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Conductance and Fluorescent Probe Studies of Sodium Dodecyi Suifateh -Butyl Aicohol/Toiuene/Brine Microemulsions Manuel Sbnchez-Rubio, Luz Mada Santos-Vidals, Dennis S. Rushforth,* Departamento de Quimica, Centro de Investigacidn y de Estudios Avanzados, I.P.N., 07000-MZxico, D.F.
and Jorge E. Puig Facultad de Ciencias Quimicas, Universidad de Guadalajara, 44430 Guadalajara, Jal., Mgxico (Received: March 7 , 1984; In Final Form: August 16, 1984) We report conductance and fluorascent probe measurements of one-phase microemulsionscomposed of sodium dodecyl sulfate (SDS), n-butyl alcohol, toluene, and 2.5% NaCl brine. We detected three patterns of microemulsion conductance as a function of toluene volume fraction and an interesting minimum in the pyrene excimer formation as a function of brine volume fraction. Our results imply that there are, at least, three types of microemulsions: water continuous, oil continuous, and bicontinuous. The last type forms in the vicinity of the three-phase region where the microemulsion is the middle phase. In this region, the water-to-oil ratio in the microemulsion is close to unity. In addition we suggest the existence of a water-continuous microemulsion at high toluene volume fractions in which the structure is a network of water-swollen inverse surfactant bilayers containing large discrete oil domains.
Introduction Since their discovery,1,2 microemulsions have attracted considerable attention, not only because of their practical applications but also because of their singular physicochemical, rheological, and structural p r ~ p e r t i e s . ~ - 'By ~ microemulsion we mean an isotropic, thermodynamically stable, microstructured fluid phase that contains substantial amounts of two ordinarily immiscible liquids (i.e., water and oil) and surfactant^.^ One important area of research is that of elucidating the structure of microemulsions. Valuable information concerning the structural behavior of microemulsion systems can be provided by techniques such as conductometry and fluorescent probes, alone or combined with other methods.*-12 In this article we present conductance and fluorescent-probe measurements on the one-phase microemulsion region of the pseudo-ternary phase diagram of the SDS/NBA/toluene/2.5 wt % NaCl brine system and interpret them in terms of microemulsion structure. We detected three patterns of microemulsion conductance as a function of toluene volume fraction and an interesting minimum in the pyrene excimer formation. These unusual data suggest that there are, at least, three types of microemulsions: water continuous, oil continuous, and bicontinuous. The last type forms in the one-phase region neighboring the three-phase region where the microemulsion is the middle phase. In this region, the water-to-oil ratio in the microemulsion is close to unity. Our data also suggest the existence of water-continuous microemulsions containing high toluene volume fractions (& > 0.7). We discuss here the possible structures of this type of microemulsion.
Experimental Section Sodium dodecyl sulfate (SDS) was pure (>99%) from Merck. Toluene and n-butyl alcohol (NBA) were 99% pure from Phillips (1) (2) (3) (4)
Hoar, T. P.; Schulman, J. H. Nature (London) 1943, 152, 102. Schulman, J. H.; Riley, D. P. J. Colloid Sci. 1948, 3, 383. Friberg, S . ChemTech 1976, 6, 124. Kaler, E. W.; Bennett, K. E.; Davis, H. T.; Scriven, L. E. J . Chem.
Phys. 1983, 79, 5673. (5) Kaler, E. W.; Davis, H. T. Scriven, L. E. J. Chem. Phys. 1983, 79, 5685. (6) Lindman, B.;Kamenka, N.;Karthopoulis, T.; Brun, B.;Nilsson, P. J . Phys. Chem. 1980,84, 2484. (7) Fabre, H.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1981,853493. (E) LaguL, M.; Sauterey, C . J. Phys. Chem. 1980,84, 3503. Griescr, F.; Thomas, J. K. J. Am. Chem.Soc. 1980,202, (9) Almgren, M.; 3188. (10) Lianos, P.; Lang, J.; Strazlelle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019. (1 1) Bennett, K. E.;Hatfield, J. C.;Davis, H. T.; Maoosko, C. W.; Scriven, L. E. In 'Microemulsions"; Robb, I. D., Ed.;Plenum Press: New York, 1981; p 65. (12) Cben, S. J.; Evans, D. F.;Ninham, B. W. J. Phys. Chem. 1984.88, 1631.
