J . Phys. Chem. 1985, 89, 633-636
633
Reactions of the Methyl and Ethyl Formate Cation Radicals: Deuterium Isotope Effects Michael D. Sevilla,* David Becker, Cynthia L. Sevilla, and Steven Swarts Department of Chemistry, Oakland University, Rochester, Michigan 48063 (Received: October 15, 1984)
Reactions of the cation radicals of a number of deuterated methyl and ethyl formates are investigated by electron spin resonance spectroscopy. Radicals are formed in CFC13 solutions of the esters by y-irradiation at 77 K. These studies are undertaken to elucidate the chemistry of small esters as well as to resolve an existing discrepancy in the literature regarding the identity of certain radicals formed in methyl and ethyl formate. We find evidence for several interesting deuterium isotope effects. First, the reaction pathway for dissociation of the a*-complexes of methyl formate is found to depend upon the state of deuteration of the methyl group but not the formyl site. Second, for ethyl-d, formate-d, the CD2CH20C(OD+)Dradical is proposed to undergo an intramolecular alkyl radical attack on the carbonyl oxygen to yield CH2CD20C(OD+)Dwhereas the reverse reaction apparently does not occur. The methyl and ethyl formate radicals which we earlier identified as isolated cations are now reassigned to H-transfer radicals. We suggest the chemistry found here for the deuterated ethyl formate is applicable to rearrangement reactions observed in the cation radicals of certain larger esters.
Introduction The structure and reactions of the cation radicals formed by y-irradiation of halocarbon solutions of various compounds at cryogenic temperatures have been the subject of a number of recent studies. With most classes of compounds, notably alkanes,' alkenes,2 aromatics,, ethers: aldehyde^,^ ketones: and acetals,' cation radicals are readily trapped at 77 K. Surprisingly, ester cations are not so easily trapped and have, therefore, become the subject of some controversy. Recently Iwasaki et a1.* have reported that y-irradiation of methyl and ethyl formate in CFCl, matrices at low temperatures results in the radical cations at 4 K but hydrogen-transfer cations when irradiated at 77 K and annealed to higher temperatures. However, we9 and Symons et a1.I0 have reported that the methyl and ethyl esters form the primary cations at 77 K. Our assignment in the case of ethyl formate was based on deuterium substitution studies as well as on the isotropic nature of the spectra found at low temperatures. In this earlier work the spectrum for irradiated ethyl formate in CFC13 at 130 K showed couplings of 22.5 G to two protons and 11-G couplings to two others, whereas the deuterated compound, ethyl-2,2,2-d, formate-d, showed a highly isotropic 1:2:1 triplet with 22.5-G separations at the same temperature, corresponding to only two protons with 22.5-G couplings. (1) (a) Wang, J. T.; Williams, F. J . Phys. Chem. 1980, 84, 3156. (b) Wang, J. T.; Williams, F. J . Chem. Phys. Lett. 1981, 82, 177. (c) Iwasaki, M.; Toriyama, K.; Nunome, K. J . Am. Chem. SOC.1981, 103, 3591. (d) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Chem. Phys. 1982,77,5891. ( e ) Shida, T.; Kubodera, H.; Egawa, Y. Chem. Phys. Lett. 1981, 79, 179. (f) Svmons. M. C. R. Annu. Reo. R. SOC.Chem.. SectC. 1981. 78. 151. l e ) S$nons,'M. C. R.; Smith, I. G. J. Chem. Res., Synop. 1h9,382. (h) T a b i g M.; Lund, A. Chem. Phys. 1983, 75, 379. (2) (a) Shida, T.; Egawa, Y.; Kubodera, H. J. Chem. Phys. 1980,73,5963. (bl Toriyama, K.; Nunome, K.: Iwasaki. M. Chem. Phvs. Lett. 1984.107.86. (c) Shidtani, M.; Nagata, Y.; Sohma, J. J . Phys. Chem. 1984, 88, 4078. (3) (a) Iwasaki, M.; Toriyama, K.; Nunome, K. J . Chem. SOC.,Chem. Commun. 