Spin Exchange Optical Pumping Enhanced 129Xe NMR Spectroscopy

Feb 21, 2002 - Department of Chemistry and National High Magnetic Field Laboratory, University of Florida, Gainesville, Florida 32611-7200 ... The spe...
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J. Phys. Chem. B 2002, 106, 2884-2891

Spin Exchange Optical Pumping Enhanced 129Xe NMR Spectroscopy of SF6/Xe and Acetone-d6/Xe Mixed Type-II Clathrate Hydrates Vincent J. Storhaug, Florian Liebig, and Clifford R. Bowers* Department of Chemistry and National High Magnetic Field Laboratory, UniVersity of Florida, GainesVille, Florida 32611-7200 ReceiVed: August 24, 2001; In Final Form: December 27, 2001

Spin exchange optical pumping enhanced 129Xe NMR is applied to the study of SF6/Xe and acetone/Xe mixed type-II clathrate hydrate formation at 223 K. The highly polarized (i.e., hyperpolarized) 129Xe gas was reacted with the type-II structure that had been preformed by reaction of SF6 or acetone-d6 with solid ice at 205 K. At 223 K, enclathration into the preformed type-II structure was observed to occur on the time-scale of several minutes. The spectra show that Xe is excluded from the larger hexakaidecahedral cavities, and that the degree of exclusion appears to be more complete in the case of acetone-d6. A comparison of the 129Xe NMR line shapes in the acetone-d6/Xe and SF6/Xe mixed clathrate hydrates at 223 K reveals a reduction of symmetry of the dodecahedral cavities in the latter, apparently due to differences in the water reorientation dynamics.

1. Introduction Clathrate hydrates are crystalline, nonstiochiometric compounds in which hydrogen-bonded water molecules assemble to form cavities occupied by small guest atoms or molecules.1-8 X-ray diffraction has shown that most clathrate hydrates have either of two cubic structures, referred to as type-I and type-II. The type-I clathrate hydrate has a unit cell edge dimension of 12.0 Å and contains 46 host water molecules, arranged to form two dodecahedral cavities (512) and six tetrakaidecahedral cavities (51262). The cavities of the type-I hydrate structure will accommodate guest molecules with van der Waals diameters up to 5.9 Å, but for guests with larger diameters, the type-II clathrate hydrate forms. The type-II clathrate hydrate unit cell contains 136 host water molecules, arranged to form sixteen dodecahedral cavities and eight hexakaidecahedral cavities (51264). Guest molecules with diameters between 5.9 and 7.0 Å will occupy only the 51264 cavities, forming a stable type-II structure with vacant 512 cavities. Guests larger than 7.0 Å may still form the type-II hydrate, but usually require the presence of a small guest, termed the help gas, to stabilize the clathrate structure by occupying 512 cavities. The average radii and symmetries of the cavities influence the motions as well as the electronic environments of guest molecules, and consequently, NMR spectral parameters such as chemical shifts, spin relaxation times, and quadrupolar couplings can be used to characterize the guest-host lattice interactions.9-16 For example, the 13C MAS NMR spectrum of sediment samples obtained from the Gulf of Mexico showed two resonance lines for methane occupying small and large cavities in a clathrate hydrate structure.9 By comparing the 13C NMR spectrum of the sediment sample with one of the type-I methane clathrate hydrates, Ripmeester and Ratcliffe9 showed that the hydrate was of the type-II variety, in accordance with X-ray powder diffraction. They proposed that this hydrate had formed by the enclathration of propane solely into the 51264 cavities, and that methane was confined primarily to the 512 cavities.

The NMR properties of 129Xe are attractive for the study of clathrates and micro/nanoporous materials in general. For instance, the 129Xe chemical shift is more sensitive than 13C in methane to the cavity size and shape. The resonance lines for 129Xe as the guest in the small and large cavities of the type-I Xe deuteriohydrate are well resolved, with a separation of 91 ppm.4,9-11,17 In type-I, the appearance of a 129Xe chemical shift powder pattern in the 51262 cavities reveals axial symmetry, whereas the symmetrical Gaussian line shape in the smaller 512 cavities is indicative of an undistorted cubic symmetry (refer to Figures 6 and 7). Notwithstanding the advantages of 129Xe as a probe of local structure in clathrate studies, there is a serious practical difficulty owing to the long spin-lattice relaxation times that are typically encountered with this spin-1/2 noble gas. The 129Xe T1 in the deuteriohydrate is on the order of several tens of minutes, so achieving adequate signal-to-noise in the NMR spectrum of thermally polarized 129Xe involves lengthy acquisition times. To address this problem, 1H-129Xe Harmann-Hahn cross polarization (CP) with proton decoupling has been employed in 129Xe NMR studies of Xe hydrate.4,12 Yet even with CP, the NMR acquisition times are still too long to permit time-resolved 129Xe NMR on the time-scale of formation of Xe clathrate hydrates at temperatures above about 173 K. A further limitation is that the CP method cannot be employed to quantify the cavity occupancy ratios because the signal integrals depend on the CP dynamics and other factors such as motion, translational and spin diffusion, the number of neighboring water protons, and the magnitude of the dipolar couplings of the water protons to the guest and each other.18 The advent of spin-exchange optical pumping to produce highly nuclear spin polarized 129Xe (i.e., hyperpolarized 129Xe)19-22 has extended the applicability of high field 129Xe NMR spectroscopy to a wide range of solid-state materials, including polymers,23 zeolites,24 nanocrystals,25 and proteins.26-28 Pietrass et al.29 first demonstrated that spin exchange optical pumping could be used to enhance the 129Xe NMR signals of the type-I hydrate, and this method was later employed to

