- Email: [email protected]
Hfcs of the C6H6Mu radical in NaY zeolites
Physica B 289}290 (2000) 603}606
Hfcs of the C H Mu radical in NaY zeolites
D.G. Fleming *, M. Shelley, D.J. Arseneau, M. Senba, J.J. Pan , S.R. Kreitzman, E. Roduner Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1 Department of Chemistry, Miami University, Oxford, Ohio, USA TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada Department of Physics, Dalhousie University, Halifax, NS, Canada Institut fu( r Physikalische Chemie, Universita( t Stuttgart, Pfawenwaldring 55, D-70569, Stuttgart, Germany
Abstract The hyper"ne interactions of the MuC H radical in NaY zeolites at low to moderate benzene loadings have been measured by both the FT-lSR and ALC-lSR techniques over a wide temperature range. From a preliminary interpretation of the data, the hyper"ne coupling constants for the muon and proton of the } CHMu methylene group in two diwerent orientations for the radical bound to the S cation site have been determined. At 322 K these values are: '' A (1)"606$2 MHz and A (1)"108$2 MHz, for the muon on the opposite side of the ring from the cation; and I A (2)"430$2 MHz and A (2)"70$5 MHz, for the muon on the same side. These results as well as their trends with I temperature demonstrate that the MuC H radical adopts a non-planar equilibrium geometry due to the strong interaction of the p electron density with the Na cation. 2000 Elsevier Science B.V. All rights reserved. Keywords: NaY zeolite; Cyclohexadienyl radical; Hyper"ne couplings
1. Introduction Zeolites are microcrystalline alumino-silicate structures, often incorporating extra frame-work cations, which have a ubiquitous presence as molecular sieves and heterogeneous catalysts in chemical industry. Nevertheless, relatively little is known about the microscopic interactions of molecules in di!erent zeolite frameworks, particularly in the case of neutral free radicals that could be formed by H-atom addition reactions involving catalytically active sites. The present paper reports on the hyper"ne coupling constants (hfcs) found for the * Corresponding author. Fax: 604-822-2847. E-mail address: #[email protected] (D.G. Fleming).
MuC H radical in NaY, at low to moderate ben zene loadings and complements our earlier study in USY at near saturation loadings [1]. Both NaY and USY are typical faujasites, an important class of zeolites consisting of &Supercages' (sc), about 13 As in diameter, connected by &Window' (W) sites of about 7 As diameter. Lower loadings facilitate the study of guest}host interactions, the theme of the present work, which complements as well-related lSR studies in ZSM-5, another important industrial zeolite, but one with a much more restricted geometry [2]. The goal of this work is to assess the dynamics of (neutral) free radical intermediates in reactive processes, with a view towards catalytically important reactions of particular relevance to the petrochemical industry.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 2 9 2 - 1
604
D.G. Fleming et al. / Physica B 289}290 (2000) 603}606
2. Experimental and lSR results The NaY zeolite was obtained from Chemie Uetikon (Switzerland), and placed in stainless-steel sample cells, dehydrated overnight at &500 K, and then loaded with degassed benzene to give 1}3 benzenes/sc, corresponding to &5}15% by weight of dehydrated sample. Most of the data was taken at a loading of 2 benzenes/sc. The data was obtained at TRIUMF with surface muon beams, mainly with the HELIOS apparatus on the M20B beam line, with some very recent FT-lSR data also taken with the LAMPF magnet on M15.
2.1. FT-lSR results Near ambient temperature, FT spectra reveal three distinct albeit broad lines, in addition to the diamagnetic frequency. At 322 K these peaks are at frequencies labelled as &B'"119.5 MHz, &C'" 82 MHz and &D'"27 MHz. The labelling is convenient and corresponds to a similar labelling in the ALC spectra to follow. Since the muon hfc of the MuC H radical is well known in the bulk phase [2}4], these FT frequencies correspond to muon hfcs, A "604$1 MHz (for peak B), 530$ I 4 MHz (C) and 425$4 MHz (D).
