J. Phys. Chem. 1992,96,4145-4148
the disappearance of the 1071-cm-l peak and the growth of the bulklike band at about 1055 cm-I. However, this interpretation does not account for the continued presence of the 1085-cm-’ band and is not our first choice. It is also appropriate to note that in Na+(CH30H)N”and CS+(CH,OH)~’Ono evidence for restructuring of the first solvent shell has been observed. It is somewhat surprising that the onset of bulklike behavior in Na+(NH3)Moccurs at the low value of M = 10. In Na+( C H 3 0 H ) , similar behavior was not observed until N = 21.” However, in comparison to the ammoniated ammonium ion, this low threshold appears to be reasonable, since Lee and co-workers’ observed ammonias in a bulklike environment for NH4+(NH3), M 1 8, differing only by two molecules from our observations. It is interesting to conjecture at this point why the onset of bulk solvent regions in the cluster ions is so different for Na+(NH3)M and Na+(CH,OH)N. When the first solvent shell of the ion is filled, the ion is somewhat screened from subsequent solvent molecules by those in the first shell. The ion now interacts with the second-shell molecules in two ways: first through residual electric field strength that is not “screened” by the first-shell molecules and second through interaction with the first-shell
4145
methanols that have been polarized by the ion. The molecular polarizability of NH, is about 30% less than that for CH30H. Hence, the ammonia molecules in the first shell of Na+ will be less strongly polarized by the ion than the methanol molecules and the interaction between first- and second-shell ammonia molecules will not be much greater than in the bulk. The influence of the ion will extend only a short distance into the solvent, and the interaction between ammonia molecules will become dominant, resulting in a net environment very similar to bulk ammonia. We are currently looking at larger cluster ions Na+(NH3)Mwith 50 1 M 1 l2I9 as well as other ammoniated alkali-metal ions23to better understand ion solvation by ammonia.
Acknowledgment. This work has been supported in part by the National Science Foundation (Grant CHE-9111930) and the University of Illinois Research Board. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. (23) Selegue, T. J.; Moe, N.; Draves, J. A.; Lisy, J. M. Work in progress.
Polymerization of Pyrrole over Pd and SnO, Supported on KL Zeolite Gustavo Larsen, Gary L. Hailer,* Department of Chemical Engineering, Yale University, P.O. Box 2159, Yale Station, New Haven, Connecticut 06520
and Manuel Marquez Chemistry Department, Yale University, P.O. Box 6666, Yale Station, New Haven, Connecticut 06520 (Received: January 28, 1992; In Final Form: March 24, 1992)
We have prepared polypyrrolezeolitesystems by oxidative polymerization over small SnOz particles and oxygen-covered Pd clusters supported in KL zeolite. The local structure of the Pd and Sn02clusters was studied by means of extended X-ray absorption fine structure (EXAFS). Electron spin resonance (ESR) and Fourier transform infrared (FTIR)spectroscopies and solvent extraction techniques were used to characterize the polymeric species in the zeolite cavities. A monomer/oxidant ratio larger than stoichiometric appears to indicate that the oxidation reaction may proceed catalytically after air exposure rather than in a redox fashion.
