim. Acta. 1984,29,521. (7)Meyer, G.; Nadjo, L.; Saveant, J. M. J.Electroanal. Chem. 1981, 119,417. (8)McIntire, G. L.;Chiappardi, D. M.; Casselbery, R. L.; Blount, H. N. J. Phys. Chem. 1982,86,2632. (9) Zana, R.; Mackay, R. A. Langmuir 1986,2,109. (10)Saji, T.;Hoshino, K.; Aoyagui, S. J. Am. Chem. SOC.1985,107, 6865. (11)Batina, N.;Cosovic, B.; Adzic, R. J. Electroanal. Chem. 1985.184. 427. (12)Kaifer, A. E.;Bard, A. J. J.Phys. Chem. 1985,89,4876
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prompted us to investigate its reductive electrochemistry in the presence of sodium decyl sulfate (SdecS) at concentration levels above and below the cmc. This surfactant shows a higher cmc than SDS (30 mM vs. 8 mM in pure wateP) thus presenting a wider concentration range where monomeric surfactant-substrate interactions can be assessed by using conventional electrochemical techniques. We report here the results of this research.
Experimental Section Materials. Methylviologendichloride hydrate (Aldrich)was
recrystallized from methanol and dried at 80 "C in vacuo. Sodium decyl sulfate was obtained from Kodak.This material was washed with ether, recrystallized from absolute ethanol, and dried at 80 "C under vacuum. All other reagents were of analytical grade. Distilled water was further purified by passage through a Barnstead Nanopure pressurized system. Equipment. Electrochemicalexperiments were performed with a Princeton Applied Research (PAR) Model 175 universal programmer, a Model 173 potentiostat, and a Model 179 digital coulometer equipped with positive feedback circuitry for IR compensation. Voltammograms were recorded on a Soltec VP-6423s X-Y recorder. Absorption spectra were measured with a Perkin-Elmer Lambda 9 UV/VIS/NIR spectrophotometer. Procedures. All of the experiments were performed under a purified nitrogen atmosphere. Nitrogen gas was also used to purge all solutions. It was found that maintaining a positive pressure of nitrogen over the stirred surfactant solutions is a fairly efficient procedure to remove oxygen, thus avoiding the extensive foaming that accompanies nitrogen bubbling through these solutions. The electrochemical measurements were carried out in cells of conventional design. Glassy carbon electrodes (Bioanalytical Systems) were employed as working electrodes for voltammetry. The electrode surface was polished with a 0.05-pm alumina-water slurry on a felt surface immediately before use. All potentials were recorded against a sodium chloride saturated calomel electrode (SSCE). Samples of the methylviologen cation radical were prepared by controlled potential electrolysis in a two-compartment cell of solutions containing adequate concentrations of the parent dication. Either carbon cloth or gold flag cathodes were used as working electrodes in these experiments. Both materials yielded good results, but the former were more efficient and allowed shorter electrolysis times. Samples of the cation radical solutions were then transferred to a nitrogen-filled 0.1-cm quartz cell through Teflon tubing under positive nitrogen pressure. The cell was then sealed and the absorption spectrum recorded. (13)Mukerjee, P.;Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactants Systems; National Bureau o f Standards. US. Government Printing Office: Washington, DC, 1971;NSRDS-NBS 36.
0 1987 American Chemical Society
Quintela and Kaifer
770 Langmuir, Vol. 3, No. 5, 1987
I
-1.3
-1.1
-0.9
POTENTIAL
1
1
-
-0.5
-a7 ( V
-0.3
- 0.1
vs S S C E )
Figure 1. Cyclic voltammograms at a glassy carbon working electrode (0.08 cm2)of 1.0 mM methylviologen in aqueous 50 mM NaCl also containing (A) 70 mM SdecS and (B)15 mM SdecS.
Scan rate 50 mV/s.
