J . Phys. Chem. 1988, 92, 7167-7168
7167
Magnetic-Fieid-Induced Orientation of Crystallites in Powders of Layered Intercalation Compounds Detected by NMR Thomas K. Halstead,? Claudia Schmidt, Hans W. Spiess, * Max-Planck-Institut fur Polymerforschung, Postfach 31 48 0-500 Mainz, West Germany
Robert Schollhorn, Institut fur Anorganische und Analytische Chemie der Technischen Uniuersitat Berlin, Strasse des 17 Juni 135, D-IO00 Berlin 12, West Germany
Werner Muller-Warmuth, and H. Moiler Institut fur Physikalische Chemie der Uniuersitat Miinster, Schlossplatz 4-7 , 0-4400 Miinster. West Germany (Received: August I , 1988: In Final Form: September 1 , 1988)
Alignment of crystallites by a magnetic field of 7.2 T has been observed by ‘H NMR in powder samples of the hydrated at room temperature, where A = Na and K. Samples loaded into the magnet layered chalcogenidesAo,33+(H20)o,67(NbS2)0.33in the normal way give well-resolved spectra consistent with the crystallites comprising the powder being oriented so that all the H-H vectors of the cointercalated water molecules are aligned parallel to the external magnetic field. The ordering can be disturbed mechanically and is not induced in a field of 1.4 T. The analysis of the asymmetric splitting of the lines yields chemical shift anisotropies uII- ul = -20 ppm for &.33(H20)o.67(NbS2), -22 ppm for Nao,33(H20)o,67(NbS2}, and -1 1 ppm for Ko,33(H20)o,67(TaSCo,s), where 11 and I respectively refer to parallel and perpendicular to the normal of the layers.
Introduction We report the observation of the alignment of crystallites in powder samples of a layered intercalation compounds induced by magnetic fields. The ordering was detected by ’H N M R techniques in samples of hydrated monolayered chalcogenides of the type A,+(H20),,{NbS2)’, A denoting Na and K, x = 0.33 and y = 0.67.14 These compounds can be regarded as polyelectrolytes consisting of two-dimensional (MS2P- macroanions with mobile solvated cations between the layers. Previous N M R studies have shown that the behavior of the water molecules in these compounds is rather unusual. The spectra reported by Muller-Warmuth et a l . ’ ~ for ~ . ~randomly oriented powder samples of Ko,33(H20)o,67(NbS2), and other compounds of this type,3 recorded at 0.19 T (8 MHz), exhibit the classical Pake line shapes expected for powder samples containing pairs nuclei. Two pairs of discontinuities are evident in all of spin of these spectra: an inner pair (H-H perpendicular to Bo) split by about 39 kHz and an outer pair (H-H parallel to Bo) by about 81 kHz. The following structural picture for the water molecules is generally accepted as being consistent with these spectra: (i) the molecules are undistorted with an H-H separation of 1.65 A, compared to the separation of 1.58 8, found for water in gypsum: (ii) all the molecules are aligned with their C2axes lying in the ab plane defined by the macroanions, and their H-H vectors parallel to the c axis: and (iii) rapid translational diffusion of the water molecules in the interlayer region leaves the proton intramolecular dipole-dipole coupling unchanged but averages the intermolecular coupling to zero. These low-field powder spectra appear symmetrical. In addition, there is usually a narrow central line of an intensity which varies from sample to sample and which increases slowly with the age of the sample. This line indicates rapid proton exchange between the water molecules within the layer^.^.^ Recently, Kanzaki et aLs have published spectra of K0.33 (HZ0)0,67(NbS2) recorded at 6 T (270 MHz) which have a different shape from those recorded at low field by Muller-Warmuth et al.’L4 The spectra consist of a well-resolved doublet with a splitting of 78 kHz and a smaller central peak. Kanzaki et al.,s assuming that the doublet corresponds to the inner pair of discontinuities
in the low-field spectra and that signals corresponding to the outer pair were undetectable, suggested three explanations for the large splitting (78 kHz) of the inner pair: (i) the water molecules are distorted with a bond angle of about 90°: (ii) all the water molecules are oriented in a fixed direction relative to the external field, irrespective of the various orientations of the crystallites in the powder: and (iii) the simple Pake theory is inadequate. All of these explanations, if proved to be correct, would call for major changes in either the generally accepted view of the nature of these intercalation compounds or even the form of the dipole-dipole interaction. None of these explanations, however, seems to be at all likely to us. Moreover, the experiments we report in this Letter provide convincing evidence that the outer peaks of the high-field spectra simply correspond to the outer discontinuities in the low-field spectra and that the crystallites comprising the powder samples are oriented by the external magnetic field. Once the origin of the line shapes observed is established, the chemical shift anisotropy can be extracted from their asymmetry.