Introduction The advent of conducting polymer/zeolite composite materials is relatively recent and has been pioneered by Bein et al.’-, and other research group^.^.^ It offers a fascinating opportunity for preparing highly oriented “wires” of molecular dimensions that may ultimately lead to simple electric circuits of the smallest conceivable size.6 Zeolite cations, generally Na+ or K+,can be ion-exchanged with aqueous solutions of metal ions that are desired to be incorporated into the zeolite lattice. The importance of the ion-exchange technique is that redox-active species can be easily loaded into the crystalline host allowing oxidative polymerization reactions to take place within cavities. We have chosen L zeolite which has a structure of parallel channels? with the goal of constraining the polymer chain growth process to one dimension, thereby minimizing the Occurrence of uncontrolled ‘coiling” throughout the whole zeolite lattice. To provide the zeolite host with the required redox-active species we Enzel, P.;Bein, T. Synth. Met. 1989, 29, E163. Enzel, P.;Bein, T. J. Phys. Chem. 1989, 93, 6270. Enzel, P.;Bein, T. J. Chem. Soc., Chem. Commun. 1989, 1326. Chao, T. H.; Erf, H. A. J. Caral. 1986, 100, 492. Dutta, P.K.;Puri, M. J. Cafal. 1988, I l l , 453. Molecular Elecfronic Devices I& Carter, F. L., Ed.; Marcel Dekker: New York, 1987. (7) Newsam, P. J. J. Phys. Chem. 1989, 93, 7689. (1) (2) (3) (4) (5) (6)
proceeded in a different (and somewhat more complex) way. Very small Pd8 and SnO, clusters are deposited by impregnation techniques and will be shown to act, in all probability, as oxygen activators rather than oxidants per se. There are at least two motivations for choosing such peculiar zeolite environments. First of all, we have reason to believe that, when performed within zeolite cavities, these five-member heterocycles polymerizations may be viewed as rather facile reactions, i.e., they may occur fairly easily without requiring very restrictive polymerization conditions. For example, Caspar et al.9 have recently reported the oligomerization of thiophene in zeolite Na-8 without intentionally introducing oxidant cations. Although in that paper the nature of the redox process appeared unclear, monomer units were not reported to remain after the zeoliteloading step, suggesting that the polymerization might not be solely due to a stoichiometric reaction of the monomer with some zeolite trace impurity. Second, one can speculate that the presence of metal and metal oxide clusters (or any other species) randomly distributed in the L-zeolite channels may play some role in the macroscopic properties of the final material, e.g., conductivity, since they might well introduce doping-like effects. The latter (8) Larsen, G.; Haller, G. L. To be published: Proc. 1OfhInr. Congr. on Cafal.,Budapest, July 1992. (9) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J . Am. Chem. SOC. 1991, 113, 600.
0022-3654,I92 12096-4145S03.00,IO 0 1992 American Chemical Society -I
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4146 The Journal of Physical Chemistry, Vol. 96, No. 11, 195
deserves some attention since it is now known that the conductivity of these encased macromolecules decreases with respect to the doped bulk polymer.1°
Experimental Section The KL zeolite (unit cell formula Na0.1K8.9A19Si27072) was purchased from Tosoh Corporation, Japan. Tin(1V) acetate from Aldrich and Pd(NHJ4(N0J2 from Alfa Products were employed as metal precursors. Pyrrole (Aldrich) was purified by two sequential distillations over Zn powder, and its purity (>99%) was verified by IH NMR. A 1.08 wt % Pd/L catalyst was prepared by the incipient wetness technique and will be described in a separate paper.8 In brief, the dried catalyst was calcined in oxygen flow at 623 K and reduced at 493 K in pure hydrogen for 1 h. The Pd metal surface is reactivated in a conventional Pyrex vacuum system for 1 h under 1 atm of static oxygen at 453 K and subsequently outgassed for another hour at the same temperature and a total pressure of about Torr. The glass cell containing the activated Pd/L catalyst was sealed off under vacuum until use. The Sn02/L sample (1.35 wt 76 Sn) was also prepared by impregnation of 10 g of KL zeolite to incipient wetness with 7.5 mL of a solution of Sn(Ac), in acetic anhydride followed by calcination at 773 K for 1 h under a O2 flow. A portion of the KL zeolite (as received) was outgassed at 773 K for 1 h in order to carry out blank experiments. The chemical analysis of samples was performed by Galbraith Labs. Two hundred milligrams of each of KL zeolite (dehydrated), 1.08% Pd/L, and 1.35% Sn02/L were suspended and stirred in 60 mL of a 0.83 mM solution of pyrrole in chloroform for 2 h at room temperature. In addition, the same amount of KL zeolite was stirred in pure HCC13 to provide a blank for spectroscopic measurements. The suspension was filtered and then washed