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J. Phys. Chem. 1996, 100, 15968-15971
NMR of 3He Dissolved in Organic Solids Martin Saunders,* Hugo A. Jime´ nez-Va´ zquez,† and Anthony Khong Department of Chemistry, Yale UniVersity, New HaVen, Connecticut 06511-8118 ReceiVed: June 17, 1996X
It has been found that 3He under pressures of a few atmospheres dissolves in the solid polymers Lucite, Nylon, Delrin, polyethylene, Teflon, and Lexan sufficiently to yield strong helium NMR spectra. The peaks range from sharp (Lucite, Lexan) to relatively broad (nylon, Delrin). The signals are interpreted as indicating rapid motion among sites. Helium NMR signals were also seen in the organic polycrystalline solids benzene, durene, naphthalene, phenanthrene, tert-butyl alcohol, acetic acid, and hexamethylbenzene. More complicated spectra with broader lines were obtained. Changes on repeated melting and refreezing are interpreted to indicate shift dependence on the orientation of individual crystallites. It is considered that 3He NMR spectroscopy is likely to yield valuable information about sites available and motion among them in a variety of solids in the future.
3He
is a superb NMR nucleus. It has spin 1/2 and a sensitivity about half of that for protons at the same field and concentration. Nevertheless, 3He NMR did not find a place as a useful tool for chemists until recently. The introduction of helium into the cages of fullerenes led to preparation of the first helium compounds.1 3He NMR was then shown to be very valuable in the study of these molecules and their chemical reactions.2 We present here some other ways in which 3He NMR can be useful. While looking at 3He in solution at a few atmospheres of pressure, we noticed a broad weak signal which we traced to helium dissolved in the glass wall of the NMR tubes. Diehl and co-workers, while studying the effect of solvents on the 3He chemical shift,3 had noticed the same signal and studied it in detail.4 It has long been known that helium diffuses through glass, polymers, and many other solid materials. There are many studies concerning solubilities and diffusion rates of noble gasessand other gases as wellsin solids. In the case of helium, its small size enables it to penetrate into very narrow spaces. If 3He is used, we can obtain structural information about the solid in question using NMR. Due to the high sensitivity of 3He NMR, even a very small amount of 3He within the solid can be seen. What might we learn from the spectra of 3He in solids? We cannot expect specific sites to bind helium since it has extremely weak van der Waals interactions. It therefore should ignore the chemical character of its surroundings and go anywhere it can fit. The helium NMR shift should simply reflect the magnetic field it sees. This field is a function of the bulk magnetic susceptibility of the solid and of local effects due to nearby molecules. Magnetic nuclei or unshared electrons could also affect the local magnetic field. Movement of the helium atom between different sites might produce broadening or averaging of the signals depending on the rate of motion. Fast relaxation could produce signal broadening as well. For the particular case of glass, Diehl found that all of the above effects are operative. The chemical shift and line shape of the helium in glass is a result of the magnetic susceptibility of the material, the diffusion rates of the gas within the glass, and the relaxation † Present address: Departamento de Quimica Organica, Escuela Nacional de Ciencias Biologicas, Prol. Carpio y Plan de Ayala, Col Santo Tomas, Mexico, D.F. 11340, Mexico. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01778-9 CCC: $12.00
time of helium. The latter is with paramagnetic Fe3+ ions or broken Si-O bonds. We have studied the NMR of 3He dissolved in organic polymers and crystalline substances. We used the polymers Lucite (poly(methyl methacrylate)), Teflon, nylon, Lexan (polycarbonate), Delrin (polyformaldehyde), and polyethylene. Rods of these plastics were machined into pieces small enough in diameter to fit inside 5 mm NMR tubes between 3 and 4 cm in length. The samples were sealed in the tubes under a pressure of 2.5 to 3 atm of 3He. Helium gas present between the rod and the wall of the NMR tube was sufficient to provide a reference signal.5 In all cases, the spectra showed the signal of 3He dissolved in the glass walls of the NMR tubes. This signal served as an additional reference. A single peak was observed for 3He dissolved in each of the polymers. The chemical shifts, relative to the helium gas signal, were -2.64 (Lucite) (Figure 1), -2.8 (nylon), -2.97 (Delrin), -3.14 (polyethylene), -3.17 (Teflon), and -3.48 ppm (Lexan). The signals of helium dissolved in Lucite and Lexan are sharp (about 10 Hz),6 and the intensities in both cases are similar.7 The signals of helium dissolved in Teflon and polyethylene are broader and less intense. On the other hand, the signals of helium dissolved in nylon and Delrin appear as small bumps on top of the signal of helium dissolved in glass. The spectra of these two samples suggest that there is less helium dissolved. Spectra taken of helium dissolved in Lucite or Lexan broadened only when the temperature was lowered to below -50 °C. The NMR of 129Xe dissolved in several polymers has been studied. It was found that it can provide valuable information about amorphous regions in these materials.7 Xenon is much larger and more polarizable than helium. It prefers sites where there are attractive van der Waals forces. Xenon NMR chemical shifts are strongly affected by these interactions. In solution or in the amorphous regions of solid polymers, space can be easily provided by motion of molecules so as to accommodate xenon. However, spaces big enough to easily contain xenon are likely to be rare within a crystal lattice. There should be many more spaces large enough for helium. Helium might also be able to move more rapidly from site to site. We verified this conjecture by observing the spectra of 3He dissolved in a variety of organic crystals: benzene (at 0 °C), (Figures 2 and 3) durene (1,2,4,5-tetramethylbenzene), naphthalene, (Figures 4 and 5) phenanthrene, tert-butyl alcohol (at 5 °C), acetic acid (at -90 °C), and hexamethylbenzene. The © 1996 American Chemical Society
NMR of 3He Dissolved in Organic Solids
J. Phys. Chem., Vol. 100, No. 39, 1996 15969
Figure 1. 3He NMR spectrum in Lucite (broad peak at 3.1 is helium in glass).
