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2D NMR of C6 Hydrocarbon Molecules in K–L Zeolites
P.A. Jacobs and R.A. van Santcn (Editors), Zeolites: Fncts, Figira, F u w c I989 Elscvicr Science Publishers B.V., Anistcrdam - Printed in Thc Netherlands
615
2D NMR OF C6 HYDROCARBON MOLECULES IN K-L ZEOLITES B.G. SILBERNAGEL, A.R. GARCIA, R. HULME, AND J.M. NEWSAM Corporate Research, Exxon Research and Engineering Co., Route 22 East, Annandale, NJ 08801, U.S.A.
ABSTRACT Deuterium nuclear magnetic resonance (2D NMR) data for the perdeuterated C6 hydrocarbons benzene, n-hexane, and cyclohexane sorbed within potassium zeolite L (K-L), are discussed in the context of similar data for these molecules in other zeolites. Their motion has been studied using various pulse sequences as a function of loading level and temperature (125K T 350K). For C6D6, spinning in the molecular plane persists at all temperatures 2 120K and there is evidence for other restricted reorientations at higher temperatures. Similar behavior for benzene has been observed previously in other zeolites. For CoD14 in K-L, two inequivalent 2D absorptions are observed, corresponding to reductions of 1/9 and 1/27 of the static values of the quadrupole coupling constant for sp3 bonds. This averaging is attributed to "reptation" of the hexane molecules in the zeolite cages. A more complex form of partial averaging is seen for CBD12, leading to significant narrowing in the vicinity of 273K.
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INTRODUCTION Constrained environments can lead to restricted motion for molecular species. One useful probe for the study for such motions is deuterium NMR, which offers some specific advantages. Since 2D has relatively low isotopic abundance, deuterated species can be distinguished in materials containing high levels of protons, for example in the study of the inclusion of small molecules in coal (ref. 1). The strength of the deuterium quadrupolar interaction is small enough to be a perturbation on the Zeeman field, yet large enough to dominate other local deuterium interactions, such as dipole-dipole interactions. As a consequence, the NMR properties are determined largely by the interaction of the deuterium nuclear quadrupole moment with the electrostatic inhomogeneities associated with the C-D chemical bond, which simplifies the interpretation of the NMR spectra. Finally, molecular motion has the effect of narrowing the static quadrupole spectrum when the rate of the motion exceeds the strength of the quadrupole interaction, i.e. when 1 / 2~ 2m2qQ/h N 1 psec. Isotropic motion completely averages the interaction t o zero, but restricted motion leads t o a partial narrowing of the spectrum, the form of which depends upon the details of the motion (ref. 2). 2D NMR studies of hydrocarbon molecules within zeolites, although represented by a still rather limited literature, can perform a dually important role. First, the nature of zeolite-hydrocarbon interactions is of wide technological significance; 2D NMR measurements provides quantitative insight about these various interactions. Secondly, the regularity of zeolite structures is a major advantage for studies of the motional
616
characteristics of hydrocarbon molecules within constrained environments. The character of the motional constraints imposed by containment within the zeolite depends on the character of the pore space, the nature of the hydrocarbon, the hydrocarbon loading level, and measurement conditions such as temperature. In the present report, these dependencies are illustrated by 2D NMR data for three perdeuterated C6 hydrocarbons, benzene, n-hexane, and cyclohexane sorbed within potassium zeolite L. These data are contrasted, where appropriate, with data for benzene and n-hexane in other zeolite hosts. The picture of molecular motion provided by the 2D NMR results is strengthened and enhanced by results from complementary techniques such as powder neutron diffraction and molecular modeling (ref. 3), relevant results from which are also mentioned. Princides of ID NMR The presence of a relatively strong quadrupolar interaction leads to a different series of considerations than those normally encountered in proton or *3C NMR problems. Since the 2D nucleus has an intrinsic spin of I = 1, the resonance spectrum encountered in the study of powdered samples has a shape of the form shown in Fig. l b , with the splitting between the singularities (Ithorns") of the spectrum being related to the quadrupole coupling constant (eaqQ/h) as: TAH, = (eZqQ/h), < (3 cos28- 1) >a", (1) where AH, is the splitting between singularities of the 2D powder spectrum, -y is the nuclear gyromagnetic factor, and < ... >a" represents the time average of the angle between the direction of the applied magnetic field, H,, and the direction of the principal axis of the electric field gradient (usually the C-D bond direction). For a molecule reorienting about an axis g,application of the addition theorem of spherical harmonics leads to the following expression: <(3 cos2O - 1)> 3 (3 cos28(,, g ) - 1)/2 x (3 cos2q_w,Eo) - 11, (2) where e(g, g) and E,) represent the angles between the axis of rotation and the electric field gradient, and the axis of rotation and the applied magnetic field respectively. This result can be generalized to multiple axes of rotation (ref. 4). Examples of some typical cases are shown in (ref. 2). The static quadrupole interaction provides a qualitative measure of the molecular reorientation time, T . A more quantitative measure is provided by the determination of the spin-lattice relaxation time (TI) of the 2D nuclei. General considerations of spin-lattice relaxation theory demonstrate that quadrupole relaxation of an I = 1 nucleus leads to Bloch relaxation (ref. 5), with:
-
.