2.2. ALC-lSR spectra Fig. 1 shows representative ALC spectra obtained at 322, 150 and 50 K, in which four clear resonances are seen. Additional data was also obtained down to 3 K. The solid lines shown are (skewed) Gaussian "ts, in the absence of a correct theoretical model to account for these complex and closely spaced line shapes. As with the FT-lSR data, we will refer to these resonances as &A' at the highest "elds (&26.5 kG), to &B', &C' and &D' at the lowest "elds (&16.0 kG). Both D (M"0) and D (M"1) ALC transitions are expected, probing the isotropic and anisotropic hyper"ne interactions, respectively. Since D ALC lSR and FT-lSR involve a muon spin-#ip process, both should give the same muon hfc, an important feature in the assignment of level crossings. The
Fig. 1. Representative ALC-lSR spectra for the MuC H rad ical in NaY at 322, 150 and 50 K. The solid curves shown are skewed Gaussian "ts to the data. Additional data was taken over a wider range of temperatures, down to 3 K, but it was not possible to obtain good-quality "ts at all temperatures.
ALC-lSR resonant "eld positions are given by
1 A !A A #A I! I I, B (D )" I (1) P 2 c !c c I I C 1A A I! I , B (D )" (2) P 2 c c I C where A and A (proton) are the isotropic muon I I and nuclear hfcs, respectively.
3. Discussion of hfcs 3.1. Accessible sites NaY is important for this initial lSR study because benzene itself has been well studied in this faujasite, by 2D-NMR [5], neutron scattering [6] and molecular dynamics studies [7]. There are only two well-known sites for the adsorption of benzene, and hence the MuC H radical: the S Na cation '' site, located within a supercage, and the W site between supercages. (In contrast NaX, for example,
D.G. Fleming et al. / Physica B 289}290 (2000) 603}606
605
has several such sites). In contrast to the two resonances seen in siliceous ZSM-5 [2] or in solid benzene [8], the observation of four ALC peaks for MuC H in NaY means that both of these two di!erent sites are occupied and/or that there are two di!erent orientations of the } CHMu group. 3.2. Peaks &A' and &B' In comparison with previous data, we expect peaks A and B in Fig. 1 to correspond to the }CHMu group of the MuC H radical in NaY. The lower "eld resonance, peak B, is unequivocally D , since, from Eq. (2), it gives A "605 MHz at I 322 K, the same value as that found from the FT-lSR spectrum (average: 606$2 MHz). From this value and the line position of peak A, the corresponding D resonance yields, from Eq. (1), A "108$2 MHz, averaged. Both of these values represent uniquely large shifts in hfcs for the MuC H radical compared to the bulk phase, or in other powder environments [2,4], with A in NaY I being about 15% higher and A correspondingly about 15% lower. This we interpret as being due to strong binding of the MuC H radical to the Na cations (at the S site) in NaY. The temperature '' dependence of both these muon and proton hfcs is plotted in Fig. 2, with the muon hfc given in &reduced units', A "(c /c ) A . Although there is I I I more scatter in the proton points, the opposite trends with temperature are reproducible and signi"cant. This we interpret as evidence for a nonplanar equilibrium geometry of the MuC H radical. 3.3. Peaks &C ' and &D': sites or orientations? These additional peaks are clear indications of either other sites or other orientations of the C}Mu bond. First consider the possibility that either or both of these peaks corresponds to the W site, the only other accessible site in NaY. On the assumption that peak D, at the lower "eld, is a D line, A "430$2 MHz, essentially the same hfc as I found for D in the FT spectrum. However, this value again represents a remarkably large (&20%) (negative) shift in muon hfc from the bulk phase, which seems inconsistent with expectations for the
Fig. 2. A plot of the temperature dependence of the &reduced' muon, A (upper), and proton, A (lower), from peaks &B' and I &A', respectively. The solid lines are linear "ts, yielding the temperature dependences !0.027$0.002 MHz/K and #0.017$0.004 MHz/K, respectively.