Results The electrochemistry of MV2+ in aqueous media is characterized by two reversible reduction steps yielding the cation radical, MV", and the neutral species, MV.14 Therefore, two reduction and two correspondingoxidation peaks are observed in cyclic voltammetry. Depending upon the concentration of MV2+and the scan rate, the wave for the oxidation of M Y to MV" can display distortions owing to the insolubility of the former in aqueous solution.12 We have recently reported the effects that SDS micelles exert on this electrochemical behavior.I2 Namely, a remarkable stabilization of the cation radical was detected as deduced from the increase of the potential separation between the two consecutive redox couples. The equilibrium constant for the conproportionation of the cation radical (eq 1)was MV2++ MV
2MV"
(1)
found to increase by a factor of 400 in the presence of SDS micelles. A similar behavior was anticipated for MV2+in solutions containing SdecS micelles due to the structural resemblance of both surfactants. This was confirmed by the experimental results. A cyclic voltammogram for a solution containing 1.0 mM MV2+and 70 mM SdecS is shown in Figure 1A. The cmc of SdecS in the presence of 50 mM NaCl (added as supporting electrolyte) has been reported to be 20.8 mM,15 so that the selected concentration is well in the micellar range. The two reduction couples were electrochemically reversible, with half-wave potentials of -0.64 and -1.10 V vs. SSCE. The equilibrium (14) Bird, C. L.; Kuhn, A. T. Chem. SOC.Reu. 1981, 10, 49. (15) Mysels, K. J.; Kapauan, P. J . Colloid. Sci. 1961, 16, 481
constant for the conproportionation of MV'+ was calculated to be 6.0 X lo', in fair agreement with the value obtained for SDS micellar solutions (8.8 X 107).12 This indicates that both types of anionic micelles give rise to a similar stabilization of the cation radical regardless of the small difference in the hydrocarbon chain lengths. The apparent diffusion coefficient of MV2+calculated from the voltammetric peak currents was 1.6 X lo4 cm2/s. The corresponding value in the absence of surfactant was 6.6 X lo4 cm2/s. This decrease reflects the association of MV2+with the micellar aggregates. The cmc of the SdecS surfactant system was determined in the presence of 1.0 mM MV2+ because the viologen dication may act as a nucleating agent for micelle formation and thus lower the effective cmc value.13 Surface tension measurements were performed, using the Du Nuoy ring method with platinum-iridium rings, were performed on solutions containing 1.0 mM MV2+,50 mM NaC1, and variable concentrations of SdecS. The cmc was found at 11 mM, while the reported value in the absence of MV2+ is 20.8 mM.I5 Thus, the viologen dication has a pronounced effect on the effective cmc of the SdecS system. Similar experiments were also carried out with solution containing 1.0 mM MV2+,200 mM NaC1, and variable concentrations of SdecS. The effective cmc was found at 7 mM, while the reported value for this concentration of NaCl in the absence of viologen is 11 mM.15 Again, the presence of 1.0 mM MV2+lowers the apparent cmc of the surfactant system. If the concentration of SdecS is kept slightly above the cmc value, the potential difference between the two reduction processes decreases. For instance, the cyclic voltammogram obtained with a solution containing 1mM MV2+ and 15 mM SdecS appears in Figure 1B. The half-wave potentials were determined to be -0.65 and -1.06 V vs. SSCE. While the first reduction occurs at a potential quite close to that found in the solutions containing a larger concentration of micelles (70 mM SdecS), the second reduction takes place at a potential ca. 40 mV more positive. The interpretation of these values is difficult since in this solution the concentration of monomeric surfactant anions is much larger (about 11 mM) than the concentration of micelles (estimated to be less than 0.1 mM). Indeed, cation radical-surfactant anion interactions have been previously reported2J6and could play an important role at these concentration levels. Samples of 1.0 mM MV" in SdecS micellar (70 mM) solutions were prepared according to the procedures described in the Experimental Section. The visible absorption spectrum of these solutions is shown in Figure 2A. Its shape closely correspondsto the spectra reported for M V + in nonaqueous solvents wherein dimerization is essentially supressed."J8 The spectrum in surfactant-free aqueous solution is clearly distorted by dimer absorption, which shows a maximum at 540 nm (see Figure 5 in ref 12). The absence of such distortions in the spectrum of Figure 2a is a clear indication that the SdecS micelles depress the dimerization equilibrium, probably by providing a hydrophobic microenvironment to the cation radical. Again, this is in perfect agreement with previously reported results in SDS micellar solutions.12 Similar experiments performed with SdecS solutions at concentration levels slightly above the cmc (15 mM) revealed the spectrum of Figure 2B, which shows extensive dimerization. In fact, the dimerization constant is considerably enhanced from its (16) Park,J. W.; Nam, H. L. Bull. Korean Chem. SOC.1984,5, 182. (17) Kosower, E. M.; Cotter, J . L. J. Am. Chem. SOC.1964,86, 5524. (18) Watanabe, T.; Honda, K . J. Phys. Chem. 1982,86, 2617.
Langmuir, Vol. 3, No, 5, 1987 771
Electrochemistry of Methylviologen
t
3.601
1
A
I
- 0.9
VOLTS
o!o
-0!9
0.0
vs
SSCE
Figure 3. Cyclic voltammograms at a glassy carbon electrode of 1.0 mM methylviologen in aqueous 50 mM NaCl solution also containing (A) 6 mM SdecS and (B) 12 mM SdecS. Scan rate 100 mV/s.