( W I ) = (37+/10) (ezqQ/h)a [ J(wo) + 4 J(2w0) I, (3) where w, is the Larmor frequency and J(w) is often assumed t o be Lorentzian in form: J(w) = T / ( 1 ~ 2 ~ 2 ) . (4) To within the approximations associated with modeling the spectral density (ref. 6), 7 can
+
617
be determined quantitatively from TI. EXPERIMENTAL CONDITIONS Samule PreDaration The preparation of samples for 2D NMR or powder neutron diffraction have been described previously (ref. 7). Briefly, a typical sample of potassium zeolite L, N ~ o . o ~ K o . ~Si26.70072 ~ A ~ ~ . ~nH20, o was dehydrated in air at ~400OCand loaded into tared vials which were crimp-sealed with aluminum-faced rubber septa. Appropriate volumes of perdeuterated hydrocarbons (from KOR isotope service, nominally: CaD6-99.96%D, C~D14-98%D, CaDlr99.5%D) were injected into the vials and the samples were homogenized prior to measurement. Loading levels were determined gravimetrically. PND data for various samples were collected on the powder diffractometer of the Missouri University Research Reactor facility and analyzed by Rietveld refinement. These measurements have already provided structural details for the parent dehydrated potassium zeolite L (ref. 8), and for the zeolite containing perdeuterobenzene at a loading level of one molecule per unit cell (ref. 9). Analyses of data from other zeolite L materials and for other zeolite L sorbate complexes are in progress. NMR Conditions The 2D NMR measurements were performed with a Bruker MSL Spectrometer system, operating at a frequency of 55.3 MHz. Temperatures ranging from 125K to 350K were obtained using a flowing gas cryostat. To achieve greatest fidelity of the spectral lineshape solid echo and saturation echo pulse sequences were employed. Spin-lattice relaxation times, TI, were measured using a saturating comb pulse sequence followed by a solid echo sequence for magnetization determination. EXPERIMENTAL DATA Benzene in K-L Typical ZD NMR spectra for a sample containing N2 molecules of CsD6 per channel lobe in the zeolite L structure (or, equivalently, per unit cell) at two different temperatures are shown in Fig. 1. At the lowest temperatures (Fig. lb), a well-articulated powder spectrum is observed with a splitting between the singularities of 69.58 kHz (ref. 7). The splitting translates into an effective quadrupolar coupling constant which is almost exactly half of the static value. The ZD NMR data thus indicate that the molecules are reorienting rapidly in the molecular plane (@, 9) = 900 in eq. 2). As discussed previously (refs. 7,9), this mode of motion is consistent with the benzene molecule location observed (at a loading of one molecule per unit cell) at 78K by PND (ref. 9). A narrow central component, of the type seen in Na-X zeolite at low C6D6loadings [0.7 of a possible 5.61 (ref. lo), is not observed under any of the loading and temperature ( T 3 350K) conditions studied to date. The 2D NMR spectra do, however, show a more subtle temperature dependence. First, although at 150K the full spectral splitting is almost exactly one half of the static value, a
618
small, but progressive reduction in the splitting between the "horns" accompanies temperature increases up to 350K. This reduced splitting from 125K to 250K suggests a small degree of rocking (em, N 60) of the benzene about the potassium cation to ring center vector (about which reorientations are rapid on the SD NMR timescale at all temperatures studied).
200kHz
Fig. 1. 2D NMR lineshapes for CoDn in K-L zeolite, a t a loading of 2 molecules per unit cell. Temperatures of observation are (a) 350K, (b) 150K. Note the rowth of intensity in the center of the resonance spectrum with increasing temperature. "fine structure" with a splitting equal to roughly half of the overall pattern, is observed at the highest temperatures.
