1564
J. Phys. Chem. 1985,89, 1564-1 567
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as [Q] was varied. The slope of the In (I(NF,b)Jvs. [Q] plot gives the rate constant, kQ,directly. Some typical results are shown in Figure 3, and all the rate constants are listed in Table I. The kclz and kBrz. values in the table are mean values from several fixed point experiments, as well as the experiments of Figure 2. The quoted error limit for the kQ values is the statistical standard deviation; the absolute uncertainty is estimated to be f30%, which arises from uncertainties in the Ar flowmeter calibration and the use of a capillary flowmeter for which the calibration must be adjusted according to the viscosities of the reagent gases. Experiments were done with N2, but no quenching could be detected and kN2.C 0.2 X lo-', cm3molecule-' s-I. The previously reported3 cm3 molecule-' s-I) quenching rate constant for NF3 (2.5 X is too large, and NF2, NF,, NZF4, and CF, all have small rate constants for quenching of NF(b). In making the quenching measurements, we discovered some systems which did not follow well-behaved first-order kinetics. In each case the fixed point method gave curved decay plots with two apparent first-order quenching regimes: one at larger [Q] with a small rate constant and one at lower [Q] with a large rate constant. Some of these cases were also studied by the movable detector method, and the results were similar; Le., good first-order constants were found for a given [Q], but the first-order rate The reagent molecules constants did not depend linearly on [QJ. showing this behavior were H2S, HBr, C2H4, C2H2,H2, D,, and perhaps NO. Separate studies for N O using a monochromator to observe the NF(b) emission gave significantly better decay plots with kQ = 5 X IO-', cm3 molecule-' s-I, and the difficulty with NO, in part, may be a consequence of very weak emission from N O + 0 (the 0 may arise from the N atoms generated from Ar* + NF2). For some cases, we strongly suspect that the kinetic problems are associated with development of a wall quenching rate that depends on reagent concentration; Le., k , is no longer small (and constant) but increases to some maximum value as [Q] increases. The gas-phase quenching for such reagents is only observed when kQ[Q] 1 k,. These poorly behaved reactions require further study with measurement of k, or, even better, elimination of k , by surface coating, before quenching rate constants can be reported. Another suspected complication for some cases is the involvement of secondary reactions arising from the presence of F atoms in the flow reactor.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. We thank Mr. P. Green and Mr. Hyungki Cha for assistance in the experimental measurements of the NF(b) quenching rate constants. We thank Dr. N. Sadeghi, Grenoble University, for a discussion of the diffusion kinetics of NF(b) and Dr. K. Schofield for a preprint of his review of NH(b) and O,(b) quenching rate constants.
Conclusions We have developed a flowing-afterglow source of NF(b), which is suitable for both kinetic and spectroscopic studies. Except for reagent molecules with low-energy electronic states, such as CI,,
(25) Pritt, A. T., Jr.; Patel, D.; Coombe, R. D. J . Chem. Phys. 1981,75, 5720. ( 2 6 ) Clyne, M. A. A.; MacRobert, A. J.; Brunning, J.; Cheah, C. T. J. Chem. Soc., Faraday Trans. 2 1983,79, 1515.
Br2, and O,, quenching of NF(b) proceeds by an E V transfer mechanism. Thus, the rather small rate constants reported for the reagents in Table I are expected. The E-V constant values for NF(b) can be compared to those for O,(b) and NH(b). The analogy between NH(b) and O,(b) quenching mechanisms was previously argued by Zetzsch and StuhLZ2 The quenching constants for NF(b) frequently are larger than for 02(b), but there are exceptions-notably NH3, N 2 0 , and COz. The E-V quenching rate constants of O,(b) are known to correlate with the magnitude of the highest vibrational frequency of the reagent molecule, and the product state is 02(a).12J3 Differences in quenching rate constants for NF(b) and O,(b) may be a consequence of the different energy separations between the a and b states, which are 6881 and 5251 cm-' for N F and 02,respectively, or a difference in final states, Le., NF(X) formation rather than NF(a). Comparison also can be made for quenchingZ5of NCl(b) by H F cm3 molecule-' s-I), HCl (1.0 X cm3 molecule-' (1.8 X s-I), and CO, (5.3 X lo-', cm3 molecule-' s-I). These reslts are similar to those for NF(b); however, the quenching rate of NCl(b) by Cl, and CIF is much slower than for NF(b), perhaps because the mechanism is chemical reactionz6 rather than excitationtransfer due to the lower energy of NCl(b). It is worth noting that Do(Clz)> E(NF(b)) and that excitation transfer may provide a source of metastable C12*molecules. N o product emission (A I850 nm) could be observed from quenching of NF(b) by either C12 or Br,. The flowing-afterglow source for NF(b) described here should be suitable for measuring a reliable and comprehensive set of total quenching rate constants and measuring the temperature dependence of a subset of interesting reactions. However, other techniques are needed to establish the product states from quenching. Both types of information will be required to develop successful models for E-V quenching.
