μSR Studies of Hyperfine Couplings and Molecular Interactions of the

ARTICLE pubs.acs.org/JPCC

μSR Studies of Hyperfine Couplings and Molecular Interactions of the Mu-Cyclohexadienyl Radical in Y-Zeolites and in Solid Bulk Benzene Donald G. Fleming,* Donald J. Arseneau, and Mee Y. Shelley† TRIUMF and Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 2Z1

Bettina Beck,‡ Herbert Dilger, and Emil Roduner* Institut f€ur Physikalische Chemie, Universit€at Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany ABSTRACT: The interaction of muoniated cyclohexadienyl radicals with zeolite environments in NaY, HY, and USY has been studied using mainly avoided level crossing muon spin rotation (ALCμSR) spectroscopy, which utilizes spin-polarized positive muons as local probes. A strong interaction of C6H6Mu with sodium cations in NaY is indicated and leads to significant distortion of the C6H6Mu structure from planarity, accompanied by large shifts in hyperfine coupling constant (hfcc) values compared to those in bulk benzene. Much weaker interactions of this radical with Brønsted acid sites in USY/HY are deduced from the measured muon and proton hfcc values, indicative of a largely planar radical, that, in contrast to those in NaY, also exhibit a strong dependence on benzene loading. The small shifts in hfcc values seen in USY/HY compared to those in the bulk relate to the nature of OH binding sites and possibly also to effects arising from varying local dielectric constants at different benzene packing densities. The muon ALC resonances in these frameworks exhibit a marked increase in widths at temperatures near 300 K and at low benzene loadings, evidence for molecular dynamics, indicating desorption and reorientation of C6H6Mu, in marked contrast to the nearly static widths seen in NaY up to 470 K.

1. INTRODUCTION Zeolites are aluminosilicate structures incorporating chargecompensating cations that have a ubiquitous presence as molecular sieves and heterogeneous catalysts in chemical industry. Nevertheless, relatively little is known at the microscopic level about the interactions of guest molecules in different zeolite frameworks,1
5 particularly in the case of neutral free radicals that might be formed, in situ, by H-atom addition reactions6 from active sites, in analogy with the commonly expected formation of carbocation intermediates by protonation from Brønsted acid (BA) sites.1
5,7,8 Even though these BA sites seem well-established and are believed to play an important role in facilitating catalysis in some zeolites, few have actually been identified.2 The highest occupied molecular orbital (HOMO) of a saturated or delocalized unsaturated diamagnetic hydrocarbon species has bonding character, whereas the lowest unoccupied molecular orbital (LUMO) is antibonding. As for many other π-type hydrocarbon radicals, the cyclohexadienyl radical has an odd number of delocalized pz centers, which has the consequence that the singly occupied molecular orbital (SOMO) has essentially nonbonding character and is located energetically in the HOMO
LUMO gap of nearby diamagnetic molecules. This means that the radical is electronically quite a different species than a related diamagnetic molecule, with an ionization potential that is significantly lower. (See, for example, ref 9, where it is established that the first ionization potential of benzene is ∼9.8 eV, whereas that of π radicals is more like 7.5 eV.) This has the effect r 2011 American Chemical Society

that the binding to the zeolite is stronger, particularly to cation sites, and hence, the radical dynamics is slower than that of diamagnetic molecules. Free-radical hyperfine coupling constant (hfcc) values can then provide detailed information about the geomety, location, and molecular dynamics of free radicals, whether formed by in situ H-atom addition to or abstraction from diamagnetic precursors in host environments and, notably, here, can provide alternate pathways as potential reactive intermediates for catalytic processes in zeolites. The present article reports on muon spin rotation (μSR) measurements of the hfcc values and dynamics of the Mucyclohexadienyl (C6H6Mu) radical in the Y-faujasites, namely, NaY, HY and USY, over a range of loadings and temperatures. C6H6Mu can be regarded as an isotopologue of the cyclohexadienyl radical (C6H7), which has not been well-studied in zeolites but can be expected to undergo similar interactions (as in the bulk phase10,11) and which might well play a role in zeolitecatalyzed processes in the petrochemical industry.1,3,4,6 To our knowledge, the only report of the non-muoniated C6H7 radical in zeolites is from the pulse radiolysis of benzene in HZSM-5, where H-atom transfer reactions were indeed indicated.12 Among the faujasites (which contain “supercages” of about 13-Å diameter, connected by “windows” of about 7-Å diameter), both Received: March 4, 2011 Revised: April 19, 2011 Published: May 18, 2011 11177

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The Journal of Physical Chemistry C USY and HY are particularly important industrial catalysts that can facilitate proton-transfer reactions to organic guests from acidic hydroxyl “onium” sites in catalytic reaction sequences in zeolite chemistry.2,4,5,13,14 The present study of C6H6Mu is intended to serve as a useful model for the similar free-radical interactions expected of C6H7. It also complements earlier studies of C6H6Mu in NaY at a loading of 2
3 benzenes per supercage (SC)15 and a much earlier and very preliminary study of C6H6Mu in USY at or above saturation loadings,16 along with studies reported elsewhere of C6H6Mu in ZSM-517 and HZSM5.18 New μSR results are also reported for pure bulk benzene in the solid phase that are compared with the results herein for the C6H6Mu radical in the Y-faujasites. The only other similar μSR study reported to date is of the C2H4Mu radical19 in the same faujasites and a study of C6H6Mu and other muoniated radicals in different zeolite frameworks by transverse-field muon spin rotation (TF-μSR) (only) but, in that case, with little or no interpretation in terms of binding sites or molecular dynamics.20

2. EXPERIMENTAL DETAILS AND THE μSR TECHNIQUE 2.1. Sample Preparation. The experiments in NaY reported here were carried out at the PSI accelerator near Z€urich, Switzerland,21 whereas those in HY and USY (dealuminated “ultrastable” HY) were studied at the TRIUMF accelerator in Vancouver, Canada. In both cases, zeolite samples were obtained from CU Chemie Uetikon (Z€urich, Switzerland) and were used as received. The HY sample had a Si/Al ratio of 3.5, as reported for NaY in ref 15, with a similar ratio for the NaY sample from ref 21. The USY sample had a Si/Al ratio of 6.8, as reported in ref 16. The samples for the TRIUMF work were prepared as described in refs 15 and 19. The PSI sample cells were about twice the diameter, to accommodate a larger beam spot. The stainless steel cells had welded thin muon entrance windows. The TRIUMF samples were dehydrated at ∼400
C overnight, cooled, loaded with the desired amount of benzene at ambient temperature, and then allowed to equilibrate at this temperature for several days prior to beam time. It is noted that the time scale for sorption of aromatic molecules from initial surface adsorption sites into zeolite pores is thought to be a few seconds.22 The PSI samples were dehydrated in vacuum at temperatures of ∼500
C for 10
12 h, prior to loading, and were then maintained at a temperature of 350 K for 24 h after loading, to faciliate a more homogeneous benzene distribution.23 Procedural differences could be expected to give rise to loading errors up to approximately (0.5 benzenes per supercage. In both sets of experiments, zeolite sample cells were mounted in a helium-flow cryostat for temperatures up to just above room temperature, or in an oven environment for the PSI samples up to higher temperatures, and placed in a superconducting magnet, with applied fields either transverse to or along the muon spin direction in the case of the TRIUMF experiments, but only in the longitudinal direction in the case of the PSI experiments, up to ∼30 kG in either case. The temperature range was ∼50
310 K in the TRIUMF experiments and ∼95
470 K for the PSI data in NaY. The sample temperature was assumed to be the same as for the metal target cell, which was monitored by two attached thermocouples that typically gave consistent readings to within a degree. The muon beam passed through the thin cell entrance window and stopped within the benzene-loaded zeolite sample, forming the muoniated cyclohexadienyl radical, most likely by

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Figure 1. Time-resolved TF-μSR Fourier-transform spectrum of the C6H6Mu radical in pure solid benzene at 250 K, in a TF of 14.4 kG. The lower and upper peaks at 65.5 and 460.5 MHz are the two radical frequencies, νR1 and νR2 of eq 1, respectively, giving the isotropic hfcc, Aμ = 526 MHz. The central frequency at 195.5 MHz arises from muons in diamagnetic environments.

muonium (Mu = μþe
) addition to benzene, as expected from the prior observation of the Mu atom in unloaded zeolites.24,25 2.2. μSR Basics and Bulk Benzene Reference Data. “Surface” muon beams of 4.1 MeV initial energy and 100% spinpolarization were utilized in these experiments. The basis of the μSR technique resides in monitoring the time evolution of the muon polarization, as manifested by the “asymmetry” seen in the radioactive decay of the muon (μþ f eþνeνhμ), with a lifetime of 2.2 μs, in which the positron is emitted preferentially along the muon spin direction.26,27 We used both transverse-field μSR (TF-μSR), in which the incident muon spin is perpendicular to the field direction, and avoided level crossing resonance μSR (ALC-μSR), where it is aligned with the field direction. In TF-μSR, the magnetic field causes precession of the muon polarization at characteristic frequencies corresponding to allowed transitions between the energy levels of the spin Hamiltonian of a muoniated radical.26
28 In the relatively high fields employed in these studies, only two characteristic signals are observed for a given radical environment, most easily seen in a Fourier transform μSR (FT-μSR) spectrum, and defined by10,15,26,27


1

1
νR 1 ¼
νm
Aμ
and νR 2 ¼ νm þ Aμ ð1Þ

2 2 where Aμ is the isotropic muon
electron hfcc and   i1=2 1 h 2 Aμ þ ðνe þ νμ Þ2 νm ¼
νe þ νμ 2

ð2Þ

with the Zeeman (Larmor) frequencies νμ = γμB for muons found in diamagnetic environments (γμ = 0.01355 MHz G
1) and νe = γeB for the electron (γe = 2.8025 MHz G
1). Implicit is the assumption of Lorentzian line shapes, which are seen in the isotropic environments of gases and liquids and in single crystals. In polycrystalline environments, such as zeolites, Mu-radical precession frequencies can be modified by the angular dependence of the dipolar coupling, which can give rise to FT-μSR spectra of distinctly broadened and asymmetric shape.15,29 An example FT-μSR spectrum for the C6H6Mu radical in solid benzene at 250 K and a TF of 14.4 kG is shown in Figure 1. At this 11178