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Petroleum and Baker Chemical Co., respectively. Pyrene was scintillation grade from Baker Chemical Co. Doubly distilled water, with conductivity less than 1 pS.cm-', was employed. All these materials were used without further purification. Sodium chloride (ACS grade from Fisher Scientific) was carefully dried in a vacuum oven before using. Samples were prepared by weighing all the components in cold glass ampules that were then sealed to prevent evaporation, gently hand shaken, and equilibrated in a constant-temperature water bath (25.0 f 0.05 "C) before making any measurement. Once the sample was in equilibrium, the number of phases was recorded. The criteria for equilibrium was the reproducibility of number and volume of the phases in shaking-and-standing cycles. Samples within the one-phase region were observed for months with no apparent visual changes. Phase boundaries were also determined by adding known amounts of toluene to one-phase samples made of 2.5% wt % NaCl brine and active mixture. The active mixture was one part by weight SDS and two parts by weight NBA. Phase boundaries determined by the two described methods coincided within the experimental uncertainty (&I%). Conductance was measured at 25.0 f 0.05 OC with a Metrohm Model 644 conductometer a t a frequency of 1000 Hz. Because the conductance of the microemulsions varies by four orders of magnitude along the one-phase region, two cells with different cell constant were used. For poorly conducting samples, a home-made immersion cell (cell constant of 0.46 cm-') was used whereas a Jones-type cell (28.03 cm-') was required for highly conducting microemulsions. Viscosities were obtained with a calibrated Ubbelhode viscometer. Uncorrected fluorescence spectra were taken with an Aminco SPF-500 ratio spectrofluorometer. The excitation wavelength was 337 nm. The intensities were taken at 395 (where molecular pyrene strongly emits) and 466 nm (where the excimer emits) with emission and excitation bandpass of 2 nm. Results The pseudotemary phase diagram in weight percent, shown in Figure 1, is similar to that reported e1se~here.I~There is a two-phase region where the microemulsion is the lower phase -(indicated as 2. in the diagram); a two-phase region, labeled as 2 in Figure 1, where the microemulsion is the upper phase; a three-phase region, represented as 3, where the microemulsion is the middle phase; and a large one-phase region where the attention of this paper is concentrated. There is also a two-phase region, denoted as g*, where the lower phase is not a fluid mi(13) Bellocq, A. M.; Biais, J.; Clii, B.;GPot, A.; Lalanne, P.; Lemanceau, B. J . Colloid Interface Sci. 1980, 74, 31 1.
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The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
Sinchez-Rubio et al.
NBA/SDS
I11
Figure 1. Pseudoternary phase diagram of the SDS/NBA/toluene/2.5 wt % NaCl brine system. The weight ratio of NBA to SDS used was 2. The letters A through M indicate the surfactant/alcohol to brine ratio used to prepare stock solutions for the conductance measurements. Other notation is explained in the text.
croemulsion but a surfactant-rich white phase which shows birefringence in the polarizing microscope. The hatched area in the toluene-rich corner of the diagram (Figure 1) is the zone where 2-phase behavior changes into 2*-phase behavior. According to the phase rule there must be a one-phase region or, perhaps in a more complicated fashion, a three-phase region (neither as yet detected) separating these two-phase region. The one- or three-phase region must be located inside the hatched region. The conductance of one-phase microemulsions can be divided into three different patterns as the toluene volume fraction increases. The plots in Figure 2 illustrate these patterns. The microemulsions were prepared by adding increasing amounts of toluene to stock samples of brine and active mixture. The composition of the stock samples used are indicated with the letters A to M in the phase diagram (Figure 1). To convert the data to volume fraction, we have taken the density of the surfactant to be equal to 1.02 g/cm3-this value was obtained by extrapolating to zero brine concentration the densities of the one-phase stock samples (C to M). The zones where the three conductance patterns were detected are also shown in the phase diagram. In zone I, above the dashed line, the microemulsion conductance decreases as the toluene volume fraction, dtol,increases. This is what is usually observed. In zone 11, between the dashed line and the dotted line, the conductance first decreases, passes through a minimum, and then increases as dtolincreases (curves F, G, and H). In zone 111, the conductance increases as increases (curves I to M). The division between zones I1 and I11 could be considered quite arbitrary in that the conductance behavior in these two zones is very similar and that only the presence of the miscibility gap makes it look different. However, we separated the two because the conductance of samples in zone I11 is significantly higher and shows no minimum in the one-phase region. Fluorescent probe measurements were taken along the arrowed line indicated in the phase diagram of Figure 1. Plots of the ratio of the intensity at 466 nm to that at 395 nm for various pyrene concentrations, and of conductance vs 1 - the volume fraction of is a brine, 1 - dB,are recorded in Figure 3. The ratio 1466/1395 relative measure of excimer formation in micro emulsion^.^^ The parameter (1 - dB) is a rough estimate of the volume accessible to pyrene since pyrene is essentially insoluble in water. Microemulsion viscosities (reported elsewhere) l5 are low (less t h a n 7 cP) in the whole one-phase region. (14) Selinger, B. K.; Watkins, A. R. Chem. Phys. Lett. 1978, 56, 99. (15) SHnchez-Rubio, M. Tesis Doctoral, Centro de Investigaci6n y de Estudios Avanzados del I.P.N., MCxico, 1984.