10.1021/jp0155260 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002

Mixed Type-II Clathrate Hydrates

J. Phys. Chem. B, Vol. 106, No. 11, 2002 2885

Figure 1. Diagram of the vacuum manifold used to prepare the typeII clathrate hydrate samples. (a) 10 mm medium wall (8 mm inner diameter) NMR tube with Teflon valve (b) sample spinner (c) pulverized D2O solid (d) reservoir containing the type-II former (acetone-d6 or SF6). The type-II former is reacted with the ice for a period of 30-45 min at a temperature of 205 K. The tube is then immersed in liquid nitrogen and the hyperpolarized Xe gas is condensed onto the solid D2O. The tube is held at 77 K in a magnetic field of 0.15 T to preserve the Xe polarization during transport to the NMR spectrometer.

monitor the formation of the Xe clathrate hydrate in real-time.30 With hyperpolarized 129Xe NMR, a spectrum with high signalto-noise can be acquired in a single scan as demonstrated in Figure 2, due to ∼103-104 fold signal enhancement. There is minimal depletion of the reservoir of 129Xe polarization if small tip angles are employed, permitting multiple spectra to be acquired in an in situ stopped-flow style kinetics experiment using a single batch of spin polarized 129Xe gas. This circumvents the necessity to use long recycle delays to accommodate spin lattice relaxation between transient acquisitions, as in conventional pulsed NMR derived from the thermal polarization of nuclear spins. Here, we demonstrate the application of hyperpolarized 129Xe NMR in two different mixed clathrate hydrates exhibiting typeII structures. The results provide information about (1) the cavity occupancies of these nonstoichiometric compounds formed on the surface of ice, (2) the time-scales of formation, (3) cavityguest interactions and (4) water reorientation dynamics. Timeresolved 129Xe NMR spectra were acquired for the type-II SF6/ Xe and the acetone-d6/Xe deuteriohydrates formed at 223 K. Under our experimental conditions, it was found that the formation occurs on the time-scale of minutes. 2. Experimental Methods Preparation of D2O Ice. For each sample, approximately 1.2 g D2O (99.9+% purity, Cambridge Isotope Laboratories) was frozen in liquid N2 and pulverized into a fine powder with a mortar and pestle at 77 K. The ice was then transferred to a 10 mm o.d. medium wall (8 mm i.d.) NMR tube with a total volume of 16.0 ( 0.2 cm3. The tube was immersed in a dry ice/isopropyl alcohol bath at 205 K and evacuated to a pressure of 10-5-10-6 Torr. Formation of the Type-I Xenon Clathrate Deuteriohydrate. For reasons that will be given below, the reactions of Xe with the preformed type-II clathrates hydrates were performed on ice that was subjected to the following pretreatment: the type-I Xe clathrate hydrate was formed by the reaction of approximately 1000 Torr (at 298 K) of hyperpolarized Xe with freshly pulverized ice in a sealed 10 mm NMR tube. The formation reaction was monitored by hyperpolarized 129Xe

Figure 2. (a) Hyperpolarized 129Xe NMR spectra acquired as a function of time following insertion of the NMR tube containing pulverized D2O ice and hyperpolarized 129Xe into the NMR probe at 223 K. (b) Summary plots of the hyperpolarized 129Xe NMR integrals for the 512, and 51262 peaks of the type-I clathrate hydrate and the gas peak. (c) Individual spectra selected from part (a) acquired at the reaction times indicated by the arrows labeled “i” and “ii”. The integrals of the 51262 and 512 peaks show that the occupancy ratio increases from a minimum of 3.1 at the earliest reaction time that yielded adequate signal to a steady-state value of 3.9.

NMR. After this pretreatment reaction was complete, the sample was removed from the spectrometer and evacuated at a temperature of 205 K until the pressure stabilized below 10-5 Torr. Formation of the Type-II SF6/Xe Clathrate Deuteriohydrate. The apparatus shown in Figure 1 was used to condense a known quantity of SF6 gas into the sample tube containing the pretreated ice at 77 K. The calculated final SF6 pressure after sublimation is 2.36 atm at 298 K (not including the reduction in pressure due to enclathration). The SF6/D2O ice compound was maintained at 205 K for a period of 30-45 min. At this temperature, the vapor pressure above solid SF6 is about 400 Torr.31 Just prior to condensing the hyperpolarized 129Xe onto the powdered ice, the sample tube was removed from the dry ice/isopropyl alcohol bath and immersed in liquid nitrogen. To preserve the 129Xe polarization during its condensation and transit to the NMR magnet, the sample was held in the magnetic field of a 0.15 T permanent magnet assembly. The sample was

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Figure 3. Hyperpolarized 129Xe NMR spectra (110.7 MHz) of the SF6/ Xe type-II clathrate deuteriohydrate at 223 K. The type-II structure was preformed in the absence of Xe by reaction with SF6 gas at two different pressures over a period of 30-45 min at 205 K. The spectra were recorded following a 240 s exposure to polarized Xe at 223 K. (a) 1.0 × 103 Torr SF6, 10 µs pulse (30°); (b) 3.5 × 103 Torr SF6, 30 µs pulse (90°). The calculated pressure of the Xe gas immediately following sublimation is 1.0 × 103 Torr. The chemical shift scale is referenced to the gas signal at 0 ppm. A signal due to Xe trapped in 51264 cavities is observed at approximately 70 ppm.

