J . Phys. Chem. 1985, 89, 3606-3609
3606
expected result if the process of localization occurs by the simultaneous development of radical and carbocation centers since hyperconjugative stabilization by a methyl group is generally considered to be more efficient at a carbocation than at a radical center.27 On the other hand, a mixture of radical centers might be expected to follow from chloride ion attack, given the less discriminate character of the products resulting from nucleophilic reactions with asymmetric allylic carbonium ions.2* There is also a correlation between the temperature needed for the formation of the localized species and the extent of methyl substitution at the radical end of the molecule. Thus, the localized forms of the radical cations from ethylene and propylene oxides with the RCH2- radical center are already produced at 77 K while the radical cations from 1,2-dimethyloxirane and trimethyloxirane with the RCHMe- radical center are generated between 95 and 105 K. Also, the RCMe2- center of the localized tetramethyloxirane cation requires an even higher temperature (105-1 10 K) for its formation. As a corollary, the strikingly different results for propylene oxide and 1,2-dimethyloxirane are difficult to rationalize on the basis of halogen attack since the complexed or substituted carbocation end of the radical cation would be virtually identical for these two molecules. The lack of supporting evidence for a halogen interaction from considerations of ESR hyperfine patterns,29 localized radical structure, and annealing studies refocuses attention on the orthogonal structure 2. If this model is correct, then the CFC12CF2CI matrix must impose this twisted structure on the radical cation30 because the planar delocalized structure, at least for the parent (27) A. G. Evans, Discuss. Faraday Sac., 10, 109 (1951). (28) R. H. DeWolfe and W. G. Young, Chem. Rev., 56, 753 (1956). (29) It is perhaps revealing that although the delocalized form of the ethylene oxide radical cation shows weak halogen hyperfine interactions in other matrices (CFCI,, CF3CC13),'*2 the localized form is not produced in these cases, even on annealing to the softening point of the matrix. This suggests that the dominant factor in bringing about the transformation is a more specific interaction of the radical cation with the CFCl2CF2CImatrix that stabilizes the localized form.
oxirane radical cation, is observed in CFCI,, CF3CC13,CC14,and SF6.'l2 In this event, the lower symmetry of the CFCl2CF2Cl molecule could be an important or even a decisive factor. Concluding Remarks
In contrast to the delocalized forms of the C-C ring-opened oxirane radical cations found in other Freon-type matrices,'.2 there is a remarkable preference for the formation of localized forms in the CFCl2CF2C1matrix, these species being produced irreversibly by irradiation at 77 K or on subsequent annealing to 110 K. The radical center in these species is always localized at the oxirane carbon with the least number of substituted methyl groups; increasing methyl substitution at this carbon raises the temperature needed for the formation of the localized radical cation.
Acknowledgment. This research has been supported by the Division of Chemical Sciences, U S . Department of Energy (Report No. DOE/ER/02968-159). _ _ I -
(30) A referee has invited us to comment on the direction of the temperature dependence of the structure observed in the CFC12CF2CImatrix, asking why is there a change from a delocalized to a localized structure as the temperature is increased? First, one must say that the explanation would be trivial if chlorine attack were responsible for the transformation since either the release of chloride ion from the matrix radical anion or the formation of a chloronium ion could be expected to occur irreversibly at higher temperature. However, the correlations discussed in the text appear to offer little independent support for a mechanism based on chlorine intervention. Now, the question is, of course, much more interesting if the localized radical cations do indeed have orthogonal structures. If we assume this to be the case, the delocalized form of the tetramethyloxirane radical cation originally present in the CFCl2CF2CImatrix below 105 K represents only a local energy minimum since it is converted irreversibly into the localized form above this temperature. This change is not observed in other matrices (CFCI,, CF3CC13, CCI4, SF,) and recent high-level theoretical calculations"-" affirm that the planar delocalized form of the oxirane radical cation is more stable than the twisted form, although the estimates for the energy difference (1.9'' and 6.712 kcal mol-I) are relatively small. We therefore surmise that the CFCI,CFICI matrix interaction inverts the ordering of the energy levels for these two structures, allowing the delocalized form of the cation to adjust to the more stable localized form on relaxation in this particular matrix.