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The Journal of Physical Chemistry C temperature, the FT line shapes are sharp and Lorentzian, indicative of a single-crystal environment.27,30 The central signal at 195.5 MHz is due to muons in unknown diamagnetic environments. The signals at 65.5 and 460.5 MHz are the νR1 and νR2 radical frequencies, respectively; the reduced intensity of the higher-frequency line is due to the counter time resolution. From eq 1 and the positions of the two lines in Figure 1, the muon hfcc is Aμ = 526 ( 1 MHz at 250 K, agreeing with a similar determination from fits to the corresponding time spectra for the muon asymmetry, A(t).15,26,27 The error of (1 MHz is higher than the precision inherent in the TF technique and is meant to account for some systematic error from different data sets at the same temperature, which might also reflect differences in preferential crystallite orientations.30 Although the observation of a Mu-radical by TF-μSR spectroscopy provides a valuable tool for the determination of the muon hfcc, it provides no information about other nuclear hfcc values (Ak) in the radical. Moreover, TF/FT line widths might be too large, due to hyperfine anisotropy,15,29 or the Mu addition rate might be too slow, causing dephasing of the μSR signal, rendering the amplitudes of the FT lines weak or even unobservable. In these cases, both muon and nuclear hfcc values are determined from measurements of ALC-μSR spectra, in a longitudinal field (LF) environment, which also provide the most direct information on molecular dynamics. In this level-crossing technique, a signal appears as a “dip” detected in the timeintegrated decay asymmetry, corresponding to a resonant oscillation of muon spin polarization between the backward and forward directions as the magnetic field is scanned.10,15,17,26,31 There are three different types of ALC resonances. A Δ0 (ΔM = 0, where M = me þ mμ þ mk) resonance represents the “flip
flop” exchange of spin polarization between the muon (μ) and a nuclear spin (k), and consequently, it is sensitive to both hfcc values. It arises primarily from the isotropic hyperfine Hamiltonian and, thus, is the only kind of ALC resonance seen in gases10,32 or liquids11,27 or in any otherwise anisotropic environments when the dipolar couplings are averaged to zero by fast rotational tumbling.27,33,34 A Δ1 (ΔM = 1) “muon flip” transition gives a direct measurement of the muon hfcc (as does FT-μSR) and is induced solely from the anisotropic (dipolar) part of the muon
electron hyperfine interaction. Consequently. it is dependent on the angles between the principal axes of the body-fixed hyperfine tensor and the field direction and, thus, provides a sensitive means to study reorientation dynamics of a muoniated radical.15,17,19,27,31,35 A further Δ2 “flip
flip” transition also arises from the anisotropic hyperfine interaction but is much weaker27 (see Figure 2) and is of little consequence here. A single-crystal environment gives Lorentzian line shapes for specific fixed crystal orientations for both the Δ0 and Δ1 resonances.27,30,31 In the polycrystalline environment of a zeolite, because the effective hfcc values for both the muon and nuclear spins are angle-dependent, a superposition of resonant fields corresponding to different possible crystal orientations leads to a powder pattern envelope for the line shape for each ALC resonance. In the case of a static radical, numerical simulations for the C6H6Mu radical in the bulk phase have shown that the line shapes for both the Δ0 and Δ1 resonances remain symmetric, albeit non-Lorentzian.17,27,30,34 As such, they are amenable to fitting phenomenologically by either Gaussian or Lorentzian line shapes, from which the isotropic hfcc of interest can be found

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Figure 2. Representative ALC-μSR plots for pure bulk benzene over a range of temperatures. The solid lines are Lorentzian fits to each resonance in the presence of a field-dependent background, fit to an eighth-order polynomial, and then removed to better display each resonance. Clear Δ1 and Δ0 resonances are seen at lower (∼19 kG) and higher (∼21 kG) fields below the bulk benzene melting point (279 K), down to ∼170 K (and below), where the Δ1 resonance splits into a pair of lines (indicated by the double red arrows) among other unidentified multiple resonances. The Δ1 resonance disappears at the bulk melting point because of isotropic reorientation in the liquid phase. A weak Δ2 resonance was also fit at all temperatures below the melting point and is indicated by the blue arrow in the scan at 277 K, where it is most noticeable.

from the fitted resonance positions of each resonance,15,17,21,26,27 according to

1

Aμ
Ak Aμ þ Ak

ð3Þ
Br ðΔ0 Þ ¼

2
γμ
γk γe


1

Aμ Aμ

Br ðΔ1 Þ ¼


2
γμ γe


ð4Þ

where γk is the gyromagnetic ratio for the specific nuclear spin and γe = 206.6γμ (as above). This assumption and eqs 3 and 4 were utilized in our previous analysis of C6H6Mu in NaY15 and of C2H4Mu in faujasites19 and are employed here as well because mainly broad symmetric ALC lines are again seen. It is worth noting that, although the muon hfcc can be determined from the position of the Δ1 resonance 9 (eq 4), this assignment is often not clear from experimental level crossing spectra. In such cases, because TF-μSR transitions and the Δ1 resonance are effectively driven by the same (Iμþ) operator, the observation of FT-μSR spectra (e.g., Figure 1) can play an invaluable role in identifying the positions of Δ1 resonances in ALC spectra, both expected to give then the same value for the muon hfcc. Knowing the value of Aμ is also required to determine Ak from eq 3, unless prior assumptions about the ratio of Aμ/Ak can be made.36 11179

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)

Fast uniaxial motion about a particular axis partially averages the (θ, φ) hyperfine anisotropy, leaving only a θ-dependent axial hyperfine tensor, with muon components Dzz = D =
2D^ =
6.8 MHz for the unperturbed C6H6Mu radical.17,27,30 In this case, the powder pattern superposition of resonant fields for a polycrystalline environment gives rise to an asymmetric cusp-like ALC line shape for both the Δ0 and Δ1 lines, from which the isotropic and dipolar muon (or nuclear) hfcc can be determined from a fit to the line shape.15,17,30,31,33,35 At the higher temperatures in the present study of C6H6Mu in NaY, there is evidence for such an axial shape for the B resonance, as discussed below. Additional motion can further average the tensor and broaden the Δ1 resonance, which disappears in the case of a random isotropic reorientation of the radical, on a time scale of τc , τALC = 1/2 πD^ (∼50 ns for C6H6Mu), leaving only the Δ0 resonance as an observable.17,27 This is an important feature seen in the data that follow. Example background-corrected ALC spectra (from fitting the general trend of the field dependence to a polynomial, as described in ref 15) are shown in Figure 2, for C6H6Mu in bulk benzene over a range of temperatures, which also serve as reference data for the discussion in zeolites to follow. Two clear resonances due to the
CHMu group are seen at temperatures below the bulk melting point (278.5 K), down to about 200 K: a Δ1 resonance around 19 kG and a somewhat narrower Δ0 resonance around 21 kG. The solid lines shown are fits to three Lorentzians, including a very weak Δ2 resonance that is most clearly seen in the scan at 277 K near 18.2 kG (blue arrow), which gives, within errors, the same proton hfcc as found from the Δ0 resonance. A similar spectrum can be seen at 263 K in refs 15 and 27. Note that, above the melting point, the Δ1 and Δ2 resonances disappear because of isotropic reorientation in the liquid phase, leaving only the Δ0 resonance. At temperatures near and below 170 K in Figure 2, multiple resonances are seen that could not be fully analyzed, although the Δ1 resonance seen at the higher temperatures appears to split into a pair of Δ1 lines, consistent with FT spectra and indicated by the red arrows on the 170 K scan in the figure. Similar features are seen in the HSY/USY data at higher benzene loadings, as discussed below. The muon, Aμ(T), and proton, Ap(T), hfcc values are found from the fitted positions of these resonances and eqs 4 and 3, respectively. Errors from the least-squares fits to such wellseparated LCR resonances are typically ∼0.5 MHz, although they are larger for broader lines and systematic errors could also be ∼1 MHz, as indicated earlier. It is convenient to define the muon hfcc in terms of reduced units, A0 μ = (γp/γμ)Aμ = Aμ/ 3.1833, which corrects for the ratio of muon and proton magnetic moments,27 allowing for a direct comparison then with similarly expected proton hfcc for C6H7.

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3. MU-CYCLOHEXADIENYL RADICAL IN NAY 3.1. Results for C6H6Mu. Figure 3 shows background-corrected (determined in this case from results obtained with a cell filled with pure zeolite) but unfitted ALC-μSR spectra for NaY at a near-saturation benzene loading of 4
5 molecules/SC,21 which extends the previous results of ref 15 to both higher loadings and higher temperatures. The alphabetic labeling distinguishes different site locations and orientations of the
CHMu group of the C6H6Mu radical seen as different Δ1 and Δ0 resonances. It is noteworthy that these data agree very well with the results of ref 15, at a loading of 2
3 molecules/SC, with

Figure 3. ALC-μSR spectra of the C6H6Mu radical in NaY over a range of temperatures and at a loading of 4
5 benzenes/SC, taken from ref 21. The data have been corrected for background from a pure zeolite sample. The labels B
D are muon Δ1 resonances from
CHMu for different site locations or orientations; those labeled A and E are the proton Δ0 resonances corresponding to the Δ1 lines for B and D. The G resonance is also believed to be a proton resonance but its origin is unclear. The weak resonance labeled F at 296 K is not clearly identified at other temperatures and might be an artifact.