with three 5-mL portions of HCC13 and oven-dried for 0.5 h at 328 K. The dried material was stored in the dark and kept at 258-263 K until use. The X-ray absorption at the Sn and Pd K-edges was measured at the Come11 High Energy Synchrotron Source (CHESS) X-ray facilities. The experimental setup and EXAFS data analysis are described elsewhere." Fourier transform infrared spectra of the loaded zeolites were obtained in a Nicolet 5X spectrometer at a resolution of 4 cm-'. Typically, 40 mg of pyrrole/zeolite sample and 80 mg of KBr were pressed into ca. 1 cmz wafers. X-band ESR spectra were collected on a Bruker Model ESP 300E spectrometer equipped with a variable temperature controller. Spectra were obtained in the range 110-300 K on the pyrroleloaded Pd/L and Sn02/L samples. Results and Discussion The use of EXAFS spectroscopy to derive structural parameters is well documented.I2 The so-called "fine structure" oscillations above the energy of the absorption edge of core electron ejection may extend as far as 1.5 keV. These fluctuations arise from the quantum interference of the outgoing photoelectron wave with its backscattered components from different neighbor shells and allow the average coordination number (N) and interatomic distance ( R ) of a given coordination shell to be extracted. Figure 1 compares the uncorrected Fourier transform (FT) magnitude functions of the Sn02/L EXAFS with that of bulk Sn02. The latter has a rutile structure and serves as a reference compound, Le., provides known Nand R values necessary for the EXAFS modeling of samples. The FT of Sn(Ac),/L (fresh precursor) is not shown in the interest of brevity. As mentioned earlier, the Pd/L preparation and characterization by EXAFS will be discussed in ref 8. Here, we simply report the Pd first shell coordination number to be 5.6. Table I summarizes the EXAFS results of the SnOZ/Lsample before and after calcination at 773 K. (IO) Dagani, R. Chem. Eng News 1991, May 27, 24. (11) McHugh, B. J.; Larsen, G.; Hailer, G . L. J . Phys. Chem. 1990, 94, 8621. (12) Teo, B. K . EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986.
3
2
4
6
8
10
Distance (A)
Figure 1. Fourier transform magnitude functions of EXAFS spectra, SnO, reference (-) and SnO,/L-zeolite (+).
TABLE I: Modeled Sn EXAFS Coordination Numbers (Nsrx), Interatomic Distances (Rs,x), and Debye-WaUer Factors (DW,,)" sample SnO, (ref) Sn(Ac),/L
Sn02/L
R"W3 Nsn4 2.24 2.32 2.25
6.0 7.2 5.7
DWsn,,
h2
RSA*n'
0.0056 0.0042
3.17 none 3.18
DWSn-Sn,
Nqn-qn h2 12.0 none none 2.9 0.0006
"The subscript denotes the scatterer atom.
The first shell N a n d R of Sn in the Sn(Ac),/L were found to be consistent with those encountered for bulk Sn(Ac), (not shown). No second-shell backscattering contribution was detected, which suggests that only Sn-O short-range order exists in both Sn(Ac), and Sn(Ac),/L, probably in the form of four acetate ions coordinated to Sn4+in a bidentate fashion (expected N = 8). After calcination, very small Sn02clusters are formed as indicated by the appearance of a small second-shell contribution in the FT as well as the contraction of the Sn-O distance toward that of bulk SnO,. Upon calcination, impregnation methods lead to both deposition of neutral crystallites, generally oxides, as well as ion exchange to some extent. The latter depends on the exchange equilibrium for the particular (zeolite and impregnating solution) ions involved. A potential problem of calcined impregnation preparations is the a priori uncertainty in the final oxide cluster and/or cation location in the zeolite lattice. Thus, there are two fundamental questions to be addressed regarding the characterization of host materials. First, the relative importance of ion-exchange processes (ionic precursor anchoring to the zeolite lattice) with respect to mere support impregnation-calcination (deposition of oxide crystallites) must be determined. Second, the size and location of metal and metal oxide clusters in the zeolite (internal vs external confinement) are of crucial importance since it appears reasonable to believe that small particles encaged in the L-zeolite channel lobes are first required to prepare the zeolite-oriented poly-pyrrole chains. As derived from EXAFS analysis, we have determined a Pd-0 coordination distance of 2.02 A in the Pd/L calcined precursor, which is in excellent agreement with that found in the PdO reference compound.8 In the case of the Sn02/L catalyst, the EXAFS data presented in Table I indicates that an exactly analogous situation is also achieved since the Sn-0 distances in Sn02/L and S n 0 2 agree within 0.01 A. The fact that both calcined systems displayed oxide-like X-ray absorption features suggests that ion exchange between ionic precursor forms and lattice K+ may not, if indeed they take place during the impregnation step, induce the formation of stable ions anchored to the zeolite lattice upon catalyst calcination.