* K
* L
Figure 2. Relative conductance of a series of microemulsions. Each curve corresponds to a stock solution of SDS/NBA/2.5% NaCl brine successively diluted by toluene. The letter identifying the curve corresponds to the same letter in the phase diagram (Figure 1) indicating the surfactant/alcohol to brine ratio. The conductance of the brine, KO,is equal to 41 mS-cm-'.
r
I
I
I
i\
10
,466 t
0 07
1
1 -
.L
i
OB
09
(I-
IO
0,)
Figure 3. Ratio of pyrene exciher emission intensity measured at 466 nm to pyrene emission intensity measured at 395 nm in SDS/NBA/ toluene/2.5% NaCl brine microemulsions as a function of (1 Curves a through d correspond to a pyrene in toluene concentration of (a) 0.05, (b) 0.02, (c) 0.01, and (d) 0.005 M.
Discussion Both conductance and excimer formation are measures of transport properties in microemulsions. Conductance measures
Conductance and Fluorescent Studies of Microemulsions the ability of ions in the aqueous domain of a microemulsion to move from one electrode to the other under the influence of an electrical field. It is expected that aqueous-continuous microemulsions ought to display fairly high electrical conductance in contrast to oil-continuous ones which should be poorly conducting. Pyrene excimer formation reflects the ability of an excited pyrene molecule within the oil domains of a microemulsion to diffuse to and interact with another pyrene molecule in the ground state during the lifetime of the excited state.I4*I6 Thus, in watercontinuous microemulsion, because the diffusion of water-insoluble pyrene molecules is restricted, excimer formation ought to be low compared to that in pure oil containing the same concentration of pyrene. In contrast, excimer formation in oil-continuous microemulsions should be comparable to that in pure oil. Conductance and pyrene excimer formation measurements made in our laboratory15 with water-continuous and oil-continuous microemulsions support these hypotheses. These results are consistent with the general picture of microemulsion structure as solution of oil- (or water-) continuous swollen (inverted) micelles a t low volume fraction of the dispersed component.4-“ As the ratio of volume fraction of water to that of oil approaches unity, there is disagreement about the structure of microemulsions. At least two views of the structure at intermediate water-to-oil ratios have been put f ~ r t h . ~ * ~ ?To ” - ~examine ’ structural transitions in microemulsions and to obtain some information about the structure at intermediate water-to-oil ratios, conductance and fluorescent probe measurements were made on a high microemulsion which was diluted successively with brine (path indicated by arrowed line in Figure 1). At low water-to-toluene ratio (1 - $B = l), the conductance is very small (0.002 mS-cm-’) whereas excimer-to-monomer emission intensity ratio, 1466/1‘95, is close to that in pure toluene (see Figure 3). This indicates a toluene-continuous, water-restricted microemulsion. By diluting this microemulsion with brine, its conductance increases; the increment is first small but, as the sample approaches the three-phase region, the microemulsion conductance increases tweorders of magnitude. The ratio on the other hand, decreases slightly as brine is added to the system (Figure 3). This result is consistent with a reduction in the volume accessible for diffusion of pyrene molecules which reduces the probability of excited pyrene molecules encounters with a ground-state pyrene molecule. Nevertheless, in the neighborhood of the three-phase region, 1466/1395 increases again demonstrating a larger accessible volume in which the pyrene molecules can diffuse. At the same time, the conductance is high indicating that the ions are free to diffuse. It is worth mentioning that F6/Ps5 is also affected by the viscosity of the sample and by the lifetime of the pyrene excited state.16 However, it is unlikely that either of these two factors is responsible for the observed changes: measured lifetimes for pyrene at low concentration ( M) are almost equal for all the