Between a Rock and a Magnetic Field: Geologists Exploit NMR
N
uclear magnetic resonance (NMR) spectroscopy, with all its complexities, has developed into a surprisingly versatile analyti cal technique. This is attributable, in part, to a host of clever modifications such as Fourier transform and multi dimensional NMR, which have in creased sensitivity and introduced methods for analyzing data. At the same time, these new developments are m a k i n g NMR accessible to a broader range of disciplines. The introduction of magic-angle spinning (MAS) for collecting usable s p e c t r a w i t h solid s a m p l e s h a s opened the door for geologists to in vestigate NMR as a means of charac terizing materials such as minerals, clays, and glasses. Although MAS NMR is expensive and complex, it competes with even more costly sur face techniques such as low-energy electron diffraction spectroscopy (LEEDS), Auger spectroscopy, and electron energy loss spectroscopy (EELS). It also offers a way of obtaining data about materials that are now determined by calculations based on chemical analysis, but with poten tially fewer problems arising from impurities. In addition, NMR probes specific environments nondestructively and does not require clean sur faces or orientations along a certain axis or surface. Finally, NMR encompasses a wide range of nuclei that can be studied by geologists looking at such questions as mineral composition and the ratio of metal ions in different coordina tion sites.
Magic-angle spinning
Without MAS, NMR spectra of solids are broad and undefined. The poor quality spectra arise primarily be cause of anisotropic chemical shifts that lead to overlapping spectra and dipolar-dipolar coupling between nu clei t h a t broaden peaks. For isotopes with / > 1/2, quadrupole interactions also adversely affect the spectra. Anisotropic chemical shifts are ob served in solids because the chemical shift depends on a molecule's orienta tion with respect to the magnetic field. In solution, rapid molecular tumbling averages the anisotropic chemical shifts into a single isotropic shift.
FOCUS Mathematically, the magnetic field's effect on the chemical shift is related to the t e r m 3 cos 2 θ — 1, where θ is the angle between the ap plied magnetic field (H 0 ) and the rel evant axis of interaction (e.g., along the bond). This term goes to zero when θ = 54.7°. Thus spinning a sol id sample at this "magic" angle with respect to H 0 and at rates that are large compared with the chemical shift a n i s o t r o p y ( t y p i c a l l y 1-10 kHz—the required rate increases as the magnetic field gets higher) reduc es or even eliminates the anisotropy. If the high-speed spinning is not fast enough to remove all the broadening, spinning sidebands may be intro duced into the spectrum. However,
these are easily identified as peaks spaced at integer multiples of the spinning frequency. The other important factor t h a t broadens spectra, dipolar coupling, is produced by through-space coupling of nuclei with spin. The coupling de pends on the intemuclear distance r between the nuclei (which varies as 1/r3), the product of the coupled nu clei's magnetogyric ratios ( γ ^ ) , and the 3 cos 2 θ - 1 term. Thus MAS can, in theory, elimi nate dipolar coupling. Unfortunately, isotopes with high magnetogyric ra tios require spinning rates as high as 70 kHz. Once again NMR techniques can help to overcome this problem. Dipolar coupling between different isotopes can be reduced by combining MAS with decoupling of one of the isotopes (e.g., saturating Ή frequen cies to measure 13 C by MAS). Homonuclear coupling can also be reduced by an appropriate FT pulse sequence (such as WAHUHA) (J) or by choos ing an NMR isotope with a low natu ral abundance (so that the isotope is in effect diluted). What follows is a brief survey of various elements that geologists have examined by NMR. NMR isotopes
Silicon-29. Like *H NMR for chem ists, 2 9 Si NMR has proved to be a workhorse for geologists. Silicon, which r a n k s as t h e second most a b u n d a n t element in the E a r t h ' s c r u s t , c o n v e n i e n t l y y i e l d s wellresolved NMR spectra. The natural abundance of 2 9 Si (/ = 1/2) is just 4.7%. Thus it requires long FT-NMR acquisition times, but the low abun-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991 · 467 A