samples that were prepared by quite different loading procedures, as described above. It is also worth noting that these spectra are very different from the reference spectra shown for pure benzene in Figure 2. It is well-known that there are two binding sites for benzene (and hence C6H6Mu) in NaY, the extraframework SII Na cations within an SC and the window (W) sites between supercages,3,37
40 leading to a nominal saturation loading of 6 benzenes/SC, although, in practice, this is more like 5 molecules/SC.38 The peaks (dips) labeled B and D in Figure 3 are Δ1 resonances due to the binding of C6H6Mu to cations, but of two different orientations arising from distortion of the radical from its normal planar geometry, with the C
Mu bond orientation “exo” and “endo”, respectively, with respect to the cation.15,41 This is shown schematically for C6H6Mu in a zeolite framework section of NaY in Figure 4. The C label is also for a Δ1 resonance but from W sites. This is stronger near room temperature at the higher loadings of Figure 3 than in the data of ref 15 at lower loadings. The identification of these peaks as Δ1 resonances was confirmed by the observation of FT spectra, as in ref 15. The A peak in Figure 3 is a Δ0 resonance, the endo proton partner of the exo muon of peak B. From the analysis reported in ref 21, the weak peaks labeled E and G are also believed to be Δ0 proton resonances, with peak E likely the exo proton of the D Δ1 resonance, also tentatively identified earlier in ref 15. Resonance G was obscured in ref 15 by a doublet structure associated with A, but it appears more clearly in Figure 3. Its origin is unclear, but it cannot be the proton partner of the muon peak C at the W site, which would be at a much lower field. The origin of peak F, seen 11180

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Table 1. Reduced Muon hfcc Values, A0 μ (peaks D, C, and B,a in MHz),b Proton hfcc Values, Ap (peak A, in MHz),b and Widths, Γ (fwhm, in Gauss),c for the Major ALC Resonances of MuC6H6 in NaYd peak D A0 μ

Γ

A0 μ

471

134.7

265

160.6

446

134.9

234

161.8

413

135.4

215

403

135.7

395 373

)

)

)

peak B

peak A

A0 μ

D

132

188.6


10.6

not observed

182

189.2


10.7

not observed

164.1

122

189.9


11.0

203

164.4

108

190.0


12.2

105.6

135.7

205

164.7

106

190.2


12.5

103.0

82.0

135.9

184

165.4

73.2

190.6


13.4

101.1

89.8

347 296

135.6 135.7

166 142

166.1 167.6

64.4 59.8

190.0 192.3


13.3
12.5

102.1 102.2

89.6 83.8

273

135.7

148

168.3

55.4

192.7


13.7

101.4

87.6

246

135.0

173

169.2

53.0

193.8


12.0

103.4

92.2

196

134.3

174

171.5

80.8

195.3


10.7

102.5

147

135.4

128

173.9

96

133.9

157

176.0

Γ

107 81.8

)

T(K)

Figure 4. Schematic diagram of the C6H6Mu radical interacting with the SII cation in NaY (shown by the green circle), which causes distortion from planarity of the C
Mu bond above (exo) and below (endo) the plane, shown in the diagram as Hexo and Hendo, respectively. The Si atoms are shown by the light gray circles, with the O-atom bridges shown by the smaller red circles. Atom sizes are not to scale.

Ap

Γ

not observed

196.6


9.7

not observed

197.2


5.8

not observed

)

a Peak B fit to an axial hyperfine tensor, with width determined by the hyperfine anisotropy (D ) given in MHz. (The corresponding fwhm is 3 /2D /γμ.) b Calculated from eq 4 for peaks D and C and from eq 3 for peak A. For peak B, see footnote a. For notation, see Figure 3. The muon hfcc values are given in reduced units, A0 μ = Aμ/3.1833. c Widths, Γ (in G), are fwhm values from Lorentzian fits to the data of Figure 3 (not shown), except for B, which was fit to an axial tensor. See footnote a. d From the thesis work of ref 21 at a loading of 4
5 benzenes/SC. Results for temperatures up to 296 K very similar to those in ref 15 at a loading of 2
3 benzenes/SC. )

very weakly near room temperature, is unknown. It could be an artifact, arising from background subtraction, although it was tentatively identified as a proton resonance in ref 21. It is possible as well that it could be a (weak) Na Δ0 resonance, discussed further below. The line shapes seen in Figure 3 are, by and large, symmetric and were fit to Lorentzians (not shown), with the muon and proton hfcc values determined from the Δ1 and Δ0 resonances and eqs 4 and 3, respectively, as described earlier. Their values are recorded in Table 1, along with the widths Γ (fwhm). The shape of peak B (exo muon) appears to be more cusp-like than D (endo muon) and was fit to an assumed axial hyperfine tensor, with the widths D given in MHz in Table 1 (Γ = 3/2D /γμ in Gauss). The negative sign indicates an oblate shape for axial rotation of the C6H6Mu radical about the SII cation.27,30,31 The temperature dependences of these hfcc values, fit to an assumed linear dependence,21 generally agree very well with the trends and slopes reported earlier at both lower loadings and temperatures,15 testimony to little or no loading dependence in the position (or number) of the ALC resonances seen. The only exception to this level of agreement is for the proton coupling constants, Ap, of peak A, which show considerably more scatter when plotted over higher temperatures, partly a legacy of the aforementioned unresolved doublet structure (peak G) seen in ref 15. The widths for the Δ1 ALC resonances reported in Table 1 are also essentially the same as those found earlier (corrected from Gaussian fits at that time).15 At the higher temperatures, there is a trend for these widths to increase, indicative of the onset of some dynamics, although this is less clear from the values of D given for peak B, which even appear to decrease at both lower and higher temperatures. 3.2. C6D6Mu and Possible Na Δ0 Resonances in NaY. In the case of the C6H6Mu radical in NaY, there are two possible nuclear hfcc values due to coupling with the ipso proton and/or with nearby Na nuclei, giving rise then, in principle, to two different Δ0 resonances for each C
Mu bond orientation. Macrae and Webster41 carried out calculations of both the muon and proton hfcc values of the C6H6Mu radical in a NaY cluster that gave excellent agreement with the earlier experiment.15 The binding energy (BE) of C6H6Mu was found to be

peak C

∼40 kJ/mol, although rather strongly dependent on methodology and basis-set choice. The effective charge on the Na cation from these calculations was found to be 0.88, suggesting a partial charge transfer of 0.12 from the radical to the Na. If the spin transfer were to scale in the same way, this would give a maximum hfcc on Na of 110 MHz.,42 and if this were partnered with the strong muon B line, a Na Δ0 resonance around 20 kG near 300 K would be expected, in the neighborhood of peak F in Figure 3. Replacing the protons to give the perdeuterated radical, C6D6Mu, is expected to have a large effect on both the positions and intensities of all proton Δ0 resonances, scaling with the D hfcc and hence with the gyromagnetic ration γd/γp = 0.154. Thus, from eq 3, the positions of the D Δ0 resonances would shift considerably, and because the intensity scales as |AμAk|,15,27 they are also expected to be some 6-fold weaker than their proton counterparts. In contrast, to first order, the position of any Na resonances, due to the presence of extraframework cations in NaY, should be in the same place and with much the same intensity in the perdeuterated radical. Comparison of the C6H6Mu and C6D6Mu spectra is therefore expected to support the assignment of these resonances. Figure 5 compares an experimental (background-corrected) ALC-μSR spectrum for the C6D6Mu radical in NaY (dotted line) with that for the C6H6Mu radical (solid line) at the same temperature, 347 K. The resonances previously identified for C6H6Mu (see Figure 3) are also labeled. Note that all of the Δ1 lines (B
D) are still present, although shifted slightly upfield (by ∼200 G), giving a shift of a few megahertz in the muon hfcc, consistent with previous comparisons of C6H6Mu and C6D6Mu 11181

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Figure 5. Comparison of the ALC-μSR spectra for the C6H6Mu (solid line and previous labels) and C6D6Mu (dotted line) radicals in NaY at 347 K. The lines are drawn just to guide the eye. The labels and arrows indicate the original positions of the C6H6Mu resonances. The strong Δ1 lines (B
D) remain, shifted slightly upfield, but the previous Δ0 lines (A, G, and E) have disappeared, consistent with their assignment as being due to proton hfc. Shoulders apparent on the sides of peaks B and D are likely the corresponding (and much weaker) D
Δ0 resonances. There is no evidence for any Na Δ0 coupling.

in the gas and liquid phases.10 Completely absent, however, are the previously assigned Δ0 lines (A, G, and E), which are therefore clearly weak proton resonances. There are noticeable shoulders on the downfield side of peak B and on the upfield side of peak D, shifted by ∼3
4 kG from where the proton resonances were and lying close to the expected positions for the D Δ0 resonances but (predictably) of very weak intensity. There is no further sign of peak F or any other candidates for a Na Δ0 resonance. 3.3. Discussion: Cyclohexadienyl Radical in NaY. A number of important points concerning the hfcc values and molecular dynamics of the cyclohexadienyl radical in NaY emerge from the present μSR studies of C6H6Mu and C6D6Mu at high loadings in comparison with the results reported earlier for C6H6Mu at lower loadings of 2
3/SC.15 First, the multiple and generally strong ALC resonances seen for C6H6Mu in Figure 3, with comparable intensities and essentially the same hfcc and T dependences as found in ref 15, demonstrate that host
guest interactions dominate in NaY, independent of loading. There is also no evidence for a nonhomogeneous distribution of benzene due to different loading procedures or sample preparation temperatures, which might have been expected from the results of ref 23, or of benzene residing in intergranular pores. That the Δ1 resonances, arising from the binding of the radical to Na cations (D and B) in the NaY supercage, remain clearly observable up to the highest temperatures measured (471 K), far above the melting point of bulk benzene (279 K), indicates a stable cation environment and a large BE of the cyclohexadienyl radical to these SII cations. Indications are, judging also from the comparisons with data for C6H6Mu in USY/HY to follow, that this BE is probably considerably higher than the ∼40 kJ/mol indicated for C6H6Mu on an NaY cluster from the calculations of Macrae and Webster,41 perhaps even approaching the BE of ∼100 kJ/mol for the cyclohexadienyl radical bound to a bare Naþ reported in these same calculations. Third, a strong binding of the C6H6Mu radical to cations in NaY is in accord with the ∼20% shifts in hfcc values reported earlier15 and seen here as well. It arises from the interaction of π-electron density with Naþ, which distorts the benzene ring from its normal planar geometry, rendering the exo and endo