x
Secondly, the center of the spectrum grows in intensity as the temperature is raised and, at the highest temperature studied (350K) discernible structure appears (Fig. la). This less broad component is most readily interpretable in terms of site exchange, primarily between the six equivalent benzene molecule locations in each channel lobe (ref. 9). The channel direction is parallel to an axis within the molecular plane, and, depending upon the exact orientation of the molecule with respect t o this axis, differing reductions in the effective quadrupolar coupling constants for subsets of the six benzene deuterons are expected. The broad width of the central component (Fig. l a ) is consistent with the presence of several such components. This type of spectral evolution for perdeuterobenzene has been observed in other systems such as Na-X and Na-Y at high loading levels (ref. ll), where a model involving combined molecule spinning and site exchange may also be most appropriate (ref. 11). The evolution of intensity in the spectral center can be traced by examining the ratio of the intensity of the line center to that of the singularities (Fig. 2). (Such an analysis is only intended to be illustrative, because the magnitude of the singularities depends sensitively on several factors, including the intrinsic broadening of the powder spectrum.) The increase in the relative proportion of the central component is minor for T < 250K, but grows dramatically at higher temperatures. The exponential increase is suggestive of a thermally activated process.
619
IOOOTT (K-' )
Fig. 2. Variation of the relative proportion of the central intensity of the resonance spectrum to that of the singularities. Note the dramatic increase which occurs for T > 250K.
A more quantitative estimate of the molecular dynamics is, as above, provided by the measurement of spin-lattice relaxation as a function of temperature. There is some ambiguity in the C6D6 case due to the width of the spectrum and the fact that the resonance line is inhomogeneously broadened. The recovery of the magnetization of the resonance line is exponential, but varies somewhat at different positions on the spectrum, presumably due to spectral spin diffusion effects. A plot of the logarithm of the average value of 1/T1 as a function of temperature (Fig. 3) shows the onset of a relaxation rate increase in the vicinity of 250K, the same region in which the increase in the intensity of the resonance center is The motional properties of benzene in potassium zeolite L in the range 150K < T < 350K can thus be interpreted in terms of reorientations in the molecular plane that have only a small activation barrier and which are therefore rapid. At progressively higher temperatures, an activated hopping between different (but crystallographically equivalent) sites develops, although without permitting isotropic molecule orientation. A similar pattern of motional behavior might be expected for benzene in other zeolite systems, and is observed to occur. 1.01
' . , . , . 2 3 4 5 6 7 8 9
-0.21
I
,
9
,
I
.
,
100OTT ( K - l )
Fig. 3. Variation of the logarithm of 1/T1 as a function of 1000/T for C6D6 in K-L. the increase in 1/Tl which occurs in the vicinity of 250K. for a t least the Na-X
Note
and Na-Y systems (refs. 10,ll). The librational freedom of the
620
preferred benzene site(s), and the activation barriers both t o reorientation and to site exchange are, however, expected to vary significantly from one system t o the next. n-Hexane The spectrum for CaD14 at 298K is shown in Fig. 4. As in the case of CeDa, there is no evidence for a narrow central signal which would indicate isotropic molecular motion. There are two well-defined components to the spectrum, with splittings that correspond to effective quadrupolar coupling constants of 7.20 kHz and 21.6 kHz. The ratio of the intensity of the narrow component to that of the broad component is approximately 3:4. As discussed previously (ref. 7), this behavior can be accounted for by assuming that the hexane molecules are reorienting about the C - C bond axes, in a motion commonly described as "reptation" by polymer scientists. In this case, each of the methylene deuterons (of which there are eight per hexane molecule) has two degrees of freedom for the reorientation process (ref. 6), while the terminal methyl deuterons (six per molecule) have three degrees of freedom as a consequence of the spinning of the methyl group. For each axis of rotation, there is a reduction factor of 1/3, corresponding to rotation about the tetrahedral bond angle (109.50). Thus the expected methylene and methyl quadrupole coupling reduction factors are 119 and 1/27 respectively. Since the accepted value for the static quadrupole interaction strength of C-D in a sp3 bond is 191.5 kHz (ref. 12), the expected values for the quadrupole couplings are 21.3 kHz and 7.09 kHz, respectively. The agreement with the observed values is excellent. Measurements of the temperature dependence of the spin-lattice relaxation time yield exponential magnetization recoveries. The T I %are thermally activated (ref. 7), with an activation energy of ~2 kcal/mole, comparable to activation energies previously reported for alkane bond rotations determined by microwave techniques (ref. 13).