Structures and Reactions of C2H, Adsorbed on Small Pt Clusters Po-Kang Wang, Charles P. Slichter,* Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
and John H. Sinfelt Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: May 20, 1985)
The authors have used NMR to determine the structure of C2H4adsorbed at room temperature on small Pt clusters supported on alumina. They observe an ethylidyne species (C-CH,), with a C-C bond length of 1.49 0.02 A. The CH3 groups rotate freely about t h e C-C direction at 77 K. Up t o 390 K, t h e C-C bond length remains the same, suggesting the same surface species. C-C bond scission takes place above 390 K, and is complete at about 480 K, forming predominantly single carbon atoms with no hydrogen attached and a small amount of CH3 species, in contrast with CH or CH2 species proposed to form on Pt( 11 1) surfaces.
*
Structures and reactions of simple molecules adsorbed on metal surfaces are of great importance in studying catalysis. There have been extensive studies of adsorption of simple molecules, such as C2H2and C2H4,on single crystal surfaces. The extent to which industrial catalysts containing small metal clusters of 10 to 100 A in diameter may be thought of as composed of single crystal surfaces is not well-known. One might question whether the structure and reaction of adsorbed molecules on small metal 0022-3654/85/2089-3606$01.50/0
particles will be different from that on single crystal surfaces. In this Letter we report N M R studies of the molecular structure, of C-C bond rupture, and of the nature of the postrupture products of ethylene (C2H4)on Pt catalysts. We compare the results with single crystal studies. The s p i e s formed from ethylene adsorbed at room temperature on Pt( 111) surfaces have been of great interest. From earlier single crystal studies three species were proposed: CH-CH3,' CH-CH2,2 0 1985 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3607
and C-CH3.3 Of the three, the ethylidyne species (C-CH,) has been favored by a dynamical LEED analysis3 and an angle-resolved photoemission study.4 A recent HREELS e ~ p e r i m e n t , ~ which gives good agreement with the IR peak assignment of the ethylidyne group in (CH3C)Co3(CO),,lent strong support for the ethylidyne species. THe ethylidyne species is also observed on Pt(100).6 At temperatures between 415 and 510 K the ethylidyne species undergoes further reaction, and is thought to undergo C-C bond rupture to form CH or CH2.5-738 N M R is a noninvasive spectroscopy with the nuclear spin excitation energy in the order of 10" eV, which, however, also makes the sensitivity poor, and a large surface area a requirement. Industrial catalysts are therefore a natural candidate for N M R studies. Our samples of small Pt clusters supported on 7-alumina are characterized by dispersion (percentage of Pt atoms on the surface), which is measured by hydrogen chemisorption. Samples in our present study have a dispersion of 51%. A typical sample of 1 g has about 10 m2 of Pt surface. Surfaces of such clusters are thought to consist primarily of (1 11) surfaces, with a small fraction of (100) surfaces. The saniples are cleaned at 570 K under alternating flow of H2 and 02,and then evacuated to 10" torr. The samples are cooled to room temperature under vacuum, ethylene gas is admitted, and the samples are sealed off in glass vials. The coverage, the number of ethylene molecules per surface Pt atom, is 15%. All data presented in this paper are for ethylene adsorption at room temperature. All NMR spectra were recorded at 77 K . We have previously used N M R to determine the structure of acetylene adsorbed a t room temperature on Pt particle^.^ The structure is found to be predominantly CCH2,with a C-C bond length of 1.44 f 0.02 A. The structure is found from measurement of the dipolar interactions among the nuclear magnetic moments. For two unlike spins,I0 the dipolar coupling causes the N M R line of each spin to split into a pair of lines separated in frequency by 0