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positions of the
CHMu group inequivalent, as shown by Figure 4, in accord with the calculations of ref 41. The W site is seen with more intensity in the present study than in the lower-loading data of ref 15, in accord with expectations.3,37 In contrast to the cation site, the muon hfcc at the W site is shifted only slightly upward from the bulk value (∼2%), because this 12-O-ring environment between supercages is largely a silicious one. In accord with the weaker BE expected for benzene at W sites in NaY, compared to cation sites39 (about half), the BE of the C6H6Mu radical to the W site (C) in NaY is also expected to be weaker, consistent with the observation that its intensity is markedly reduced at the highest temperatures (Figure 3). By and large, the strong ALC lines that remain broad and symmetric for the C6H6Mu radical in NaY over the temperature range now measured, from ∼5 to 470 K, are indicative of a largely static radical, although some motional effects are indicated by the larger line widths seen for the D and C resonances at the higher temperatures (Table 1). Even so, there is no indication of any dramatic broadening associated with large-amplitude radical motion, recently reported for the Mu-ethyl radical in faujasites19 and seen also in the present data for C6H6Mu in USY and HY discussed below. In NaY, the Na cations in particular appear to act as effective traps for cyclohexadienyl radicals, at least on the μSR time scale of τALC ≈ 50 ns.17 Nor is there any evidence for appreciable motional narrowing, as seen for C6H6Mu in ZSM5,17 even though the B cation resonance shows some sign of reduced widths at higher temperatures from the results of the axial fits in Table 1. This, in turn, suggests that the BE of the cyclohexadienyl radical is much weaker in that largely silicious environment. That cations effectively trap the C6H6Mu radical in NaY is in marked contrast to the case for benzene itself, where jumps between adsorption sites are well-established in NaY, by D NMR, on a much longer time scale;43,44 by neutron scattering;45 by molecular dynamics simulations;40,46 and by calculations of diffusion rates.47 If the BEs of benzene and C6H6Mu to cations are similar, as the calculations of ref 41 suggest, this contrasting behavior must relate to the appreciable dipolar interaction energy of ∼22 kJ/mol between C6H6Mu (calculated dipole moment μD = 0.47 D41) and zeolite framework charges, restricting hexad rotation and “locking” the radical orientation in place, in marked contrast to the much smaller energy of only ∼1 kJ/mol39,43,48 for the similar (symmetric, C6) rotation of benzene. For benzene, such a small activation energy guarantees facile in-plane rotation, with a correlation time of τc ≈ 10 ps near room temperature. Assuming a similar value for the time scale for unhindered in-plane rotation of C6H6Mu in NaY at high temperatures, the aforementioned ∼22 kJ/mol dipolar activation energy would correspond to τc ≈ 10 ns near 400 K, comparable to τALC and thus consistent with the small motional effects (only) that can be inferred from the ALC line widths given in Table 1. Finally, there is no convincing evidence for Na Δ0 resonances in either the C6H6Mu or C6D6Mu data in NaY, suggesting that the degree of spin transfer to the Na cation must be much less than the maximum of 12% that could have been expected from the calculations of ref 41.

4. HFCC VALUES OF THE MU-CYCLOHEXADIENYL RADICAL IN USY, HY, AND BULK BENZENE Unlike the NaY case, where there are clearly defined SII Na cations to which benzene and hence C6H6Mu can bind, the 11182

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Table 2. Reduced Muon and Proton hfcc Values,a A0 μ and Ap (MHz), and Widths,b fwhm (G), for ALC Resonances of C6H6Mu in USY(2), USY(4), and HY(4) at Representative Temperatures, down to 150 K USY(2)c A0 μ

USY(2)c

USY(4)c A0 μ

USY(4)c

HY(4)c A0 μ

Ap,2e

fwhm(2)

∼2000f

129(4)

500(50)

162.5(8)

1000(300)

129(1)

200(20)

550(70)

162.8(8) 163(1)

800(150) 600(60)

133(1)

400(100)

129(1) 128(1)

250(20) 300(60)

162.5(10)

900(100)

127(2)

490(60)

164.5(5)

500(40)

133(1)

300(60)

128(1)

300(60)

164.6(4)

430(40)

164.8(2)

480(40)

134(1)

600(150)

128(1)

200(80)

700(200)

126.3(10)

600(70)

165.7(5)

540(60)

135(1)

∼400g

128(1)

∼300g

165.3(5)

520(60)

162.5(5)

550(150)

125.9(8)

710(60)

166(1)@218 K

500(80)

134(1)

∼300g

128(2)

300(100)

162.5(5)

600(150)

125.5(10)

1100(200)

166(1)

900(250)

134(1)

∼700g

125(2)

∼500g

166.7(8)

400(100)

170

163(1)

∼1000

150h

165.5(2)

∼1000h

A0 μ = 166(1)

∼800h

Td (K)

fwhm

Ap

fwhm

162(2)

∼2000f

129(4)

480(50)

162(2)

295

161(1)

∼1200

126(2)

600(90)

286 282

162(1)

800(300)

127.2(15)

275

162(1)

∼1000f

250

162.5(5)

220 200

310

f

260

f

126(2)

1200(500)

0

fwhm

A μ = 167(1)

∼1200

A0 μ = 168(2)

∼1000g

h

Ap,1e

USY(4)c

fwhm(1)

161.8(5)

fwhm

900(200)

h

Calculated from eqs 3 and 4 from the positions of the fitted ALC resonances. See also footnote b to Table 1. Errors shown in parentheses, from the leastsquares fits but including effects of reproducibility as well. Further systematic errors could be (1 MHz. b Widths (fwhm), in G, from Lorentzian fits to the data. Fits mainly determined with the same background (eighth-order polynomial), as described in ref 15. Errors from the fits, if well determined, shown in parentheses; otherwise, approximate values are indicated. c Loadings of benzene/SC given in parentheses. d Temperature listed for USY(2) study but similar to other loadings, unless otherwise indicated. Systematic errors due to differences in thermocouple readings and hysterisis effects estimated at (2 K. e Split Δ0 resonances of the
CHMu group observed in USY(4) and HY(4) at intermediate temperatures. The lower resonance of the pair (Ap,1) is believed to be due to intergranular benzene, whereas the upper resonance (Ap,2) is due to pore-confined benzene in the SC, in roughly equal amounts. At temperatures of j170 K, these lines could no longer be identified. f Typically broad or very broad Δ1 ALC lines of weak intensity and with widths that were difficult to fit. In some cases, stable fits for resonance positions could not be found, so these were fixed at expected values from trends or from Aμ values determined from FT data. g Widths poorly determined because only partially resolved lines of the doublet were seen. h For the higher-loading data, at temperatures of j170 K, multiple ALC resonances observed. In general, these spectra were too complex to analyze, although split Δ1 resonances could be identified, as indicated by the red arrows shown for the scans at 170 K plotted in Figures 8, 9, and 2. The average muon hfcc values of these split lines, A0 μ, are given in the table at 170 and 150 K for USY(4) and also at 150 K for HY(4). Estimated widths also reported as average values at these temperatures. a

natures and locations of binding sites in HY and particularly in USY are less clear.5,7,49,50 It is well-established that the Brønsted acid (BA) sites are generally bridging hydroxyl Si
(OH)
Al groups, for example, from17O NMR studies,51 and as well, D NMR studies52 have shown that accessible OH sites in HY and HUSY are similarly populated. For USY, the degree of Brønsted acidity depends quite sensitively on the location of framework Al,53,54 with the number of extraframework AlOH Lewis acid sites also depending on the Si/Al ratio.14,55
57 Because the USY sample in the present study has a Si/Al ratio of ∼7 and is believed to be in the proton form, consistent with the essentially identical results found for the hfcc values in USY and HY discussed below (Tables 2 and 3), there should be appreciable BA activity in the USY sample. Some Na cations could also be present, either in the supercage, and hence accessible to benzene, or in the smaller beta cages. There are believed to be four distinct BA OH sites, but only two of these, the so-called O1H and O4H sites5,7,49,50,58,59 (there does not appear to be uniformity of notation), roughly centered on the 4-T and 6-T rings, respectively, are in the HY SC and could accommodate benzene, but with somewhat different BEs,49 in accord with D NMR52,60 and neutron diffraction60 data. This suggests that one could expect a reduced level of saturation loading in HY/USY compared to NaY, of ∼4 benzenes/SC (two at OH sites, two at W sites), in contrast to the maximum posible value of 6 benzenes/SC in NaY noted earlier. 4.1. Spectra and hfcc Values at Low Loading: USY(2). Representative ALC-μSR spectra from 35 to 310 K are shown in Figure 6, for a loading of 2 benzenes/SC, USY(2). Low loadings facilitate a high “sticking probability”61 and are most characteristic

then of host
guest interactions of the C6H6Mu radical confined to supercage pores. In each case, Lorentzian fits are shown for spectra that were background-corrected following the procedure described in ref 15 and implemented earlier in the reference spectra for pure benzene plotted in Figure 2. At first glance, these ALC data for USY(2) look very similar to those of pure benzene, in that only two clear resonances are seen from the
CHMu group, a Δ1 resonance at lower fields and a Δ0 resonance at higher fields. However, in contrast, the Δ1 resonance is seen over the whole temperature range and, in particular, is observed well above the bulk melting point of 279 K, demonstrating pore-confined benzene in USY supercages. In addition, the lines are considerably broader, indicative of powder-pattern spectra in a more polycrystalline environment than in solid benzene. Note the systematic shifts in the resonance positions in Figure 6, increasing with decreasing temperature, a trend that corresponds to increasing values of Aμ(T) and decreasing values for Ap(T), as recorded (for the reduced muon hfcc, A0 μ = Aμ/3.1833) in Table 2, along with the fwhm resonance widths. Similar entries for higher loadings in USY, discussed later, are also recorded in this table, as well as in Table 3, with the results for pure benzene referred to earlier (Figure 2) also given in Table 3. The values for both A0 μ(T) and Ap(T) for USY(2) are reduced by a few megahertz compared to those for pure benzene in the solid phase, as can be seen from Figure 7. In contrast to the rather similar-looking ALC-μSR spectra for the C6H6Mu radical in USY(2) in Figure 6, and for pure benzene in Figure 2, these spectra differ dramatically from the multipleline ALC spectra seen for NaY in Figure 3 at comparable temperatures (and independent of loading), which shows immediately 11183