4
300
460
500
600
70 0
c
8 0 0 nm
Figure 2. Absorption spectra of 1.0 mM M V in aqueous 50 mM NaCl also containing (A) 70 mM SdecS and (B) 15 mM SdecS. Optical pathway 0.1 cm.
aqueous value, as suggested by the large absorption at 540 nm. This surprising result clearly establishes two facts: (i) SdecS micellar aggregates are responsible for the depression observed in the dimerization equilibrium at 70 mM SdecS concentration; (ii) a concentration of micellar aggregates similar to that of the cation radical is required for the complete inhibition of dimerization. However, if the concentration of micelles is very low relative to the MV'+ concentration, the dimerization is clearly enhanced. These conclusions prompted us to examine carefully the electrochemistry of MV2+with added SdecS at concentrations under the cmc. The cyclic voltammogram for the first reduction of methylviologen in the presence of 6 mM SdecS is shown in Figure 3A. The large parabolic peak on scan reversal corresponding to the oxidation of the cation radical to the dication is the result of MV'+ adsorption,l9 which is probably triggered by the adsorption of decyl sulfate ions onto the electrode surface. The voltammetric response returns to diffusion control, as seen in Figure 3B, if the surfactant concentration is increased above 10 mM. An appropriate way to follow the dependence of this adsorption phenomenon upon SdecS concentration is to plot the ratio of cathodic over anodic peak currents (ipc/ipa)vs. surfactant concentration. Furthermore, it is well-known that the cmc usually decreases when the concentration of added electrolyte is increased. Hence, we obtained data at two different concentrations of NaC1, 50 and 200 mM, to ascertain the effect of the cmc upon this behavior. The results are plotted in Figure 4. The region where ad(19) Wopschall, R. H.; Shain, I. Anal. Chem. 1967, 39, 1514.
l
5
'
l
10
'
l
15
'
i
20
'
l
25
[SdecS], mM
Figure 4. Dependence of the ratio of cathodic t o anodic peak currents for the fiit reduction of methylviologen at a glassy carbon electrode on SdecS concentration: (A) 50 mM NaCl; (B) 200 mM
NaC1.
sorption of MV" takes place is characterized by a value of i /i, well below unity due to the parabolic distortion of tge anodic peak. In solutions containing 50 mM NaCl (Figure 4A), the adsorption region starts around 2-3 mM SdecS and continues up ta 10 mM. From 5 to 9 mM of surfactant concentration a plateau is observed with an i,,/ip, ratio of 0.35. A t 200 mM NaCl (Figure 4B),this plateau is narrower, although the minimum ip,/ipa ratio is also about 0.35; however, the adsorption region starts
772 Langmuir, Vol. 3, No. 5, 1987 at slightly lower concentrations and ends at about 7 mM SdecS. These differences seem to arise from the lower cmc in the more concentrated NaCl solutions. The voltammetric and spectral results for the micellar region (vide supra) clearly suggest that the cation radical interacts with the micellar aggregates. It is thus reasonable to postulate that the adsorption of MV'+ is eliminated by the onset of SdecS micelles. These arguments seem to explain the shape of the graph for the 50 mM and 200 mM NaCl solutions. An inspection of Figure 4 shows that the elimination of the M V + adsorption and the return to diffusion-controlled electrochemistry occur at surfactant concentrations which are very close to the cmc values obtained in the presence of the viologen (11and 7 mM for 50 and 200 mM NaCI, respectively). At these levels of surfactant the cation radical may interact with (a) the adsorbed decyl sulfate ion, (b) the monomeric decyl sulfate ion, and (c) the premicellar and micellar decyl sulfate aggregates. Indeed, the dication also interacts with all of these species, but the interactions seem to be of smaller magnitude and less relevant to the understanding of the overall electrochemical response, perhaps reflecting the more pronounced hydrophobic character of the cation radical. Our results suggest that the interaction of the cation radical with the adsorbed decyl sulfate ions predominates in the absence of micelles, while the association in solution of M V with the micellar aggregates appears to become of primary importance above the cmc. Notice that this is consistent with the extensive dimerization of the cation radical detected by visible spectroscopy at 15 mM SdecS; M V and decyl sulfate anions form micellar aggregates in which dimers are quite abundant, probably because of the small concentration of micelles at these concentrations of SdecS.