where y l and y2are the nuclear gyromagnetic ratios, h , Planck's constant divided by 2?r, r, the distance between the two nuclei, and 6,the angle between the internuclear axis and the external magnetic field." Since yl, y2,and h are known constants, and cos 6 has a uniform distribution between -1 and 1 for a powdered sample, measurement of the dipolar coupling, w, directly yields the internuclear distance, r. As explained in ref 9, the splittings of the N M R line described above are completely obscured by magnetic field inhomogeneity due to the magnetic susceptibility of the Pt and I3Cchemical shift anisotropy. We thus have to resort to pulse techniques to measure the dipolar couplings. W e have measured the C-C bond length of the adsorbed ethylene from the I3C spin echos as shown in Figure 1. In the experiment we apply two rf pulses, separated by a time interval T , and observe a spin echo a t a time T after the second pulse.I2 The decay of the amplitude of the spin echoes
(1) H. Ibach and S. Lehwald, J . Vac. Sci. Technol., 15, 407 (1978). (2) J. E. Demuth, Surf. Sci., 80, 367 (1979). (3) L. L. Kesmodel, L. H. Dubois, and G. A. Somorjai, J . Chem. Phys., 70, 2180 (1979). (4) M. R. Alkrt, L. G. Sneddon, W. Eberhardt, F. Greuter, T. Gustafsson, and E. W. Plummer, Surf. Sci., 120, 19 (1982). (5) H. Steininger, H. Ibach, and S.Lehwald, Surf. Sci., 117,685 (1982). (6) H. Ibach in 'Proceedings of the International Conference on Vibrations in Adsorbed Layers", Julich, 1978, p 64. (7) A. M. Baro and H. Ibach, J . Chem. Phys., 74, 4194 (1981). (8) J. R. Creighton and J. M. White, Surf. Sci., 129, 327 (1983). (9) Po-Kang Wang, Charles P. Slichter, and John H. Sinfelt, Phys. Reo. Lett., 53; 82 (1984). (10) Footnote 12 of ref 9 explains how one knows whether to treat two "C nuclei as 'like" or "unlike". (1 1) Charles P. Slichter, "Principles of Magnetic Resonance", SpringerVerlag, New York, 1980, 2nd ed, Chapter 2.
T(@
Figure 1. I3C spin echo amplitude vs. pulse separation T at 77 K for I3C2H4on Pt clusters. The circles are data. The solid line is a fit with a C-C bond length of 1.49 A; the dotted and the dashed lines are that of simple single bond length of 1.54 A and double bond length of 1.34
A, respectively.
vs. the pulse separation T consists of two components. One is a simple exponential attributed to isolated I3C nuclei. The other is an oscillating component, which also decays exponentially, attributed to I3C-I3C pairs. The oscillations, which are called "slow beats", are given by (COS
(UT))average
over all orientations
(2)
From the relative intensities of the two components, one obtains the amount of isolated I3Cnuclei relative to I3C-l3Cpairs. From the frequency of the oscillation, one directly obtains the C-C distance. For groups of more than two 13Cnuclei, eq 2 becomes the real part of the Fourier transform of the total l3C-I3Cdipolar coupling. The solid line in Figure 1 is a fit of 23%isolated I3C, and 77% of I3Cin pairs with a C-C bond length of 1.49 f 0.02 A. Predictions for a single bond length of 1.548,and double bond length of 1.34 A are also shown in Figure 1 . The ethylene gas is I3C enriched to 90% (Le. the ethylene and 1% molecules are 81% I3CH2=I3CH2,18% 13CH2=12CH2, I2CH2=l2CH2). If there is no C-C bond rupture, the isolated 13C should account for 10% of the total intensity. Since the experimental value is 23%,the extent of rupture of C-C bonds may be as high as 13%. However, corrections for the nonideal nature of the rf field (which is not infinite and uniform) must be taken into account to obtain a more precise value of the extent of C-C rupture, as will be discussed later. The C-C bond length of 1.49 f 0.02 A agrees with that of 1.51 f 0.02 8, for the ethylidyne group in H3Ru3(C0),(CCH3),"and that of 1.50 f 0.05 A for the ethylidyne species on Pt( 1 1 1) from a dynamical LEED a n a l y ~ i s . ~ The structure is further determined by measuring the I3C-lH dipolar coupling, in a I3C-lH spin echo double resonance (SEDOR) experiment. In this experiment we employ a new form of SEDOR in which we hold the time between the I3C pulses fixed. We apply an 'H pulse between the two I3C pulses, and observe the reduction in the I3Cspin echo amplitude as a function of the time delay between the 'H pulse and the first "C pulse. The reduction in I3C spin echo amplitude divided by the original amplitude is called the SEDOR fraction. For I3C nuclei not coupled to 'H the extra 'H pulse makes no difference to the I3C signal and the SEDOR fraction is therefore zero. For I3C-*H pairs the SEDOR fraction varies with the time interval between the first I3C pulse and the 'H pulse, T', as SEDOR fraction = a( 1 - cos(W7') )average
over a11 orientations
(3)
(12) We actually employ an add-subtract method in which we alternate between 90,-180, and 90_,-180, pulse pairs, subtracting the echoes from the second sequence from those of the first. (13) G. M. Sheldrick and J. P. Yesinowski, J . Chem. Soc., Dalton Trans., 873 (1975).