Storhaug et al.

Figure 5. Comparison of the hyperpolarized 129Xe NMR spectra (110.7 MHz) of the 512 cavity signals in the (a) acetone-d6/Xe and the (b) SF6/Xe type-II mixed clathrate deuteriohydrates recorded at 223 K. There is an observable CSA of ∆δ ≈ -19 ppm in the case of acetoned6. Chemical shielding parameters and line widths are summarized in Table 1.

Figure 4. Hyperpolarized 129Xe NMR spectrum (110.7 MHz) of the acetone-d6/Xe type-II clathrate deuteriohydrate at 223 K. The spectrum was recorded following a 240 s exposure to polarized Xe at 223 K with a 15 µs (45°) pulse. The calculated pressure of the Xe gas immediately following sublimation is 1.0 × 103 Torr. The asterisk indicates the peak due to Xe dissolved in liquid acetone-d6. The chemical shift scale is referenced to the gas signal at 0 ppm.

then removed from the permanent magnet assembly and quickly inserted into the 10 mm high-resolution NMR probe pre-cooled to 223 K. Xenon-129 spectra were acquired at 110.7 MHz on a Bruker Avance 400 MHz spectrometer. A series of free induction decays were recorded using 24°, 8 µs pulses to monitor the formation of the SF6/Xe clathrate hydrate. Acetone-d6/Xe Type-II Clathrate Hydrate. A 1.2 g sample of pulverized D2O ice was subjected to the type-I pretreatment described above. A volume of 0.3-0.4 mL of acetone-d6 (100% pure, Sigma-Aldrich) was introduced to the sample tube containing the ice at a temperature of 205 K by distillation from an attached reservoir of liquid. The acetone-d6/D2O (ice) mixture was maintained at 205 K for a period of 30-45 min during which the hyperpolarized 129Xe gas was prepared by spin exchange optical pumping. It should be noted that the vapor pressure of acetone at 205 K is < 1 Torr. The sample tube was then immersed in liquid nitrogen and hyperpolarized 129Xe was condensed into the sample tube as described above. After insertion into the Bruker NMR spectrometer, a series of free induction decays were recorded using 45°, 15 µs pulses to follow the Xe/acetone-d6 clathrate hydrate formation. Spin Exchange Optical Pumping of 129Xe. Approximately 200-250 Torr of natural abundance Xe gas was combined with

Figure 6. 138.2 MHz hyperpolarized 129Xe NMR spectra of the type-I Xe clathrate hydrate acquired at a series of temperatures, as indicated. The clathrate was initially formed at 233 K and spectra were subsequently recorded after each temperature jump. Two batches of hyperpolarized Xe were required to obtain spectra over the entire temperature range while maintaining acceptable signal intensities. At least 420 s was allowed prior to acquisition of the free induction decay with a single 6 µs pulse which corresponds to a tip angle of 20°. The sharp peak at about 186 ppm corresponds to 129Xe dissolved in liquid D2O. The line width of the 512 peak increased from 1.0 to 2.1 kHz when the temperature was reduced from 273 to 203 K due to freezing out of the water disorder.

Rb metal droplets in a cylindrical borosilicate glass pumping cell with a volume of 77.6 cm3. The optical pumping cell was maintained at 373-383 K in a magnetic field of approximately 100 G. The Rb vapor was excited with circularly polarized light (1.75 W) at the wavelength corresponding to the D1 line (794.7 nm). The excitation beam was produced by an argon-ion (Coherent I-200) pumped titanium-sapphire (Coherent 899) ringlaser. The 129Xe was polarized over a period of 30-45 min. Prior to transferring the Xe gas from the pumping cell to the sample tube, the cell temperature was reduced to condense the Rb.

Mixed Type-II Clathrate Hydrates

Figure 7. Expansion of the hyperpolarized 129Xe spectrum about the 51262 peak at 263 K. The solid line represents the least-squares fit of the experimental line to a single chemical shift powder pattern with a cyclindrically symmetric shielding tensor, yielding δiso ) 148 ppm, ∆δ ) - 29.1 ppm, and a Gaussian line broadening of 1400 Hz.

Acquisition of 19F NMR Spectra. Fluorine-19 NMR spectra were acquired on a Varian Innova 500 MHz spectrometer equipped with a 10 mm high resolution double-resonance probe. The probe was maintained at 223 K and tuned to the 19F NMR detection frequency of 470.1 MHz. A series of free induction decays were recorded using 90°, 22.5 µs pulses, with a recycle time of 4s to allow complete spin-lattice relaxation. 3. Results and Discussion 129Xe