I W.LMW
t
/@' 50kHz
I
Fig. 4. ZD NMR spectrum of C,$14 in K-L. The narrow and broad spectra observed are associated with methyl and methylene deuterons respectively. For any alkane in which deuteron (or proton) reorientation occurs via C-C bond torsional rotations, spectra essentially similar to Fig. 4 might be anticipated, with the
62 1
relative proportions of the two spectral components suitably weighted to allow for the relative numbers of methylene and methyl deuterons. Realization of this mode of motion at ambient temperature, however, apparently requires a relatively favorable zeolitealkane interaction geometry ar significant steric constraints on certain motions. Room temperature data for n-hexane in zeolite Z K J (with a pore structure of supercages linked through 8-ring windows), for example, shows an isotropic signal, somewhat broader than that of liquids at the same temperature. Cvclohexane Cyclohexane exhibits more complex behavior in its dynamical properties. Perhaps the most striking feature, shown in Fig. 5, is the fact that a relatively narrow ( N3 kHz) resonance line is observed at temperatures ~280K,while the line is broader at higher and lower temperatures (ref. 7). This behavior is unexpected, since it is generally observed that resonance lines n4wozu as the temperature is increased. Similar behavior is observed with samples containing one and two cyclohexane molecules per channel lobe, implying that the narrowing is not associated with loading effects. Measurements on perdeutero-methylcyclohexane over the same temperature range reveal that such narrowing does not occur, but rather that the 2D NMR spectrum broadens continuously on lowering the temperature from 350K. We attribute this difference in behavior to the influence of the methyl group on the activation barriers to ring conformation changes. We plan further investigations of the motion of cyclohexane in a series of other zeolite materials.
b
1 OOkHz
I
Fig. 5. Representative spectra for C6Dii near 280K show a narrowing of the resonance line at intermediate temperat ures.
DISCUSSION For CEDE,the molecular motion is dominated by spinning in the molecular plane (i.e. about the 6-fold symmetry axis. However at higher temperatures, and particularly above 250K other motions are operative. As previously observed (ref. 7), the geometry of the lobe in the zeolite L channel offers little hindrance to such spinning, given the location of the
622
C6D6 molecules above the potassium cation sites on the walls of the channel, as observed by PND (ref. 9). The increase in intensity of the central region of the powder spectrum can be explained by invoking an activated hopping of the C6D6 molecules between the cation sites on the channel walls. For CeD14, the absence of isotropic motion is apparently a consequence of the van der W a d s length of the hexane molecule being comparable to the dimensions of the channel lobe. While "reptation" is sterically allowed, the pore geometry inhibits tumbling of the long axis of the molecule. Isotropic averaging of the quadrupole interaction, such as observed in the compositionally related zeolite ZK-5, therefore does not occur. The CeDla dynamics are more complex. The key to interpreting the principal forms of motion in C6D6 and C6D14 is the fact that the observed effective quadrupole constants are simple ratios of the static quadrupole interaction strength: 1/2 for C6De and (1/3)n for CeD14. In C6D12, the motion of the direction of the electric field gradient is constrained by the cyclic nature of the molecule, and a detailed quantitative treatment of the narrowing effects expected for the various possible modes of molecular reorientation requires more sophisticated modeling. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13
B.G. Silbernagel, L.B. Ebert, R.H. Schlosberg, and R.B. Long, in M.L. Gorbaty and K . Ouchi (Editors), Coal Science, American Chemical Society, Washington, 1981, (Advances in Chemistry Series v. 192) pp. 23-35. B. Boddenberg, in G.R. Castro, M. Cardona (Editors), Lectures on Surface Science, Springer, Berlin, 1987, pp. 226-243. J.M. Newsam, B.G. Silbernagel, M.T. Melchior, T.O. Brun, and F. Trouw, in J.L. Atwood (Editor) Proceedings of the Vth International Symposium on Inclusion Phenomena and Molecular Recognition, Plenum, New York, 1989 (in press). See, e.g. J.G. Powles, and H.S. Gutowsky, J. Chem. Phys. 21 (1953) 1704-1709. A. Abragam, Principles of Nuclear Magnetism, Oxford U. Press, Oxford, 1961, p. 314. B.G. Silbernagel, Z. Physik. Chemie, Neue Folge 151 (1987) 85-92. B.G. Silbernagel, A.R. Garcia, J.M. Newsam, and R. Hulme, submitted for publication, 1988. J.M. Newsam, J. Chem. SOC.Chem. Comm. (1987) 123-124. J.M. Newsam, B.G. Silbernagel, A.R. Garcia, and R. Hulme, J. Chem. SOC.Chem. Comm. (1987) 664-666. D.L. Hasha, V.L. Miner, J.R. Garces, and S.C. Rocke, in M.L. Deviney and J.L. Gland (Editors), Catalyst Characterization Science, American Chemical Society, Washington, 1985, (ACS Symposium Series v. 288) pp. 4 8 5 4 9 7 . B. Zibrowius, J. Caro, and H. Pfeiffer, J. Chem. SOC. Faraday Trans. I, 84 (1988) 2347-2356.