Figure 2. 3He in liquid benzene (sharp peak on the side of the glass peak at 0).
Figure 3. 3He in solid benzene (+glass at 0).
samples were prepared by introducing the solid or liquid material in the NMR tube, degassing under high vacuum, and sealing the tubes under a pressure of 2.5-3 atm of 3He. The solid samples were melted and then cooled in order to obtain the solid phase. The samples which are liquids at room temperature were solidified inside the magnet of the NMR instrument. For all the samples, except for hexamethylbenzene and phenanthrene, 3He spectra were also obtained of helium dissolved in the corresponding liquid. The signals of helium in the liquids were very sharp (line width of 2 Hz or less6). They appeared
in the vicinity of -3.0 ppm,9 in the region where the signal of helium dissolved in substances that are liquids at room temperature (and the usual broad signal of helium in glass) usually show up. In marked contrast with the spectra in polymers, the spectra of 3He in the crystalline substances consisted of broad lines. The large areas under the peaks seem to indicate that the amount of helium dissolved in these solids is much larger than in the liquid. Solid benzene and durene show single broad peaks. Naphthalene shows up in three distinct regions within the same
15970 J. Phys. Chem., Vol. 100, No. 39, 1996
Saunders et al.
Figure 4. 3He in solid naphthalene (+glass at 3).
Figure 5. 3He in solid naphthalene (after melting and resolidification +glass).
very broad signal. Hexamethylbenzene and solid tert-butyl alcohol seem to have at least two different regions although, in the latter, the signals appear around -2.8 ppm. Acetic acid yields a single, asymmetric, very broad signal. It is notable that the NMR signals of helium in crystals of the aromatic compounds are shifted two or more ppm downfield from the positions in the liquids. The chemical shift of 3He dissolved in liquid benzene, after correcting for the magnetic susceptibility of the solvent and the shape of the cell, is upfield of where it is in most other oganic solvents.3 This shift can be attributed to strong ring current effects experienced by the helium atoms when they are near the faces of the aromatic rings. The aromatic molecules which we analyzed crystallize in two different ways. Hexamethylbenzene and durene form stacked layers. Naphthalene and benzene feature T-shaped packing; each ring has the edges of two other rings pointed at its faces. In both cases, the helium atoms should therefore be excluded from the volume near the faces and be exposed mostly to the edges of the rings, hence the net downfield shift. Spectra of the same substance taken after melting and solidifying more than once show that the peaks have approximately the same overall shape, although the chemical shifts and relative intensity of the peaks vary from run to run. These differences seem to be due to the way the sample crystallizes. When similar crystallization conditions are used, the shape and position of the 3He signals are more or less reproducible. If different conditions of crystallization are used, the spectra of the same substance look less similar. This may indicate that the 3He NMR shift depends on the orientation of the crystal with respect to the magnetic field. We also expect that a solidification in which a large number of small, randomly oriented crystals are formed, might give a spectrum different from that of a sample consisting of mostly a single crystal.