NMR Spectra of Mixed Type-I Clathrate Hydrate. When freshly prepared ice surfaces are exposed to a gaseous type-I hydrate former such as Xe, there is an initial induction period during which the reaction proceeds at a relatively slow rate. This induction period is indicative of a crystallite nucleation or surface structure rearrangement process that is initially rate limiting, although a definitive mechanism has yet to be elucidated. Whatever the nature of this structural rearrangement, a residual template of the formation of the clathrate hydrate persists even after evacuation of the guest gas.32,33 In Xe gas uptake experiments conducted at 220 K, the induction period is practically eliminated if the Xe clathrate hydrate sample is evacuated and then reexposed to Xe. It has been proposed that small cavity structures remain intact, serving as preformed nucleation sites when the ice is re-exposed to Xe or CH4.3 In our hyperpolarized 129Xe NMR experiments, we consistently observed a higher initial reaction rate and more intense NMR signals for the enclathrated 129Xe when repeating the experiment with “recycled” ice. It appears that this type-I pretreatment also provides nucleation sites for the formation of the type-II SF6/ Xe mixed clathrate hydrates, a finding that we have exploited in order to maximize the hyperpolarized 129Xe NMR signals in these compounds. A stacked plot of the hyperpolarized 129Xe NMR spectra obtained at 15-s intervals following insertion of the NMR tube containing solid hyperpolarized 129Xe and freshly pulverized D2O ice at 233 K is shown in Figure 2a, and the signal integrals of each NMR peak are summarized in Figure 2b. The spectra labeled i and ii presented in Figure 2c were recorded at the times indicated by the arrows in Figure 2b during the formation. The initial growth of the 129Xe gas NMR peak due to the sublimation of the solid Xe is followed by a monotonic decrease in intensity with time due to the uptake of the Xe by the ice. Simultaneously, 129Xe signals of the large and small cages grow in. These selected single-scan spectra at positions i and ii have

J. Phys. Chem. B, Vol. 106, No. 11, 2002 2887 the requisite signal-to-noise for accurate integration of the peak areas. The integrals of the 51262 and 512 peaks show that the occupancy ratio increases from a minimum of 3.1:1 (measured at the earliest reaction time that yielded adequate signal) to a steady-state value of 3.9:1. 129Xe NMR Spectra of Mixed Type-II Clathrate Hydrates. Hyperpolarized 129Xe NMR spectra of the SF6/Xe mixed clathrate hydrate after reaction for approximately 240 s at 223 K are shown in Figure 3a,b. The spectra were obtained with 1000 Torr and 3500 Torr initial pressures of SF6, respectively. The resonance peak at 66.5 ppm relative to the Xe gas reference is associated with Xe enclathrated in the 51264 cavities, whereas the 129Xe resonance attributed to the 512 cavities appears at 233 ppm with a Gaussian line shape. The observed ratio of the 512 signal to the 51264 signal is 40:1 and 20:1 in Figure 3a and 3b, respectively. Recall that the ratio in the number of 512 to 51264 cavities is 2:1 in the fully occupied type-II unit cell. This is clear evidence that only type-II material has formed and that Xe has been almost completely displaced from the 51264 cavities. Furthermore, the pressure dependence of the ratio suggests that the extent of exclusion is reduced at higher SF6 pressure. An alternate explanation for the observed suppression of the large cage peak signal is that the large cages remain occupied by residual nonpolarized Xe that remains following the pretreatment procedure in which the type-I Xe hydrate was formed. This can be ruled out on the basis of the following three independent observations. First, the amount of residual Xe gas that remains after evacuation was found to be only a few percent of the enclathrated Xe, as determined by measuring the pressure after melting the solid ice following gas uptake trials. Hence, there is a negligible amount of residual unpolarized Xe left over from the pretreatment of the ice. Second, the 51262:512 occupancy ratio of the type-I formation on freshly pulverized ice was found to be within experimental error the same as the ratio obtained for type-I formation on pretreated ice. Finally, experiments were performed whereby a sample of the type-II mixed clathrate was formed by the procedure described above, evacuated, and then re-reacted with hyperpolarized Xe. This resulted in a type-I spectrum with the same cage occupancy ratio as obtained in freshly pulverized ice. Therefore, residual, unpolarized Xe plays no role in displacing polarized Xe from the large cages, and the observed signal ratios lead to the conclusion that the typeII material has formed exclusively, with Xe displaced from the large cages. The hyperpolarized 129Xe NMR spectrum of the acetone-d6/ Xe mixed hydrate at 223 K is shown in Figure 4. The small sharp peak observed at 198 ppm corresponds to 129Xe dissolved in liquid acetone. A resonance peak that would correspond to 129Xe enclathrated into 51264 cavities is not visible in this spectrum or any other acetone-d6/Xe hydrate spectra that were acquired, suggesting that the occupancy of the 51264 cavities is more complete for acetone that for SF6. The 129Xe resonance in the 512 cavities appears at δiso ) 230.5 ppm, a shift that is within experimental error the same as in a previous report for several other mixed type-II compounds.16 Fitting of the line shape to a chemical shift powder pattern using a cylindrically symmetric shielding tensor yields a good fit with a chemical shielding anisotropy (CSA) of ∆δ ) δ| - δiso ) -19.4 ppm, a value slightly larger in magnitude than the 129Xe CSA values of -16.6 and -15.7 ppm that have been previously reported with tetrahydrofuran (THF) and 1,1,1 dichlorofluoroethane (DCFE), respectively, occupying the 51264 sites.16 Differences in the observed 129Xe line shape with acetone, THF or TCFE occupying the 51264 cavities versus SF6 occupying