W site, far removed as it is from strong interactions with the Na cations. Thus we are inclined to the view that peak D cannot be due to the W site, and so likely represents the opposite orientation of the methylene C}Mu bond at the cation site, from that of peaks A and B. Given this result, it would seem reasonable to associate peak C as the D resonance due to the methylene proton in this di!erent orientation, giving A "67$2 MHz from its posi tion in Fig. 1. However, this interpretation con#icts with the observation of a peak C in the FT. Although the evidence for this FT peak is not compelling, it does appear to be present, with a weak intensity, also in the ALC-lSR spectra (Fig. 1), consistent with both a weaker binding energy [7] and fewer W sites [6,7]. If peak C in the FT is due to the muon hfc at the W site, then its position in the ALC spectra of Fig. 2 should yield the same hfc as in the bulk, which it does, within errors, 525 MHz. This seeming conundrum could be resolved by the possibility of close-lying or overlapping peaks. Indeed, inspection of the region around peak C in Fig. 1 does
606
D.G. Fleming et al. / Physica B 289}290 (2000) 603}606
indicate additional structure but the widths involved render further separation problematic, at best. In view of this uncertainty we adopt A "70$5 MHz for the (C) proton hfc, at 322 K. 3.4. Non-planarity of the MuC H radical There are two important points from the plot in Fig. 2 that are worth emphasizing: "rst, at a given temperature, the very size of both hfcs, being shifted &15% higher and lower for the muon and proton, respectively, is unprecedented and points to an unusually strong interaction between MuC H and the sodium cation site in NaY; secondly, the trends with temperature, wherein the proton hfc actually increases, albeit modestly, with increasing temperature (#0.017$0.004 MHz/K), in marked contrast to that of A (!0.027$0.002 MHz/K), has I also not been reported before for the MuC H radical. Both of these results point towards a nonplanar equilibrium geometry for the MuC H rad ical interacting with the Na cation, with the }CHMu group above the plane of the ring, wherein the muon (peak B) adopts a more axial orientation and the proton (A) an equatorial one. This conclusion is supported by the ab initio calculations of Chipman [9] and we expect to see the same result emerging from current high-level calculations of Macrae and Webster [10]. At 322 K, the values of both the muon (A (1)"606 MHz) and the proton I (A (1)"108 MHz) hfcs are in good agreement with these calculations for the muon in the }CHMu group being on the opposite side of the cation. The fact that the muon and proton hfcs have the opposite temperature dependence can be explained if, with increasing temperature, the muon e!ectively moves away from the source of p electron density at the ortho carbons, while the proton moves towards this source. It is not yet known if the calculations of Macrae and Webster [10] can account for these trends. Other arguments have been brought to bear in the past for some out-of-plane motion, in the bulk phase, but with an equilibrium planar geometry [11]. It remains to be seen also if theory can con"rm our preliminary interpretation above for the sites
labelled C and D, in which (at 322 K) the proton (A (2)"70 MHz) and muon (A (2)"430 MHz) I hfcs are both considerably reduced for the muon in the } CHMu group pointing towards the Na cation, but already indications are that this is indeed the case [10]. Note that the average of the muon hfcs for both positions (518 MHz) is essentially the same as in other environments [2}4]. The positions of the muon and the proton in this (&down') position would be reversed, so we expect the ¹-dependence of A (2) and A (2) to show the opposite trends to I those plotted in Fig. 2. From the little change in positions of peak &D' seen in Fig. 1, it can be seen that A (2) is essentially ¹-independent. However, I the position of (the doublet?) peak C noticeably increases with decreasing temperature leading to a much more dramatic e!ect for A (2) than that plotted in Fig. 2 and, contrary to expectations, in the same direction. This again suggests that peak C at the lower temperatures could be mainly the W D resonance with the true position of the sec ond D resonance corresponding to the muon coupling of peak D shifted somewhat down"eld and too weak to detect. Further guidance from theory would be most helpful in resolving this conundrum. References [1] M. Shelley, D.J. Arseneau, D.G. Fleming, E. Roduner et al., Studies Surf. Sci. Catal. 94 (1994) 469. [2] E. Roduner, M. Stolmar, H. Dilger, I.D. Reid, J. Phys. Chem. 102 (1998) 7591. [3] D.G. Fleming, D.J. Arseneau, J.J. Pan, P.W. Percival et al., Appl. Mag. Res. 13 (1997) 181. [4] M. Schwager, H. Dilger, E. Roduner, P.W. Percival et al., Chem. Phys. 189 (1994) 697. [5] G. Vitale, L.M. Bull, A.K. Cheetham et al., J. Phys. Chem. 99 (1995) 16 087. [6] A.N. Fitch, H. Jobic, A. Renouprez, J. Phys. Chem. 90 (1986) 1311. [7] S.M. Auerbach, N.J. Henson, A.K. Cheetham, H.I. Meitu, J. Phys. Chem. 99 (1995) 10 600. [8] E. Roduner, Hyper"ne Interactions 65 (1990) 857. [9] D.M. Chipman, J. Phys. Chem. 96 (1992) 3294. [10] R. Macrae, B. Webster, Contribution to this Conference, B. Webster, private communication. [11] D.Yu, P.W. Percival, J.-C. Brodovitch, S.F.J. Cox et al., Chem. Phys. 142 (1990) 229.