samples examined and the viscosity does not change appreciably along the chosen dilution line.I5 Thus one-phase microemulsions near the three-phase region have both high conductance and high pyrene mobility. This result strongly supports the existence of bicontinuous microemulsion. Moreover, the data imply that a toluene-continuous (oil-continuous) microemulsion is transformed into a bicontinuous one by adding brine. The latter forms in the vicinity of the three-phase region where the brine-to-toluene ratio is close to unity. The unusual conductance behavior reported here (Figure 2) can also be explained consistently. In zone I (Figure 1) the microemulsion should be oil continuous, i.e., toluene (or at least toluene and NBA) continuous at all but the lowest toluene volume N
(16) Thomas, J. K. Chem.Rev. 1980,80,283. (17) Scriven, L. E. In ‘Miccllization Solubilization, and Microemulsions”; Mittal, K. L., Ed.; Plenum Press: New York, 1977; p 877. (18) Clausse, M.; Peyrelasse, J.; Heil, J.; Boned, C.; Lagourette, B. Nature (London)1981, 293, 636. (19) Ober, R.; Taupin, C. J. Phys.Chem.1980,84, 2418. (20) Cebula, D. J.; Ottewill, R. H.; Ralston, J.; Pusey, P. N . J. Chem.Soc., Faraday Trans.1, 1981, 77,2585. (21) Gulari, E.; Bedwell, B.; Alkhafaji, S. J. Colloid Interface Sci. 1980, 77, 202.
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 413 fraction. As increases, the water domains become smaller and more restricted, and so the ions can no longer diffuse freely. Consequently, the conductance decreases. Interestingly, the lowest conductance measured in the one-phase region was that of samples at the toluene-rich corner neighboring the Z*-phase region. In zone I1 the microemulsion should be water continuous at low &, Le., a solution of oil-swollen micelles. This is demonstrated by the high conductance (Figure 2) and low excimer formation15 shown by the microemulsions in this region. Diluting these samples with toluene causes a rapid reduction in their conductance. This is expected since the volume fraction of the noncontinuous oleic phase increases whereas the water paths become more restricted. At around 0.6 to 0.72 volume fraction of toluene the conductances of the various samples reach their minimum values. If the oil phase is roughly considered to be formed by the toluene plus the surfactant tails, the oil volume fraction at the minimum is around 0.70-0.74. The latter value corresponds to the closest sphere packing,22 provided that the oil droplets are monodisperse. However, close sphere packing, i.e., droplets in contact, is rarely achieved because the droplets repel each other as a result of electrostatic, steric, and other forces. Thus, values lower than 0.74 can be expected.23 At this point it is usually assumed that a reversion to an oil-continuous microemulsion will occur upon addition of more toluene with a consequent conductance reduction. Instead, in this system, the conductance rises suggesting that the water connectivity increases. One possible explanation is provided by a model first proposed by LissantZZfor high internal phase ratio emulsions, and adopted by Chen et a1.12 for microemulsions. According to this model, an internal (noncontinuous) phase can exist at volume fractions in excess of 74%. Above this value and up to 96.6%,22the noncontinuous phase can be accommodated in a rhomboidal dodecahedron (RDH) packing. Thus, instead of reversing, the microemulsion takes a RDH-like structure. In this structure the brine is located between the RDH flattened sheets of reversed surfactant bilayers. These flattened sheets provide connected conducting water path for the ions. Of course this model is very simplistic because it requires extreme curvature at the edges and corners of the rhombic dodecahedra and this is unlikely. It also implies a long-range geometrical order which may not be consistent with the low viscosity (less than 5 cP) ob~erved.’