Letters
3608 The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 I .o
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Figure 4. The fraction of broken C-C bonds, with no correction (0)and with maximum correction (e),vs. the temperature to which the sample
T'(/LS)
Figure 2. SEDOR fraction vs. r' at 77 K for I3C2H4on Pt clusters. The solid line is the theoretical prediction for the ethylidyne species (C-CH,), with the methyl group rotating about the C-C axis, the dashed line is that of frozen C-CH,, and the dotted line is that of CH-CH, with rotating methyl groups.
has been heated. The crosses are the result of a simulation assuming an activation energy of 36 kcal/mol. I .2 I .o C
.? V e u-
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Figure 3. I3Cspin echo amplitude vs.
r'(,us) Figure 5. SEDOR fraction vs. 7' at 77 K after the sample has been heated to 479 K. The solid line is a fit of 85% C and 15% rotating CH,. The dotted and the dashed lines are predictions for CH and CH2species, respectively.
where a is ideally unity, but a constant close to unity for any real spectrometer. We have calibrated our spectrometer with the model compound 13CH31and have determined the constant a to be 0.85. For I3CH2and I3CH3groups, cos (UT') in eq 3 is replaced by the real part of the Fourier transform of the total I3C-IH dipolar interaction. In general as 7' increases the SEDOR fraction approaches an asymptotic value, which is roughly the fraction of 13C coupled to IH. Our SEDOR result in Figure 2 shows that only about half of the I3C are attached to 'H, thus ruling out immediately the models of CH-CH2, CH-CH3, and CH2-CH2. The data furthermore agree well with the model of the ethylidyne species (C-CH,) with the methyl group at 77 K undergoing rotation about the C-C axis at a rate much faster than the N M R line width of 10 kHz. The theoretical prediction of frozen C-CH3 is also shown in Figure 2. The rotation of the methyl group at 77 K is also observed in the model compound I3CH3I. We thus conclude that the structure of ethylene adsorbed at room temperature on Pt particles is primarily the ethylidyne species (C-CH,), identical with that observed on Pt( 111) and Pt( 100) single crystal surfaces. The C-C bond length is determined to be 1.49 k 0.02 A. The fraction of C-C bond rupture is 13% or lower. We have also studied the reaction of adsorbed ethylene at elevated temperatures. The samples as contained in sealed glass vials are heated in an oven, held at a constant temperature for 3 h, and then quenched in liquid nitrogen for N M R measurements. As the temperature is increased the relative intensity of isolated
13C nuclei in the I3C slow beat data also increases, indicating breaking of the C-C bonds (see Figure 3). We can estimate the fraction of ruptured C-C bonds assuming no correction due to the nonideal rf magnetic field in one extreme, and assuming maximum correction in the other. The reality must lie somewhere in between. The factor for the maximum correction can be obtained by assuming that there is no C-C bond rupture at room temperature. The results are shown in Figure 4, open circles for no correction, and solid circles for maximum correction. The crosses are results of a simulation in which we assume that there is some C-C scission in the initial chemisorption step. We further assume that the probability of breaking the remaining C-C bonds per unit time is given by vo exp(-E/kg, with uo of lo1, Hz and an activation energy, E , of 36 kcal/mol (or 1.6 eV). If the uo is off by a factor of 10, the estimated activation energy will be off by only 2 kcal/mol (or 0.1 eV). We have also measured the 13C-IH dipolar couplings after the ethylidyne species breaks up. The SEDOR result in Figure 5 shows that the I3C--lH coupling has greatly diminished after the C-C bonds break. The data show that most carbons are no longer bonded to hydrogens, as would be were there C H or CH2 groups. The small SEDOR fraction can be accounted for by about 15% of rotating CH3 species. We therefore propose that, after the C-C bonds break, most single carbon atoms are completely dehydrogenated on small Pt clusters. I3C slow beat data also show that C atoms are at least 3 A away from each other. In conclusion we have observed ethylidyne species (C-CH,) from adsorption of ethylene on small Pt particles at room temperature, agreeing with single crystal studies. At about 470 K
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r(,us) 7 at 77 K for I3CzH4 on Pt, after adsorption at room temperature ( O ) , and after the sample has been heated to 391 (A),453 (O), and 479 K ( 0 ) for 3 h.