S.C. Wofsy, J.S. Muenter, and W.J. Klemperer, J. Chem. Phys. 53 (1970) 40054014. See, e.g. C.H. Townes and A.L. Schawlow, Microwave Spectroscopy, Mc Graw-Hill, New York, 1955, p.323.
615
2D NMR OF C6 HYDROCARBON MOLECULES IN K-L ZEOLITES B.G. SILBERNAGEL, A.R. GARCIA, R. HULME, AND J.M. NEWSAM Corporate Research, Exxon Research and Engineering Co., Route 22 East, Annandale, NJ 08801, U.S.A.
ABSTRACT Deuterium nuclear magnetic resonance (2D NMR) data for the perdeuterated C6 hydrocarbons benzene, n-hexane, and cyclohexane sorbed within potassium zeolite L (K-L), are discussed in the context of similar data for these molecules in other zeolites. Their motion has been studied using various pulse sequences as a function of loading level and temperature (125K T 350K). For C6D6, spinning in the molecular plane persists at all temperatures 2 120K and there is evidence for other restricted reorientations at higher temperatures. Similar behavior for benzene has been observed previously in other zeolites. For CoD14 in K-L, two inequivalent 2D absorptions are observed, corresponding to reductions of 1/9 and 1/27 of the static values of the quadrupole coupling constant for sp3 bonds. This averaging is attributed to "reptation" of the hexane molecules in the zeolite cages. A more complex form of partial averaging is seen for CBD12, leading to significant narrowing in the vicinity of 273K.
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INTRODUCTION Constrained environments can lead to restricted motion for molecular species. One useful probe for the study for such motions is deuterium NMR, which offers some specific advantages. Since 2D has relatively low isotopic abundance, deuterated species can be distinguished in materials containing high levels of protons, for example in the study of the inclusion of small molecules in coal (ref. 1). The strength of the deuterium quadrupolar interaction is small enough to be a perturbation on the Zeeman field, yet large enough to dominate other local deuterium interactions, such as dipole-dipole interactions. As a consequence, the NMR properties are determined largely by the interaction of the deuterium nuclear quadrupole moment with the electrostatic inhomogeneities associated with the C-D chemical bond, which simplifies the interpretation of the NMR spectra. Finally, molecular motion has the effect of narrowing the static quadrupole spectrum when the rate of the motion exceeds the strength of the quadrupole interaction, i.e. when 1 / 2~ 2m2qQ/h N 1 psec. Isotropic motion completely averages the interaction t o zero, but restricted motion leads t o a partial narrowing of the spectrum, the form of which depends upon the details of the motion (ref. 2). 2D NMR studies of hydrocarbon molecules within zeolites, although represented by a still rather limited literature, can perform a dually important role. First, the nature of zeolite-hydrocarbon interactions is of wide technological significance; 2D NMR measurements provides quantitative insight about these various interactions. Secondly, the regularity of zeolite structures is a major advantage for studies of the motional
616
characteristics of hydrocarbon molecules within constrained environments. The character of the motional constraints imposed by containment within the zeolite depends on the character of the pore space, the nature of the hydrocarbon, the hydrocarbon loading level, and measurement conditions such as temperature. In the present report, these dependencies are illustrated by 2D NMR data for three perdeuterated C6 hydrocarbons, benzene, n-hexane, and cyclohexane sorbed within potassium zeolite L. These data are contrasted, where appropriate, with data for benzene and n-hexane in other zeolite hosts. The picture of molecular motion provided by the 2D NMR results is strengthened and enhanced by results from complementary techniques such as powder neutron diffraction and molecular modeling (ref. 3), relevant results from which are also mentioned. Princides of ID NMR The presence of a relatively strong quadrupolar interaction leads to a different series of considerations than those normally encountered in proton or *3C NMR problems. Since the 2D nucleus has an intrinsic spin of I = 1, the resonance spectrum encountered in the study of powdered samples has a shape of the form shown in Fig. l b , with the splitting between the singularities (Ithorns") of the spectrum being related to the quadrupole coupling constant (eaqQ/h) as: TAH, = (eZqQ/h), < (3 cos28- 1) >a", (1) where AH, is the splitting between singularities of the 2D powder spectrum, -y is the nuclear gyromagnetic factor, and < ... >a" represents the time average of the angle between the direction of the applied magnetic field, H,, and the direction of the principal axis of the electric field gradient (usually the C-D bond direction). For a molecule reorienting about an axis g,application of the addition theorem of spherical harmonics leads to the following expression: <(3 cos2O - 1)> 3 (3 cos28(,, g ) - 1)/2 x (3 cos2q_w,Eo) - 11, (2) where e(g, g) and E,) represent the angles between the axis of rotation and the electric field gradient, and the axis of rotation and the applied magnetic field respectively. This result can be generalized to multiple axes of rotation (ref. 4). Examples of some typical cases are shown in (ref. 2). The static quadrupole interaction provides a qualitative measure of the molecular reorientation time, T . A more quantitative measure is provided by the determination of the spin-lattice relaxation time (TI) of the 2D nuclei. General considerations of spin-lattice relaxation theory demonstrate that quadrupole relaxation of an I = 1 nucleus leads to Bloch relaxation (ref. 5), with:
-
.