Hfcs of the C H Mu radical in NaY zeolites
D.G. Fleming *, M. Shelley, D.J. Arseneau, M. Senba, J.J. Pan , S.R. Kreitzman, E. Roduner Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1 Department of Chemistry, Miami University, Oxford, Ohio, USA TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada Department of Physics, Dalhousie University, Halifax, NS, Canada Institut fu( r Physikalische Chemie, Universita( t Stuttgart, Pfawenwaldring 55, D-70569, Stuttgart, Germany
Abstract The hyper"ne interactions of the MuC H radical in NaY zeolites at low to moderate benzene loadings have been measured by both the FT-lSR and ALC-lSR techniques over a wide temperature range. From a preliminary interpretation of the data, the hyper"ne coupling constants for the muon and proton of the } CHMu methylene group in two diwerent orientations for the radical bound to the S cation site have been determined. At 322 K these values are: '' A (1)"606$2 MHz and A (1)"108$2 MHz, for the muon on the opposite side of the ring from the cation; and I A (2)"430$2 MHz and A (2)"70$5 MHz, for the muon on the same side. These results as well as their trends with I temperature demonstrate that the MuC H radical adopts a non-planar equilibrium geometry due to the strong interaction of the p electron density with the Na cation. 2000 Elsevier Science B.V. All rights reserved. Keywords: NaY zeolite; Cyclohexadienyl radical; Hyper"ne couplings
1. Introduction Zeolites are microcrystalline alumino-silicate structures, often incorporating extra frame-work cations, which have a ubiquitous presence as molecular sieves and heterogeneous catalysts in chemical industry. Nevertheless, relatively little is known about the microscopic interactions of molecules in di!erent zeolite frameworks, particularly in the case of neutral free radicals that could be formed by H-atom addition reactions involving catalytically active sites. The present paper reports on the hyper"ne coupling constants (hfcs) found for the * Corresponding author. Fax: 604-822-2847. E-mail address: #[email protected] (D.G. Fleming).
MuC H radical in NaY, at low to moderate ben zene loadings and complements our earlier study in USY at near saturation loadings [1]. Both NaY and USY are typical faujasites, an important class of zeolites consisting of &Supercages' (sc), about 13 As in diameter, connected by &Window' (W) sites of about 7 As diameter. Lower loadings facilitate the study of guest}host interactions, the theme of the present work, which complements as well-related lSR studies in ZSM-5, another important industrial zeolite, but one with a much more restricted geometry [2]. The goal of this work is to assess the dynamics of (neutral) free radical intermediates in reactive processes, with a view towards catalytically important reactions of particular relevance to the petrochemical industry.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 2 9 2 - 1
604
D.G. Fleming et al. / Physica B 289}290 (2000) 603}606
2. Experimental and lSR results The NaY zeolite was obtained from Chemie Uetikon (Switzerland), and placed in stainless-steel sample cells, dehydrated overnight at &500 K, and then loaded with degassed benzene to give 1}3 benzenes/sc, corresponding to &5}15% by weight of dehydrated sample. Most of the data was taken at a loading of 2 benzenes/sc. The data was obtained at TRIUMF with surface muon beams, mainly with the HELIOS apparatus on the M20B beam line, with some very recent FT-lSR data also taken with the LAMPF magnet on M15.
2.1. FT-lSR results Near ambient temperature, FT spectra reveal three distinct albeit broad lines, in addition to the diamagnetic frequency. At 322 K these peaks are at frequencies labelled as &B'"119.5 MHz, &C'" 82 MHz and &D'"27 MHz. The labelling is convenient and corresponds to a similar labelling in the ALC spectra to follow. Since the muon hfc of the MuC H radical is well known in the bulk phase [2}4], these FT frequencies correspond to muon hfcs, A "604$1 MHz (for peak B), 530$ I 4 MHz (C) and 425$4 MHz (D).
