J . Phys. Chem. 1991, 95, 7575-7579 spectroscopic studies of polyatomic photodissociation, do not utilize the equipment optimum for resonance Raman studies and clearly can be improved upon in other laboratories.
Conclusion We have obtained 266-nm resonance Raman spectra of transand gauche-n-propyl iodide. All the normal modes with detectable Raman intensity have C-I stretch character, but it appears that in the dissociation there is significant motion along the CCC bend as well. The importance of bend contributions to the dissociative motion seems greater for the trans conformer than for the gauche
7575
conformer, suggesting that the dissociation dynamics for the two conformers are different. Also apparently indicative of differentiation in the photodissociation dynamics of the two conformations of n-propyl iodide is the high intensity of the trans "C-I stretch" progression compared with the gauche "C-I stretch" progression. Some part of the apparent difference between the two conformations can be understood in terms of the different geometries of the two forms.
Acknowledgment. This research has been supported by Grant CHE-88 10557 from the National Science Foundation.
*@SiMAS NMR of the Aluminosilicate Mineral Kyanite: Residual Dipolar Coupling to *'AI and Nonexponential Spin-Lattice Relaxation J. Stephen Hartman* Department of Chemistry, Brock University, St. Catharines, Ontario L2S 3A1, Canada
and Barbara L. Sherriff Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada (Received: December 27, 1990; In Final Form: May 3, 1991)
The 29SiMAS NMR signal of the mineral kyanite (AI2SiOS)is split into two closely spaced resonances corresponding to the two nonequivalent silicon sites, but only at high magnetic field strengths. Broadening and disappearance of this splitting at lower field (4.7 T), under conditions that give a very sharp signal from quartz present in the same sample, is consistent only with residual dipolar coupling to 27Al,hitherto an elusive phenomenon in 29SiMAS NMR, and demonstrates the value of very high magnetic field strength, even with I = nuclei. This small line broadening effect is usually overwhelmed by larger effects, such as broadening arising from Si,A1 disorder, which are absent in kyanite. 29Sispin-lattice relaxation in natural kyanite samples with a range of paramagnetic impurities does not follow the commonly assumed exponential decay pattern, and this is consistent with lack of spin diffusion.
Introduction The mineral kyanite, the high-pressure form of the A12Si05 polymorphs, has a structure based on cubic close-packed oxygens with 10%of the tetrahedral sites filled with Si and 40% of the octahedral sites filled with A1.I The triclinic structure can be described as having chains of A106 octahedra parallel to the c axis, supported by Si04tetrahedra and further AI06 octahedra (Figure 1). There are two very similar but crystallographically nonequivalent Si sites, with each of the oxygens in the SiO, group being linked to two AI atoms as well as to the Si atom. Kyanite is classified with the orthosilicates (isolated SiO, tetrahedra, i.e., no oxygen bridging to other four-coordinate atoms). However, the bridging of the Si04oxygens to octahedral AI in a complex edge-sharing arrangementlb gives kyanite the closest packing known in the aluminosilicates. Of special relevance to the present work is that each Si has eight octahedral AI next-nearest neighbors. The z9SiMAS N M R spectrum of kyanite has been reported to consist of a single Accidental superposition of the resonances of the two environments, as proposed by Smith et al.? did seem plausible. However, some time ago4 we observed two ( I ) (a) Burnham, C. W. Z . Krystallogr. 1963,118, 337. (b) Deer, W. A.; Howie, R. A.; Zussman, J. Rock Forming Minerals, Volume IA, Orthosilicutes, 2nd ed.; Longman Group Limited: London, 1982. (c) Winter, J. K.; Ghose, S.Am. Mineral. 1979, 64, 573. (2) Smith, K. A.; Kirkpatrick, R. J.; Oldfield, E.; Henderson, D. M. Am. Mineral. 1983, 68, 1206. (3) Engelhardt, G.;Michel, D. High Resolution Solid State NMR of Silicates and Zeolites; John Wiley and Sons: Chichester, 1987; Chapter V , especially pp 159-161. (4) (a) Hartman, J. S.;Williams, B. L. Presented at the Sixth International Meeting on NMR Spectfoscopy, Edinburgh, Scotland, July 1983. (b) Williams, B. L. MSc. Thesis,Brock University, 1984.