( W I ) = (37+/10) (ezqQ/h)a [ J(wo) + 4 J(2w0) I, (3) where w, is the Larmor frequency and J(w) is often assumed t o be Lorentzian in form: J(w) = T / ( 1 ~ 2 ~ 2 ) . (4) To within the approximations associated with modeling the spectral density (ref. 6), 7 can
+
617
be determined quantitatively from TI. EXPERIMENTAL CONDITIONS Samule PreDaration The preparation of samples for 2D NMR or powder neutron diffraction have been described previously (ref. 7). Briefly, a typical sample of potassium zeolite L, N ~ o . o ~ K o . ~Si26.70072 ~ A ~ ~ . ~nH20, o was dehydrated in air at ~400OCand loaded into tared vials which were crimp-sealed with aluminum-faced rubber septa. Appropriate volumes of perdeuterated hydrocarbons (from KOR isotope service, nominally: CaD6-99.96%D, C~D14-98%D, CaDlr99.5%D) were injected into the vials and the samples were homogenized prior to measurement. Loading levels were determined gravimetrically. PND data for various samples were collected on the powder diffractometer of the Missouri University Research Reactor facility and analyzed by Rietveld refinement. These measurements have already provided structural details for the parent dehydrated potassium zeolite L (ref. 8), and for the zeolite containing perdeuterobenzene at a loading level of one molecule per unit cell (ref. 9). Analyses of data from other zeolite L materials and for other zeolite L sorbate complexes are in progress. NMR Conditions The 2D NMR measurements were performed with a Bruker MSL Spectrometer system, operating at a frequency of 55.3 MHz. Temperatures ranging from 125K to 350K were obtained using a flowing gas cryostat. To achieve greatest fidelity of the spectral lineshape solid echo and saturation echo pulse sequences were employed. Spin-lattice relaxation times, TI, were measured using a saturating comb pulse sequence followed by a solid echo sequence for magnetization determination. EXPERIMENTAL DATA Benzene in K-L Typical ZD NMR spectra for a sample containing N2 molecules of CsD6 per channel lobe in the zeolite L structure (or, equivalently, per unit cell) at two different temperatures are shown in Fig. 1. At the lowest temperatures (Fig. lb), a well-articulated powder spectrum is observed with a splitting between the singularities of 69.58 kHz (ref. 7). The splitting translates into an effective quadrupolar coupling constant which is almost exactly half of the static value. The ZD NMR data thus indicate that the molecules are reorienting rapidly in the molecular plane (@, 9) = 900 in eq. 2). As discussed previously (refs. 7,9), this mode of motion is consistent with the benzene molecule location observed (at a loading of one molecule per unit cell) at 78K by PND (ref. 9). A narrow central component, of the type seen in Na-X zeolite at low C6D6loadings [0.7 of a possible 5.61 (ref. lo), is not observed under any of the loading and temperature ( T 3 350K) conditions studied to date. The 2D NMR spectra do, however, show a more subtle temperature dependence. First, although at 150K the full spectral splitting is almost exactly one half of the static value, a
618
small, but progressive reduction in the splitting between the "horns" accompanies temperature increases up to 350K. This reduced splitting from 125K to 250K suggests a small degree of rocking (em, N 60) of the benzene about the potassium cation to ring center vector (about which reorientations are rapid on the SD NMR timescale at all temperatures studied).
200kHz
Fig. 1. 2D NMR lineshapes for CoDn in K-L zeolite, a t a loading of 2 molecules per unit cell. Temperatures of observation are (a) 350K, (b) 150K. Note the rowth of intensity in the center of the resonance spectrum with increasing temperature. "fine structure" with a splitting equal to roughly half of the overall pattern, is observed at the highest temperatures.
x
Secondly, the center of the spectrum grows in intensity as the temperature is raised and, at the highest temperature studied (350K) discernible structure appears (Fig. la). This less broad component is most readily interpretable in terms of site exchange, primarily between the six equivalent benzene molecule locations in each channel lobe (ref. 9). The channel direction is parallel to an axis within the molecular plane, and, depending upon the exact orientation of the molecule with respect t o this axis, differing reductions in the effective quadrupolar coupling constants for subsets of the six benzene deuterons are expected. The broad width of the central component (Fig. l a ) is consistent with the presence of several such components. This type of spectral evolution for perdeuterobenzene has been observed in other systems such as Na-X and Na-Y at high loading levels (ref. ll), where a model involving combined molecule spinning and site exchange may also be most appropriate (ref. 11). The evolution of intensity in the spectral center can be traced by examining the ratio of the intensity of the line center to that of the singularities (Fig. 2). (Such an analysis is only intended to be illustrative, because the magnitude of the singularities depends sensitively on several factors, including the intrinsic broadening of the powder spectrum.) The increase in the relative proportion of the central component is minor for T < 250K, but grows dramatically at higher temperatures. The exponential increase is suggestive of a thermally activated process.