1983,320. (b) Symons, M. C. R.; Harris, L.J Chem Res., Synop. 1982, 268. (c) Tabata, M.; Lund, A. Z . Naturforsch., A 1983, 38A, 428. (4) (a) Wang, J. T.; Williams, F. J. Am. Chem. Soc. 1981, 103, 6994. (b) Snow, L. D.; Wang, J. T.; Williams, F. Chem. Phys. Lett. 1983,100, 193. (c) Symons, M. C. R.; Wren, B. W. J. Chem. SOC.,Perkin Trans. 2 1984, 511. (d) Kubodera, H.; Shida, T.; Shimokoshi, K. J . Phys. Chem. 1981,85, 2583. (5) (a) Symons, M.C. R.; Boon, P. J. Chem. Phys. Lett. 1982,89, 516. (b) Chem. Phys. Lett. 1983, 100, 203. (c) Snow, L. D.; Williams, F. Chem. Phys. Lett. 1983, 100, 198. (6) Snow, L. D.; Williams, F. J. Chem. Soc., Chem. Commun. 1983, 1090. (7) (a) Snow, L.D.; Wang, J. T.; Williams, F. J . Am. Chem. SOC.1982, 104, 2062. (b) Symons, M. C. R.; Wren, B. W. J. Chem. SOC.,Chem. Commun. 1982, 817. (c) Ushida, K.; Shida, T. J . Am. Chem. Soc. 1982, 104, 7332. (8) Iwasaki, M.; Muto, H.; Toriyama, K.; Nunome, K. Chem. Phys. Left. 1984,105, 586, 592. (9) (a) Becker, D.; Plante, K.; Sevilla, M. D. J . Phys. Chem. 1983, 87, 1648. (b) Sevilla, M. D.; Becker, D.; Sevilla, C. L.; Swarts, S. J . Phys. Chem. 1984, 88, 1701. (10) Rao, D. N. R.; Rideout, J.; Symons, M. C. R. J. Chem. Soc., Perkin Trans. 2, in press.
0022-3654/85/2089-0633$01.50/0
This was considered good evidence that the major coupling in ethyl formate arose from the methylene protons in its structure and that the structure was the original cation and not the H-transfer radical, CHZCH20C(OH+)H. In light of the recent work of Iwasaki et al. on the methyl and ethyl formate esters, we have investigated several deuterated methyl and ethyl formate esters not studied earlier. We find evidence for some new and interesting chemistry which verifies that Iwasaki et al. are correct in their 77 K assignment. We suggest that this chemistry is applicable to rearrangement reactions found in certain larger esters.",12
Experimental Section Deuterated and protonated esters were synthesized from coommercially available compounds following standard procedures. Generally, transesterification of available esters or esterification of acids was used. Samples were purified by GLC, and their structure was confirmed by N M R spectroscopy. Samples of esters (0.1-1 mol %) in CFC1, were irradiated in Spectrosil quartz tubes at 77 K for doses of 0.2 Mrd. A Varian Century ESR spectrometer with a dual cavity was employed. Hyperfine splittings and g values were measured vs. Fremy's salt with A = 13.09 G and g = 2.0056. Other experimental details have been explained in our previous work.g Results Deuterated Ethyl Formate Esters. A number of deuterated ethyl formate esters were investigated in this work. It was found that deuteration of the formyl proton had little effect on the ESR spectra other than to give slightly better resolution persumably due to the loss of a small coupling; however, deuteration at different sites on the ethyl group had a profound effect on the spectra and chemistry found for the deuterated species studied. In Figure 1A,B we show the spectra found for ethyl-l,l-d2 formate-d in CFC1,. In Figure 2A,D we show results for ethyl-2,2,2-d3 formate-d a t the same temperatures. The results for ethyl-l,l-d2 formate-d can readily be interpreted on the basis of the hydrogen-transfer radical, .CH2CD20CD(OH+)(11), formed through reaction 1. In this reaction and others that follow, we assume
I*
II
that a hydrogen has transferred to the carbonyl oxygen; however, we do not have definitive experimental evidence that this is the (11) Sevilla, M. D.; Becker, D.; Sevilla, C. L.; Plante, K.; Swarts, S. Faraday Discuss. Chem. SOC.,in press. (12) Sevilla, M. D.; Becker, D.; Swarts, S., in preparation.
0 1985 American Chemical Society
634
Sevilla et al.