2.2. ALC-lSR spectra Fig. 1 shows representative ALC spectra obtained at 322, 150 and 50 K, in which four clear resonances are seen. Additional data was also obtained down to 3 K. The solid lines shown are (skewed) Gaussian "ts, in the absence of a correct theoretical model to account for these complex and closely spaced line shapes. As with the FT-lSR data, we will refer to these resonances as &A' at the highest "elds (&26.5 kG), to &B', &C' and &D' at the lowest "elds (&16.0 kG). Both D (M"0) and D (M"1) ALC transitions are expected, probing the isotropic and anisotropic hyper"ne interactions, respectively. Since D ALC lSR and FT-lSR involve a muon spin-#ip process, both should give the same muon hfc, an important feature in the assignment of level crossings. The
Fig. 1. Representative ALC-lSR spectra for the MuC H rad ical in NaY at 322, 150 and 50 K. The solid curves shown are skewed Gaussian "ts to the data. Additional data was taken over a wider range of temperatures, down to 3 K, but it was not possible to obtain good-quality "ts at all temperatures.
ALC-lSR resonant "eld positions are given by
1 A !A A #A I! I I, B (D )" I (1) P 2 c !c c I I C 1A A I! I , B (D )" (2) P 2 c c I C where A and A (proton) are the isotropic muon I I and nuclear hfcs, respectively.
3. Discussion of hfcs 3.1. Accessible sites NaY is important for this initial lSR study because benzene itself has been well studied in this faujasite, by 2D-NMR [5], neutron scattering [6] and molecular dynamics studies [7]. There are only two well-known sites for the adsorption of benzene, and hence the MuC H radical: the S Na cation '' site, located within a supercage, and the W site between supercages. (In contrast NaX, for example,
D.G. Fleming et al. / Physica B 289}290 (2000) 603}606
605
has several such sites). In contrast to the two resonances seen in siliceous ZSM-5 [2] or in solid benzene [8], the observation of four ALC peaks for MuC H in NaY means that both of these two di!erent sites are occupied and/or that there are two di!erent orientations of the } CHMu group. 3.2. Peaks &A' and &B' In comparison with previous data, we expect peaks A and B in Fig. 1 to correspond to the }CHMu group of the MuC H radical in NaY. The lower "eld resonance, peak B, is unequivocally D , since, from Eq. (2), it gives A "605 MHz at I 322 K, the same value as that found from the FT-lSR spectrum (average: 606$2 MHz). From this value and the line position of peak A, the corresponding D resonance yields, from Eq. (1), A "108$2 MHz, averaged. Both of these values represent uniquely large shifts in hfcs for the MuC H radical compared to the bulk phase, or in other powder environments [2,4], with A in NaY I being about 15% higher and A correspondingly about 15% lower. This we interpret as being due to strong binding of the MuC H radical to the Na cations (at the S site) in NaY. The temperature '' dependence of both these muon and proton hfcs is plotted in Fig. 2, with the muon hfc given in &reduced units', A "(c /c ) A . Although there is I I I more scatter in the proton points, the opposite trends with temperature are reproducible and signi"cant. This we interpret as evidence for a nonplanar equilibrium geometry of the MuC H radical. 3.3. Peaks &C ' and &D': sites or orientations? These additional peaks are clear indications of either other sites or other orientations of the C}Mu bond. First consider the possibility that either or both of these peaks corresponds to the W site, the only other accessible site in NaY. On the assumption that peak D, at the lower "eld, is a D line, A "430$2 MHz, essentially the same hfc as I found for D in the FT spectrum. However, this value again represents a remarkably large (&20%) (negative) shift in muon hfc from the bulk phase, which seems inconsistent with expectations for the
Fig. 2. A plot of the temperature dependence of the &reduced' muon, A (upper), and proton, A (lower), from peaks &B' and I &A', respectively. The solid lines are linear "ts, yielding the temperature dependences !0.027$0.002 MHz/K and #0.017$0.004 MHz/K, respectively.