619
IOOOTT (K-' )
Fig. 2. Variation of the relative proportion of the central intensity of the resonance spectrum to that of the singularities. Note the dramatic increase which occurs for T > 250K.
A more quantitative estimate of the molecular dynamics is, as above, provided by the measurement of spin-lattice relaxation as a function of temperature. There is some ambiguity in the C6D6 case due to the width of the spectrum and the fact that the resonance line is inhomogeneously broadened. The recovery of the magnetization of the resonance line is exponential, but varies somewhat at different positions on the spectrum, presumably due to spectral spin diffusion effects. A plot of the logarithm of the average value of 1/T1 as a function of temperature (Fig. 3) shows the onset of a relaxation rate increase in the vicinity of 250K, the same region in which the increase in the intensity of the resonance center is The motional properties of benzene in potassium zeolite L in the range 150K < T < 350K can thus be interpreted in terms of reorientations in the molecular plane that have only a small activation barrier and which are therefore rapid. At progressively higher temperatures, an activated hopping between different (but crystallographically equivalent) sites develops, although without permitting isotropic molecule orientation. A similar pattern of motional behavior might be expected for benzene in other zeolite systems, and is observed to occur. 1.01
' . , . , . 2 3 4 5 6 7 8 9
-0.21
I
,
9
,
I
.
,
100OTT ( K - l )
Fig. 3. Variation of the logarithm of 1/T1 as a function of 1000/T for C6D6 in K-L. the increase in 1/Tl which occurs in the vicinity of 250K. for a t least the Na-X
Note
and Na-Y systems (refs. 10,ll). The librational freedom of the
620
preferred benzene site(s), and the activation barriers both t o reorientation and to site exchange are, however, expected to vary significantly from one system t o the next. n-Hexane The spectrum for CaD14 at 298K is shown in Fig. 4. As in the case of CeDa, there is no evidence for a narrow central signal which would indicate isotropic molecular motion. There are two well-defined components to the spectrum, with splittings that correspond to effective quadrupolar coupling constants of 7.20 kHz and 21.6 kHz. The ratio of the intensity of the narrow component to that of the broad component is approximately 3:4. As discussed previously (ref. 7), this behavior can be accounted for by assuming that the hexane molecules are reorienting about the C - C bond axes, in a motion commonly described as "reptation" by polymer scientists. In this case, each of the methylene deuterons (of which there are eight per hexane molecule) has two degrees of freedom for the reorientation process (ref. 6), while the terminal methyl deuterons (six per molecule) have three degrees of freedom as a consequence of the spinning of the methyl group. For each axis of rotation, there is a reduction factor of 1/3, corresponding to rotation about the tetrahedral bond angle (109.50). Thus the expected methylene and methyl quadrupole coupling reduction factors are 119 and 1/27 respectively. Since the accepted value for the static quadrupole interaction strength of C-D in a sp3 bond is 191.5 kHz (ref. 12), the expected values for the quadrupole couplings are 21.3 kHz and 7.09 kHz, respectively. The agreement with the observed values is excellent. Measurements of the temperature dependence of the spin-lattice relaxation time yield exponential magnetization recoveries. The T I %are thermally activated (ref. 7), with an activation energy of ~2 kcal/mole, comparable to activation energies previously reported for alkane bond rotations determined by microwave techniques (ref. 13).