Infrared Studies of Metal Additive Effects on CO Chemisorption Modes on Si0,-Supported Rh-Mn, -Ti, and -Fe Catalysts Masaru Ichikawa*+and Takakazu Fukushima Sagami Chemical Research Center, 4-4- 1 Nishi- Onuma, Sagamihara, Kanagawa, 229, Japan (Received: November 26, 1984; In Final Form: February 25, 1985) IR spectra of CO chemisorption revealed an unusually large reduction of stretching frequency of bridging carbonyls on Rh in contact with Mn,Ti, and Zr ions, indicating a tilted CO chemisorption mode with its carbon bonded to Rh atoms and its oxygen to the promoter ions. Fe added to Rh strongly suppresses the abundance of bridging carbonyl on Rh, proving a large contribution of Fe basically as a geometric ensemble effect due to the alloy formation of Rh-Fe on the particle surface. Introduction Recent studies have demonstratedlJ that early transition metal ions and their oxide supports have a significant effect on the activity and the selectivity of the Rh catalysts in the CO-H, Pressent address: Ipatieff Laboratory, Department of Chemistry, Northwestern University, Evanston, IL 60201
0022-3654/85/2089- 1564$01 S O / O
reaction. Mn,3 Ti, and Zr4 which are present as oxides under the prevailing reaction conditions increase the activity of Rh/SiO, (1) Ellgen, P. C.; Bartley, W. J.; Bhasin, M. M.; Wilson, T. P. Adu. Chem. Ser. 1919,No. 178, 147. (2) Ichikawa, M. Bull. Chem. SOC.Jpn. 1982,513 2213; CHEMTECH 1982,674.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1565
Letters 2200
2000
1900
1800
1700
1600
I
I
I
I
I ,,,/
2200 2000
1500 cm-'
1900
1800
1700
1600
1500
1400
I
-". ....
.... background
1
I1
sample' 1. 2.
5. 3.
4.
Rh : Mn
1 1 1 1 1
C O a d s o r p t i o n peaks
0
2040
0.05
2040 2035 2030 2030
0.1 1 2
1880(S) 1870(s) 1710(w) 1820( s ) 1870 (m) 1720(w) 175O(s) 1520(w) 1700(s) 1 5 2 0 ( s )
Figure 1. IR spectral changes in CO chemisorption on Rh-Mn/SiO, catalysts by changing the Mn contents (Mn:Rh = 0-2.0 atomic ratios). p(C0) = 150 torr at 25 OC.
+
by 10-50 times in keeping the selectivity of conversion of CO Hz into Czoxygenates, e.g. CzH50H,CH3CH0, and CH3COOH. Moreover, the addition of Fe to Rh/SiOz5 basically influences the selectivity of oxygenates, enhancing the ethanol yield besides the marked formation of methanol.6 The C O conversion is less strongly affected. In this letter, we report the IR observation of unusual CO chemisorption modes, indicative of specific promoting roles based on the activation of C O by interaction of the promoter Mn, Ti, and Zr ions with carbonyls chemisorbed on Rh. In addition, the effect of Fe added to Rh on the depressive influence of bridging CO on Rh will be discussed in terms of geometric ensemble effects due to Rh-Fe alloy formation on the Rh particle surface.