A referee suggested that the signals we see in crystalline solids might be due to bubbles of gaseous helium forced out of solution when the crystals grow (this assumes that helium is much less soluble in the solid than in the liquid). We carried out an additional experiment, the results of which indicate that the above possibility is less likely. We degassed a sample of phenanthrene by repeated cycles of melting, solidifying, and evacuating, finally allowing the sample to solidify from the melt under vacuum. 3He was then introduced and the NMR spectrum taken without melting the sample. A very strong helium signal was obtained after a few hours. Since the solidification occurred with a degassed sample, there should be no bubbles. We assume that each crystallite grows from the melt until it runs into another growing crystal. Little or no space should be left between crystallites. If the helium is within the crystal lattice, it could intrude into spaces between molecules. Another possibility is that it occupies defects left by missing molecules. We do not have evidence to distinguish these cases. In conclusion, the sharp lines in Lexan and Lucite suggest that the helium atoms are moving very rapidly through the spaces between the polymer chains. If the helium atoms moved more slowly, dipole-dipole magnetic interaction between the 3He and 1H nuclei would be expected to produce substantial broadening of the signal. The observation that low temperatures result in broader signals is in accord with this idea because the motion of the helium atoms should slow down. The fact that in the other polymers the lines are broader suggests that the space between the polymer chains is smaller. In the case of the crystals, the broad signals might be due to several possible causes. As noted above, random crystal orientation is one possibility. There might be regions with different magnetic environments within the same crystal. If the motion of the
NMR of 3He Dissolved in Organic Solids 3He
atoms is relatively slow, incomplete averaging of magnetic dipole-dipole interaction would cause broadening. We have observed 3He NMR spectra in all the solids we have examined so far. The signal intensity is an indication of how much helium is inside. The rate at which it goes in or out could be monitored by observing the spectra over time after changing the helium pressure. The chemical shift contains information about where the helium is in the polymer or in the crystal lattice. The line width and its changes with temperature contain information about motion of the helium in the solid. Standard NMR pulse methods should be able to quantitatively measure diffusion rates of helium. 3He NMR is likely to be a valuable tool for studying a wide variety of solid materials. Acknowledgment. We thank Prof. R. H. Crabtree, Yale University, for providing us with the phenanthrene samples. References and Notes (1) (a) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Poreda, R. J. Science 1993, 259, 1428. (b) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Mroczkowski, S.; Gross, M. L.; Giblin, D. E. Poreda, R. J. J. Am. Chem. Soc. 1994, 116, 2193. (c) Saunders, M.; Cross, R. J.; Jime´nez-Va´zquez, H. A.; Shimshi, R.; Khong, A. Science 1996, 271, 1693. (2) (a) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Mroczkowski, S.; Freedberg, D. I.; Anet, F. A. L. Nature 1994, 367, 256. (b) Saunders, M.; Jime´nez-Va´zquez, H. A.; Bangerter, B. W.; Cross, R. J.; Mroczkowski, S.; Freedberg, D. I.; Anet, F. A. L. J. Am. Chem. Soc. 1994,
J. Phys. Chem., Vol. 100, No. 39, 1996 15971 116, 3621-3622. (d) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Billups, W. E.; Gesenberg, C.; McCord, D. J. Tetrahedron Lett. 1994, 35, 3869-3872. (e) Smith, A. B., III; Strongin, R. M.; Brard, L.; Romanow, W. J.; Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J. J. Am. Chem. Soc. 1994, 116, 10831-10832. (f) Saunders, M.; Jime´nez-Va´zquez, H. A.; Cross, R. J.; Billups, W. E.; Gesenberg, C.; Gonzalez, A.; Luo, W.; Haddon, R. C.; Diederich, F.; Herrmann, A. J. Am. Chem. Soc. 1995, 117, 9305-9308. (3) Seydoux, R.; Diehl, P.; Mazitov, R.; Jokisaari, J. J. Magn. Reson., Ser. A 1993, 101, 78-83. (4) (a) Mazitov, R. K.; Diehl, P.; Seydoux, R. Chem. Phys. Lett. 1993, 201, 543. (b) Seydoux, R.; Diehl, P. J. Magn. Reson., Ser. A 1996, 119, 76-81. (5) All the spectra mentioned here were obtained in a Bruker AM500 spectrometer at 381 MHz using a 3He probe made by the Nalorac Co. The samples were not spun. When possible, shimming was done using the FID signal of the sample, although the intensity of the FID did not allow for a good adjustment of the field homogeneity. (6) The line width may actually be better than this. In many cases we were unable to achieve good shimming due to the low intensity of the FIDs in a single pulse. (7) This approximate comparison was made using the signal of the helium dissolved in glass as a reference, although field inhomogeneities and differences in the pressure of the gas in the NMR tubes may change the actual amounts. (8) (a) Stengle, T. R.; Williamson, K. L. Macromolecules 1987, 20, 1428. (b) Kentgens, A.; van Boxtel, H. A.; Verweel, R.-J.; Veeman, W. S. Macromolecules 1991, 29, 3712. (9) This chemical shift is relative to the signal of 3He gas and was estimated by obtaining a spectrum of an NMR tube filled with 3He, setting the signal to 0 ppm, and then introducing the sample with 3He in solution.
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