2888 J. Phys. Chem. B, Vol. 106, No. 11, 2002 the 51264 cavities can be attributed to differences in the water reoreientation dynamics in these compounds. For example, the 129Xe NMR line shapes in the SF and acetone-d mixed 6 6 hydrates are directly compared in Figure 5. Note that the activation energy, Ea, for the reorientation of water in the typeII acetone hydrate is 6.5 kcal mol-1, whereas in the SF6 hydrate, Ea ) 12.3 kcal mol-1. At 233.2 K the dielectric relaxation times of water in the acetone and SF6 type-II hydrates are 0.57 s and 80 s, respectively,15 indicating that the water reorientation rate is significantly greater in the case of acetone. At 223 K, fast reorientation motion of the water in the acetone hydrate results in an average environment characteristic of the true crystallographic symmetry, whereas the slow water dynamics in the SF6 hydrate would yield a reduced local symmetry due to freezing in of the water disorder. Thus, the 129Xe line shape in the SF6 hydrate represents a superposition of cage environments that are effectively static on the NMR time-scale. The effect of water dynamics on the 129Xe NMR line shape was originally reported in the context of the large cages in the type-I Xe hydrate, where the broadening of the CSA line shape at low temperature (77 K) was attributed to the freezing out of the 6-fold disorder in the orientations of each water molecule.10,12 To illustrate how the water reorientation dynamics can be studied in detail by hyperpolarized 129Xe NMR, Figure 6 presents the 138.2 MHz 129Xe NMR spectra of the type-I Xe clathrate hydrate at a series of temperatures ranging from 203 K to above 300 K. To obtain this series of spectra at different temperatures, the Xe hydrate clathrate was initially formed at 233 K, and spectra were subsequently recorded after each temperatures jump. Two exposures of the ice to hyperpolarized 129Xe were necessary to cover the temperature range while maintaining acceptable signal intensities. A period of at least 420 s was allowed at each temperature for sample equilibration prior to acquisition of the free induction decay with a single 6 µs pulse (tip angle, 20°). The variation of the isotropic shifts over this range of temperatures, as referenced to the gas peak, was found to be negligible. However, substantial changes in the broadening and line shape of both the small and large cage line shapes are clearly apparent. Attempts to fit the line shape of Xe enclathrated into the large cavity to a single chemical shift powder pattern (η ) 0 or otherwise) did not yield a good match at temperatures below 253 K, where line broadening obscures the CSA. Good correspondence of the theoretical fit to the observed CSA was obtained at temperatures of 253 K or greater, as shown in Figure 7. At these higher temperatures, the 129Xe NMR line shape from Xe in the large cavity is well described by a single cylindrically symmetric chemical shielding powder pattern. The principal values of the chemical shielding tensor were determined to be δiso ) 147.6 ppm and ∆δ ) - 29.1 (2 ppm with respect to the 129Xe gas signal, in agreement with the original report of ∆δ ) - 32 (3 ppm.10 The best fits were obtained with a Gaussian line broadening of 1400 Hz. At temperatures lower than about 253 K a satistfactory fit to the line shape cannot be obtained with a single chemical shift powder pattern because the spectrum results from the statistical superposition of spectra resulting from all possible configurations of the water molecules. Over the temperature range from 203 to 273 K, the line width of the small cage doubles from 1.0 to 2.1 kHz and is well described as Gaussian. The chemical shielding parameters for the type-I Xe, the type-II SF6/Xe and type-II acetone-d6/Xe deuteriohydrates are summarized in Table 1.

Storhaug et al. TABLE 1: Summary of 129Xe NMR Chemical Shielding Parameters ((2 ppm Error), Referenced to the Gaseous 129Xe Signal, in the Type-I Xe Deuteriohydrate and the Type-II SF6/Xe and Acetone-d6/Xe Deuteriohydrates. Note: ∆δ ) δiso - δzz Deuteriohydrate

δiso(512), ppm

∆δ(512) or fwhm, ppm

type-I Xea type-II SF6b type-II Acetoneb

239 233 231

Gaussian, 8.7 Gaussian, 26.4 -19.4

a

δ(51264) or ∆δ(51264) or δ(51262), ppm ∆δ(51262), ppm 148 66.5

- 29.1 Gaussian, 2.36

T ) 263 K, 138.2 MHz. b T ) 223 K, 110.7 MHz.

Figure 8. Time-resolved hyperpolarized 129Xe NMR spectra (110.7 MHz) following the in situ formation of the acetone-d6/Xe clathrate hydrates. The reaction was run at 223 K with 0.4 mL acetone-d6, and at a calculated initial Xe pressure of 1.5 × 103 Torr. The free induction decay signals were acquired using 15 µs pulses (45°) at 12 s intervals, with the origin of time being defined as the time of insertion of the sample into the NMR magnet.

Time Dependence of the Formation of the Type-II Mixed Clathrate Hydrates. The use of hyperpolarized 129Xe NMR has facilitated real-time observation of the formation of both the SF6/Xe and acetone-d6/Xe deuteriohydrates on the surfaces of powdered ice crystals. Figure 8 shows individual single-shot 129Xe NMR spectra of the acetone-d /Xe deuteriohydrate 6 recorded at the times indicated. The resonance at 198 ppm corresponds to 129Xe dissolved in acetone. As is evident from the summary plot of Figure 9b, this signal decays as 129Xe either relaxes or leaves this phase to become incorporated into 512 cavities. At longer reaction times, a decay of the gaseous 129Xe is observed. Figure 9a,b presents the magnetization decay of 129Xe in the 512 cavities for both the SF /Xe and acetone-d /Xe 6 6 deuteriohydrates. The curves have been normalized at the peak of their signal integrals to facilitate direct comparison of the signal decays. The single-shot acetone-d6/Xe hydrate NMR spectra were acquired every 12 s with 45° rf pulses, whereas the SF6/Xe hydrate spectra were acquired with 23° rf pulses every 20 s. Notice that even with more frequent pulsing with a greater flip angle, the 129Xe signal decays more slowly in the case of the acetone-d6 clathrate hydrate. The factors affecting the observed time dependence of the 129Xe signal amplitudes include the rate of Xe enclathration, the spin-lattice relaxation times in the cavities or on the ice surface, magnetization destruction due to the application of successive rf pulses, and possibly the chemical exchange between the gaseous and clathrate phases.