Experimental Section Si02-supported catalysts of Rh, Rh-Mn, Rh-Ti, Rh-Zr, and Rh-Fe were prepared by conventional coimpregnation of RhC13-3Hz0, MnCl2.4HZ0,TiC14, ZrC14, and FeC13 (99.99% purity) from methanol solutions with SiOz gel (Aerosil-300, purchased from Nihon Aerosil Co.Ltd, surface area 280 m2/g). The samples were dried and pressed into a disk wafer (20-mm diameter, 70 mg). They were subsequently reduced with flowing H2 (99.999% purity, 1 atm, 100 mL/min) by temperature-programmed heating up to 400 OC. For the IR spectroscopic measurements, each sample disk was rereduced in an IR cell with Hz flow a t 400 OC for 2 h and evacuated at lo4 torr a t the same temperature for 10 min. The IR spectra were recorded by means of a Hitachi560A I R spectrophotometer with reference to a S i 0 2 disk wafer (20-mm diameter, 70 mg) to compensate for contributions from (3) Wilson T. P.;Kasai, P. H.; Ellgen, P. C. J . Catal. 1981, 69, 193. (4) (a) Ichikawa, M.;Sekizawa K.; Shikakura, K.; Kawai, M. J . Mol. Catal. 1981, l l , 167. (b) Kawai, M.; Uda, M.; Ichikawa M. J. Phys. Chem., in press. ( 5 ) (a) Niernansverdriet, J. W.; van der Kraan, A. M.; van Loef, J. J.; Delgass, W. N. J. Phys. Chem. 1983,87, 1292. (b) Minai, Y.; Fukushima T.; Ichikawa M.;Tominaga T. J . Radioanal. Nucl. Chem. 1984, 87, 189. (6) Bhasin, M.M.;Bartley, W. J.; Ellgen, P.C.; Wilson, T. C. J . Catal. 1978, 54, 120.
Figure 2. IR spectra of CO chemisorption on Rh, Rh-Ti ( l : l ) , Rh-Zr ( l : l ) ,and Rh-Mn (1:l) supported on SO2. Rh 4.0 wt % loading,p(CO) = 150 torr at 25 OC.
the SiOz support. The particle sizes of the catalyst samples were independently measured by transmission electron microscopy to find the following averaged particle sizes: Rh/Si02, 40 A; RhMn/Si02, 30 A; Rh-Ti/Si02, 20 A; and Rh-Fe/Si02, 20 A.
Results and Discussion Infrared spectra (2200-1400 cm-') for C O chemisorption were recorded on each reduced sample upon admission of C O (10-1 50 torr) at 25 'C. Figure 1 shows the I R spectra in the carbonyl region for Rh/SiOz (4.0 wt % Rh loading) and Rh-Mn/Si02 (4.0 wt. % Rh) with the different Mn contents (1:O.l-2.0 Mn:Rh atomic ratios). The two strong bands at 2040 and 1880 cm-' on Rh/Si02 are reasonably assigned to linearly bonded carbonyl and bridged carbonyl on reduced Rh,' respectively. A surprising result in C O chemisorption was obtained on Rh-Mn/SiO, catalysts, where the stretching frequency of the bridged carbonyl (lower frequency band) was dramatically shifted from 1880 cm-' on Rh/SiOz to 1520 cm-' on Rh-Mn (1:l and 1:2)/Si02. Frequency reduction of the bridged carbonyl bands is substantially affected by the amounts of Mn added with respect to Rh when changing from 0.1 to 2.0 atom ratios of Mn:Rh, while those of the linear carbonyl (high-frequency band) remain virtually unchanged at 2030-2040 cm-I even with the addition of Mn to Rh. Similarly, on the addition of Ti and Zr to Rh, the lower frequency bands were reduced considerably to 1830 and 1680 cm-I on Rh-Ti (l:1)/Si02 and to 1830 and 1670 cm-' on Rh-Zr (l:l)/Si02, with only a minute change of their high-frequency bands, as shown in Figure 2. As it has been previously reported in XPS4 and EPR3s studies, all three promoter elements are not reduced to zero valence. Mn is present as Mn2+ and Ti and Zr exist as Ti4+/Ti3+and Zr4+/Zr3+,even after the strong H2 reduction of the Si02-supported Rh bimetals. The reduction of promoter metal ions and oxides on the SiOz support results in oxygen vacancies and partially reduced ions, e.g. Ti3+, Zr3+,and Mn2+,which can reside at the Rh interface. It is extremely likely that promoter metal ions and oxygen vacancies may exhibit strong oxophilic (7) Yao, H. C.; Rothschild, W. G. J . Chem. Phys. 1978, 68,4774. (8) Ichikawa, M.; Kawai, M. Shokubai 1981, 23, 93.