‘Permanent address: Department of Chemistry, University of York, York YO1 5DD, England.
Phys.
0022-3654/88/2092-7167%01.50/0
Experimental Section The preparation and analysis of the intercalation compounds followed the procedures described previously.’**The samples were dried by evacuation for about 12 h, giving free-flowing powders which were sealed in glass tubes of external diameter 5 mm. The high-field N M R spectra were taken at room temperature with a Bruker MSL 300 spectrometer operating at 300.13 MHz. The sample tube was located horizontally in a wide-line probe with the powder loosely filling only the lower half of the tube. The magnetic field of the superconducting solenoid (Oxford Instruments, 7.2 T, 89 mm diameter) was vertical. Solid echoes were excited with a 90°,-7 hs-9Oo,, sequence. The 90° pulse length was 2.5 hs. Quadrature detection was used and 128 signals were averaged with a cycle time of 2 s. The spectrum was obtained (1) Riider, U.; Miiller-Warmuth. W.; Schollhorn, R. Chem Phys. 1979, 70,2864. (2) Riider, U.; Miiller-Warmuth, W.; Schollhorn, R. J. Chem. Phys. 1981,
-.(3) Riider, U.; k
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R. J .
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0 1988 American Chemical Society
Letters
7168 The Journal of Physical Chemistry, Vol. 92, No. 26, 1988
h
( b i Disoriented
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la! Oriented II
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I
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-u~!/’kHz
Figure 1. IH N M R spectrum of K,,33(H20)o.67(NbS2)powder (a) completely oriented by the magnetic field; the outer doublet is ascribed to intercalated water molecules with their H-H vectors aligned virtually parallel to the external magnetic field; (b) deliberately disoriented to some extent, the outer and inner doublets are ascribed to intercalated water molecules whose H-H vectors are aligned, respectively, virtually parallel and normal to the external magnetic field. In (a) and (b) the central peak is ascribed to mobile protons. Both spectra were recorded at 7.2 T and room temperature.
by Fourier transformation of the echo, setting the time zero to coincide with the top of the echo.
Results and Discussion The high-field (7.2 T) spectrum of &,33(H@)o,67{NbS2], shown in Figure la, is similar to that in ref 5 , but is strikingly different from any of the low-field spectra reported for this and related c o r n p o ~ n d swhich ’ ~ ~ show the classical powder line shape expected for crystal hydrates. Instead it consists only of three narrow peaks: the splitting of the outer pair of peaks is 82.8 kHz and is asymmetrical with respect to the central peak. This spectrum is entirely consistent with an oriented powder in which the crystallites are oriented in such a way that virtually all the H-H vectors are aligned parallel to the external magnetic field giving a unique dipole-dipole splitting of 82.8 kHz corresponding to a H-H distance of 1.63 A. The absence of any detectable signals with splitting greater than 82.8 kHz rules out the possibility of distorted water molecules with a smaller H-H distance. No deliberate attempt was made to orient the powder and it appeared that the ordering occurred spontaneously during the normal sample-loading procedure. If the bulk diamagnetic susceptibility of a crystallite is anisotropic, it is conceivable that mechanical disturbances, occurring as the loosely packed sample was moved into the field, could have facilitated the ordering. It should be noted that for our aligned samples slight misalignments of the crystallites do not strongly affect the line shape because the angular dependence of the dipolar split lines vanishes when the H-H vector is parallel to the field. In addition, the fact that the inner singularities, vide infra, are not detected indicates that there are virtually no crystallites present for which the H-H vector is perpendicular to the field.