well-resolved sharp 29Sisignals, separated by 0.9 ppm, from a sample of white kyanite. Under the same conditions we obtained only a single, broader resonance from a sample of blue kyanite. The characteristic blue color of kyanites has been reported to arise from a charge-transfer interaction between trace amounts of iron and titanium,5 so we attributed the broadening to the effects of paramagnetic centers, consistent with our %i MAS NMR study of feldspars.6 We now report a wider study of the 29Sispectra of kyanites, including the effects of paramagnetic impurities. In the course of this work we have determined that resolution of the two kyanite peaks is possible only at very high magnetic fields, consistent with broadening due to residual dipolar coupling to quadrupolar "AI. This effect, best known in I3CCP/MAS spectra of solids containing quadrupolar I4N,' cannot be entirely removed by magic angle spinning and arises as follows. In the absence of an external magnetic field, quantization of spin states of a quadrupolar nucleus occurs along an axis determined solely by the electric field distribution at the nucleus; this is the tiasis of nuclear quadrupole resonance. As stronger and stronger magnetic fields are applied, the axis of quantization shifts closer and closer to the magnetic field axis. Since even a t high magnetic field quantization of the quadrupolar-nucleus spin states is not exactly along the magnetic field axis, dipolar coupling effects cannot be entirely removed by magic angle spinning.' The result, in the spectrum of a spin nucleus adjacent to a quadrupolar (5) Parkin, K. M.; Loeffler, B. M.; Burns, R. G. Phys. Chem. Miner. 1977, I , 301. ( 6 ) Sherriff, B. L.; Hartman, J. S.Can. Mineral. 1985, 23, 205. (7) (a) Hexem, J. G.; Frey, M. H.; Opella. S. J. J . Chem. Phys. 1982,77, 3847 and references therein. (b) Olivieri, A. C.; Frydman, L.; Diaz, L. E. J . Magn. Reson. 1987, 75, 50.
0022-3654/9 1 /2095-7575%02.50/0 0 1991 American Chemical Society
7576 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Hartman and Sherriff
n Figure 1. Kyanite, showing polyhedral chains parallel to the c axis.
nucleus, is quadrupole-perturbed residual dipolar coupling: nuclei asymmetric splittings or broadened resonances for spin near the quadrupolar nucleus. The effect diminishes at higher magnetic fields as the quadrupolar-nucleus quantization axis becomes more closely aligned to the magnetic field axis. Although residual dipolar coupling is most commonly encountered in 13C CP/MAS NMR where it gives asymmetric splittings or broadened resonances for carbons near I4N,’ there have been a limited number of observations of the effect with spin nuclei other than I3C and quadrupolar nuclei other than 14N.* The effect has been shown to be negligible compared to other broadening effects in 29SiMAS NMR spectra of aluminosilicates containing fourcoordinate AI and Si, at magnetic fields of 4.7 T or above.9 The effect has recently been reported in 29SiMAS N M R spectra of silicon nitride, in which the quadrupolar nuclei are first rather than second neighbors of silicon.I0 Experimental Section
29Simagic angle spinning NMR spectra were obtained on Bruker AC-200 (4.7 T), WH-400 (9.4 T), and AM-500 (1 1.7 T) multinuclear Fourier transform NMR instruments, using homebuilt magic angle spinning probes of the Andrew-Beams type” and Delrin rotors. Powdered samples were spun at 3.0-3.6 kHz at an angle of 54’44’ to the magnetic field. Typical spectra were obtained by using 30’ pulses and relaxation delays of between 5 s and 5 min between pulses. Spectra were obtained on the AC-200 instrument with 8K data points, a frequency of 39.76 MHz, and a spectral