Letters
1566 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
properties toward oxygen-containing molecules such as chemisorbed CO. In fact, Rh-Ti/SiOz and Rh-Zr/Si02 catalysts after H2 reduction show a strong Lewis acidity for ammonia chemisorption On the other hand, Shriver et aL9 have recently proposed a bifunctional activation of bridged C O in the adducts of carbonyl metal clusters with Lewis acid reagents such as AlBr, and BF3, e.g. [Fe4(CO),Cp,] [AlBr3],-3 and C O ~ ( C O ) ~ . B FThey ~ . exhibit a markedly large reduction of bridged carbonyl IR frequencies from 1640 and 1820 cm-' in clusters to 1390 and 1548 cm-' in the adduct complexes, respectively. A distinct characteristic of metal carbonyl adduct formation is the large decrease observed for uc0 of the carbonyl bridging to the Lewis acid and a small increase for uco of all remaining linear carbonyls, affording good evidence for adduct formation at the bridging carbonyl oxygen. This suggestion is in excellent agreement with our IR spectroscopic results of CO chemisorption on Rh in contact with Mn, Ti, and Zr promoter additives. It is interesting to note that a CO stretching mode at the same frequency on Rh-Mn/SiO, has been observed for CO chemisorbed on stepped surfaces of Ni 5(111) X (170) and Fe(lll).Io Moreover, unusually lower carbonyl stretching frequencies of 1350 and 1490 cm-I were recently reported by Hoffman et al." in C O chemisorption on the K-Ru(OO1) surface. They suggest that C O lies on the Ru surface in a a-CO chemisorption mode by bifunctional interaction with the potassium cation. Since Rh/Si02 alone does not give rise to those new lower frequency bands, the effect should be attributed to the oxophilic sites of promoter ions and oxides rather than the stepped surface of Rh. Consequently, our IR observation of the significant decrease of bridging CO frequencies could be explained by a tilted CO chemisorption mode which has been recently proposed by Sachtler.12 The carbon of chemisorbed C O is bonded to two or more Rh atoms and its oxygen to the promoter metal ions. A schematic representation of this type of bonding is depicted in the following: .498
M=Mn*'. Ti3'/Ti4+,
...
This effect may be ascribed to a lowering of the activation energy for C O diss~ciation'~ which is reflected by a large enhancement of CO dissociation on the supported Rh catalysts with such oxophilic promoter ions and their suboxides. An essentially similar model has been previously proposed for certain reducible oxides to interpret the enhanced activities for C O hydrogenation over Ti02-supported Pt,14Ni,I5 and Fe.I6 Bell et al." have observed broad and weak peaks at 1700-1500 cm-' in C O chemisorption on Ru/AlzO, which are much lower than normally associated with bridging carbonyls. They attribute the large difference in these frequencies to p-bonded carbonyls with Lewis acid sites on A1203 support. Recently, Vannice18 has suggested the direct attractive (9) Shriver, D. F. Adu. Chem. Ser. 1981, No. 252, 1 . Horwitz, C. P.; Shriver, D. F. Ado. Orgunomer. Chem. 1984, 23, 219. Kristoff J. S.;Shriver, D. F. Inorg. Chem. 1914, 23, 499. (10) Erley, W.; Ibach, H.; Lehwald S.; Wagner H. Surf.Sci. 1979, 83, 585. (11) Hoffman, F. M.; Hrbek J.; Depola, R. A. Chem. Phys. Let?. 1984, 206, 83. (12) Sachtler, W. M. H. Proc. In?. Congr. Curd., 8rh, 1984, I, 151. (13) Ichikawa, M.; Fukushima, T.; Shikakura, K. Proc. In?.Congr. Curd., 8rh, 1984, 2, 69. (14) Vannice, M. A.; Twu, C. C.; Moon, S. H. J . Curul. 1983, 79, 70. Vannice M. A.; Twu, C. C. Ibid. 1983, 82,213. (15) Takatani, S.;Chung, Y.-W. J . Card 1984, 90, 75. Burch, R.; Flambard, A. R. Ibid. 1982, 78, 389. (16) Santos, J.; Phillips, J.; Dumesic, J. A. J. Carol. 1983, 81, 147. (17) Kellner, C. S.;Bell, A. T. J. Cutu2. 1981, 72, 296. (18) Vannice, M. A.; Sudhakar, C. J . Phys. Chem. 1984, 88, 2429.