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Table 3. Reduced Muon and Proton hfcc Values,a
e A0 μ and Ap (MHz), and Widths,a
e fwhm (G), for ALC Resonances of C6H6Mu in HY(4), USY(6), and Pure Benzene at Representative Temperatures, down to 150 K HY(4)a
e T(K)a
e

Ap,1a
e

fwhm(1)

HY(4)a
e Ap,2a
e

USY(6)a
e A0 μ

fwhm(2)

313

161.2(3)f

295

161.6(3)

f

162.6(3)

f

126(1) 127(1) 128.3(4)

220(20) 160(10) 270(50)

USY(6)a
e fwhm

Ap

fwhm

127.3(5)

210(30)

127.6(5)

160(10)

benzenea
e A0 μ

fwhm

benzenea
e Ap

fwhm

161.2(3)g 161.6(3)g

125.9(5)

140(5)

162.0(3)g 164.1(2)

127.6(5) 127.8(3)

600(150) 260(20)

280 277

134(1)

310(90)

275

133(1)

260(60)

128.4(4)

340(40)

163.9(2)

460(25)

127.8(3)

350(50)

164.2(5)

250

136(2)

700(100)

128(1)

500(100)

164.7(2)

420(30)

128.2(3)

220(20)

165.3(6)

165.4(2)@224K

520(30)

128.4(3)

390(50)

166.2(2)

500(20)

129.1(3)

370(30)

133(2)

∼500

128(2)

∼500

165.9(2)

610(50)

128.8(8)

700(100)

166.5(2)

450(20)

129.5(5)

220(40)

∼1000h

127(3)

900(200) A0 μ = 168(2)h

380(50)

220 200 190

165(1) 0

h

170

A μ = 166(1)

600(200)

150

A0 μ = 168(2)h

400(150)

400(15)

a
e

See corresponding footnotes for Table 2. For footnote c, the last (four) columns here are for neat benzene, mainly for temperatures in the solid phase from the present study. f TF/FT data only, from the present study. Errors taken to be (0.3 MHz. g Muon hfcc values, A0 μ(T), above the bulk melting point (279 K) taken from ref 11. Errors also taken to be (0.3 MHz. h See footnote h for Table 2.

that any residual Naþ in the USY sample is inaccessible to benzene. This comparison reveals as well a marked difference in the host
guest interactions of the Mu-cyclohexadienyl radical in these two environments. In the case of NaY, strong interactions due to the binding of C6H6Mu at cations give rise to a pair of Δ1 and Δ0 resonances for inequivalent exo and endo orientations (Figure 4), as well as a further pair of resonances expected from C6H6Mu at W sites. In contrast, in the more silicious USY/HY environment, weaker interactions at framework OH sites are expected.5,7,49 For reference, the calculated BE for benzene in HY is about half that of binding to the SII cations in NaY,39,40,49 which is consistent with the much simpler ALC spectra seen for the muoniated radical in the USY/HY environment. Two important points from comparing Figures 3 and 6 and the relevant data in Table 2 deserve emphasis. First, the observation of just two and quite broad ALC resonances seen in USY(2) over the whole temperature range, in contrast to the multiple resonances seen for NaY, is a strong indication of a planar cyclohexadienyl radical in the polycrystalline supercage environment. Spectra similar to those seen in Figure 6 are found for C6H6Mu in other silica-like environments, in silicious ZSM-517 and in silica powder and gels,34,62 and in bulk benzene as well at temperatures below its melting point (Figure 2), although in that case with sharper lines indicative of a single-crystal environment. Second, although perhaps somewhat broader at the lowest temperatures, the Δ1 muon resonance exhibits an essentially constant width (∼600
800 G) from ∼200 K to near room temperature and then begins to broaden dramatically, such that, by the highest temperature measured of only 310 K, although well above the 279 K melting point of bulk benzene, it is barely visible above background (Figure 6). This is clear evidence of enhanced molecular motion of the C6H6Mu radical, which is discussed in more detail below. In contrast, the Δ1 lines in NaY, although tending to exhibit comparable widths for the same loading at lower temperatures15 (see also Table 1), remain clearly visible up to 471 K (Figure 3). Note also the opposite behavior exhibited by the Δ0 resonances in USY(2), which tend to become narrower at these same temperatures, a pattern exhibited by C6H6Mu in silica powder and gels as well.34,62

Figure 6. ALC-μSR spectra over a range of temperatures, taken with variable step sizes, from 35 to 310 K, for USY at a loading of 2 benzenes/ SC, USY(2). The lines are Lorentzians fitted to each ALC resonance from background-corrected spectra, as described in ref 15. See also the caption to Figure 2. Only two such resonances are seen at all temperatures. Note the persistence of the Δ1 resonance (lower fields) to well above the bulk melting point (279 K). Results for the hfcc values and widths (fwhm) from fits to both the muon Δ1 and proton Δ0 (upperfield) resonances of the
CHMu group are recorded in Table 2. The markedly enhanced broadening seen for the Δ1 resonance at the highest temperatures is noteworthy.

The reduced muon and proton hfcc values, A0 μ(T) and Ap(T) from Table 2, for the C6H6Mu radical in USY(2) are plotted as a function of temperature in Figure 7, both fit to a straight-line dependence. Error bars are typically the size of the plotted points unless indicated otherwise. For the muon hfcc, the slope is dA0 μ/dT =
0.023 MHz/K, the same as for peak B (exo muon 11184

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Figure 7. (Top) Temperature dependence of the muon hfcc, A0 μ(T), in USY(2) from Table 2 (open blue circles and fitted straight line), compared with A0 μ(T) for the exo muon of C6H6Mu at the cation (peak B, uppermost dashed green line) and W site (C, middle dashed green line) in NaY, from fits to the data reported for the C6H6Mu radical in NaY at a comparable loading in ref 15. The A0 μ(T) data for USY(4) exhibit a similar trend but are not plotted. Also shown are the data for A0 μ(T) for pure benzene from Table 3 (smaller solid brown circles and fitted line). The points plotted at 170 and 140 K are average muon hfcc values from the split Δ1 resonances seen in Figure 2. See discussion in the text. The upper three data points for benzene above the melting point (279 K) are from ref 11 and were not included in the fit. (Bottom) Temperature dependence of the proton hfcc, Ap(T), for USY(2) (open blue squares) compared with that for peak A in NaY (lowest dashed green line) from ref 15. The proton hfcc values for pure benzene from Table 3 are also plotted (solid brown sqares), with straight-line fits again assumed. The change in slope compared to the USY(2) data is noteworthy.

orientation at SII in NaY,15 recall also Figure 4), although somewhat shallower than for peak C (W site) in NaY, both shown for comparison at a comparable loading as the labeled upper dashed green lines, taken from fits to the data reported in ref 15. The corresponding proton hfcc, Ap(T), for USY(2) gives the opposite slope, dAp/dT = 0.024 MHz/K, but also the same as that found earlier for peak A in NaY15 (the endo proton partner to peak B), shown by the lower dashed green line. Also shown in Figure 7 are the results for A0 μ(T) and Ap(T) for pure benzene (plotted brown points), for data mainly below the melting point. The three highest-temperature A0 μ(T) points are taken from ref 11 for C6H6Mu data in liquid benzene, which exhibit a reduced hfcc at the phase transition. (These data were not included in the fit.) This reduced hfcc for C6H6Mu in the liquid is likely the result of solvent polarization by the dipole moment of C6H6Mu in the differing dielectric constants of solid and liquid benzene.36 Note that the fitted line for pure benzene has the same slope as for A0 μ(T) in USY(2), but not for the proton hfcc. Although there is some scatter in the data, the opposite nature of the slopes seen for A0 μ(T) and Ap(T) in USY(2) is worth noting. Several other features also emerge from the comparisons of hfcc values shown in Figure 7. First, A0 μ(T) for the C6H6Mu radical in USY(2) lies well below that due to cation binding in NaY, reflecting a much weaker interaction in the more silicious USY/HY environment.

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Somewhat surprisingly, however, the slope of dA0 μ(T)/ dT =
0.023 MHz is essentially the same in the two environments. These muon hfcc values also lie below those for bulk benzene in the solid phase, although by only a small amount (∼2%), and have a similar slope (Figure 7). This result is similar to that from previous μSR studies of C6H6Mu in other silicious-like environments, ZSM-5,17 and silica powder and gels,34,62 where the muon hfcc values at different loadings for C6H6Mu below the bulk melting point also fall below bulk benzene values. Surprisingly, the values for A0 μ(T) for USY(2) fall even further below those for the window site (C) in NaY in Figure 7, also a largely silicious environment. These observations suggest that a small fraction of electron spin density might be transferred from the radical to hydroxyl groups in the USY supercage. In addition, because there are two hydroxyl sites in the USY supercage to which benzene could bind, O1H and O4H, which are reported to have different BEs,49 multiple-line ALC resonances might have been expected, as seen in NaY. The observation of just a single pair of resonances, which is also evidence for a largely planar C6H6Mu radical bound at these OH sites, suggests that either both sites are magnetically equivalent to C6H6Mu or the data reflect some kind of dynamic average between the two. That C6H6Mu in USY(2) exhibits the same slopes for the T dependences of A0 μ(T) and Ap(T) in Figure 7 as seen for the exo orientation at cation sites in NaY, both opposite to each other, in contrast to the same slopes seen in other studies in silica-like environments,17,34,62 suggests that the C6H6Mu radical cannot be completely planar, despite the characteristic two-line ALC spectra seen (Figure 6). Evidently, however, the bridging Brønsted acid groups in HY (USY)2,49 are not strong enough to break the symmetry between exo and endo nuclei that is seen at cation sites in NaY. Finally it can be remarked that, because proton-transfer reactions to organic guest molecules from Brønsted acid zeolites are well-established,2
4,13 one might expect the formation of the radical cation of Mu-cyclohexadienyl, C6H7Muþ, in the acidic HY environment, previously identified in HZSM-5 from ALCμSR spectra at lower temperatures (j250 K).18 However, there appears to be no evidence for this in the present study of USY(2), or in the USY(4)/HY(4) data below, given the widths of the Δ1 resonances seen. 4.2. Spectra and hfcc Values at High Loadings: USY/HY(4) and USY(6). Figure 8 presents a plot similar to Figure 6 but for a loading of 4 benzenes/SC, showing both similarities and differences with the USY(2) data. [Data taken for HY(4) were very similar to those for USY(4) but were obtained over more restricted temperature and scan ranges and are not plotted.] The A0 μ(T) data for both USY(4) and HY(4) are recorded in Table 2 and are seen to be the same, within errors, consistent with earlier arguments of essentially the same environments and binding sites. In like manner to USY(2), the strong (lower-field) Δ1 resonance, believed to be due to C6H6Mu bound to OH
SC sites, also persists far above the bulk melting point (Figure 8 and Table 2). It is worth noting that the intensity of this Δ1 resonance in USY(4) is about twice that for USY(2) in Figure 6 over the whole temperature range (note the scale change in Figure 8), consistent with the change in loading. As for USY(2), the observation of this resonance above the melting point can only be due to C6H6Mu within SC pores. The muon hfcc values in USY(4) are about 2 MHz higher than those in USY(2) at temperatures below the melting point (Table 2), but otherwise, they exhibit a trend for A0 μ(T) very similar to that seen in Figure 7. In accord 11185