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Figure 10. Fluorine-19 NMR spectra (470.1 MHz) of (a) gaseous SF6, (b) solid SF6, and (c) SF6 enclathrated into the deuteriohydrate at chemical and thermal equilibrium at 223 K. All spectra were signal averaged over 8 transients, acquired with 90° (22.5 µs) pulses and a recycle delay of 4 s.

129

Figure 9. Time dependence of the hyperpolarized Xe NMR absorption signal integrals acquired during the in situ formation of the type-II deuteriohydrate of SF6/Xe and acetone-d6/Xe. The Xe is introduced by sublimation as the sample tube warmed. Thermal equilibrium is estimated to have been achieved within 60-90 s after insertion into the temperature controlled NMR probe. Spectra were recorded immediately following insertion. The data have been normalized to the area of the maximum observed 512-cavity signal, and the origin of time has been shifted to coincide with the time at which the maximum observed 512-cavity signal was obtained. In each plot, the function S(tn) ) cosnθ, as described in the text, is plotted as a solid curve. (a) The SF6/Xe type-II deuteriohydrate was formed at 223 K, with a calculated initial SF6 pressure of 1.6 × 103 Torr (at 298 K) and Xe gas pressure of 1.5 × 103 Torr. Spectra were acquire every 20 s with an 8 µs (θ ) 23°) pulse. (b) The acetone-d6/Xe deuteriohydrate was formed at 223 K. The sample tube initially contained 0.4 mL acetone-d6, and a calculated initial pressure of 1500 Torr spin polarized Xe gas. Spectra were acquired every 12 s with a 15 µs (θ ) 45°) pulse. The Xe gas signal and Xe in acetone signal integrals are also plotted.

The solid curves of Figure 9a and 9b represent the calculated loss of magnetization due only to the successive rf pulses for the SF6/Xe hydrate and the acetone-d6/Xe hydrate. The experimental time dependence of the 129Xe NMR signal for the SF6/ Xe deuteriohydrate matches well with the calculated curve, suggesting that (1) the hydrate has been fully formed within the initial 240 s of the experiment, and (2) the decay of 129Xe magnetization in the hydrate is dominated by the destruction of magnetization by repeated pulsing as opposed to 129Xe spin lattice relaxation. The signal follows the S(tn) ) Scosnθ dependence where θ ) 23° and n is the number of pulses applied. In contrast, the experimental NMR signal time dependence in the acetone-d6/Xe hydrate exhibits a substantial deviation from the calculated curve representing depletion of magnetization by rf pulses. One cannot rule out the possibility that there may be chemical exchange between Xe in the gas

phase and enclathrated Xe. Because the gaseous Xe occupies the entire volume of the NMR tube and not only the region within the rf coil, the destruction of the gaseous Xe magnetization due to rf pulses would be mitigated. However, it also appears that the formation of acetone/Xe hydrate continues to occur at times well beyond 240 s. Note that the sharp peak at 198 ppm due to 129Xe dissolved in acetone initially increases, presumably as the Xe gas dissolves into the solvent, and then decreases at longer reaction time as both the acetone and Xe are consumed to form the type-II mixed hydrate. Simultaneously, the signal attributed to the 512 cages increases. The lower vapor pressure of acetone-d6 compared to SF6 at the reaction temperature could explain the apparent slow but continued formation of new hydrate that compensates for the loss of signal due to magnetization destruction by the rf pulses. Time-Resolved 19F NMR of the Type-II SF6 and the SF6/ Xe Deuteriohydrate Formation. The validity of the assumption that the type-II SF6 clathrate hydrate structure is already present prior to exposure to Xe gas under our reaction conditions has been confirmed by recording the 19F NMR spectra of SF6 as a function of time of exposure of the gas to ice which has undergone the same preconditioning with Xe gas as described above. The 19F NMR spectra of SF6 gas, SF6 solid and SF6 in the presence of preconditioned ice after reaching chemical and thermal equilibrium at 223 K are shown in Figure 10. The broad resonance centered at 4.5 ppm is consistent with SF6 in a typeII clathrate hydrate environment that is quite distinct from the gaseous and solid-state resonance lines. Furthermore, the time dependence of the SF6 19F spectra can be monitored during the reaction of Xe gas with a preexisting SF6 type-II structure (prepared using the same procedure described earlier) by thermally polarized 19F NMR, as shown in Figure 11. As the NMR tube warms the Xe sublimes and then reacts to form the mixed type-II SF6 clathrate deuteriohydrate, as is observed in the hyperpolarized 129Xe experiment presented in Figure 9a. The broad signal at 4.5 ppm is attributed to the mixed SF6/Xe type-

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Figure 11. Monitoring the formation of the type-II SF6/Xe deuteriohydrate by Fluorine-19 NMR at 223 K. The spectra were acquired at the times indicated following removal of the sample tube containing a preformed type-II SF6 hydrate and thermally polarized Xe from liquid nitrogen and insertion in the NMR spectrometer. Individual spectra were acquired using a 90° (22.5 µs) pulse with signal averaging over 8 transients and with a recycle delay of 4 s.