2040
Fe/Rh=@ .1 2028
2000
Figure 3. IR spectral changes in CO chemisorption on SO,-supported Rh-Fe catalysts in varying the Fe contents (Fe:Rh = 0-1.0 atomic ratO C , Rh 4.0 wt % loading.
ios). p ( C 0 ) = 150 torr at 25
interaction of chemisorbed C O oxygen with Ti3+and/or oxygen vacancies generated on the supported Ti02/Pt interface. On Rh/SiOZ, the abundances of high- and low-frequency carbonyl bands in C O chemisorption are comparable, but when the Fe content is increased in Rh-Fe/Si02 catalysts, the intensities of low-frequency carbonyl bands sharply decreased, compared with those of high-frequency carbonyl bands as shown in Figure 3. The drastic change in low frequency to high frequency ratios is analogous to the result reported by Sachtler et a1.I9 on Pd-Ag/Si02 and Ni-Cu alloy catalysts. The relative concentrations of the two adsorption sites for CO (linear and bridging) are determined predominantly by the geometric condition of the bimetal alloy surface since every Rh atom can form a linear complex with C O but pairs of adjacent Rh atoms are required for the formation of the bridged carbonyl. As we have previously reported on the basis of Mossbauer spectroscopic studies,5blarge amounts of Fe in Rh-Fe/Si02 catalysts form bimetallic clusters with Rh of the same compositions as for the present IR samples. On the other hand, both high- and low-frequency carbonyl bands on all the Rh-Fe/Si02 catalysts shifted to lower frequencies (Auto = 30-50 cm-I) with increasing Fe content with respect to Rh, and the amounts of C O adsorption were also found to decrease: C 0 : R h = 0.45 at Fe:Rh = 0.10 and C 0 : R h = 0.15 at Fe:Rh = 1.0 in the manometric measurements for C O at 25 OC. Despite the decrease in C0:Rh stoichiometry, TEM observations revealed that the particle sizes of all the catalyst samples, which consisted of 15-30 A, remained basically unchanged. Fe added to Rh is, in fact, enriched at the surface layer of the particles of Rh-Fe catalysts, as implied from EXAFS and Mossbauer data. 5b.20 Accordingly, the effect of adding Fe to Rh and exerting a large decrease in the bridging carbonyl chemisorption on Rh can be (19) Soma-Noto, Y.; Sachtler, W. M. H. J . Cural. 1974, 32, 315. Primet, M.; Matthieu, M. V.; Sachtler, W. M. H. Ibid. 1976, 44,324. ( 2 0 ) Ichikawa, M.; Fukushima, T.; Yokoyama, T.; Kosugi, N.; Kuroda, H., to be submitted to publication.
1567
J . Phys. Chem. 1985,89, 1567-1568 explained in terms of geometric ensemble requirements of Rh-Fe alloy formation on the Rh surface. Such a surface impedes CO dissociation, e.g. methanation, while enhancing the hydrogenation of nondissociatively chemisorbed CO into methanol. Variation in surface coverage of CO causes a frequency shift due to dipole-dipole interaction of only 10 cm-I,l9 so most of the shifts in both carbonyl bands could be attributed to an electronic effect in the supported Rh-Fe catalysts, possibly by increasing the back-donation of electron density from Fe to CO adsorbed on Rh.
Conclusions IR spectra of CO chemisorption demonstrated unusually large reduction of stretching frequencies of bridged carbonyls adsorbed on Rh in contact with oxophilic Mn, Ti, and Zr ions, e.g. vc0 = 150-350 This indicates that the ions of electropositivemetals and/or oxygen-deficient metal oxides on the Rh particle surface provide sites at which CO is activated by a tilted CO chemisorption
mode with its carbon bonded to Rh and its oxygen to the promoter ions and/or oxygen vacancies similar to adducts with Lewis acids. These observations of IR frequency reduction suggest that the adsorbed C- and 0-bonded CO dissociates to Cadsand Oads with lower activation energies, which is reflected in the strong enhancement of C O conversion on such oxophilic metal-promoted Rh catalysts. On the other hand, we find that Fe added to Rh highly suppresses the abundance of bridged carbonyl chemisorption on Rh surface. This is basically interpreted in terms of a geometric ensemble effect where multibridged CO chemisorption is preferentially prevented by the formation of Rh-Fe alloy by clustering on the Rh particle surface, similarly with Pd-Ag and Cu-Ni alloy catalysts.