The Journal of Physical Chemistry, Vol. 96,No. 11, 1992 4147
Letters
15
I
I
a
1;'
7 b
\
I
2140
1870
1600
lJJ0
1060
Wavenumbers (cm-') Figure 2. Fourier transform infrared spectra of (a) pyrrole-loaded L zeolite, (b) "washed" L zeolite, and pyrrole-loaded (c) Sn02/L and (d)
Pd/L. Perhaps the most interesting information extracted from the EXAFS analysis of the calcined SnOz/L is the indication that very small oxide clusters are apparently located in the L-zeolite channel lobes. By inspecting the SnO, unit cell (rutile structure), it was found that in order to satisfy both N values for SnOZ/L in Table I, we should start by predicting an average oxide cluster with five Sn ions. Two possible geometries with this number of metal ions gave calculated Nsnsnvalues of 2.4 and 3.2, yielding an average coordination number that closely resembles our experimental value of 2.9 (Table I). However, when these small clusters were forced to satisfy charge neutrality (SnsOlo),Sn-0 coordination numbers of about 4 were always found. The N s n ~ in bulk cassiterite (Sn02) is 6, very close to the 5.6 calculated value for SnO2/L. Saturation of the first Sn-0 sphere is achieved if further Sn-O coordination of the stoichiometric cluster to the zeolite internal walls is postulated, requiring the presence of an estimated average cluster like Sn5OI8.Alternatively, a Sn-0 first shell more populated than expected may also be achieved if isolated Sn4+ions bind to certain zeolite sites of high oxygen coordination. However, the latter does not appear to be a good explanation since this would require the zeolite-anchored Sn cations to have exactly the same coordination distance as that of bulk SnO,. In addition, it does not seem reasonable to think that the rather flat zeolite externai surface would also be able to supply this extra number of 0 atoms. We conclude that small, zeoliteencased SnOzclusters are generated upon calcination. We do not have strong evidence to conclude that Pd clusters in the Pd/L host are entirely confined to the zeolite cavities. However, we suspect that the Pd particle growth is restricted by the zeolite pores since significantly larger crystallites are almost invariably obtained when depositing Pd over more open structures, e.g., A1203or S O z , by similar impregnation techniques.13 The low Pd first-shell coordination number as extracted from EXAM suggests that extremely small Pd clusters of about 13 atoms are formed,14 slightly larger than those reported by Bein et al.Is on Pd/X-zeolite. Four selected infrared spectra in the range of 2140-1060 cm-l are presented in Figure 2. The spectrum in Figure 2a was taken on a pure KL sample upon pyrrole adsorption, while Figure 2b shows the effect of a liquid chromatography-like extraction of the adsorbed material with CH2Clzfrom the same sample in Figure 2a. The other two spectra are from the pyrrole-loaded hosts, and they show essentially the same features observed in Figure 2a but (13) Benson. J. E.: Wann. H. G.: Boudart. M.J. Catal. 1973. 30. 146. (14j Kip, B.'J.; DdvenvGrden, F: B. M.; Koningsberger, D. C.; Prins, R. J . Caral. 1987. 105. 26. (15) Molier; K.; Koningsberger, D. C.; Bein, T.J. Phys. Chem. 1989.93, 6116.
3260
I
I
3300
I
I
3340
I
I
3380
I
I
3420
[GI Figure 3. Electron spin resonance spectra of polypyrrole polarons at 110 K in (a) Pd/L and (b) Sn02/L. The latter was taken at 2 orders of magnitude higher gain.