W site, far removed as it is from strong interactions with the Na cations. Thus we are inclined to the view that peak D cannot be due to the W site, and so likely represents the opposite orientation of the methylene C}Mu bond at the cation site, from that of peaks A and B. Given this result, it would seem reasonable to associate peak C as the D resonance due to the methylene proton in this di!erent orientation, giving A "67$2 MHz from its posi tion in Fig. 1. However, this interpretation con#icts with the observation of a peak C in the FT. Although the evidence for this FT peak is not compelling, it does appear to be present, with a weak intensity, also in the ALC-lSR spectra (Fig. 1), consistent with both a weaker binding energy [7] and fewer W sites [6,7]. If peak C in the FT is due to the muon hfc at the W site, then its position in the ALC spectra of Fig. 2 should yield the same hfc as in the bulk, which it does, within errors, 525 MHz. This seeming conundrum could be resolved by the possibility of close-lying or overlapping peaks. Indeed, inspection of the region around peak C in Fig. 1 does
606
D.G. Fleming et al. / Physica B 289}290 (2000) 603}606
indicate additional structure but the widths involved render further separation problematic, at best. In view of this uncertainty we adopt A "70$5 MHz for the (C) proton hfc, at 322 K. 3.4. Non-planarity of the MuC H radical There are two important points from the plot in Fig. 2 that are worth emphasizing: "rst, at a given temperature, the very size of both hfcs, being shifted &15% higher and lower for the muon and proton, respectively, is unprecedented and points to an unusually strong interaction between MuC H and the sodium cation site in NaY; secondly, the trends with temperature, wherein the proton hfc actually increases, albeit modestly, with increasing temperature (#0.017$0.004 MHz/K), in marked contrast to that of A (!0.027$0.002 MHz/K), has I also not been reported before for the MuC H radical. Both of these results point towards a nonplanar equilibrium geometry for the MuC H rad ical interacting with the Na cation, with the }CHMu group above the plane of the ring, wherein the muon (peak B) adopts a more axial orientation and the proton (A) an equatorial one. This conclusion is supported by the ab initio calculations of Chipman [9] and we expect to see the same result emerging from current high-level calculations of Macrae and Webster [10]. At 322 K, the values of both the muon (A (1)"606 MHz) and the proton I (A (1)"108 MHz) hfcs are in good agreement with these calculations for the muon in the }CHMu group being on the opposite side of the cation. The fact that the muon and proton hfcs have the opposite temperature dependence can be explained if, with increasing temperature, the muon e!ectively moves away from the source of p electron density at the ortho carbons, while the proton moves towards this source. It is not yet known if the calculations of Macrae and Webster [10] can account for these trends. Other arguments have been brought to bear in the past for some out-of-plane motion, in the bulk phase, but with an equilibrium planar geometry [11]. It remains to be seen also if theory can con"rm our preliminary interpretation above for the sites
labelled C and D, in which (at 322 K) the proton (A (2)"70 MHz) and muon (A (2)"430 MHz) I hfcs are both considerably reduced for the muon in the } CHMu group pointing towards the Na cation, but already indications are that this is indeed the case [10]. Note that the average of the muon hfcs for both positions (518 MHz) is essentially the same as in other environments [2}4]. The positions of the muon and the proton in this (&down') position would be reversed, so we expect the ¹-dependence of A (2) and A (2) to show the opposite trends to I those plotted in Fig. 2. From the little change in positions of peak &D' seen in Fig. 1, it can be seen that A (2) is essentially ¹-independent. However, I the position of (the doublet?) peak C noticeably increases with decreasing temperature leading to a much more dramatic e!ect for A (2) than that plotted in Fig. 2 and, contrary to expectations, in the same direction. This again suggests that peak C at the lower temperatures could be mainly the W D resonance with the true position of the sec ond D resonance corresponding to the muon coupling of peak D shifted somewhat down"eld and too weak to detect. Further guidance from theory would be most helpful in resolving this conundrum. References [1] M. Shelley, D.J. Arseneau, D.G. Fleming, E. Roduner et al., Studies Surf. Sci. Catal. 94 (1994) 469. [2] E. Roduner, M. Stolmar, H. Dilger, I.D. Reid, J. Phys. Chem. 102 (1998) 7591. [3] D.G. Fleming, D.J. Arseneau, J.J. Pan, P.W. Percival et al., Appl. Mag. Res. 13 (1997) 181. [4] M. Schwager, H. Dilger, E. Roduner, P.W. Percival et al., Chem. Phys. 189 (1994) 697. [5] G. Vitale, L.M. Bull, A.K. Cheetham et al., J. Phys. Chem. 99 (1995) 16 087. [6] A.N. Fitch, H. Jobic, A. Renouprez, J. Phys. Chem. 90 (1986) 1311. [7] S.M. Auerbach, N.J. Henson, A.K. Cheetham, H.I. Meitu, J. Phys. Chem. 99 (1995) 10 600. [8] E. Roduner, Hyper"ne Interactions 65 (1990) 857. [9] D.M. Chipman, J. Phys. Chem. 96 (1992) 3294. [10] R. Macrae, B. Webster, Contribution to this Conference, B. Webster, private communication. [11] D.Yu, P.W. Percival, J.-C. Brodovitch, S.F.J. Cox et al., Chem. Phys. 142 (1990) 229.