Discussion In a 70 mM SdecS and 50 mM NaCl solution also containing I mM MV2+the concentration of free surfactant is about 11 mM according to our determination of cmc values in this surfactant system. The remaining decyl sulfate ions are associated forming micellar assemblies. In a similar solution containing 70 mM SDS and 50 mM NaCl the concentration of free surfactant is only 2.3 mM, and most of the surfactant is found in the form of aggregates or micelles.12 This is the result of the higher cmc exhibited by the shorter chain surfactant and establishes an important difference in the solution composition of these two amphiphilic compounds; that is, SdecS micellar solutions contain roughly 5 times'more free surfactant than micellar SDS solutions (at 50 mM NaCl concentration). The fact that the results presented here for SdecS micellar solutions are quite close to those obtained previously with SDS micelles indicates the predominance of substrate-micelle interactions over substratesurfactant interactions for each of the three viologen redox forms. The interpretation of redox potential data in micellar solutions must be done very carefully due to the possibility of preferential partitioning of some of the redox species into the micellar aggregates.lp3y4In cases where this is of importance, shifts in the voltammetric half-wave potentials may result from the rather large differences in the effective diffusion coefficients of the reduced and oxidized forms of the electroactive substrate under study. Furthermore, Eddowes and GratzelZ0have shown that for multistep electron-transfer processes in which one of the redox forms partitions extensively into the micellar phase, the observed peak currents are not determined primarily by the diffu(20)Eddowes, M. J.; Gratzel, M. J. Electroanal. Chem. 1983,152,143.
Quintela and Kaifer
-1.11
c f
-0.651
t
3
j
/
Figure 5. Effect of the methylviologen concentration on its half-wave reduction potentials and apparent diffusion coefficient. AU data points were obtained in solutions containing 70 mM SdecS and 50 mM NaC1.
sion coefficient of the initial redox species. Thus, they found remarkable differences for the two consecutive oxidation peak currents of tetrathiafulvalene in the presence of cationic micelles.20 The system under study here does not show this type of behavior, as evidenced by the shape of the cyclic voltammograms in the presence of micelles (see Figure lA), which is reasonably close to the shape predicted by the theory of multistep electron-transfer processes. Nonetheless, to asses the extent to which the potentials determined in this work are affected by partitioning into the micelles, we performed measurements in solutions containing 70 mM SdecS and varying concentrations of MV2+. The results are plotted in Figure 5. It is clear that the redox potential for both MV2+reduction processes are constant (within error margin) and independent of the ratio of MV2+/SdecSconcentrations over the tested range. This strongly indicates that partitioning between the bulk aqueous and the micellar phases does not play an important role in the observed redox potentials. This can be easily explained if we assume that all three methylviologen redox forms interact with the SdecS micelles, which would render the effective diffusion coefficients of these species close to one another. In the following paragraphs we address each of these interactions on the basis of our experimental results. The interaction of the methylviologen dication with SDS micelles has been reported to be mostly electrostatic in nature.21,22It is reasonable to expect such an interaction between an organic dication and the negatively charged surface of the anionic micellar aggregates. Our results with SdecS point toward a similar type of interaction as evidenced by the decrease in the apparent diffusion coefficient of the dication as the concentration of surfactant is increased. Furthermore, the plot in Figure 5 shows that the diffusion coefficient of MV2+in solutions containing 70 mM SdecS increases slowly as its concentration goes up from 0.5 to 1.5 mM. Further MV2+additions give rise to a sharp increase in the effective diffusion coefficient. This is consistent with an interaction mechanism presenting a saturation limit, as would be the case with elec(21) Schmell, R. H.; Whitten, D. G . J. Am. Chem. SOC.1980,102,1938. (22) Schmell, R. H.; Whitesell, L. G.; Whitten, D.G. J. Am. Chem. SOC.1981,103, 3761.