width of between 4200 and 25000 Hz. Spectra were obtained on the WH-400 and AM-500 instruments under similar conditions, at frequencies of 79.46 and 99.32 MHz, respectively. Chemical shifts are reported in ppm to low field of tetramethylsilane and were determined by using BLW036 oligoclase6 as a secondary chemical shift standard; its highest field peak was set at -1 04.6 ppm. Spin-lattice relaxation behavior was studied by using the inversion-recovery pulse sequence (1 80’7-90’) on the Bruker AC-200 instrument; the 90’ pulse length was 25.1 ps. The 2.35-T 29SiMAS NMR spectrum of Figure 2a was obtained on a Bruker MSL- 100 multinuclear Fourier transform (8) (a) Menger, E. M.; Veeman, W. S. J. Magn. Reson. 1982,46,257. (b) Komoroski, R. A.; Parker, R. G.; Mazany, A. M.; Early, T. A. J. Mugn. Reson. 1987, 73, 389. (c) Harris, R. K. J. Mugn. Reson. 1988, 78, 389 and references therein. (9) Fyfe, C. A,; Gobbi, G. C.; Murphy, W. J.; Ozubko, R. S.; Slack, D. A. J. Am. Chem. SOC.1984, 106,4435. (10) Carduner, K. R.; Blackwell, C. S.; Hammond, W. B.; Reidinger, F.; Hatfield, G. R. J. Am. Chem. SOC.1990, 112,4676. (1 1) Fyfe, C. A.; Gobbi, G. C.; Hartman, J. S.; Lenkinski, R. E.; OBrien, J. H.; Beange, E. R.; Smith, M. A. R. J . Mugn. Reson. 1982, 47, 168.
I
- 80
I
- 90
I
I
-100
-I IO
PPM
Figure 2. 29SiMAS NMR spectra of E2912 kyanite at (a) 2.35, (b) 4.7, and (c) 11.7 T (30° pulses, 300-s relaxation delay between pulses). The signal at -107.6 ppm arises from quartz which comprises about 50% of this sample.
TABLE I: Kyanite Samples samde no. color E29 12 white M3428 gray M 1462 light blue M3425 light blue M1459 M5420 SL 1 M 1464 M3522
locality
Pfitschthal, Tyrol, Austria Foster, Rhode Island, U S A . Chester Co., Pennsylvania, U.S.A. Death Rapids, Columbia River, British Columbia, Canada medium blue Guilford Co., North Carolina, U.S.A. Creston, British Columbia, Canada light blue Anderson Mine, Snow Lake, British gray Columbia, Canada Sultan Hamud, Kenya dark blue Velingrad, Bulgaria light blue
NMR instrument using a Bruker magic angle spinning probe with a cylindrical sample spinner and a spinning speed of 4 kHz. The localities and colors of the kyanite samples are listed in Table I. Sample E2912 is on loan from the Royal Ontario Museum, and the rest are from the mineral collection of the Department of Geological Sciences, University of Manitoba. The electron microprobe analyses given in Table I1 were done on grain mounts in the wavelength-dispersive mode on a Cameca Camebax SX50 instrument, using Cu Kcu radiation and the following standards: spodumene (Si, AI), almandine (Fe), NaScSi206(Na), Mn2Si04 (Mn), diopside (Ca), olivine (Mg), sphene (Ti), orthoclase (K). Data reduction was done using the “PAP” routine of Pouchou and Pichoir.I2 Results and Discussion Magnetic Field Dependence. The most striking spectra have been obtained from a white kyanite sample, E2912, shown by powder pattern X-ray diffraction to include approximately 50%
quartz. Electron microprobe analysis of grains of kyanite from this sample show an exceptionally low level of iron, a common impurity in kyanites, and typical levels of other impurities (Table 11). 29SiMAS NMR spectra of this sample, obtained at magnetic field strengths of 2.35, 4.7, and 11.7 T, are shown in Figure 2. The excellent resolution of the two kyanite peaks at 1 1.7 T contrasts with the broad single peak obtained at the lower field strengths. The quartz peak at -107.6 ppm is narrow in all spectra, and this provides evidence of similar good instrument resolution at all three field strengths. These spectra were obtained with 5-min (12) Pouchou, J. L.; Pichoir, F. Rech. Aerosp. 1985, 1984, 13.