If our conjecture is correct, then any misalignment of the sample should be detectable as an increase in intensity between the outer peaks of the spectrum shown in Figure l a (in addition to that associated with mobile protons). We were able to demonstrate this effect by shaking the sample tube while it was outside the magnetic field to randomly orient the powder and then, taking precautions not to disturb the powder, placing the sample in the probe coil and inserting the probe into the magnet. The spectrum of this deliberately disoriented sample is shown in Figure 1b. It reveals an increase in intensity for signals with splittings less than 82.8 kHz, confirming that some of the crystallites had, in fact, remained unaligned with respect to the field. In particular, discontinuities with a splitting of 41.3 kHz corresponding to some H-H vectors being perpendicular to the applied field are now evident. All our attempts to record a spectrum of K,,33(H20)o,67{NbS2] in a completely randomly oriented state in a field of 7.2 T were unsuccessful. On the other hand, judging from its spectrum, a sample of Ko,33(H20),(NbS2]which contained more water, prepared by drying under vacuum for only 1 h instead of 12 h, was only partially oriented by the field. We found that the orientation phenomenon is not restricted only to this particular system; for example, we also observed it for samples of Nao,33(H20)o,67(NbS2). Line shapes, similar to that shown in Figure 1b, are characteristics of partially oriented solids6 and have, of course, been frequently observed, for example, in 2H N M R of drawn polymers,’ liquid crystalline polymers,* lipid bilayer^^^'^ and in low molar mass crystalline materials with a tendency for crystal growth in preferred directions, e.g., benzene.6 The centers of gravity of the two pairs of discontinuities, evident in the spectrum of the partially oriented sample, Figure lb, do not coincide because of chemical shift anisotropy. Since the water molecules rapidly diffuse within the layers, all intra- and intermolecular second rank tensorial interactions governing the N M R line shape are averaged to axially symmetric tensors with a common unique axis perpendicular to the layers. The principal axes of the averaged dipoledipole coupling and chemical shielding tensors are thus assumed to be collinear. Consequently, the spectra could be analyzed as described by Kaplan et al.” yielding Au = u,,- uI = -6.1 kHz (-20 ppm) for &.33(H20)0.67(NbS2] (Figure lb) and -22.3 ppm for Nao.33(H20)o.67{NbS2}. These values are similar to those observed in ice.I2 The value of -1 1 .O ppm determined for the compound &,33( H20)o,67(TaSCo,5], however, suggests that the intercalation of the water strongly influences the proton anisotropy which depends more on the type of macroanion than the kind of alkali metal. A systematic study of the dependence of these effects is currently being undertaken. Acknowledgment. T.K.H. thanks the Max-Planck-Gesellschaft for financial support and the Royal Society for a travel grant. (6) Hentschel, R.; Schlitter, J.; Sillescu, H.; Spiess, H. W. J . Chem. Phys. 1978. 48. - - ,-56. ~1
~
(7) Hentschel, R.; Sillescu, H.; Spiess, H. W. Polymer 1981, 22, 1516. (8) Boeffel, C.; Spiess, H. W.; Hisgen, B.; Ringsdorf, H.; Ohm, H.; Kirste, R. G. Makromol. Chem. Rapid Commun. 1986, 7, 777. (9) Lewis, B. A.; Rosenblatt, C.; Griffin, R. G.; Courtemanche, J.; Herzfeld, J. Biophys. J . 1985, 47, 143. (10) Speyer, J. B.; Sripoda, P. K.; Das Gupta, S. K.; Shipley, G . G.; Griffin, R. G . Biophys. J . 1987. 51, 687. ( 1 1 ) Kaplan, S.; Pines, A.; Griffin, R. G.; Waugh, J. S. Chem. Phys. Lett. 1974. 25. 78. ( 1 2 ) Haeberlen, U. Magn. Reson. Rev. 1985, 10, 81.