FOCUS dance minimizes homonuclear dipole broadening. Studies have shown t h a t 29Si chemical shifts are sensitive to the local structural environment. Octah e d r a l l y c o o r d i n a t e d Si f e a t u r e s chemical shifts (referenced to TMS) in the range of -180 to - 2 2 1 ppm, whereas peaks for tetrahedral Si ap pear between - 6 0 and -126 ppm. As chemists might suspect, of the two geometries, tetrahedral coordi nation in minerals is the more com mon. Chemical shifts for tetrahedrally coordinated Si can f u r t h e r be correlated to the degree of tetrahe dral polymerization, average Si-O-Si or Si-O-Al bond angle, number of nearest-neighbor aluminum atoms, number of bridging oxygen atoms ver sus nonbridging atoms, and, in gener al, electronegativity of constituents. For instance, in a study of related silicates a strong linear correlation was found between the chemical shift for tetrahedrally coordinated Si and the mean Si-Si distances of the four nearby Si atoms or the mean secant of the Si-O-Si bond angle (2). Chem ical shifts for tetrahedral silicon at oms ranged over 15 ppm, reflecting average bond lengths and bond an gles that varied by < 0.1 A and - 10°, respectively. From this correlation, semiempirical formulae have been derived that can be used to predict bond distances or angles from NMR data alone. Data for this study were collected on a commercial FT-NMR spectrome ter tuned to 39.7 MHz for 2 9 Si. Sam ples were spun at 3.5 kHz and pulsed every 10.2 s. The authors reported that the chemical shifts were accu rate to at least 0.1 ppm. Correlations of NMR and structur al details have been extended by comparing silicates with a series of aluminosilicates (3). Because alumi n u m deshields silicon, the 2 9 Si chem ical shifts can be grouped according to the number of nearest-neighbor aluminum atoms. Thus silicates free of aluminum atoms feature peaks in t h e region of - 1 0 5 to - 1 1 5 ppm, w h e r e a s aluminosilicate m i n e r a l s with four Al per Si shift the 2 9 Si peaks into the range of - 8 0 to - 9 0 ppm. Once again a formula for chem ical shifts was derived, which in this case included the number of nearby aluminum atoms. With these correlations established it is possible to look at even more complex structures by NMR. Silicon MAS NMR of a series of layered sili cates and aluminosilicates found cor relations between chemical shifts and distortions in the sheet-like lay
ers and between chemical shifts and the total layer charge (4). The m i n e r a l s in this study are composed of alternating layers of tet rahedral and octahedral sheets with oxygen atoms crosslinking the differ ent layers. Distortions in the sheets develop naturally as the two layers try to fit together. Those distortions also vary as aluminum atoms substi tute for silicon atoms in the sheets. For instance, in a series of minerals where the only change is the substi tution of Al for tetrahedral Si, the 29 Si resonances become progressively deshielded as distortion increases. In addition to the varying concen trations of Al and Si, these layers also contain hydroxide, oxygen, mag nesium, and in some, a small amount of iron. Taken together, this can lead to a total layer charge t h a t deshields 29 Si peaks as the charge increases. Thus layer charge correlates linearly with the chemical shift. A l u m i n u m - 2 7 After 2 9 S i , 27 A1 ranks as the most common NMR iso tope studied by geologists. This is the only naturally occurring isotope of Al and thus it is easily detected. How ever, the nuclear spin (/ = 5/2) leads to broadening of the spectrum. The effect arises because nuclei with / > 1/2 have an asymmetric charge dis tribution about the nucleus. The re sulting nuclear quadrupole moment interacts with the electric field gradi ent (EFG) at the nucleus to generate the quadrupole effect. Broadening caused by aluminum's quadrupole effect can be greatly re duced by MAS at high m a g n e t i c fields and by observing solely the metal's (1/2,-1/2) spin transition. Relying on this spin transition elimi n a t e s the first-order t e r m for the quadrupole interaction because it contains a 3 cos 2 θ - 1 factor. However, a second-order term re mains that displaces resonances from their isotopic chemical shift positions and broadens peaks. The secondorder term is related to (QCC) 2 (1 + η 2 /3)
(1)