The Journal of Physical Chemistry, Vol. 95, No. 20, I991 7577
29SiMAS N M R of Kyanite
TABLE 11: %i MAS NMR Peak Widths at Half-Height, %i Spin-Lattice Relaxation Behavior, and Electron Microprobe AMIYS~SResults for Kyanites E2912 MI462 M3428 MI459 M3425 M5420 SLI M3522 MI464 peak width at half-height: Hz 73 79 82 111 1I7 1 I7 169 263 264 T’(eq 3h s >240 1.6 1 .o 0.74 0.67 0.34 0.17 0.072 0.045
percentagesb SiOl A1203 Fe203
Ti02 MnO Na20
CaO
36.66 62.44 0.01 0.03 0.01 0.00 0.01 0.00 0.00
36.81 62.95 0.12 0.01 0.02 0.00 0.01
36.16 63.06 0.15 0.00 0.01 0.00 0.00
36.17 63.23 0.21 0.02 0.02 0.00
36.76 62.98 0.18 0.00 0.02 0.00
0.01
0.01
36.95 62.43 0.34 0.01 0.02 0.00 0.00 0.01 0.00
36.76 62.32 0.34 0.02 0.00 0.00 0.00 0.00 0.00
36.84 61.09 0.93 0.01 0.01 0.02
36.61 62.36 0.96 0.03 0.04 0.00
0.01
0.01
0.01 0.00 0.01 0.00 0.03 0.00 MgO 0.01 0.00 0.00 0.02 0.00 0.01 K20 ‘At 4.7 T (39.76 MHz); corrected for applied exponential line broadening. Estimated errors increase from &5 Hz for E2912 to A20 Hz for MI464 and A40 Hz for M3522. bBy electron microprobe analysis.
relaxation delays between 30° pulses; more rapid pulsing (e.g., every 5 s) gave similar kyanite peaks but no quartz signal, indicating much less efficient spin-lattice relaxation for quartz than for kyanite. Spectra obtained at 9.4 T also showed two well-resolved kyanite peaks. All mechanisms of broadening except residual dipolar coupling predict that the degree of broadening of spin nucleus resonances would either stay the same or increase at higher magnetic field. Therefore, the sharpening of the unresolved single peak as the magnetic field increases, to give well-resolved two-peak spectra at 9.4 and 11.4 T, is conclusive evidence for residual dipolar coupling interaction with 27AI. AI,Si disorder commonly occurs in many aluminosilicates (including zeolites) which have four-coordinate aluminum as well as silicon. The disorder causes 29Sipeak broadening because replacement of Si for AI, even at sites remote from the 2%i being observed, has appreciable effects on chemical shift. Each signal is thus a superposition of a large number of unresolved signals from similar but not identical 29Sienvironments. Kyanite is a particularly favorable aluminosilicate mineral for observation of the effects of residual dipolar coupling because Si cannot enter the octahedral AI sites, and the structure cannot accommodate the additional charge-balancing cations that would be required if AI were to enter the tetrahedral Si sites. Thus, the broadening due to residual dipolar coupling is not masked by more pronounced broadening due to disorder. Iron is virtually the only impurity element that can enter the kyanite structure in significant amounts,Ib through the isomorphous replacement of AI(II1) by Fe(II1). Hence, in the low-iron sample E2912 the 29Sipeaks are intrinsically sharp due to the absence of iron as well as the absence of AI,Si disorder, and except for residual dipolar broadening the line width should in principle be similar to that of silica p o l y m ~ r p h s . ~ JThis ~ is consistent with Figure 2c in which the kyanite and quartz peaks have similar peak widths at 11.7 T. At low magnetic fields the residual dipolar broadening is amplified by Si having eight AI next-nearest neighbors, all of which contribute, making residual dipolar broadening unequivocally detectable in the absence of other broadening effects (Figure 2a,b). Most reported cases of splittings due to quadrupole-induced residual dipolar coupling involve directly bonded atoms, in accord with its inverse cube dependence on distance. It is however well established that the effect can occur between atoms separated by two bonds,“ although one claim of a long-range effect has recently been shown to be erroneous.I5 In previous cases, the quadrupolar nuclei have had I = I 7 * l 0 and I = 3/2.8 There have apparently been no reports of splittings caused by residual dipolar coupling to an I = 5/2 nucleus such as 27Al,although in one publication
1
- 60
I
-70
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(14) Harris. R. K.; Jonsen, P.; Packer, K. J. Magn. Reson. Chem. 1985,
. 565.