Langmuir 1987,3, 773-777 trostatic binding to the micellar surface. The interaction of the methylviologen cation radical with the SdecS micelles stabilizes this species not only toward disproportionation but also toward dimerization. The experimental data available in the chemical literature on viologen chemistry suggest that this might result from an increased hydrophobicity of the cation radical's microenvironment. Indeed, this requires at least partial penetration of MV'+ into the hydrophobic core of the micelles. The data presented here for solutions with surfactant concentrations slightly above the cmc indicate that the cation radical is also stabilized by interactions with the monomeric surfactant anions (or with the scarcely abundant micellar aggregates). This is in good agreement with Park and Nam's report? of a strong interaction between the methylviologen cation radical and monomeric dodecyl sulfate anions. Most interestingly, the extensive micellization of the surfactant was found to depress the dimerization equilibrium while the opposite effect, an increase in the extent of dimerization, was observed for solutions containing a very low concentration of SdecS micelles. This constitutes an interesting example of the influence of the relative abundance of micellar aggregates on the dimerization equilibrium of a cation radical. The reasons for this effect are probably related to the statistical distribution of the cation radicals among the aggregates. Finally, the interaction of the neutral species MV with the SdecS micelles, although not addressed in detail in this work, can be simply described as the solubilization of a hydrophobic molecule by micellar assemblies. This is evidenced by the diffusional shape of the peak associated
773
with MV oxidation to M V . As discussed above, this peak appears frequently distorted in micelle-free aqueous solution owing to the insolubility of this uncharged molecule in water. Since these distortions are not observed in the presence of SdecS micelles, it seems reasonable to postulate that the neutral viologen species is solubilized by the micellar aggregates. Conclusions We have shown that the reductive electrochemistry of MV2+in SdecS micellar solutions is quite similar to that previously reported in SDS micellar solutions. This is so despite small differences in surfactant chain length and large differences in the free surfactant concentration in equilibrium with the micelles, pointing to a predominance of substrate-micelle over substratesurfactant interactions even in experimental conditions wherein the monomeric surfactant is present in concentrations 10-fold larger than those of the micelles and the electroactive substrate. The dimerization of the methylviologen cation radical was found to be enhanced by small concentrations of SdecS micelles and suppressed in the presence of micellar concentrations similar to that of the cation radical. Acknowledgment. This work was partially supported by the University of Miami Research Council. We express our gratitude to Richard Reno for performing the surface tension measurements and to one of the reviewers for offering several helpful suggestions. Registry No. SdecS, 142-87-0; MV2+, 4685-14-7; MV', 25239-55-8; MV, 25128-26-1.
Excimer Formation of a Water-Soluble Fluorescence Probe in Anionic Micelles and Nonionic Polymer Aggregates Nicholas J. Turro* and Ping-Lin Kuo Chemistry Department, Columbia University, New York, New York 10027 Received January 14, 1987. I n Final Form: March 10, 1987 The excimer/monomer emission of the water-soluble fluorescence probe, sodium pyrene-&sulfonate(Py-S), has been investigated in aqueous solutions containing anionic micelles of sodium dodecyl sulfate (SDS), nonionic micelles of a series of polyethylene glycol n-nonyl phenyl ethers (C,PhE,), and aggregates of poly(ethy1ene oxide-propylene oxide-ethylene oxide) copolymers (EPE). The influences of temperature, pressure, and added salt on the intensity of the excimer to monomer emission (Ie/Im) in the various systems were investigated. Dramatic differences were observed in the intensity of the excimer to monomer emission (Ie/Im) under the varying environmental conditions. The results are interpreted in terms of a dynamic equilibrium of Py-S between the aqueous phase and the aggregate phases in response to variations of aggregate structure that are induced by the experimental variations. Introduction Investigations of the solution behavior of molecules solubilized in micelles or in bilayer vesicles provide information and generate methods and concepts that can be transferred to experimental systems involving biological molecules such as en2ymes.l Photophysical techniques have been developed which make photoluminescence probes a very convenient means for investigating the nature and structure of molecules solubilized in hydrophobic aggregates in aqueous solution.2 For example, excimer (1)(a) Barton, J. K.; Kumar, C. V.; Turro, N. J. J. A h . Chem. Soc. 1986,108,6391.(b) Fox, M.A. Adu. Photochem. 1986,13,237.
formation of pyrene has been employed to investigate the microscopic viscosity of micelles and to determine aggregation numbers of mi~elles.~Since pyrene is a strongly hydrophobic molecule, it has low water solubility and tends (2) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. (b) Thomas, J. K. Acc. Chem. Res. 1977,10, 133. (c) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Znt. Ed. Engl. 1980,19, 675. (3)(a) Hirayama, F. J. Chem. Phys. 1965,42,3163. (b) Klopffer, W.; Liptay, W. 2.Naturforsch., A.: Phys., Phys. Chem., Astrophys. 1970, %A, 1091. (c) Kordas, A. J.; El-Bayoumi, M. A. Chem. Phys. Lett. 1974, Emert, J.; Morawetz, H. J. Am. Chem. Soc. 26,273. (d) Goldenberg, M.; 1979,101,771. (e) Ito, S.;Yamamoto, M.; Nishijima, Y. Bull. Chem. SOC. Jpn. 1981,54,35.
0743-7463/87/2403-0773$01.50/0 0 1987 American Chemical Society