they do not change at all when the same extraction procedure as that performed on the loaded KL sample is attempted. We assign the bands around 1390 and 1760 cm-'to ring stretching and -NH bending, respectively, in both pyrrole and polypyrrole as previously reported.4 Usually, other regions in the polypyrrole infrared spectrum are also studied, but the one we report here showed the smallest degree of overlapping with L-zeolite infrared peaks. The 1620-cm-' band seems to be a property of the starting KL zeolite since it was found to be present in all pyrrole-free samples. The retention of the infrared bands in polypyrrole-loaded Pd/L and SnO,/L is very unlikely to be due to two different complexation mechanisms but of comparable M-pyrrole bond strengths (where M = Sn4+or Pd). We note that such Pd complexes are unstable while, to the best of our knowledge, no pyrrole-Sn complex has been reported. We ascribe this band in Figure 2c,d to the formation of polypyrrole in the L zeolite which is supported by ESR spectroscopy as discussed below. There seems to be agreement that at least two charged forms exist in these conducting macromolecules. In the doped bulk polymer the dication, or "bipolaron", is known to be more stable than the singly charged (polaron) species by about 0.45 eV,I6 the latter being ESR active. In general, this energy gap is always small enough to normally allow the presence of polymer to be detected by this technique. Figure 3 presents the ESR spectra of the two pyrrole-loaded catalysts at 110 K showing the typical polaron signal of polypyrrole around g = 2.0025 in both cases. The spin population of the polypyrrole-Pd/L sample is about 100 times larger than that of polypyrrole-Sn02/L. However, we believe that this may be due to a change in the polaron/bipolaron relative stabilities induced by the different host environment since the 1390-cm-I infrared band is of comparable intensity in both cases and not removable by extraction with CH2ClZ. Another important observation is the lack of hyperfine structure in the ESR spectra as previously reported in the polypyrrole-Fe(II1)Y-zeolite system: but generally observed in polypyrrole free radicals." It was found that no apparent change takes place in our ESR line shapes when cooling the samples from ambient to 110 K. In our particular case, the unidimensionality of the L-zeolite channels makes spin exchange processes between bulky organic species a very unlikely situation. It can be speculated that the polypyrrole radicals may interact with metal or metal oxide clusters giving the observed lineshape features. The asymmetry of the polypyrrole-SnOz/L line shape is not an artifact. When collecting the spectrum of the same sample over an expanded range, there (16) Bredas, J. L.; Scott, J. C.; Yakoshi, K.; Steet, G. B. Phys. Reu. B 1984, 30, 1023. (17) Lloyd, R. V.; Di Gregorio, S.;Wood, D. E. J. Chem. Phys. 1978,68, 1813.
4148
J . Phys. Chem. 1992, 96, 4148-4151
seems to be a weak interaction between the organic radical and some other species, probably zeolite impurities. This is clearly not detected in the Pd/L signal due to its significantly higher intensity. To have a rough estimate of the actual polypyrrole content in our zeolitic materials, a portion of the loaded Pd/L was evacuated at 373 K for 1 h and then heated at 923 K for another hour under 1 atm of pure 0,in the same vacuum line used for oxygen chemisorption. The glass system was equipped with a Baratron pressure transducer and a cold microtrap held at 150 K. After calcination, all organic carbon is assumed to be collected in the trap as CO, while excess O2can be totally evacuated from the adsorption line. Using this method, we determined the polymer formed in the zeolite to be 73 f 15% of the total amount of pyrrole initially present during the loading step. The large inaccuracy arises mainly from the fact that we do not know whether or not the organic nitrogen had also been retained in the cold trap. Nevertheless, it indicates that our monomer/metal ratios in the zeolite are roughly 1.5-2.5. Several implications of the above results are worth discussing. The standard Sn4+/Sn2+redox potential, Eo(Sn), is 0.16 V and in principle would not be sufficient to cause the oxidative polymerization of pyrrole to occur. One possibility would be that the redox potential of the Sn pair is significantly increased by the zeolite environment. It was mentioned above that thiophene oligomers within the Na-&zeolite cavities were synthesized by Caspar et al.9 These authors reported that the redox potential of this zeolite (in its sodium form) with respect to the saturated calomel electrode had been found to be about 1.6 V, high enough to induce thiophene polymerization. An alternative explanation would be that the small SnO, oxide clusters not only experience a perturbation in their redox properties induced by the host but also act as catalytic sites for oxidation of pyrrole by oxygen during the drying step, which may hold for the oxygen-covered Pd crystallites in the Pd/L catalyst as well. We choose the latter
since the pyrrole/metal loading ratio was found to be larger than 1.5 for the 0,-activated Pd/L catalyst, above the value of one required for the formation of a long polypyrrole chain. It should be noted that only in the extreme case of very short chain oligomer formation, Le., dimers or trimers, the reaction might still be stoichiometric. However, some fraction of dimers and trimers should be expected to diffuse out of the zeolite pores upon solvent extraction, but this is not what our infrared-extraction results indicate in the case of pyrrole adsorption reaction on the two catalysts. For example, it is possible to actually load a narrow-pore zeolite such as ZSM-5 with thiophene trimers in liquid media.9 In addition, the absence of hyperfine structure that we observe in Figure 3 has also been associated with the formation of longchain polymer^.^,^^ We conclude that rather high molecular weight species are formed catalytically by the supported particles on L zeolite. A number of questions still arise regarding the macroscopic properties of these composite materials as well as the chemical nature of the polymerization process and its role in determining the polymer (oligomer) chain length. Although it can be speculated that either “creeping” of short oligomer chains along the channels or oxygen spillover from active sites would provide the necessary mobility for reaction completion, the very modest temperatures used during the loading-drying steps do not seem, in principle, to allow such simple interpretations. Further work is needed to clarify the above-mentioned ideas.