II structure. When both chemical and thermal equilibrium have finally been reached after approximately 12 min, only a broadened gaseous signal due to unreacted SF6 and a type-II SF6 clathrate hydrate signal are evident in the 19F spectrum. Note that the intensity of the resonance corresponding to enclathrated SF6 remains essentially constant throughout the experiment. This result confirms that the preformed type-II clathrate hydrate is already present and that there is minimal additional incorporation of SF6 during the second reaction with Xe. These 19F NMR results demonstrate that the type-II SF6 hydrate clathrate is already preformed when it is subsequently reacted with polarized 129Xe to form the mixed SF6/Xe type-II hydrate as monitored by hyperpolarized 129Xe NMR. 4. Conclusions In summary, the formation of SF6/Xe and acetone-d6/Xe mixed type-II clathrate hydrate on the surface of ice has been monitored at the relatively high temperature of 223 K using time-resolved hyperpolarized 129Xe NMR. In the SF6/Xe deuteriohydrate, the NMR signal amplitude for the 129Xe enclathrated into the 51264 cavities was less than one-twentieth that of Xe trapped in the 512 cavities. The lack of any signal due to Xe in the 51264 cavities of the acetone-d6/Xe deuteriohydrate indicates an even higher propensity of acetone to occupy the large cages to the greater exclusion of Xe. These hyperpolarized 129Xe NMR data clearly demonstrate that there is a negligible amount of residual nonpolarized Xe resulting from pretreatment of the ice. Time-resolved 19F NMR has also been used to monitor the formation of type-II SF6 deuteriohydrate under reaction conditions identical to those in the polarized 129Xe formation studies. The 19F data clearly supports the conclusion that the type-II SF6 hydrate is preformed prior to addition of polarized Xe, and that the Xe reacts directly with this preformed type-II SF6 deuteriohydrate. Additional proof that the type-II has formed

Storhaug et al. in the case of the SF6/Xe mixed hydrate comes directly from the spectra in Figure 3. The peak having a chemical shift of 66.5 ppm has been previously attributed to Xe in a type-II clathrate hydrate structure, and does not correspond to the chemical shift of the 51262 cavity in the type-I clathrates. The 5:1251264 signal ratio of >20:1 is very much larger than the stoichiometric ratio of 2:1. This further supports the conclusion that type-II material has formed exclusively and that the Xe has been displaced from the large cages. The observed time dependence of the 129Xe signal amplitude leads to the conclusion that the reaction to form SF6/Xe deuteriohydrate has reached completion within about 240 s. The decay of the signal is in accordance with the dependence expected if the loss of magnetization is primarily due to the application of successive rf pulses. A qualitatively different result was obtained in the case of the acetone-d6/Xe deuteriohydrate formation. In this reaction, the signal due to 129Xe in the 512 cages decayed more slowly than that which would be expected from the destruction of magnetization by rf pulses. Two possible explanations for this behavior are offered. Due to the low vapor pressure of the liquid acetone in the sample tube, the formation of the type-II compound appears to continue over a longer time-scale. As the acetone evaporates, it combines with ice and Xe to form new mixed clathrate hydrate material. The incorporation of polarized 129Xe gas into the solid compensates for the depletion of magnetization due to the applied rf pulses. Alternatively, chemical exchange of Xe between the enclathrated phase at the surface with the gas phase could explain the relatively slow decay of the 512-cavity signal. A comparison of the line shapes due to Xe enclathrated in the 512 cages of the SF6 and acetone-d6 type-II clathrate deuteriohydrates shows significant differences that can be attributed to differences in the reorientation dynamics of each guest. Fast water dynamics in the acetone hydrate will result in an average environment characteristic of the true crystallographic symmetry, whereas the slow water dynamics in the SF6 hydrate would give rise to a reduced local symmetry due to freezing in of the water disorder. The 129Xe line shape in the SF6 hydrate represents a superposition of cage environments that are effectively static on the NMR time-scale. It has been demonstrated that the temperature dependence of these effects, which have been previously observed by conventional (thermally polarized) 129Xe NMR in the context of the large cages in the type-I Xe hydrate, can be explored using temperature-jump, hyperpolarized 129Xe NMR. Finally, we suggest that the method presented here should be applicable to a wide range of type-II and type-H Xe clathrate hydrate systems. The hyperpolarized 129Xe NMR technique combines the advantages of increased signal, as is necessary to monitor enclathration at the ice surface at relatively high temperatures where the reaction time-scale is on the order of seconds to minutes, with the high chemical shift dispersion of 129Xe atom. The 129Xe spectrum contains information about the size and symmetry of the cavities, and the signal integrals are directly proportional to the cage occupancy numbers. All in all, the method extremely well suited to the study of Xe clathrate structure, kinetics, and mechanism. Registry No. Acetone-d6 666-52-4; sulfur hexafluoride 2551-62-4; deuterium oxide 7789-200; xenon 7440-63-3. Acknowledgment. This work was funded by NSF Award CHE-9724635, the University of Florida, and the NHMFL InHouse Research Program. A portion of this work was performed using facilities of the National High Magnetic Field Laboratory,