where QCC is the quadrupole cou pling constant, which measures the EFG at the nucleus, and η is the a s y m m e t r y p a r a m e t e r t h a t deter mines how much the EFG deviates from axial symmetry. This secondorder effect decreases with increas ing H 0 . The second-order quadrupole effect is demonstrated in a recent study by Donald Woesnner of Mobil Research and Development in Dallas. He in vestigated several of the same lay
468 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
ered aluminosilicate clay minerals t h a t were previously described (5). The article reported 27A1 MAS NMR recorded on three commercial instru ments that operate at magnetic fields of 2.35 T, 6.35 T, and 11.74 T. Spin ning rates were 3.5, 8.9, and 5.3 kHz, respectively. Under these conditions a peak as signed to a tetrahedrally coordinated Al in the mineral Black Jack beidellite ( M L 10 [(Al 3 . 91 Mg 0 . 02 Fe 0 0 6 ) ( S i 6 9 5 Ali o 5 )0 2 0 (OH) 4 ]} shifts from 64.8 ppm at 6.35 Τ to 70.0 ppm at 11.74 T. (Chemical shifts are measured with respect to the external s t a n d a r d , A1(H 2 0) 6 C1 3 in water.) The full linewidth of the peak at half height also v a r i e d w i t h t h e m a g n e t i c field, shrinking from 11.9 to 4.2 ppm with the increasing magnetic field. Other minerals displayed similar changes in spectra. By recording each mineral's spec trum at two different magnetic fields it is possible to determine the isotro pic chemical shift of the resonances. From the shifts in peak positions it is possible to calculate t h e value of Equation 1 and the isotropic chemi cal shift. (By simulating the spec trum, it is also possible to determine the values of QCC and η.) W h e n t h i s c a l c u l a t i o n is per formed, the resonance for tetrahedral Al for beidellite is determined to fall at 72.2 ppm and the second-order quadrupole effect is calculated as 2.54 MHz. Other techniques for cor recting for the second-order effect in volve measuring the spinning side bands or, if distinct peaks from the quadrupole splitting exist, simula tions to match the spectra. As this example suggests, chemical shifts for 27A1 NMR, like 2 9 Si NMR, correlate strongly with coordination site. Tetrahedral Al falls in the range of 50-80 ppm, whereas octahedral Al is found between -10 and 15 ppm. Five-coordinate Al appears in be tween these ranges, at 35-40 ppm. Thus it is possible to determine the ratio of aluminum atoms in different coordination sites. For example, from the NMR of beidellite at 11.74 Τ the calculated ratio of A l t e t r / ( A l t e t r + Al oct ) is 0.219, compared with a value of 0.212 from chemical analysis. Other correlations can be drawn from the spectra of these aluminosili cates. For instance, the Al t e t r secondorder quadrupole effect values corre late linearly with the total charge in that layer of the silicate and with the percentage of Al t e t r relative to Al oct . Furthermore, like 2 9 Si, 27A1 chemical shifts vary linearly with these factors. Boron-11. Boron is found in glasses
FOCUS and as an oxide. Like 27A1, it has a quadrupole effect nucleus (7 = 3/2) with a large n a t u r a l abundance (80.42%). MAS distinguishes three-co ordinate planar boron (12-19 ppm rel ative to BF 3 · E t 2 0 , QCC of- 2.5 MHz) from tetrahedral boron (2 to - 4 ppm, QCC of 0 to 0.5 MHz). A British study of coal and its com bustion residues offers a unique ap plication of MAS 1XB NMR (6). Both trigonal and tetrahedral boron were observed in the coal samples, and samples from different locales dis played different NMR patterns. The spectra were recorded at 5.9 Τ with a spinning frequency of- 4.7 kHz. P h o s p h o r u s - 3 1 . As chemists al ready know, 3 1 P is an ideal NMR iso tope, offering 100% n a t u r a l abun dance and 7 = 1/2. F u r t h e r m o r e , chemical shift variations for phos phorus in m i n e r a l s m i r r o r those trends already established for 2 9 Si. A joint University of Illinois-Sandia National Laboratory study of a se ries of phosphate glasses illustrates how 3 1 P MAS NMR is being applied (7). Unlike the previous examples, these glasses are manufactured ma terials that are produced by melting appropriate mixtures of phosphates and carbonates. The glasses are de scribed with the general formula: x(Na 2 0 + H 2 0 ) (1 - * ) P 2 0 5 . A de tailed chemical composition for each glass was determined by inductively coupled p l a s m a atomic emission spectroscopy. NMR spectra of