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
are consistent with deuterium couplings of ca. 3 G. At 120 K, Figure 2B, the second (?-proton shows coupling and the fine structure associated with the deuterium couplings is well resolved on the end components. Upon further annealing to 129 K there occurs a collapse of the two large proton couplings from 30 and 14 G to a poorly resolved spectrum suggesting two protons at ca. 8 G (Figure 2C). Spectra found between 77 and 140 K (Figure 2A-D) show evidence for an irreversible change; Le., upon cooling from 140 to 77 K a spectrum identical with that in Figure 1A is found, rather than one similar to that in Figure 2A. The spectra for ethyl-2,2,2-d3 formate-d, therefore, indicate that a dramatic change in radical structure has taken place upon annealing. There are two possible explanations for these observed changes. The first and most reasonable explanation for these results is that the radical observed at 77 K is the carbon-centered radical IV produced by deuterium atom loss from the terminal methyl group of 111 (reaction 2). PI+.
CD3 I
DAL.0A H 2
-
fD+ ,,C\WCH,
111 *
Figure 1. (A) First-derivative ESR spectrum found at 77 K after the y-irradiation of ethyl-l,l-d, formate-d (0.5%) in CFC1,. (B) Spectrum found on annealing to 128 K. The change observed is reversible, Le., cooling the sample in (B) to 77 K gives the spectrum found in (A). Both spectra are assigned to the H-transfer radical, .CH2CD20CD(OHt). The three markers are separated by 13.09 G. The central marker is at g = 2.0056.
Figure 2. First-derivativeESR spectra found for ethyI-Z,2,2-d3formate-d (0.5%) in CFC13after y-irradiation and annealing to various temperatures shown in the figure. The conversions noted at 129 and 140 K are not reversible. Cooling the sample from 140 to 77 K results in the same spectrum as found in Figure 1A. The spectra in (A-C) are tentatively
assigned to the D-transfer radical, .CD2CH20CD(OD+).The spectrum in (D) is clearly due to .CH2CD20CD(OD+)and likely results from a
rearrangement of the D-transfer radical (see text). case. Transfer to a matrix molecule or the ether oxygen is also possible. Those structures marked with an asterisk are suggested to be unstable at 77 K. The spectrum in Figure 1A shows the 22.5-G triplet expected for a C H I group whose anisotropic structure has been largely but not completely removed by tumbling or other motional averaging. In Figure 1B at 128 K the radical is undergoing rapid tumbling (but not internal rotationsee below) on an ESR time scale, and as a consequence all of the anisotropic structure is lost and a nearly perfect isotropic 1:2:1 triplet of 22.5-G couplings is found. Since Figures 1B and 2D show identical 22.5-G 1:2:1 triplets, we must reassign the radical found at 140 K for CD,CH,OCO(D) to CH2CD20CD(OD+)(V). Although the spectrum found at 140 K for ethyl-2,2,2-d3 formate-d is the same as that found for ethyl-l,l-d, formate-d at the same temperature, the spectra at 77 K for the two deuterated compounds are clearly not due to the same species. The spectrum in Figure 2A shows a large 40-G doublet which presumably arises from a (?-proton coupling. The spectrum also shows fine structure whose splittings
(2)
IV
Structure IV explains the (?-proton coupling at 77 K and the 3-G deuterium splittings. A small conformational shift at 120 K for IV gives coupling to both (?-protons. The decrease in the (?-proton couplings at 129 K is associated with a conformational change in the D-transfer radical IV, whereas the final radical V found at 140 K is apparently a result of .CD, attack on the carbonyl oxygen with concomitant C - O bond cleavage (reaction 3).
IV J
7%
V
The second possible explanation is that the spectra found at 77 and 115 K are due to the original cation which decays to IV at 135 K followed by reaction 3. With the techniques available to us we cannot eliminate this possibility although we feel it is less likely. In either case, clearly the process occurring is irreversible and occurs only in ethyl-2,2,2-d3 formate-d and not ethyl-l,l-d, formate-d. Thus, the .CH2CD20C(OD+)Dform of the radical appears energetically favored over the .CD2CH20C(OD+)Dform. Secondary thermodynamic isotope effects13 predict that t h e trigonal -CHI group is energetically favored over the trigonal -CDz group and also that the tetrahedral -CD2- group is favored over the tetrahedral -CH2- group; hence, such effects may partially explain the conversion of one form of the radical to the other. However, this effect is likely to be too small to explain the fact that the process is so effectively driven in one direction only; thus we feel some other stabilization factor may also be involved. These new results allow us now to correctly assign the hyperfine couplings previously found in ethyl formate at 77 K to the following specific sites.