~ Nevertheless, the essential aspects of this model still may be true if one can take into account the curvature required for such high noncontinuous phase volume fractions. Here we envision a more likely structure as a network of water-swollen inverse surfactant bilayers containing large discrete oil domains. The excimer formation data in zone I1 is consistent with this possibility: pyrene diffusion appears to be more restricted in this zone than in zone I and, interestingly, more than in zone 111 (compare Figures 1 and 3). In zone 111, excimer formation increases by adding brine (Figure 3), suggesting that the discrete oil domains of the RDH-like structures of region I1 connect to allow an increased pyrene diffusion. At the same time, the fact that the conductance is high everywhere in this zone and increases as brine is added (Figure 3) argues in favor of bicontinuous structures. Moreover, the conductance of microemulsions in zone I11 also rises upon toluene addition (Figure 2) indicating that the water connectivity of the bicontinuous region increases as the volume fraction of brine decreases. This is what Chen et a1.12 observed. In conclusion, excimer formation and conductance measurements in the one-phase region of the microemulsion system SDS/NBA/toluene/2.5 wt % NaCl brine indicate differences in the microemulsion structure. Our data are consistent with the formation of, a t least, three types of microemulsions: water continuous, bicontinuous, and oil continuous. We postulate, in addition, the existence of water-continuous RDH-like structures at high toluene volume fractions. To obtain further information about the structure of these microemulsions, X-ray, time-dependent (22) Lissant, K. J. J. Colloid InterfaceSci. 1966,22, 462. (23) Princen, H. M.; Aronson, M. P.; Maser, J. C. J. Colloid Interface Sci. 1980, 75, 246.
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fluorescent-probe, fluorescent depolarization, viscosity, and conductance experiments are under way to study in more detail the one-phase region as well as the multiphase region of this system. Acknowledgment. We thank Prof. L. E. Scriven for his useful comments. We also thank Dr. G. Poillerat for the loan of his conductivity cell. The aminco spectrofluorometer was obtained
through a CONACYT-BID Grant No. PCCBBNA 001787. M.S.R. was supported, in part, by a scholarship from CONACYT. H e also acknowledges considerations from the Instituto Mexican0 in the Of this work' Registry No. SDS,151-21-3; BuOH, 71-36-3; PhMe, 108-88-3; NaCl, 7647-14-5.
Infrared Multiphoton Absorption and Decomposition for CF3CH,Br, CH,CHBrCH,, CH,CHCICH,, CH,CHCiCH,CH,, CH3CF3,and CH,CH,F J. C. Jang-Wren, D. W. Setser,* Chemistry Department, Kansas State University, Manhattan, Kansas 66506
and J. C. Ferrero Chemistry Department, Cordoba University, 501 6- Cordoba, Argentina (Received: April 25, 1984;
In Final Form: September 24, 1984)
The energy absorbed from a C02 laser pulse has been measured over the 0.05-5.0 J/cm2 fluence range for the six halogenated molecules mentioned in the title. Reaction probabilities also were measured for CF3CH3,CH3CH2F,and CF3CH2Br. For the propyl and butyl halides little reaction occurred up to a fluence of 5 J/cm2 because the energy absorption cross sections are intrinsically low rather than because of bottlenecking. The CF3CH3and CH3CH2Fmolecules show strong rotational bottlenecking for both the energy absorption and the induced reaction probability, whereas the larger molecules show less restriction that can be associated with rotational effects on the energy level structure. The energy absorption data for CF3CH2Br, CH3CHBrCH3,and ethyl acetate (previously published by the laboratory) were modeled by the master equation formulation to assign microscopic (molecular) absorption cross sections. These experimental molecular cross sections were compared to those calculated from the Goodman and Stone formulation for multiphoton absorption. The agreement with theory was poor. Some possible reasons that the theoretical model fails for these large molecules are discussed. The importance of incorporating rotational states into the photon absorption steps in the low-energy regime in a realistic way is emphasized.