Mixed Type-II Clathrate Hydrates which is supported by NSF Cooperative Agreement DMR9527035 and the State of Florida. The authors wish to thank Joe Caruso for providing expert glassblowing services. References and Notes (1) Von Stackelberg, M.; Muller, H. R. Z. Elektrochem. 1954, 58, 25.; von Stackelberg, M.; Jahns, W. Elektrochem. 1954, 58, 162; von Stackelberg, M.; Meuthen, B. Elektrochem. 1958, 62, 130. (2) McMullan, R. K.; Jeffrey, G. A. J. Chem. Phys. 1965, 42, 2725; Mak, T. C. W.; McMullan, R. K. J. Chem. Phys. 1965, 42, 2732. (3) Sloan, E. Dendy, Jr. Clathrate Hydrates of Natural Gases, Marcel Dekker: New York, 1990. (4) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 8773. (5) Clausen, W. F. J. Phys. Chem. 1951, 19, 662; J. Phys. Chem. 1951, 19, 1425. (6) Mu¨ller, H. R.; von Stackelberg, M. Naturwissenchaften 1952, 39, 20. (7) Pauling, L.; Marsh, R. E. Proc. Natl. Acad. Sci. U.S.A. 1952, 38, 112. (8) Davidson, D. W.; Ripmeester, J. A. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1984, Vol. 3, Chapter 3. (9) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1988, 92, 337. (10) Ripmeester, J. A.; Davidson, D. W. J. Mol. Struct. 1981, 74, 67. (11) Ripmeester, J. A.; Ratcliffe, C. I. Energy Fuels 1998, 12, 197. (12) Davidson, D. W.; Handa, Y. P.; Ripmeester, J. A. J. Phys. Chem. 1986, 90, 6549. (13) Garg, S. K.; Gough, S. R.; Davidson, D. W. J. Chem. Phys. 1975, 63, 1646. (14) Jacobs, D. M.; Zeidler, M. D.; Kanert, O. J. Phys. Chem. A 1997, 101, 5241. (15) Ripmeester, J. A.; Ratcliffe, C. T. In Inclusion Compounds; Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1991, Vol. 5, Chapter 2. (16) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 2000, 123, 2066. (17) Handa, Y. P.; Ratcliffe, C. I.; Ripmeester, J. A.; Tse, J. S. J. Phys. Chem. 1990, 94, 4363.

J. Phys. Chem. B, Vol. 106, No. 11, 2002 2891 (18) Pines, A.; Gibby, M. G., Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (19) Bhaskar, N. D.; Happer, W.; McClelland, T. Phys. ReV. Lett. 1982, 49, 25. (20) Happer, W.; Miron, E.; Schaefer, S.; van Wijngaarden, W. A.; Zeng, X. Phys. ReV. A 1984, 29, 3092-3110. (21) Zeng, X.; Wu, Z.; Call, T.; Miron, E.; Schreiber, D.; Happer, W. Phys. ReV. A 1985, 31, 260-278. (22) Raftery, D.; Long, H.; Meersman, T.; Gradinetti, P. J.; Reven, L.; Pines, A. Phys. ReV. Lett. 1991, 66, 584-587. (23) Raftery, D.; Reven, L.; Long, H.; Pines, A.; Tang, P.; Reimer, J. A. J. Phys. Chem. 1993, 97, 1649. (24) Brunner, E.; Seydoux, R.; Haake, M.; Pines, A.; Reimer, J. A. J. Magn. Res. 1998, 130, 145-148. (25) Bowers, C. R.; Pietrass, T.; Barash, E.; Pines, A.; Grubbs, R. K.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 9400-9404. (26) Bowers, C. R.; Storhaug, V.; Webster, C. E.; Bharatam, J.; Cottone III, A., Gianna, R.; Betsey, K.; Gaffney, B. J. J. Am. Chem. Soc. 1999, 121, 9370-9377. (27) Landon, C.; Berthault, P.; Vovelle, F.; Desvaux, H. Protein Science 2001, 10, 762. (28) Desvaux, H.; Gautier, T.; Le Goff, G.; Petro, M.; Berthault, P. Eur. Phys. J. D 2000, 12, 289. (29) Pietrass, T.; Bowers, C. R.; Gaede, H. C.; Pines, A.; Ripmeester, J. A., unpublished results. (30) Pietrass, T.; Gaede, H. C.; Bifone, A.; Pines, A.; Ripmeester, J. A. J. Am. Chem. Soc. 1995, 17, 7520. (31) From the CRC Handbook of Chemistry and Physics; Weast, R. C., Lide, D. R., Astle, M. J., Beyer, W. H., Eds.; CRC Press: Boca Raton, 1989, pp D-194. (32) Rodger, P. M. Ann. N. Y. Acad. Sci. 2000, 912, 474. (33) Uchida, T.; Ebinuma, T.; Narita, H. J. Cryst. Growth 2000, 217(1/ 2), 189. (34) Garg, S. K.; Davidson, D. W. Chem. Phys. Lett. 1972, 13, 73. (35) Majid, Y. A.; Garg, S. K.; Davidson, D. W. Can. J. Chem. 1968, 46, 1683.