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Figure 8. Representative ALC-μSR plots from background-corrected spectra for USY(4) over a range of temperatures from 170 to 294 K. It is noteworthy that the (lower-field) Δ1 resonance again persists to above the bulk melting point (279 K), as in USY(2). Note the doublet at intermediate temperatures (double green arrows in the scan at 275 K), near where a single Δ0 line is seen at 286 K and at a similar position in USY(2) (Figure 6). The upper line of this doublet is attributed to a Δ0 resonance for C6H6Mu from SC-confined benzene, and the lower line is a Δ0 resonance for C6H6Mu in intergranular regions. At lower temperatures, j170 K, multiple ALC resonances are seen down to 50 K, exemplified by the scan shown at 170 K. These spectra are too complex to analayze, although it was possible to identify split Δ1 resonances, indicated by the red arrows in the 170 K scan.

with the USY(2) data as well, the Δ1 lines in USY(4) exhibit marked increases in widths at the highest temperatures. However, there are two major differences in the data for C6H6Mu in USY(4)/HY(4) compared to USY(2) that can be seen from comparing their ALC spectra and from the entries in Tables 2 and 3. First, these data show a splitting of what was first thought to be an overly broad Δ0 resonance at temperatures near the bulk melting point (279 K) and below, at around the same position as found in USY(2). The expected single-line Δ0 resonance is seen for USY(4) in the 286 K scan in Figure 8, at 20.8 kG, but just below, at 282 K, it splits into a doublet with similar features that appear down to 200 K, as indicated by the double green arrows plotted on the scan at 275 K. Although not fully resolved, this splitting is clearly revealed by the Lorentzian fits shown (and more so on an expanded scale), but by 200 K, it has become much weaker and is lost at temperatures of j170 K, where a series of multiple ALC resonances are observed. Although seen convincingly in the data (Figure 8), the origin of this doublet is unclear. The upper-field resonance of the pair gives a proton hfcc value very similar to that found from the Δ0 resonance for
CHMu in USY(2) at SC sites at comparable temperatures and listed as Ap,2(T) in Table 2 [with essentially the same values, although at fewer temperatures, listed for HY(4) in Table 3]. Accordingly, the temperature dependence for these Ap,2(T) hfcc values has much the same slope as plotted in Figure 7 for Ap(T) in USY(2), although with more scatter and over a more limited temperature range here.

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Thus, taking the upper resonance of this doublet to be Δ0 for C6H6Mu at SC sites in USY(4), what gives rise to the lower resonance? One possibility is that this could be the Δ1 resonance for C6H6Mu bound at W sites between supercages, not seen in USY(2) but reasonably expected at this higher loading, as was the case discussed earlier for NaY (Figure 3). However, the muon hfcc values A0 μ(T) would then be some 10 MHz above those for the W sites C of NaY (dashed green line in Figure 7), whereas, being largely a silicious environment of similar size, one would expect these values to be much the same, independent of cation. Another possibility is that this could be the Δ1 line for C6H6Mu formed from benzene trapped in intergranular regions, which one could also expect to be more likely at this higher loading, but again, the muon hfcc seems far too high compared to bulk values (Table 3). Moreover, this line is also seen at 282 K in Figure 8, above the bulk melting point of 279 K. We are persuaded then that both lines in this doublet are most likely Δ0 resonances. The higher-field resonance, Ap,2(T), as commented above, is associated with the proton hfcc for
CHMu of C6H6Mu within the SC, and the lower-field resonance, denoted by Ap,1(T) in Tables 2 and 3, is believed to be due to
CHMu from intergranular benzene. It is noteworthy that this fraction appears to have the same trend in hfcc values with temperature as in the bulk, tending to decrease with increasing temperature (Figure 7), as might be expected, in contrast to the opposite temperature dependence seen for Ap,2(T). Because the amplitudes of the two resonances in this doublet are about equal, we conclude that roughly half of the benzene at a nominal loading of 4 molecules/SC in USY/HY resides in SC pores and about half resides in intergranular regions. This is also in accord with the observation that part of the Δ1 resonance survives above 282 K (Figure 8) when the intergranular fraction of benzene has melted. The magnitude of the hfcc found for Ap,1(T), however, ∼134 MHz for both USY(4) and HY(4), is some 5 MHz higher than the proton hfcc in the bulk over the same temperature range (Tables 2 and 3 and Figure 7), indicating a different environment in the intergranular regions, similar perhaps to that reported by Masierak et al. from D NMR studies of benzene in mesoporous silica environments.63 Differing dielectric effects on the hfc of the C6H6Mu radical at different benzene densities36 might also be playing a role. However, there is also some experimental uncertainty as to which is the correct value of Aμ(T) to use in eq 3 here, that from the Δ1 resonance in Figure 8 for USY(4), as assumed, or that from pure benzene. If the latter, the proton hfcc Ap,1(T) would be ∼1
2 MHz smaller. It might well be that there is a small difference in muon hfcc values between the bulk and the intergranular region that cannot be distiguished, given the widths of the Δ1 resonances seen. The second major difference between the ALC-μSR spectra in Figure 6 for USY(2) and in Figure 8 for USY(4) is seen at temperatures near 170 K and below, where the separate resonances at the higher temperatures evolve into a series of multiple and overlapping resonances of generally unknown origin. This is indicative of a freezing of rotation so that discrete orientations are observed within intercyrstalline benzene, with similar results seen in USY(6) (Figure 9) and particularly in bulk benzene (Figure 2), as shown by the representative scans at 170 K for all loadings. Quite similar patterns are observed down to about 50 K in all of these cases. These spectra are too complex to analyze, although, as noted earlier, it was possible to identify a pair of Δ1 resonances by comparing with FT spectra at the same or nearby temperatures, as shown by the red arrows for the 170 K scans in 11186

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Figure 9. Representative ALC-μSR plots from background-corrected spectra for USY(6) over a range of temperatures down to 170 K. Note the disappearance of the Δ1 resonance just above the melting point of bulk benzene (279 K), in contrast to both USY(2) (Figure 6) and USY(4) (Figure 8). Note also the absence of any doublet near the position of the Δ0 resonance, in contrast to the USY(4) data. At temperatures j170 K, multiple resonances are again seen (down to 44 K), represented by the scan shown at 170 K, with split Δ1 resonances also indicated by red arrows, as in Figure 8. See entries in Table 2 and caption to Figure 8.

Figure 2, 8, and 9. These lines are split roughly equally about where the position of the principal Δ1 resonance associated with C6H6Mu at SC sites in USY would otherwise be, as can be seen from nearby higher temperatures. The average muon hfcc for this pair of Δ1 resonances, Aμ0 , is given in Tables 2 and 3 at 170 K and at 150 K for USY(4) and USY(6), in order to get a sense of the trend. This increases with decreasing temperature in like manner to the A0 μ(T) data for USY(2) and also for pure benzene, as plotted in Figure 7. The USY(6) data in Figure 9 are different from the USY(4)/ HY(4) data above 170 K in two important aspects. First, the Δ1 resonance disappears within a degree of the bulk melting point of 279 K, as can be inferred from the scans at 274 and 281 K. It is noteworthy that the highest-temperature scan at 313 K exhibits the same single Δ0 resonance as at 281 K, just above the melting point, demonstrating no further evidence for a Δ1 resonance, in contrast to the broadening seen for the Δ1 resonances at comparable temperatures in both USY(4) and USY(2). This “melting” behavior seen in the loss of the Δ1 resonance for USY(6) in Figure 9 is the same as that found in the reference spectra for pure bulk benzene in Figure 2 and indicates that benzene, at this nominally above-saturation loading, is found mainly in intergranular regions. Second, there is no evidence for a doublet in the neighborhood of 21 kG. Because we interpreted this doublet as indicating roughly equal fractions of benzene in SCs and in intergranular regions in USY(4), the presence of only a single line here is consistent with the previous statement that most of the benzene is in fact in intergranular regions in USY(6) and, notably, with essentially the same hfcc values as for Ap,2(T) in USY(4)/HY(4), as can be seen from Tables 2 and 3. (Small