J. Phys. Chem. 1985, 89, 3609-361 1 the C-C bonds break, also in agreement with single crystal s t u d i e ~ . ~The * ~ species formed are predominantly single carbon atoms (no hydrogen attached) and a small amount of CH3, in contrast with the situation on P t ( l l 1 ) surfaces in single crystal studies, in which it has been proposed that the carbon is in C H
3609
or C H 2 groups.
Acknowledgment. This research was supported in part by the Department of Energy, Division of Materials Sciences, Contract No. DE-AC02-76ER01198.
State-Resolved Photofragmentation of OCS Monomers and Clusters N. Sivakumar, I. Burak,t W.-Y. Cheung,* P. L. Houston,* Department of Chemistry, Cornell University, Ithaca, New York 14853
and J. W. Hepburn Center for Molecular Beams and Laser Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada (Received: June 3, 1985)
Dissociation of the OCS monomer at 222 nm yields S('D) and S(3P) in the ratio 0.85/0.15 and CO which is >98% in u = 0 but with very high rotational energy. The peak in the rotational distribution occurs at J = 55 and corresponds to about 56% of the available energy for the S('D) channel. These results support the contention that the upper OCS 'A state is bent. Dissociation of OCS clusters leads to a completely different photochemistry: at least some of the CO is formed rotationally cold (Trot= 50 K), and Sz is also produced.
Introduction State-resolved photofragmentation of both triatomic moleculesId and van der Waals is currently receiving much attention. In the former case, the dynamics of the dissociation process can be used to infer the shape of the dissociative potential surface, while in the latter case the distribution of energy in the fragments gives insight into the mechanism by which electronic and vibrational energy migrates between strong and weak bonds. In this paper we report the distribution of energy in the S and C O fragments formed from dissociation of OCS monomers and (OCS), clusters at 222 nm. Excitation of the monomer at this wavelength is through a weak IB' transition absorption band that has been assigned to a l A from the linear ground state to a bent upper Previous work has shown that both S(lD) and S(3P) are primary photoproducts; studies of the secondary photochemistry suggest that the branching ratio of S(')/S(3P) is 0.75/0.25.13314 N o direct measurements have been performed either on the distribution of the S atom states or on the distribution of internal energy in the C O fragment. Photodissociation of van der Waals clusters of OCS in this region of the ultraviolet has never before been reported. We find that dissociation of (OCS), yields significantly different products than dissociation of the monomer. Specifically, C O is formed in very low rotational levels (TrotE 50 K). S2is also detected as a product of the dissociation, suggesting that a chemical reaction occurs in the cluster as a result of the dissociation. To our knowledge, this is one of the few examples in which excitation has been observed to result in chemical reaction between the weakly bound components of a van der Waals c l ~ s t e r . ' ~
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Experimental Section Photolysis of the OCS monomers and clusters was performed in a pulsed supersonic jet apparatus similar to that described earlier.16 Mixtures were prepared by flowing helium over OCS which was held in a trap at a specified temperature. The seeded gas was then expanded from a total pressure of 1300 torr through a 0.5-mm-diameter pinhole by using a pulsed nozzle assembly Permanent address: Department of Chemistry, University of Tel Aviv, Tel Aviv, Israel. 'Current address: Geo-Centers, P.O. Box 523, Wharton, N J 07885.