I. Introduction Infrared multiphoton absorption (IRMPA) and subsequent decomposition (IRMPD) of polyatomic molecules have been the subject of much recent study.'-' The mechanism of IRMPA leading to IRMPD is extremely complex because it involves numerous phenomena. The lack of a detailed understanding of the internal energy level structure of polyatomic molecules is especially troublesome. According to the general theory of IRMPA, the vibrational manifold can be divided into two regions: the discrete levels at lower energy and the quasi-continuum or true-continuum region at higher energy. In the low-energy regime the molecules absorb photons through a set of near-resonant steps. Anharmonicities, which may cause energy mismatch between the vibrational spacing and the laser frequency, may be compensated
by power broadening,8 vibrational-rotational interaction,8 anharmonic splittings9 of degenerate levels, Stark shifting induced by the laser field,Ib and fortuitous anharmonic compensation.1° At higher energy, the density of states is sufficient for the system to be treated as a quasi-continuum. The vibrational levels of the pumped mode are presumed to be coupled with background levels of the quasi-continuum, and the energy formally identified within the pumped mode is redistributed among the other coupled modes on a picosecond time The coupling mechanism between the pumped mode and the dense vibrational background levels is not well understood. If the total density of vibrational states was the critical factor for energy relaxation from the pumped mode, a rapid increase of overtone bandwidths with excitation energy would be expected due to very rapid increase of the density of total vibrational states. However, a relatively modest increase
(1) (a) Danen, W. C.; Rio, V. C.; Setser, D. W. J . Am. Chem. Soc. 1982,
104, 5431. (b) Jang, J. C.; Setser, D. W.; Danen, W. C. J . Am. Chem. SOC.
1982,104,5440. (c) Danen, W. C.; Jang, J. C. In 'Laser Induced Chemical Processes"; Steinfeld, J. I., Ed.; Plenum Press: New York, 1981. (2) Schulz, P. A.; Sudbci, Aa. S.; Krajnovich, D. J.; Kwok, J. S.; Shen, Y. R.; Lee, Y. T. Annu. Rev. Phys. Chem. 1919, 30, 379. (3) King, D. In "Advances in Chemical Physics"; Lawley, K., Ed.; Wiley-Interscience: New York, 1981. (4) Letokhov, V. S.; Moore, C. B. In "Chemical and Biochemical Applications of Laser''; Moore, C. B., Ed.; Academic Press: New York, 1977; Vol. 111. (5) (a) Cantrell, C. D.; Freund, S . M.; Lyman, J. L. In "Laser Handbook"; North-Holland: Amsterdam, 1978; Vol. 36. (b) Aldridge, J. P.; Birely, J.
H.; Cantrell, C. D.; Cartwright, D. C. In 'Physics of Quantum Electronics"; Jacobs, S . F., Sargent, 111, M., Scully, M. O., Walker, C. T., Eds.; Addison-Wesley: Reading, MA, 1977; Vol. 4. (6) Ben-Shaal, A.; Haas, Y.; Kompa, K. I.; Levine, R. 0. "Lasers and Chemical Change"; Springer-Verlag: New York, 1981. (7) Hudgens, J. W.; McDonald, J. D. J . Chem. Phys. 1982, 76, 173.
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(8) Ambartmmian, R. V.; Letokhov, V. S . In "Chemical and Biochemical Applications of Laser"; Moore, C. B., Ed.; Academic Press: New York, 1977; VOl. 111. (9) (a) Cantrell, C. D.; Galbraith, H. W. Opt. Commun. 1976, 18, 513. (b) Jensen, C. C.; Person, W. B.; Krohn, B. J.; Overland, J. Opt. Commun.
1911, 20, 215. (10) Dai, H. L.; Kung, A. H.; Moore, C. B. J. Chem. Phys. 1981,73,6124. (11) Kwok, H. S.; Yablonovitch, E. Phys. Reu. Lett. 1978, 41, 745. (12) (a) KO,A. N.; Rabinovitch, B. S. Chem. Phys. 1978.29, 271; 1978, 30, 361. (b) Walters, F. C.; Rabinovitch, B. S.; KO,A. N. Chem. Phys. 1980, 49, 65. (13) (a) Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J. Chem. Phys. 1980, 72, 5039,5049, 5921. (b) Beck, S . M.; Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J . Chem. Phys. 1980, 73, 2019. (c) Hopkins, J. B.; Langridge-Smith, P. R. R.; Msalley, R. E. J . Chem. Phys. 1983, 78, 3410. (14) Moss, M. G.; Ensminger, M. D.; Steward, G. M.; Mordaunt, D.; McDonald, J. D. J. Chem. Phys. 1980, 73, 1256. (15) Nesbitt, D. J.; Leone, S. R. Chem. Phys. Lett. 1982, 87, 123.
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