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differences in hfcc values could be the result of agglomeration effects due to collective interactions.) 4.3. Comparisons of C6H6Mu in USY and Bulk Benzene. The “reference” ALC spectra shown in Figure 2 and the hfcc data in Table 3 extend the results for C6H6Mu in neat benzene previously measured in the gas10 and liquid11 phases and in the solid at 263 K27 to a range of lower temperatures in the solid phase. Data obtained here above the melting point also agree well with the earlier determinations in the liquid phase.11 Pure solid benzene forms a single crystal at low temperatures,27,63
65 with four benzenes per unit cell, each with a different orientation with respect to an applied field direction. There are two possibilites for Mu addition to each of six carbons, although with symmetry above and below the plane, giving 24 different directions of the C
Mu bond. Each C6H6Mu radical so formed would have the same isotropic hfcc, Aμ, but different dipolar couplings and, hence, different angle-dependent resonance positions and widths,27,30 allowing for the possibility of 24 separate Δ1 and Δ0 resonances at the lowest temperatures. It is these different orientations that lead to the complex μSR spectrum of overlapping resonances seen at 170 K (and lower) for pure benzene in Figure 2 and also for USY(4) in Figure 8 and for USY(6) in Figure 9, indicating very similar environments. With increasing temperature, benzene molecules undergo hopping motions, first by anisotropic rotational diffusion about the 6-fold axis, as measured by D NMR spectroscopy.63,66 At temperatures between ∼170 K and the bulk melting point (278.5 K), this complex spectrum collapses to the predominant pair of single Δ1 and Δ0 resonances seen for C6H6Mu in Figure 2 and for USY(6) in Figure 9. Remarkably, bulk benzene remains in a single-crystal environment up to its melting point, as shown previously by measurements of orientation-dependent line positions for C6H6Mu.27,30 It is perhaps somewhat surprising to realize just how similar the muon and proton hfcc values are for C6H6Mu in USY and in the bulk. These differ by only a few megahertz over the full range of loadings and temperatures (above ∼170 K) studied and are mainly distinguished by the slopes of their T dependences. As seen from Figure 7, the muon and proton hfcc values for C6H6Mu in USY(2) have the opposite slopes for A0 μ(T) and Ap(T), indicative of some distortion of the radical from planarity, with similar results seen for the muon and proton hfcc values of the SC fraction, Ap,2(T), in USY/HY(4). Although the proton hfcc values of the intergranular fraction, Ap,1(T), seem anomalously high, they arguably have the same temperature dependence as found in the bulk. However, for USY(6), the muon and proton hfcc values for C6H6Mu are virtually indistinguishable from the bulk, and tellingly, both also have the same slopes for A0 μ(T) and Ap(T). This evolution of benzene environment from being mainly in SC pores in USY(2) to about being evenly split in USY/HY(4) to being mainly in intergranular regions in USY(6) indicates that it is energetically more favorable for benzene to coalesce in droplets outside the zeolite than to disperse inside USY supercages, where there are no strong binding sites. Similar results have been reported for benzene from D NMR data,63 from calorimetry and neutron scattering data,67 and from FTIR spectroscopy68 in mesoporous silica, depending on benzene loading, temperature, and porosity. Further support for this claim is provided by a recent Raman spectroscopy study of benzene adsorbed in silicious MFI, which also indicates that benzene clustering does not occur within zeolite pores.69 An earlier μSR study of C6H6Mu in silica gel at high loadings also revealed a clear 11187

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The Journal of Physical Chemistry C melting phenomenon associated with the loss of the Δ1 resonance, although at temperatures of ∼260 K, well below the bulk melting point,34 consistent with established expectations of freezing point depression in pore-confined environments.70 The few-megahertz changes in both muon and proton hfcc values seen in USY, in comparison with bulk values, indicate rather subtle changes in interaction energies with benzene loading and are in marked contrast to the results in NaY, where much larger shifts in hfcc values are seen, in accord with both much larger BEs expected for C6H6Mu at cation sites and the fact that little or no effect is seen in ALC spectra or in hfcc values with changes in benzene loading in NaY, from ∼2 molecules/SC in ref 15 to ∼5 molecules/SC (Figure 3). In contrast, for USY, over a similar range of loadings, the subtle changes in interaction energies arising from much weaker binding to OH sites in USY have a much greater impact on observed spectra, because of the likelihood that it is energetically more favorable for benzene to coalesce within intergranular regions at high loadings, rather than remaining in SC pores. This implies that the environment of benzene in intergranular regions at high loadings must be essentially the same as that in the bulk. Support for this claim is provided by the D NMR work of benzene in mesoporous silica reported in ref 63, where the spectra reveal a superposition of an amporphous surface phase and a crystalline inner bulk phase, which occupies most of the pore volume. Thus, C6H6Mu in USY(6), although nominally in a polycrystalline environment, might well exhibit a largely singlecrystal environment for benzene in intergranular regions at high loadings. The somewhat higher proton values found from the Δ0 resonance for what is believed to be the intergranular component, Ap,1(T) in Table 2 [and for HY(4) in Table 3], could be an indication of interaction with the intergranular surface.

5. ALC LINE WIDTHS AND DYNAMICS: C6H6MU IN USY For molecular motion to occur, thermal energy has to compete with binding energy due to host
guest interactions. For Mu radicals, this is manifest by a change in the ALC line widths with temperature, particularly for the Δ1 resonances, which are most sensitive to changes in hyperfine anisotropy. The previous discussion showed that host
guest interactions of the C6H6Mu radical in USY predominate at low loadings, so the clearest interpretation of motional effects can be expected from the USY(2) data. The widths of the Δ1 resonances are relatively constant at ∼600
800 G from about 200 to 290 K (Table 2), indicative of a largely static radical.15,17,31,34 Broadening then sets in and is seen dramatically by the highest temperature of 310 K, where the Δ1 line width is twice its value at the lower temperatures, rendering the signal barely visible above background in Figure 6. (The position was fixed at the value of Aμ determined from an FT spectrum.) Although USY(2) should present the clearest case, similar behavior is also seen for the Δ1 resonance at comparable temperatures for USY(4) in Figure 8, perhaps even more dramatically so because the lines at this higher loading are typically narrower at the lower temperatures (Table 2). Similarly so for HY(4). In contrast, no such effects are seen in the bulk intergranular environment of USY(6), the dependence of line width on temperature being essentially indistinguishable from that in the bulk. The markedly enhanced broadening and almost complete loss of signal seen for the Δ1 resonances in both USY(2) and USY(4) at temperatures not far above room temperature, coupled

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with the observation of sharper Δ0 resonances at the same temperatures, is a clear indication of motion of the C6H6Mu radical on a time scale faster than the ALC time scale of τALC ≈ 50 ns.15,17,31 From the trend seen in Figure 6 (and Figure 8), one can expect that, at modestly higher temperatures (j350 K), the Δ1 resonance would disappear completely, because of either desorption of the radical and its isotropic reorientation within the supercage or hopping between sites in the cubic environment of the zeolite. This behavior is in stark constrast to that for the more strongly cation-bound C6H6Mu radical in NaY, even at saturation loading, where the Δ1 resonances remain clear and strong up to 471 K (Figure 3). Such enhanced motion apparent for the C6H6Mu radical in USY/HY, compared to the static line shapes seen in NaY up to much higher temperatures, is also qualitatively consistent with D NMR studies of benzene in these same environments,43,52 where isotropic motion is observed in HY at temperatures down to 180 K, in contrast to the clearly anisotropic motion of C6 hexad rotation seen in NaY at the same temperatures. From ref 43, the activation energies for benzene diffusion are correspondingly found to be markedly different in NaY and DAY, a more silicious form of USY: Ea(NaY) = 23.5 ( 0.9 kJ/mol, reasonably consistent with theory,46 compared to Ea(DAY) = 10.2 ( 0.8 kJ/ mol, with the diffusion coefficients then differing by 2 orders of magnitude, reflecting the much more tightly bound nature of benzene to Na cations in NaY. Similar values were reported in ref 52, with intracage hops dominating at lower temperatures in the USY environment (Ea = 10
15 kJ/mol) but intercage hops and longer-range diffusion dominating at higher temperatures (Ea = 24
30 kJ/mol), qualitatively consistent as well with theory for cage-to-cage hopping of benzene in HY.49 It is important to distinguish here between uniaxial rotation of C6H6Mu at its binding site, similar to the hexad rotation of benzene, and reorientational motion that can arise from site-tosite hopping, leading to isotropic averaging and the disappearance of the Δ1 resonance, both of which are activated processes. As previously remarked, for the C6H6Mu radical in NaY, an activation energy of EMu a (hexad) ≈ 22 kJ/mol can be expected from the dipolar coupling between the radical and the zeolite lattice.41 Hexad rotation can then give rise to a change in the line shape for the Δ1 resonance (seen as a cusp-like shape from the axial tensor fits to peak B of Figure 3, as recorded in Table 1), with a correlation time τc ≈ 10 ns near 400 K (see section 3.3), comparable to τALC, but cannot cause the loss of the signal, which arises from an isotropic reorientational motion. It is this latter motion that is at play in the USY environment. Taking the lower value of 10
15 kJ/mol mentioned above for intracage motion of benzene in USY/HY to also account for the motion seen for the C6H6Mu radical in the present USY data at low loadings, assuming as well a rotational reorientation time for a freely desorbed radical at high temperatures of ∼1 ps, and using the fact that the Δ1 resonance has broadened significantly by 310 K, we expect, from a simple Arrhenius dependence, a correlation time for this broadening of τc ≈ 0.1 ns at 310 K. Although only a rough estimate, we emphasize that this is 2 orders of magnitude less than both the ∼50 ns ALC time scale and the τc ≈ 10 ns estimated above for hexad rotation in NaY, supporting the argument here that desorption and isotropic reorientation33 of the C6H6Mu radical are responsible for the marked enhancement of the line width seen for the Δ1 resonance in both USY(2) and USY(4) near ∼300 K. This is the first time that this kind of line-broadening motion has been seen for the C6H6Mu radical in zeolites. 11188

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The Journal of Physical Chemistry C The much shorter correlation time estimated for the C6H6Mu radical in USY(2) and in USY/HY(4), in contrast to that in NaY, where the constant line widths reflect a much more static situation, is consisent as well with expected differences in binding energies for C6H6Mu in these different faujasite environments. In fact, as stated earlier, the relative stability of the Δ1 resonance in NaY over such a wide temperature range (∼5
470 K, Figure 3 and ref 15) suggests that the BE of the C6H6Mu radical in NaY might be close to the ∼100 kJ/mol given in ref 41 for binding of the radical to a bare Naþ. Although both further studies in USY/ HY at higher temperatures and theoretical calculations of the BE and barriers to diffusion for the C6H6Mu radical in different zeolites are needed here, the implication from the present study of a highly mobile isotopomer of C6H7, even near 300 K, is that the cyclohexadienyl radical could well be an important reactive intermediate in the catalytic USY/HY environment.