23. .... -.
(IS) Hollingsworth, M. D.; Cyr, N. J. Chem.Soc., Chem. Commun. 1990,
578.
-90
PPM
Figure 3. The 11.7-T%SiMAS NMR spectra of (a) M5420 kyanite and (b) E291 2 kyanite, showing signal broadening due to paramagnetic impurities in the M5420 kyanite (30° pulses, 5-s relaxation delay between
pulses). the I = 5/2 case has been mentioned briefly.I6 A further increase in complexity, such as occurs in going from an I = 1 to an I = 3/2 quadrupolar n u c l e u ~ , ’would ~ be expected. However, with greater distances between the interacting atoms, broadening rather than splitting is likely. Si and AI are separated by an oxygen bridge, and the second-neighbor Si-AI distances in kyanite vary from 3.099 to 3.241 A. In comparison, typical first-neighbor C-N distances giving splittings due to residual dipolar coupling in 13C spectra are in the range 1.46-1.50 A.7a Residual dipolar coupling has recently been invoked to explain the simplification of the fine structure in the 29SiMAS N M R spectrum of Si,N4 at very high magnetic field.IO This is a more typical case than kyanite, in that the quadrupolar nucleus is “N and is directly bonded to silicon. No predictions were made on the expected 2%i splitting pattern, since each silicon is surrounded by four I4N atoms. The present situation would be expected to be even more complex, since in kyanite silicon has eighr nextnearest-neighbor 27AIatoms, all with I = 5 / 2 . All we detect is broadening and loss of resolution at 4.7 T, consistent with (i) the larger interatomic distance, (ii) the more complex pattern expected for I = 5 / 2 than for I = 1 and I = 3/2 quadrupolar nuclei, and (iii) the presence of eight next-nearest-neighbor AI atoms. Effects of Paramagnetic Impurities: Peak Widths and SpinLattice Relaxation. 29SiMAS NMR peaks of minerals can also
be broadened due to the effects of paramagnetic i m p u r i t i e ~ , ~ ~ ~ ~ ~ ~ ~~~
(13) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Kennedy, G. J.; Barlow, G. E. J . Am. Chem. Soc. 1988, 110, 3373.
I
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(16) Bohm, J.; Fenzke, D.; Pfeifer, H. J. Magn. Reson. 1983. 55, 197. (17) Zumbulyadis, N.; Henrichs, P. M.; Young, R . H. J. Chem. Phys. 1981, 75, 1603. (18) (a) Grimmer, A.-R.; Von Lampe. F.; Magi, M.; Lippmaa. E. 2. Chem. 1983, 23, 343. (b) Oldfield, E.; Kinsey, R. A.; Smith, K. A,; Nichols, J. A.; Kirkpatrick, R. J. J. Magn. Reson. 1983, 51, 325. (c) Watanabe., T.; Shimizu, H.; Masuda, A.; Saito, H. Chem. Left. 1983, 1293.