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(Newport). A KrCl excimer laser (Lumonics TE861-4) dissociated the molecular jet roughly 1.25 cm from the nozzle source. The S(lD), S(3P), and CO(X'Z,v,J) products were probed by laser-induced fluorescence using a tunable vacuum-UV source based on four-wave mixing in magnesium vapor.17 S('D) and S(3P)were detected by the 'Pol +- ID2and 3D03 3P2or 'Do2 3P2transitions, respectively, while C O was detected by the A'II X'B transition. The vacuum-UV source has been described in detail in a recent study of the photodissociation of glyoxalIs and is very similar to that used previously for detection of Br and C0.1s24 The S2products were probed with a frequency-doubled
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(1) R. Bersohn, J . Phys. Chem., 88, 5145 (1984). (2) H. Okabe, "Photochemistry of Small Molecules", Wiley-Interscience, New York, 1978. (3) S. R. Leone, Adu. Chem. Phvs., 50, 255 (1982) (4) R. Bersohn, ZEEE J . Quantum Electron.;QE-16, 1208 (1980). ( 5 ) C. H. Greene and R. N . Zare, Annu. Rev. Phys. Chem., 33, 119 (1982). (6) M. Shapiro and R. Bersohn, Annu. Reu. Phys. Chem., 33,409 (1982). (7) D. H. Levy, C. A. Haynam, and D. V. Brumbaugh, Faraday Discuss. Chem. SOC., 73, 137 (1982). (8) N. Halberstadt and J. A. Beswick, Faraday Discuss. Chem. SOC.,73, 357 (1982). (9) D. S. King, J . Chem. Phys., to be published. (10) W. H. Breckenridne and H. Taube. J . Chem. Phvs., 52. 1713 (1970). i l l i B. M. Ferro and BYG. Reuben. Trans. Faradav Soc.. 67.2847 f1971j. (12j J. W. Rabalais, J. M. McDonald, V. Scheh, andS.'P. McGlynn, Chem. Rev., 71, 73 (1971). (13) H. E. Gunning and 0. P. Strausz, Adu. Photochem., 4, 133 (1966). (14) W. H. Breckenridae and H. Taube, J . Chem. Phvs.,53, 1750 (1970). (15) For another receit example see S. Buelow, G: Radhakrishnan, J. Catanzarite, and C. Wittig, to be published. (16) R. D. Bower, R. W. Jones, and P. L. Houston, J . Chem. Phys., 79, 2799 (1983). (17) S. C. Wallace and G. Zdasiuk, Appl. Phys. Lett., 28, 449 (1976). (18) J. W. Hepburn, N. Sivakumar, and P. L. Houston in 'Laser Techniques in the Extreme Ultraviolet", S. E. Harris and T. B. Lucatorto, Eds., American Institute of Physics, New York, AIP Conf. Proc. No. 119, pp 126-1 34. (19) J. W. Hepburn, D. Klimek, K. Lui, J. C. Polanyi, and S. C. Wallace, J . Chem. Phys., 69, 4311 (1978). (20) J. W. Hepburn, D. Klimek, K. Liu, R. G. Macdonald, F. J. Northrup, and J. C. Polanyi, J. Chem. Phys., 74, 6226 (1981). (21) J. W. Hepburn, K. Liu, R. G. Macdonald, F. J. Northrup, and J. C. Polanyi, J . Chem. Phys., 75, 3353 (1981). (22) J. W. Hepburn, F. J. Northrup, G. L. Ogram, J. M. Williamson, and J. C. Polanyi, Chem. Phys. Lert., 85, 227 (1982).
0 1985 American Chemical Society