the glasses were determined at 145.7 MHz in a 8.45 Τ magnetic field. Spinning rates varied from 4.5 to 9.2 kHz. Shifts are mea sured relative to 85% phosphoric acid. Chemical 3 1 P shifts for the tetrahe dral phosphates easily identify the number of bridging oxygens per tet rahedron. Following the nomencla ture of the geologists, these sites are identified as Q w , where m is the number of bridging oxygens. This and other studies have shown the follow ing trends: Q° sites - 15 ppm, Q 1 sites - 0.0 ppm, Q 2 sites — 2 0 ppm, and Q 3 sites — 4 0 ppm. In this study all of the coordination types were found except Q°. The relative frac tion of sites depended on composition (the value of x), which demonstrates the validity of an earlier theoretical model. A n o t h e r factor influencing t h e chemical shift was the extent of π-bond character in the Ρ—Ο bonds. For Q 2 sites a strong correlation was found between the bond order for the nonbonding oxygen atoms (estimated by an equation weighing the contri bution of various Q sites) and 3 1 P
chemical shifts over a range of - 10 ppm. As the nonbonding oxygen bond order increases (and thus the extent of π-bonding decreases) the 3 1 P reso nances appear at more deshielded values. Other correlations are found with bond angle and electronegativi ty of ligands and cations. Oxygen-17. Given the central role of oxygen in minerals, 1 7 0 would be an ideal NMR probe. However, its low abundance (0.037%) and its qua drupole moment (7 = 5/2) have lim ited its use. Some correlations have been recorded for bridging oxygens in Si-0-X M where X = Si, Al, or P. Sodium-23. Often overlooked by chemists, sodium has a ubiquitous nature that makes it an ideal probe. This isotope is sodium's only natural isotope, but with a quadrupole mo ment (7 = 3/2) 2 3 Na NMR requires high magnetic fields and calculation of the second-order quadrupole effect to adjust chemical shifts. 23 Na NMR has been used to exam ine alkali feldspars, showing some expected and unexpected trends (8). Chemical shifts for the ion become slightly less shielded with decreasing Si/(Si + Al), following trends in 2 9 Si
• *
·
Often overlooked by chemists, sodium has a ubiquitous nature that makes it an ideal probe." and 27A1 NMR. However, depending on t h e type of feldspar, opposite trends have been seen for 2 3 Na peaks with variation of Na/(Na + K) values. Further studies are needed to sort out these effects. Sodium ions are also useful for looking at surface properties of mate rials such as zeolites and silicates. For example, researchers from the University of Wisconsin—Milwaukee introduced Na + into vermiculite to probe hydration states (9). In vermiculites, negative charges are localized to a tetrahedral layer. As water hydrates the material it shields Na + from the oxygen anions. The oxygens contribute less electron density, and the cation becomes in creasingly deshielded. Thus corrected 23 Na chemical shifts (QCC was calcu
470 A · ANALYTICAL CHEMISTRY, VOL 63, NO. 8, APRIL 15, 1991
lated as < 1 MHz and η—»1) for ver miculite in the dehydrated phase ap peared at - 1 8 ppm (relative to a 0.01 M NaCl solution), whereas the 2 3 Na resonance for the two-layer hydrate shifts down to 4 ppm. Carbon-13. Organics also find a place in geology, and because of that 13 C (7 = 1/2, 1.1% abundance) MAS NMR has been used to characterize soils, peat, coal, oil shale, and even plant and microbial r e m a i n s . 1 3 C chemical shifts correlate with func tional groups. For instance, in the solid state chemical shifts for alkyls are at 50-100 ppm, for alkenes and aromatics at 110-160 ppm, and for carboxyls at 160-200 ppm. With this information, Australian researchers investigating 13 C NMR of soil sam ples found that wheat produces a soil higher in aromatics t h a n p a s t u r e land or native vegetation. To deal with the low natural abun dance of 13 C and to overcome a H - 1 3 C coupling in the solid state, research ers conducting experiments such as the Australian soil study employ the cross-polarization (CP) technique to improve signal-to-noise ratios. To ob tain CPMAS spectra, a standard 90° pulse is applied to the XH nuclei that sends the net magnet moment into what is defined as the xy plane. The spins are now irradiated in the plane so that the precession frequencies (or Larmor frequencies) for 13 C and a H nuclei become equal. This is labeled