The 16- and ca. 4-G (?-protoncouplings average to two couplings at 10 G at 140 K by a reversible conformational interconversion. In our previous papergba detailed analysis showed that the barrier (13) (a) Lowry, T. H.; Richardson, K. S. 'Mechanism and Theory in Organic Chemistry"; Harper and Row: New York, 1981, pp 205-212. (b) Halevi, E. L. In "Progress in Physical Organic Chemistry"; Cohen, S. G . , Streitwiser, A., Jr., Taft, R. W., Eds.; Interscience: New York, 1963; Vol. 6.
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985 635
Deuterated Methyl and Ethyl Formates TABLE I: ESR Parameters for Deuterated Methyl Formate us-Complexes at 77 K
(CD3OCDOCFC13)'. (CD3OCHOCFC13)+* (CHIOCDOCFC13)+* (CH3OCHOCFCI,)'.
83.3
69.5
a
2.0040 this work
82.0
68.4
17
2.0040 this work
84.0
70.6
u
2.0032 6
84.4
70.3
17
2.0032 6
B
A
4The expected deuterium coupling of 2.6 G could not be resolved. *Reference 9a.
for this process was approximately 1.7 kcal. Since couplings of near 25 G to the two methylene protons would be expected for a rotational averaging process, the averaging appears to be due to a small torsional oscillation which interchanges the proton couplings. INDO calculations were performed for a variety of both elongated and ringlike (VI) structures with and without the transferred proton present in the structure. Calculations for elongated and ring structures gave similar hyperfine couplings. The location, orientation, as well as the presence or absence of the transferred proton on the structure were found to have little effect on the spin density distribution and hyperfine couplings of the remaining protons. Thus, the INDO calculations give no support to the argument that free rotation is occurring and that protonation of the ester group has caused an unusual reduction in the 8-proton couplings. These INDO calculations are consistent only with the model that the 16- and 4-G 8-proton couplings found at low temperatures are a result of a fixed orientation of the methylene protons to the p orbital containing the unpaired electron, i.e. 0 = 50 (16 G) and 0 = -70 (4 G). The 10-G average couplings found at higher temperatures are then a result of a torsional oscillation that interchanges these proton orientations. Deuterated Methyl Formate Esters. In our previous work we investigated methyl formate (A) and methyl formate-d (B) and found that the cation radicals of each of these compounds formed a o*-complex with a CFC1, matrix molecule.9a We reported that these complexes dissociated to the free methyl formate cation when warmed or photobleached. However, Iwasaki and co-workers* have recently pointed out (correctly we now believe) that this dissociation produced an H-transfer cation radical, .CH20CH(OH'). In hopes of trapping the isolated cation radical of methyl formate and in order to further elucidate the chemistry of methyl formate cation radical, we have in this work investigated two new deuterated compounds, methyl-d3 formate (C) and methyl-d, formate-d (D).
B
A
0
0
D C a*-complexes, analogous to that described in our earlier work on methyl formate, are formed after y-irradiation at 77 K on samples containing 0.5 mol % C or D in CFCI, (Figure 3A,C). The hyperfine parameters (Table I) found for these o*-complexes are very close to those found previously for complexes of A and B. However, the complexes found here for the methyl-deuterated compounds (C and D) are far more stable than that found for the methyl-protonated compounds (A and B). With the protonated compounds we previously reported that the complexes thermally dissociate at 110 K,9a whereas with the deuterated compounds, the complexes are stable to temperatures near the melting point of the matrix, approximately 150 K. The meth-
Figure 3. First-derivative ESR spectra found for deuterated methyl formates (0.5%) in CFCI, after y-irradiation at 77 K (A, C). (A) u*-complex formed from methyl-d, formate and CFCI,. (B) The methoxyoxomethyl-d3 radical formed by photobleaching the sample in (A) with visible light. (C) a*-complex formed from methyl-d, formate-d and CFCI,. (D) The methoxyoxomethyl-d3 radical formed by photobleaching the sample in (C) with visible light.