6. CONCLUDING REMARKS The adsorption and motional behavior of the muoniated cyclohexadienyl radical has been investigated in the faujasitic zeolites NaY, USY, and HY over a range of loadings and temperatures using both the FT-μSR and ALC-μSR techniques, but mainly ALC-μSR spectroscopy. Comparisons have also been made with similar results obtained in pure benzene, mainly in the solid phase. Data are reported in NaY at both higher loadings of 4
5 benzenes/SC and to much higher temperatures than reported earlier at loadings of 2
3/SC in ref 15, including results for the perdeuterated radical. Both the measured hfcc values and ALC line widths exhibit little or no loading dependence. These data also confirm the conclusions of the earlier analysis of a distorted nonplanar C6H6Mu radical due to strong host
guest interactions arising from binding at extraframework cations, with a pair of Δ1 and Δ0 resonances due to specific exo and endo bond orientations of the
CHMu group, giving large (∼20%) shifts in hfcc values compared to bulk values. The population of W sites is seen also with higher intensity than in the lower-loading data of ref 15. The (muon) Δ1 resonances for C6H6Mu are seen strongly up to the highest temperature of 471 K, with no dramatic changes in line shape, demonstrating a largely static radical that exhibits little evidence of molecular motion on the μSR time scale of ∼50 ns, confirming as well the claim made earlier15 that the sodium ions in NaY in particular act as effective traps for cyclohexadienyl radicals. The situation is very different in almost all respects for C6H6Mu in USY/HY, which also exhibits a pronounced loading dependence. At the lowest loading of 2 molecules/SC, a classic pattern of single broad Δ1 and Δ0 resonances for a largely planar Mu-cyclohexadienyl radical in a polycrystalline environment is seen over the whole temperature range, attributed to host
guest interactions of the C6H6Mu radical within the USY supercage, both above and below the bulk melting point. Both the muon and proton hfcc values at temperatures below the melting point are slightly (∼2%) below bulk values, suggesting a small degree of spin transfer from the radical to OH binding sites, not inconsistent with data for C6H6Mu in silica powder at low loadings.62 Some distortion from planarity is indicated, however, from the T dependences of both the muon, A0 μ(T), and proton, Ap(T), hfcc values, which exhibit the same slopes and opposite signs as seen in NaY (Figure 7), but of insufficient interaction energy at OH sites to break the inherent symmetry between endo and exo

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nuclei, in marked contrast to the situation for C6H6Mu in NaY with its much stonger interaction with cations. Both ALC resonances are broader in USY(2) than in the bulk at comparable temperatures but are of largely constant width up to near room temperature, again indicative of a mainly static radical, as in NaY. However, by ∼310 K, the Δ1 line has broadened dramatically, indicating enhanced molecular motion of C6H6Mu due to isotropic reorientation of the radical within the USY supercage, with a correlation time τc estimated to be ∼0.1 ns, and leading to the expectation of complete desorption and loss of signal at only modestly higher temperatures. This is the first time that such dynamics has been seen for any cyclohexadienyl radical in zeolites, due to a much weaker BE of the C6H6Mu radical to OH sites within SC pores in USY compared to cation sites in NaY. Very similar behavior is seen in USY/HY(4) at comparable temperatures. At the nominal saturation loading of 4 benzenes/SC, in both USY and HY (which behave essentially identically), even though the results for desorption of the C6H6Mu radical from OH sites at the higher temperatures are very similar to those in USY(2), they are very different at the lower temperatures. Thus, in contrast to USY(2), from temperatures of ∼285 to ∼200 K, an additional resonance is apparent in USY(4), believed to be a further Δ0 line attributed to C6H6Mu from benzene in intergranular regions. The amplitudes of this pair of Δ0 resonances are roughly equal, indicating that benzene is also roughly equally distributed beetween SC and intergranular pores at a loading of 4 molecules/SC in both USY and HY. For USY(6), however, the data indicate that benzene resides primarily in intergranular regions. This trend suggests that the guest
guest interactions between benzenes at higher loadings are stronger than the guest
host interactions within SCs, thereby facilitating benzene agglomeration between grains. This agglomeration effect is also seen in an additional and rather striking difference between C6H6Mu in USY(2) and at the higher loadings in USY/HY(4) and USY(6), where the ALCμSR spectra at lower temperatures, j170 K, exhibit multiple and overlapping resonances, with similar patterns seen in pure benzene, indicating that benzene localized in intergranular regions at high loadings in USY individually exhibits singlecrystal-like behavior, despite the polycrystalline nature of USY. Although such a conclusion seems a bit surprising at first glance, it is consistent with D NMR data that show two regions for benzene adsorption in mesoporous silica, a surface interaction region of a few layers that surrounds most of the benzene that forms in a crystalline region.63 It is this latter region that μSR spectroscopy appears to be probing in USY at low temperatures and high loadings. The described behavior for USY is in marked contrast to that in NaY, where strong binding of C6H6Mu at sodium cations effectively immobilizes the radical on the ALC time scale of ∼50 ns. The much higher degree of mobility seen for C6H6Mu in USY at low loadings of 2
4 molecules/SC is attributed to its much weaker binding to OH sites within a supercage and reinforces a principal conclusion of the present study that the cyclohexadienyl radical could well play an important role as a reactive intermediate in certain catalytic steps in acid-active zeolites.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: fl[email protected] (D.G.F.), [email protected]. de (E.R.). 11189

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The Journal of Physical Chemistry C Present Addresses †

Schrodinger Inc., 101 SW Main Street, Suite 1300, Portland, OR 97204, USA. ‡ Bettina Teschner, B. Braun Avitum AG, 34212 Melsungen, Germany.

’ ACKNOWLEDGMENT We thank CU Chemie Uetikon AG of Switzerland for generously providing the zeolite samples. We also thank Dr. Syd Kreitzman for his management of the μSR Facility at TRIUMF and for the excellent technical support it provides. We also gratefully acknowledge the assistance of Dr. Masayoshi Senba (recently deceased) in recording the data at TRIUMF and of Dr. Robert Scheuermann at PSI. One of us (D.G.F.) also thanks the National Research Council of Canada for its financial support and the Alexander von Humboldt Foundation for its “Wiedereinladung” to the FU Berlin and to the Institute of Physical Chemistry, Universit€at Stuttgart, Stuttgart, Germany, facilitating ongoing collaborative research. ’ REFERENCES (1) Catlow, C. R. A. Modelling of Structure and Reactivity in Zeolites; Academic Press: London, 1992. (2) Haw, J. F. Phys. Chem. Chem. Phys. 2002, 4, 5431. (3) Barthomeuf, D. Catal. Rev. 1996, 38, 521. (4) Venuto, P. B. Stud. Surf. Sci. Catal. 1997, 105, 811. (5) Yoda, E; Kondo, J. N.; Domen, K. J. Phys. Chem. B 2005, 109, 1464. (6) Carley, A. F.; Edwards, H. A.; Mile, B.; Wyn Roberts, M.; Rowlands, C. G.; Hancock, F. E.; Jackson, D. S. J. Chem. Soc., Faraday Trans. 1994, 90, 3341. (7) Zheng, X; Blowers, P. J. Phys. Chem. A 2006, 110, 2455. (8) Hansen, N.; Br€uggemann, T.; Bell, A. T.; Keil, F. J. Phys. Chem. C 2008, 112, 15402. (9) Pignataro, S.; Cassuto, A.; Lossing, F. P. J. Am. Chem. Soc. 1967, 89, 3693. (10) Fleming, D. G.; Arseneau, D. J.; Pan, J. J.; Shelley, M. Y.; Senba, M.; Percival, P. W. Appl. Magn. Reson. 1997, 13, 181. (11) Yu, D.; Percival, P. W.; Brodovitch, J.-C.; Leung, S.-K.; Kiefl, R. F.; Venkateswaran, K; Cox, S. F. J. Chem. Phys. 1990, 142, 229. (12) Werst, D. W.; Han, P.; Chousse, S. C.; Vinokur, E. I.; Xu, L.; Trifunic, A. D.; Erikson, L. A. J. Phys. Chem. B 1999, 103, 9219. (13) Sauer, J. Chem. Rev. 1989, 89, 199. (14) Li, S; Huang, S-J; Shen, W; Zhang, H; Fang, H; Zheng, A; Liu, S-B; Dong, F. J. Phys. Chem. C 2008, 112, 14486. (15) Fleming, D. G.; Shelley, M. Y.; Arseneau, D. J.; Senba, M; Pan, J. J.; Roduner, E. J. Phys. Chem. B 2002, 106, 6395. (16) Shelley, M.; Arseneau, D. J.; Senba, M.; Pan, J. J.; Snooks, R.; Kreitzman, S. R.; Fleming, D. G.; Roduner, E. Stud. Surf. Sci. Catal. 1994, 94, 469. (17) Roduner, E.; Stolmar, M.; Dilger, H.; Reid, I. D. J. Phys. Chem. A 1998, 102, 7591. (18) Roduner, E; H. Dilger, H. J. Am. Chem. Soc. 2001, 123, 7717. (19) Bridges, M. D.; Arseneau, D. J.; Fleming, D. G.; Ghandi, K. J. Phys. Chem. C 2007, 111, 9979. (20) Rhodes, C. J.; Dintinger, T. C.; Scott, C. A. Magn. Reson. Chem. 2000, 38, 62. Rhodes, C. J.; Butcher, E. C.; Morris, H; Reid, I. D. Magn. Reson. Chem. 1995, 33, 8134. (21) Beck, B. Ph.D. Thesis, Institut f€ur Physikalische Chemie, Universit€at Stuttgart, Stuttgart, Germany, 2003. (22) Reitmeier, S. J.; Mukti, R. R.; Jentys, A; Lercher, J. A. J. Phys. Chem. C 2008, 112, 2538. (23) Chmelka, B. F.; Pearson, J. G.; Liu, S. B.; Ryoo, R; Menorval, L. C.; Pines, A. J. Chem. Phys. 1991, 95, 303.

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