7578 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 and we have explored this effect in kyanite samples from various sources (Table I). Broadening is greater in typical samples than in the low-iron E2912 sample of Figure 2. Moderate iron impurity levels diminish, but do not totally obscure, resolution of the two peaks at 11.7 T (Figure 3). Peak widths at half-height of the single 4.7-T kyanite peak (Table 11) show the expected4.b1*increase with increasing concentration of paramagnetics, principally iron. The iron is probably present almost entirely as Fe(II1) since only Fe(II1) can enter the kyanite structure by isomorphous replacement of AI(III), although the presence of some Fe(I1) at similar concentration levels as other divalent ions is not excluded. (The electron microprobe analyses of Table I1 do not determine oxidation state; iron concentrations are by convention reported as Fe20,.) There is little variation in peak width among the samples with the lowest impurity levels, since here the broadening due to residual dipolar coupling predominates. Insights from the relatively few MAS N M R studies of paramagnetic solidsI9 should be applicable to aluminosilicate mineral systems in which paramagnetic ions are present as randomly distributed impurities. In paramagnetic solids, paramagneticsinduced line broadenings can result from large anisotropic bulk magnetic susceptibilities and also from short spin-spin relaxation times.20 Spin-lattice relaxation becomes more efficient as the concentration of paramagnetic iron increases a c r m the series of kyanites, in accord with previous studies of other However, in the kyanites the relaxation process is not exponential. Spinlattice relaxation normally follows an exponential decay process with time constant MA0 = exP(-r/TJ
(1)
Following the 180° pulse in the inversion recovery pulse sequence ( 18Oo-7-9O0), the z magnetization should recover according to
M , = M , [ 1 - 2 exp(-r/ T,)]
(2)
where M , is the z magnetization following the variable delay time 7 of the inversion recovery sequence, M , is the equilibrium z-axis magnetization, and TI is the spin-lattice relaxation time. Exponential decay gives a linear relationship between In (1 - M,/M,) and 7 , from which the TIvalue can be determined. However, in our kyanite samples the usually assumed exponential decay of magnetization does not occur and the plot is not linear (Figure 4A). Since TI is defined only for exponential processes,22true TI values cannot be obtained for these samples. Instead of eq 1 , the data for our kyanite samples fit a ‘stretched-exponential” relationship
M,(r) = exp[-(t/T’)’/2]
(3)
where the time constant is designated as T’. (The term TI should not be used as it is the universally accepted term for the time constant for exponential decay.22 There seems to be no standard symbol for the time constant for spin-lattice relaxation that is not exponential.) In the inversion recovery sequence, eq 3 leads to z magnetization recovery as
M , = M-11 - 2 exp[-(~/T’)I/*]]
(4)
This corresponds to a linear plot of In (1 - M , / M , ) vs the square root of the delay time 7,rather than vs 7 itself, with a slope of (19) (a) Grey, C. P.; Smith, M.E.; Cheetham, A. K.;Dobson, C. M.; Dupree, R.J . Am. Chem. Soc. 1990,112,4670. (b) Grey, C. P.; Dobson, C. M.;Cheetham, A. K.;Jakeman, R.J. B. J . Am. Chem. Soc. 1989, 111.505. (c) Nayeem, A.; Yesinowski, J. P. J. Chem. Phys. 1968,89,4600. (d) Walter, T. H.; Oldfield, E. J. Chem. SOC.,Chem. Commun. 1987, 646. (20) (a) Vanderhart, D. L.; Earl, W. L.; Garroway, A. N . J. Magn. Reson. 1981, 44. 361. (b) Alla, M.;Lippmaa, E. Chem. Phys. Lett. 1982, 87, 30. (21) (a) Barron, P. F.; Frost, R.L.; Skjemstad, J. 0. J . Chem. Soc., Chem. Commun. 1983,581. (b) Barron, P. F.; Slade, P.; Frost, R. L. J. Phys. Chem. 1985, 89, 3305. ( 2 2 ) Fukushima, E.; Roeder, S. B. W. Experimental Pulse N M R . A Nuts and Bolts Approach; Addison-Wesley: Reading, M A , 1981; Chapter 111, especially pp 164-165.
Hartman and Sherriff
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fl (seC’R) Figure 4. Semilog plots for M3428 kyanite: (A) plot of In ( 1
.