Acknowledgment. We are grateful to Prof. T. Randolph for the ESR measurements and his assistance concerning the interpretation of spectra. Partial support by the Office of Basic Sciences, DOE, and beam time a t the Cornell High Energy Synchrotron Source (C2 station) are also acknowledged. (18) Ramamurthy, V.; Caspar, 1991, 113, 590.
J. V.; Corbin, D. R. J . Am. Chem. SOC.
Molecular Structures of M20 (M = B, AI, Ga) Suboxides. Bent or Linear? Jerzy Leszczyiiski* and Jdzef S. Kwiatkowskit Department of Chemistry, Jackson State University, Jackson, Mississippi 3921 7 (Received: January 31, 1992; In Final Form: April 13, 1992)
The molecular structures and properties of the B20,AI20, and Ga20suboxides were studied by the ab initio method at the HF, MP2, and CISD levels with the triple-{ valence basis set augmented by d-polarization functions. For all these species the linear conformers are the global minimum structures at the MP2/TZP level. The calculated bond distances, rotational constants, and harmonic vibrational frequencies are in a very good agreement with the available experimentaldata. Predicted by calculations, but still experimentally elusive, low-energy vibrational bands for the boron and gallium suboxides may be used for identification of the studied species in further experiments.
Introduction There has been increasing interest in molecular structure and properties of the group I11 oxides. Boron oxides were studied recently as the products of boron interactions with water in the gas phase and in argon matrix isolation experiment^.'-^ Similar experiments were carried out with Al.4 Likewise, technological importance of aluminum oxides as products of the oxidation reactions stimulated numerous experimental studies on these species.5-8 For example, simple diatomic A10 was detected in different high-temperature sources such as stellar spectra and also in vapors over heated A1203.9J0 Additionally, aluminum oxides are important elements of coal fly and volcanic ashes. Furthermore, an application of metal oxides in the semiconductor industry ‘Permanent address: Institute of Physics, N. Copernicus University, 87100 Torui. Poland.
has stimulated research on matrix reactions of molecular oxygen and ozone with aluminum, gallium, indium, and thallium atoms.’-’’ In spite of a number of experimental and theoretical ~
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~
~~~~
~~~~
(1) Gole, J. L.; Pace, S . A. J. Phys. Chem. 1981, 85, 2651. (2) Jeong, G. H.; Boucher, R.; Klabunde, K. J. J . Am. Chem. SOC.1990, 112, 3332. (3) Andrews, L.; Burkholder, T. R. J . Phys. Chem. 1991, 95, 8544. (4) Oblath, S. B.; Gole, J. L. J . Chem. Phys. 1979, 70, 581. (5) Snelson, A. J. Phys. Chem. 1970, 74, 2574. (6) (a) Makowiecki, D. M.; Lynch, D. A., Jr.; Carlson, K. D. J . Phys. Chem. 1971, 75, 1963. (b) Lynch, D. A,, Jr.; Zehe, M.J.; Carlson, K. D. J . Phys. Chem. 1974, 78, 236. (7) Srebrennikov, L. V.; Osin, S . B.; Maltsev, A. A. J. Mol. Srrucr. 1982, 81, 25. (8) Sonchlik, S. M.; Andrews, L.; Carlson, K. D. J . Phys. Chem. 1983,87, 2004. (9) Babcock, H. D. Asrrophys. J . 1945, 102, 154. (10) Lindsay, D.; Gole, J. J . Chem. Phys. 1977, 66, 3886.
0022-365419212096-4148$03.00/0 0 1992 American Chemical Society