yl-deuterated complexes dissociate when photobleached at 77 K or upon prolonged standing at higher temperatures (reaction 4), as did the methyl-protonated complexes (reaction 5). However, the products of this dissociation differ for the two types of compounds. The spectra found after photobleaching (or warming) of the methyl-deuterated complexes, in each case a singlet at g = 2.0017 (Figure 3B,D), are characteristic of the methoxyoxomethyl radical, CD30C0.14J5 Since the dissociation of the u*-complexes in the methyl-protonated compounds A and B results principally in the H-transfer radicals, there is a clear difference in reaction pathways for the methyl-deuterated complexes (VII) and the methyl-protonated complexes (IX), This difference in reaction pathways we believe is an example of the "all or none" deuterium isotope effect first described by Williams and coworkers.I6 Interestingly, while deuteration at the methyl group has a marked effect on the stability of the complex and on the dissociative reaction path, deuteration at the acyl site has no observable effect on either.
x
= matrix
molecule
S
= solute molecule
We believe reaction 4 is intermolecular (as shown) while reaction 5 is intramolecular (as shown). Further evidence for this comes from results found for the protonated compound A when neat or at high concentrations in CFC13. While the H-transfer radical is found in dilute solutions, we find that as the concentration of A is increased, the methoxyoxomethyl radical (CH,OCO) is formed upon irradiation at 77 K. In fact,for y-irradiated neat A, the only radical observed at 77 K is C H 3 0 C 0 . Thus, in concentrated solutions intermolecular H transfer of the formyl hydrogen to another solute molecule occurs, whereas in dilute solutions intramolecular H transfer from the methyl group (14) Sevilla, M. D.; Morehouse, K. M.; Swarts, S.J . Phys. Chem. 1981, 85, 923. (15) We have noted a strong dependence in the g value of the CH,OCO radical with matrix. For example, it is 2.0017 in CFCI,, 2.0012 in neat methyl formate, and 2.0002 in an aqueous environment (12 M LiC1). (16) Wang, J. T.; Williams, F. J . Am. Chem. SOC.1972, 94, 2930.
636
J. Phys. Chem. 1985,89, 636-641 formate-d) is a likely mechanism for reaction of certain neopentyl, butyl, and tert-butyl esters.l2 Our results for ethyl-2,2,2-d3 formate (and larger esters) point to the importance of reactivity at the carbonyl functional group in the intramolecular rearrangement chemistry of ester cation radicals in the solid state. The results for the various methyl formate species indicate the sensitivity of the choice of reaction pathways to deuteration at a specific site. Finally, our results clearly show that the ester cation radicals are unstable and highly reactive. They undergo a remarkable diversity of solid-state reactions, including complexation with the matrix, intramolecular and intermolecular H (or proton) transfer, fragmentation, and intramolecular alkyl attack at the carbonyl.
to the carbonyl oxygen occurs, analogous to reactions 4 and 5, respectively.
Relation to Larger Esters and Conclusions In recent work with a number of esters we," Iwasaki,8 and SymonsIo have separately shown that larger ester radical cations tend to undergo fragmentation reactions such as the McLafferty rearrangement at low temperatures. For example, fragmentation of tert-butyl acetate cation to form isobutene cation and presumably acetic acid was reported to occur at 4 K by Iwasaki and at 77 K by Symons and ourselves. From the results of our own work and that of others, we now believe that at this time the only isolated ester cations known to be stable at 77 K are those of neopentyl esters of formic, acetic, and propionic acids and in these cases the spin density is localized mainly on the neopentyl alkyl group.]' The results found in this work have bearing on the chemistry of larger ester cation radicals. Very recent work in our laboratory indicates that alkyl attack on the carbonyl functional group, with concomitant C-0 or C-C bond scission (as found for ethyl-2,2,2-d3
Acknowledgment. We thank Ffrancon Williams for helpful and encouraging comments. Acknowledgment is made to the Office of Health and Environmental Research of the U.S. Department of Energy and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
ESR and Electron Spin Echo Modulation Spectroscopic Studies of Molybdenum-Adsorbate Interactions on Supported Mo/SiO, M. Narayana, R. Y. Zhan,+ and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: June 19, 1984)
Interactions of Mo5+on Mo/Si02 prepared by impregnation and hydrogen reduction at high temperatures have been studied with electron spin resonance and electron spin echo modulation (ESEM) methods. Two Mo5+species denoted as Mo(A) and Mo(B) are observed. Mo(A) with gll = 1.865 disappears on adsorption of H20, CH30H, NH3, and CH$N and a new species, Mo(C), is formed. Mo(A) seems to be- the prime site for molecular adsorption. Analyses of ESEM data for samples with adsorbed D20, CD30H, and CH30D indicate coordination of Mo5+in Mo(C) with one hydroxyl group or with one methanol molecule with an Mo-O distance of 0.22 nm. One farther noncoordinated water and methanol are also indicated by the ESEM data. For the samples with adsorbed NH3 or ND3, strong Mo-N interactions are seen with significantquadrupole effects.