spin-locking and results in the higher energy protons transferring energy to the 13 C nuclei. Typically, a system is spin-locked for several millisec onds, during which the spin systems equilibrate. Following the spin-lock, the system is allowed to relax with proton decoupling on, providing the free induction decay for 1 3 C. The resulting energy transfer pro vides an enhanced signal for the 13 C spectrum. This technique can be ap plied to other NMR nuclei. A d d i t i o n a l n u c l e i . Like N a + , Cd 2+ ions also probe surface proper ties. In the previously described ver miculite study, 1 1 J Cd (7 = 1/2, 12.75% abundance) was introduced into the mineral to examine hydration states. Cadmium is particularly sensitive to its environment, displaying a chemi cal shift range of > 50 ppm with changes in hydration (8). 113 Cd (7 = 1/2, 12.26% abundance), 1 1 3 Cs (7 = 7/2, 100% abundance), and 7 Li (7 = 3/2, 92.58% abundance) have also been used for probing surfaces. Other techniques In addition to CPMAS, geologists have exploited several variations of
the FT-NMR technique. In an experi ment that many chemists will appre ciate, 2D 2 9 Si NMR exchange spec troscopy (EXSY) was employed to investigate aqueous alkaline silicate solutions or "water glass" (10). The experiments detailed six exchange p a t h w a y s for t h e silicate anion, which was enriched in 2 9 Si for the study. Another clever use of NMR takes advantage of a quadrupole moment. A clay sample was stirred in suspen sion for 24 h with D 2 0 and the 2 H NMR (/ = 1) recorded for the slurry. As with t h e vermiculite, D 2 0 hy drates t h e surface anions and the metal cations. Quadrupole splittings for the 2 H peaks varied with the ra tio of either Ca 2 + /Na + or Mg 2+ /Na + ions in the clay. The results are ex plained in terms of the different cat ions affecting the mode of reorienta tion for the water of hydration. Finally, although NMR h a s been used successfully to determine ratios of different coordination sites in ma terials, 2D shift-correlated (COSY) NMR spectroscopy is now being used to determine how these various sites are interconnected. COSY employs a
pulse sequence t h a t modulates the resonance of each nucleus in the nor mal NMR spectrum with the chemi cal shift of any nuclei that are cou pled to it. The resulting 2D spectrum contains the ID spectrum along the diagonal w i t h off-diagonal p e a k s (crosspeaks) corresponding to spinspin or dipolar coupled nuclei. For example, 29 Si enriched silicate glasses were examined by COSY MAS 2 9 Si NMR (11). The 2D experi ment indicated that, depending on the glass, either the Q 3 and Q 4 sites or the Q 3 and Q 2 sites are linked as nearest neighbors. As this overview suggests, geolo gists have found that NMR is a use ful and powerful analytical tool. Fur t h e r m o r e , t h e n u m b e r of n u c l e i accessible to NMR and variations in the method, such as CP and COSY, suggest that geological NMR is in its infancy. Alan R. Newman
(3) Ramdas, S.; Klinowski, J. Nature 1984, 308, 521. (4) Weiss, C. Α.; Altaner, S. P.; Kirkpatrick, R. J. Am. Mineral 1987, 72, 935. (5) Woessner, D. E. Am. Mineral 1989, 74, 203. (6) Burchill, P.; Howarth, O. W.; Richards, D. G.; Sword, B. J. Fuel 1990, 69, 421. (7) Brow, R. K.; Kirkpatrick, R. J.; Turner, G. L. / Non-Cryst. Solids 1990, 116, 39. (8) Oestrike, R.; Wang-Hong, Y.; Kirk patrick, R. J.; Hervig, R. L.; Navrotsky, Α.; Montez, B. Geochim. Cosmochim. Acta 1987, 51, 2199. (9) Laperche, V.; Lambert, J. F.; Prost, R.; Fripiat, J. J. /. Phys. Chem. 1990, 94, 8821. (10) Knight, C.T.G.; Kirkpatrick, R. J.; Oldfield, E.J. Magn. Reson. 1988. 78, 31. (11) Knight, C.T.G.; Kirkpatrick, R. J.; Oldfield, E.J. Non-Cryst. Solids 1990, 116, 140. Suggested reading
Kirkpatrick, R. J. In Reviews in Mineralogy, F.C. Hawthorne, Ed.; Mineralogy Soci ety of America: Washington, DC; 1988; Vol. 18, p. 341. Clague, A.D.H.; Alma, N.C.M. In Analyti References cal NMR; Field, L. D.; Sternhell, S., Eds.; John Wiley & Sons, Ltd.: New York, (1) Wilson, M. A. NMR Techniques and Ap plications in Geochemistry and Soil Chemis 1989; p. 116. try; New York: Pergamon Press, 1987. Wilson, M.A. NMR Techniques and Applica tions in Geochemistry and Soil Chemistry; (2) Smith, J. V.; Blackwell, C. S. Nature Pergamon Press: New York, 1987. 1983, 303, 223.
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