1
5
- M,/M,)
vs T , which would be linear if spin-lattice relaxation were exponential; (e) plot of In (1 - M,/M.) vs T I / * . (-l/T’)l/z, and our kyanite samples fit this relationship well (Figure 4B). The time constant T’ for all samples is included in Table 11. T‘becomes smaller as the iron content of the samples increases, consistent with interaction of 29Siatoms with paramagnetic impurity centers being the dominant mode of spin-lattice relaxation. The correlation with iron concentration is not exact since other paramagnetic impurities are present as well. The almost paramagnetics-free E29 12 white kyanite has very inefficient spin-lattice relaxation, and about a week of instrument time would be required to obtain a reliable T’ value. Since the single peak observed at 4.7 T is a composite signal arising from two sites, the relaxation behavior observed is a composite of two contributions. A preliminary inversion-recovery experiment at 11.7 T on the M3428 sample shows that both peaks are nulled at essentially the same variable delay value and hence that the two sites have very similar relaxation behavior. Although spin-lattice relaxation is usually assumed to be exponential, deviations from simple exponential behavior are not infrequent, especially in solids. Biexponential decay may be observed if a solid sample has two different environments, e.g. amorphous and crystalline regions of a polymer. IH relaxation in biological systems has even been analyzed in terms of continuous distributions of relaxation times.23 Even homogeneous crystalline mineral samples such as our kyanites can have local regions that differ in their relaxation behavior, because in such systems the principal 29Sispin-lattice relaxation mechanism is believed to involve interaction with paramagnetic impurity centers. Different 29Siatoms will be located at different distances from the paramagnetic centers, and this can in principle lead to a distribution of local %i relaxation environments. Hence, a crystallographically unique site in a structure need not be characterized by a single 29Sispin-lattice relaxation time. We have noted some practical applications of such effects p r e v i ~ u s l y . ~ ~ ~ ~ Relaxation via paramagnetic impurities has been shown to follow the usual exponential decay pattern, provided that spin (23) Krwker, R. M.; Henkelman, R. M. J. Magn. Reson. 1986,69,218. (24) Hartman, J. S.; Richardson, M. F.; Sherriff, B. L.; Winsborrow, B. G.J . Am. Chem. SOC.1987, 109, 6059.
29SiMAS N M R of Kyanite diffusion from dilute, randomly distributed and immobile paramagnetic centers is efficient and is the principal spin-lattice relaxation mechanism.25 This is the case when the nucleus being observed is abundant, but need not apply to magnetically dilute nuclei such as 29Si, since spin diffusion is based on dipolar interaction between the exchanging spins, which drops off very rapidly with increasing distance between the atoms undergoing the mutual spin flips. A number of studies have been carried out on spin-lattice relaxation in solids in the absence of spin diffusion and in the presence of inefficient spin diffusion.2628 Equation 3 has been found to be applicable in the absence of spin diffusion and has been applied for example to I3C relaxation in a paramagnetic solid2’ and in industrial diamond powders containing paramagnetic impuritiesa2* The situation with 29Siis less well documented, but it is clear from the present work that eq 3 applies to 29Sispin-lattice relaxation in at least some silicates. We are studying %i spin-lattice relaxation in a range of natural29and syntheticm paramagneticsdoped silicate minerals and find that ‘stretched-exponential” relaxation is widespread. Previous reports on 29Sispin-lattice relaxation times in silicate minerals1Bf21did not note any deviation from the normally assumed exponential decay pattern. At least some of the previously studied systems contained interlayer water, and mobile cations, which affected spin-lattice relaxation2’ and would be likely to make the relaxation behavior more “liquidlike” and favor exponential decay. In zeolites also, 29Sispin-lattice relaxation appears to be exponential, at least with proton-containing molecules occupying the