Introduction Supported molybdenum catalysts such as Mo/Si02 and Mo/A1203 have received considerable attention in recent years because of their importance in organic molecule oxidation, hydrodesulfurization, and coal hydrogenation processes. The paramagnetic species observable in these catalysts upon reduction and/or other treatments have been the subject of numerous electron spin resonance (ESR) studies '-I7 and have been interpreted as Mo5+ in square-pyramidal (C&), octahedral, or distorted tetrahedral configurations. There is also considerable difference of opinion as to what are the catalytically active sites and the sites of formation of the superoxide anion 0,. Hall and co-workers'*'2 suggested that the molybdenum ion in bismuth and other molybdates as well as on supported catalysts is in tetrahedral arrangement before reduction and stays in the same configuration with a hydroxyl ion replacing one of the ligand oxygens on reduction. Che et al.7389'3and other worker^^^'^-^^ interpreted the Mo5+to be formed by loss of one oxygen from Mo6+06octahedra on reduction resulting in Mo5+05square pyramidal geometries. Kazansky and co-workers14 reported a new species of Mo5+ by photoreduction at 77 K which they claimed to be in a tetrahedral environment. While the spin Hamiltonian parameters of a paramagnetic species do give a certain amount of information regarding its Permanent address: Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, The People's Republic of China.
0022-3654/85/2089-0636$01 SO10
geometry, usually it is difficult to obtain unambiguous details about short-range order from continuous wave ESR methods alone. In recent years a powerful pulsed magnetic resonance technique, (1) Masson, J.; Nechtschein, J. Bull. SOC.Chim. Fr. 1968, 3933. (2) Peacock, J. M.; Sharp, M. J.; Parka, A. J.; Ashmore, P. G.; Hocken, A. J. J. Catal. 1969, 15, 379. (3) Seshadri, K. S.; Petrakis, L. J. Phys. Chem. 1970, 74, 4102. (4) Seshadri, K. S.; Massoth, F. E.; Petrakis, L. J. Cafal. 1970, 19, 95. (5) Dufaux, M.; Che, M.; Naccache, C. J. Chim. Phys.-Chim. Biol. 1970, 67, 527. (6) Naccache, C.; Bandiera, J.; Dufaux, M. J. Cafal. 1972, 25, 334. (7) Che, M.; Tench, A. J.; Naccache, C. J . Chem. SOC.,Faraday Trans. 1 1974, 70, 263. (8) a. Burlamacchi, L.; Martini, G.; Ferroni, E. J . Chem. Soc., Faraday Trans. 1 1972, 68, 1586. b. Martini, G. J. Magn. Reson. 1974, 15, 262. (9) Howe, R. F.; Leith, I. R. J. Chem. SOC.,Faraday Trans. 1 1973.69, 1967. (10) Abdo, S.; LoJacono, M.; Clarkson, R. G.; Hall, W. K. J. Cafal. 1975, 36, 330. (1 1) Hall, W. K.; LoJacono, M. Proc. 6th In?. Congr. Catal. 1977, 246. (12) Abdo, S.; Clarkson, R. B.; Hall, W. K. J. Phys. Chem. 1976,80,2431. (13) a. Che, M.; Figueras, F.; Forissier, M.; McAteer, J.; Perrin, M.; Portefaix, J. I.; Praliaud, H. Proc. 6th Int. Congr. Catal. 1977, 261. b. Che, M.; McAteer, J. C.; Tench, A. J. J. Chem. SOC.,Faraday Trans. 1 1978, 7 4 , 2378. c. Lunsford, J. H. Coral. Reu. 1973, 8, 135. (14) Pershin, A. N.; Shelimov, B. N.; Kazansky, V. B. Kine?. Kafal. 1979, 20, 1298. (15) Petrakis, L.; Meyer, P. L.; Debies, T. P. J. Phys. Chem. 1980, 84, 1020. (16) Fricke, R.; Hanke, W.; Ohlman, G. J . Catal. 1983, 79, 1. (17) Machiels, C. J.; Sleight, A. W. J . Catal. 1982, 76, 238.
0 1985 American Chemical Societv