I W.LMW
t
/@' 50kHz
I
Fig. 4. ZD NMR spectrum of C,$14 in K-L. The narrow and broad spectra observed are associated with methyl and methylene deuterons respectively. For any alkane in which deuteron (or proton) reorientation occurs via C-C bond torsional rotations, spectra essentially similar to Fig. 4 might be anticipated, with the
62 1
relative proportions of the two spectral components suitably weighted to allow for the relative numbers of methylene and methyl deuterons. Realization of this mode of motion at ambient temperature, however, apparently requires a relatively favorable zeolitealkane interaction geometry ar significant steric constraints on certain motions. Room temperature data for n-hexane in zeolite Z K J (with a pore structure of supercages linked through 8-ring windows), for example, shows an isotropic signal, somewhat broader than that of liquids at the same temperature. Cvclohexane Cyclohexane exhibits more complex behavior in its dynamical properties. Perhaps the most striking feature, shown in Fig. 5, is the fact that a relatively narrow ( N3 kHz) resonance line is observed at temperatures ~280K,while the line is broader at higher and lower temperatures (ref. 7). This behavior is unexpected, since it is generally observed that resonance lines n4wozu as the temperature is increased. Similar behavior is observed with samples containing one and two cyclohexane molecules per channel lobe, implying that the narrowing is not associated with loading effects. Measurements on perdeutero-methylcyclohexane over the same temperature range reveal that such narrowing does not occur, but rather that the 2D NMR spectrum broadens continuously on lowering the temperature from 350K. We attribute this difference in behavior to the influence of the methyl group on the activation barriers to ring conformation changes. We plan further investigations of the motion of cyclohexane in a series of other zeolite materials.
b
1 OOkHz
I
Fig. 5. Representative spectra for C6Dii near 280K show a narrowing of the resonance line at intermediate temperat ures.
DISCUSSION For CEDE,the molecular motion is dominated by spinning in the molecular plane (i.e. about the 6-fold symmetry axis. However at higher temperatures, and particularly above 250K other motions are operative. As previously observed (ref. 7), the geometry of the lobe in the zeolite L channel offers little hindrance to such spinning, given the location of the
622
C6D6 molecules above the potassium cation sites on the walls of the channel, as observed by PND (ref. 9). The increase in intensity of the central region of the powder spectrum can be explained by invoking an activated hopping of the C6D6 molecules between the cation sites on the channel walls. For CeD14, the absence of isotropic motion is apparently a consequence of the van der W a d s length of the hexane molecule being comparable to the dimensions of the channel lobe. While "reptation" is sterically allowed, the pore geometry inhibits tumbling of the long axis of the molecule. Isotropic averaging of the quadrupole interaction, such as observed in the compositionally related zeolite ZK-5, therefore does not occur. The CeDla dynamics are more complex. The key to interpreting the principal forms of motion in C6D6 and C6D14 is the fact that the observed effective quadrupole constants are simple ratios of the static quadrupole interaction strength: 1/2 for C6De and (1/3)n for CeD14. In C6D12, the motion of the direction of the electric field gradient is constrained by the cyclic nature of the molecule, and a detailed quantitative treatment of the narrowing effects expected for the various possible modes of molecular reorientation requires more sophisticated modeling. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13
B.G. Silbernagel, L.B. Ebert, R.H. Schlosberg, and R.B. Long, in M.L. Gorbaty and K . Ouchi (Editors), Coal Science, American Chemical Society, Washington, 1981, (Advances in Chemistry Series v. 192) pp. 23-35. B. Boddenberg, in G.R. Castro, M. Cardona (Editors), Lectures on Surface Science, Springer, Berlin, 1987, pp. 226-243. J.M. Newsam, B.G. Silbernagel, M.T. Melchior, T.O. Brun, and F. Trouw, in J.L. Atwood (Editor) Proceedings of the Vth International Symposium on Inclusion Phenomena and Molecular Recognition, Plenum, New York, 1989 (in press). See, e.g. J.G. Powles, and H.S. Gutowsky, J. Chem. Phys. 21 (1953) 1704-1709. A. Abragam, Principles of Nuclear Magnetism, Oxford U. Press, Oxford, 1961, p. 314. B.G. Silbernagel, Z. Physik. Chemie, Neue Folge 151 (1987) 85-92. B.G. Silbernagel, A.R. Garcia, J.M. Newsam, and R. Hulme, submitted for publication, 1988. J.M. Newsam, J. Chem. SOC.Chem. Comm. (1987) 123-124. J.M. Newsam, B.G. Silbernagel, A.R. Garcia, and R. Hulme, J. Chem. SOC.Chem. Comm. (1987) 664-666. D.L. Hasha, V.L. Miner, J.R. Garces, and S.C. Rocke, in M.L. Deviney and J.L. Gland (Editors), Catalyst Characterization Science, American Chemical Society, Washington, 1985, (ACS Symposium Series v. 288) pp. 4 8 5 4 9 7 . B. Zibrowius, J. Caro, and H. Pfeiffer, J. Chem. SOC. Faraday Trans. I, 84 (1988) 2347-2356.
S.C. Wofsy, J.S. Muenter, and W.J. Klemperer, J. Chem. Phys. 53 (1970) 40054014. See, e.g. C.H. Townes and A.L. Schawlow, Microwave Spectroscopy, Mc Graw-Hill, New York, 1955, p.323.