channel^.^' The quartz signal in the E2912 kyanite sample is not detected at all without a long relaxation delay, so spin-lattice relaxation is much less efficient in quartz than in kyanite in this sample. This is consistent with partitioning of the traces of iron into the AI(II1) sites of kyanite, rather than into quartz, when the separate mineral phases crystallized. Lowest iron content corresponds to the best-resolved spectra and the least efficient spin-lattice relaxation, while highest iron content gives the broadest signals and the most efficient spin-lattice relaxation. Silicate mineral samples that give the best-resolved N M R spectra tend to be colorless, consistent with the absence of defects and transition-metal ions. For the intermediate-impurity-concentration kyanite samples, however, there is no simple correlation of color with impurity concentration, spin-lattice relaxation, or peak broadening. It does seem that the Fe/Ti electron-transfer process5 cannot be the sole origin of the blue color of kyanites, since M3425 is light blue despite containing negligible Ti. In some kyanites, a greenish-blue color has been related to the presence of chromium.32 Chromium is present in our samples (25) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1961; Chapter IX. (26) (a) Blumberg, W. E. Phys. Reu. 1960, 119, 79. (b) Tse, D.; Hartmann, S.R. Phys. Reu. Lea. 1968,21,51 I. (c) McHenry, M. R.; Silbemagel, B. G.; Wernick, J. H. Phys. Rev. B 1972,5,2958. (d) Cheung, T. T. P. Phys. Reu. B 1981, 23, 1404. (27) Maiti, B.; McGarvey, B. R. J. Magn. Reson. 1984, 58, 37. (28) Henrichs, P. M.; Cofield, M. L.; Young, R. H.; Hewitt, J. M. J. Magn. Reson. 1984, 58, 85. (29) Hartman, J. S.; Sherriff, B. L.; Bancroft, G . M. Work in progress. (30) (a) Hartman, J. S.; Sliwinski, D. R.; Cherniak, E. A. 32nd Experimental NMR Spectroscopy Conference, St. Louis, April 1991; poster P134. (b) Sliwinski, D. R . MSc. Thesis, Brock University, 1991. (31) Challoner, R.; Harris, R. K.; Packer, K. J.; Taylor, M. J . Zeolites 1990, IO, 539.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1519 in the ppm range, probably as Cr(II1) which is intensely paramagnetic and would have pronounced N M R effects. An important advantage of N M R in the study of complex systems such as ceramics is the possibility of obtaining quantitative information on sample composition that may not be available by other methods.” The effective use of quantitative NMR of such systems requires an understanding of spin-lattice relaxation behavior. Nonexponential relaxation behavior may occur frequently in such systems and can be easily overlooked since standard computer fitting programs will provide a T I value even if decay is nonexponential. Such values can be meaningless or misleading. Chemical Shifts. Chemical shifts of the two well-resolved kyanite resonances at -82.3 and -83.2 ppm differ by 0.9 ppm, much larger than the difference of 0.1 ppm calculated by an empirical approach based on the McConnell equation” but still marginally within the range of accuracy of the calculations (h0.5 ppm for each chemical shift value). In retrospect, it is not surprising that the two kyanite %i peaks were not resolved by Smith et ala2Although their magnetic field strength (8.45 T ) should have been adequate, their acquisition parameters (50-kHz spectral window; 2048 data points) and processing parameters (100-Hz line broadening; zero filling to 8192 data points) are not conducive to resolution of splittings as small as 0.9 ppm (64 Hz at 8.45 T), even if paramagnetic impurity concentrations were sufficiently low in their samples. We note in passing that a magnetic-field-independent 29Si splitting, attributed to scalar coupling to 27Al,has been reported in the aluminosilicate mineral low albite,35but we were unable to confirm this in our feldspar 29SiN M R work.6
Conclusion This work illustrates the importance of high magnetic field in MAS NMR, even with spin nuclei, when the possibility exists of residual dipolar coupling to quadrupolar nuclei, and confirms residual dipolar coupling to 27Al as a broadening factor in 29Si MAS NMR spectra. %i spin-lattice relaxation is nonexponential in these systems, consistent with lack of spin diffusion among %i atoms. Note Added in Proof: Stretched-exponential 29Sispin-lattice relaxation consistent with eq 3 has recently been reported for zeolites containing varying amounts of Fe3t.36 Acknowledgment. We thank that Natural Science and Engineering Research Council of Canada for financial support, Mr. Tim Jones and Mr. Brian Sayer for assistance with the N M R instrumentation, McMaster University for use of the Bruker AM-500 and MSL-100 instruments, the South Western Ontario High Field N M R Centre and Dr. R. E. Lenkinski for the use of the Bruker WH-400 instrument, Mr. Ron Chapman for assistance with the electron microprobe analysis, Dr. J. A. Mandarin0 of the Royal Ontario Museum for providing the E2912 kyanite sample, and Prof. A. D. Bain of McMaster University for helpful comments on the manuscript. Registry No. Kyanite, 1302-76-7; quartz, 14808-60-7. (32) Rost, F.; Simon, E. Neues Jahrb. Mineral., Monafsh. 1972, 383. (33) Hatfield, G . R.; Carduner, K. R. J. Mater. Sci. 1989, 24, 4209. (34) (a) Sherriff, B. L.; Grundy. H. D. Nature 1988, 332, 819. (b) Sherriff, B. L.; Grundy, H. D.;Hartman, J. S.Eur. J. Mineral., in press. (35) Woessner, D. E.; Trewella, J. C. J. Magn. Reson. 1984, 59, 352. (36) Thangaraj, A.; Ganapathy, S. Indian J . Chem. 1990, 29A, 1080.