Acknowledgment. This work was supported by National Research Development Program of the Ministry of International Trade and Industry.
Magnetic Isotope Effects in the Photolysis of Dibenzyl Ketone on Porous Silica. 13C and "0 Enrichments Nicholas J. Turro,* Chen-Chih Cheng, Peter Wan, Chao-jen Chung, Department of Chemistry, Columbia University, New York, New York 10027
and Walter Mahler The Central Research Laboratories,? E.I. duPont de Nemours and Company, Wilmington, Delaware 19898 (Received: December 4, 1984)
The photolysis of dibenzyl ketone (DBK) on porous silica has been investigated. Both 13Cand I7O isotopic enrichment in the ketone remaining after partial photolysis is demonstrated. The efficiency of "C enrichment was found to be relatively insensitiveto the average pore diameter of the silica host, to the percent coverage by DBK, and to the application of an external magnetic field. A significant dependence of "C enrichment with temperature, with a' maximum in the enrichment-temperature profile, was observed. The results are interpreted in terms of the competition between pathways available to the triplet C,H,CH2C0 CH2C6HSradical pair produced by photolysis of DBK.
Introduction We1 and others2 have shown that it is possible to obtain substantial and efficient separation of I3C from 12C based on a mechanism in which the key step involves a magnetic isotope effect (mie). The influence of the mie is found to be maximal3 when a geminate triplet pair is produced in a "supercage" environment which (1) allows diffusional separation of the radical fragments to a distance which allows reduction of electron exchange and for efficient interactions between nuclear magnetic moments and electron magnetic moments, and (2) encourages a high probability of geminate radical pair reencounters. In addition, the radical pair must possess an 'escape" process which allows sorting of nuclei which differ in their magnetic properties. Under the proper conditions magnetic nucleic embedded in the triplet radical pair can enhance the rate of tripletsinglet intersystem crossing during the diffusional excursions of the radical fragments. Upon reencounter radical pairs possessing magnetic nuclei have a higher probability of being in a singlet state than do radical pairs which do not contain magnetic nuclei. Since, in general, only singlet radical pairs are capable of undergoing cage reactions (combination and disproportionation), a mechanism is available for enriching nuclei via a mechanism based on differences in nuclear magnetic properties. The photolysis of dibenzyl ketone (DBK) in a micellar environment is a prototype system for 13Cisotopic enrichment.lq3 The conventional mechanism is outlined in Figure 1. According to
TABLE I: 13C and I'O Enrichment of Dibenzyl Ketone Recovered from Photolvsis on Silica at Room Temwrature
isotopic enrichment factor
pore diameters
A
22 8,
40 A
a13a
1.21 f 0.03 (1.18 f 0.02)c
1.22 f 0.02
1.21 f 0.01
alla
1.10 f 0.02b (1.05 f 0.02)'
1.08 f 0.036
1.06 f 0.03b
95
Conversions were typically 30-80% without significant variation in The coverage by ketone was 2.5%. (At 10% coverage there was no significant variation in C Y . ) The initial DBK was 30% enriched in 13C at the carbonyl carbon for measurement of a I 3 . The initial DBK was 34% enriched in 170at the carbonyl oxygen for measurement of a l l . The values of are the average of four points. bAverage value of coverage below 5%. CIn 2-kG magnetic field. a
CY.
this mechanism, 170isotopic enrichment should also be possible (I6O and I8O have no nuclear spin, 1 7 0 possesses spin However, we found that technical problems hampered a definitive
5/2).
(1) (a) Turro, N. J. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 609. (b) Turro, N. J.; Chow, M.-F.; Chung, C.-J.; Kraeutler, B. J . Am. Chem. SOC. 1981, 103, 3886. (2) (a) Sterna, L.; Ronis, D.; Wolfe, S.;Pines, A. J . Chem. Phys. 1980, 73, 5493. (b) Buchachenko, A. L.; Tarasov, V. F.; Maltsev, V. I. Russ. J . Phys. Chem. 1981, 55,936. (c) Epling, G. A,; Florio, E. J . Am. Chem. SOC.
---.
1981. --,103. 1237. ~ - ~
(3) Tarasov, V. F.; Buchachenko, A. L.; Maltsev, V. I. Russ. J . Phys.
Contribution no. 3478.
Chem. 1981,55, 1095.
0022-3654/85/2089-1567$01 .SO10 0 1985 American Chemical Society
