626
J . Phys. Chem. 1988, 92, 626-630
Deuterium Solid-State NMR Study of Pyridine Dynamics in Intercalated 2H-TaS2 P.L. McDaniel, T. M. Barbara,+and J. Jonas* Materials Research Laboratory and Department of Chemistry, University of Illinois, Urbana, Illinois 61801 (Received: July 14, 1987)
A quadrupole echo deuterium NMR line-shape study on polycrystalline samples of perdeuteriated pyridine and selectively deuteriated pyridine-dz intercalated into 2H-TaS2 was performed between 280 and 393 K. Above 333 K, a slow growth in intensity of three motionally narrowed components coincident with depletion in intensity of the rigid pyridine pattern was observed for the perdeuteriated sample. Deuterium spin-lattice relaxation times were also recorded over the same temperature range. A short T1 component, which was attributed to the three motionally narrowed peaks, showed a sharp decrease in correspondencewith their appearance in the line shapes. The narrowing motion was determined to be rapid rotational diffusion of the pyridine molecule about an axis perpendicular to the C2 symmetry axis and parallel to the molecular plane. Since theoretically determined motionally reduced quadrupole coupling constants support this interpretation, it was possible to determine the orientation of the pyridine in the van der Waals gap.
I. Introduction Intercalation compounds involving transition-metal dichalcogenides and Lewis bases have been extensively studied due to their potential use in catalysis,' battery technology as a result of the reversibility of the intercalation reaction,, and in their simulation of a two-dimensional ~ y s t e m . ~A variety of techniques have been employed in an attempt to understand the dynamics of the guest molecule and hence its interaction with the host lattice. X-ray diffra~tion,~ neutron diffracti~n,~ electron microscopy,6 and conductivity measurements' can provide useful information about these layered systems. Much of the work has been concentrated in the temperature regime at and below room temperature. This is partly due to the fact that intercalation with Lewis bases such as pyridine results in an elevation of the superconducting temperature.8 However, in order to develop a more thorough knowledge of the guest-host interaction, it is also important to investigate the higher temperature region between ambient and deintercalation temperature. A study of guest molecular motion as a function of temperature could provide new insight into the orientation of the intercalated molecule which has been the subject of much dispute for the pyridine/TaS2 intercalation complex. Schollhorn et aL9 postulated, that in the pyridine/TaS2 intercalation compound, there are three possible configurations for the pyridine in the van der Waals gap. Using X-ray diffraction data, which gives the change in host layer spacing after intercalation, they suggested the following pyridine orientations (see Figure 1): (a) the C, axis perpendicular to the TaS, layers, (b) the C, axis and molecular plane parallel to the layers, forming a bilayer, and (c) the C, axis parallel and molecular plane perpendicular to the host layers. Neutron diffraction studiesSa on a similar system, (CSHsN)o,SNbS2, suggest that configuration (c), shown in Figure 1, is the most likely. In contrast to the orientation of the pyridine in the NbS2 host lattice, single-crystal X-ray suggest the perpendicular diffraction data on (CSHSN)o.sTaS210 configuration, thus allowing a direct intercalation between the lone pair electrons on the nitrogen and the dZ2 orbital of the tantalum. Also, below room temperature Lifshitz et al." reported the parallel plane orientation with rotation about the axis perpendicular to the molecular plane in pyridine intercalated Cd2P,S, using deuterium magnetic resonance (zH NMR) on single crystals. Due to conflicting information about the molecular orientation within the van der Waals gap and the lack of information about intercalated pyridine motion, we chose to perform a 2H NMR line-shape study to provide new insight into the dynamics of the intercalated Lewis base molecule. ,H N M R line shapes have proven to be very effective tools for the elucidation of dynamical processes in a variety of pseudotwo-dimensional systems such as intercalation compounds" and 'Present address Department of Chemistry, SUNY, Stony Brook, NY 11794.
0022-3654/88/2092-0626$01.50/0
inclusion compounds.12 A deuterium N M R line-shape study was performed because of the extreme sensitivity of the line shape to C-,H bond motions, thereby providing insight into both molecular motion as well as rotational rates.I3 Fast rotational diffusion, rigidity, and wobbling or 180' flipping motions can be readily differentiated by using line-shape ana1y~is.l~ In this study deuterium N M R line shapes were collected for both perdeuteriated and selectively deuteriated pyridine from 280 to 393 K. Thrxz overlapping, motionally narrowed Pake patterns, indicative of rapid diffusional rotation, were observed above 333 K. In addition, the deuterium spin-lattice relaxation time showed a rapid decrease with increasing temperature coinciding with the appearance of the motionally narrowed components. This new data collected in our laboratory supports case (c) (see Figure I ) , a parallel C, axis/perpendicular molecular plane configuration of pyridine in (CS2HSN)o,sTaS2. Experimental details are summarized in section 11. This includes a discussion of both synthetic methods and deuterium solid-state N M R techniques. Experimental results are reported and discussed in detail in section 111, which also includes the main conclusions of this study. 11. Experimental Section
Polycrystalline 2H-TaSz was prepared by reacting stoichio~~
~
(1) (a) Harris, S.; Chianelli, R. R. Chem. Phys. Lett. 1983,101, 603. (b) Weisser, 0.; Landa, S . Sulfide Catalysts: Their Properties and Applications; Pergamon: Oxford, U.K., 1973. (2) (a) Whittingham, M. S . Science 1976, 192, 1126. ( b ) Whittingham, M. S. Prog. Solid State Chem. 1978, 12, 41. (3) Silbernagel, B. G.;Gamble, F. R. Phys. Reu. Lett. 1974, 32, 1436. (4) (a) Gamble, F. R.; Osiecki, J. H.; DiSalvo, F. J . J . Chem. Phys. 1971, 55, 3525. (b) Parry, G. S.;Scruby, C. B.; Williams, P. M. Philos. Mag. 1974, 29, 601. (5) (a) Riekel, C.; Hohlwein, D.; Schollhorn, R. J . Chem. Soc., Chem. Commun. 1976, 863. (b) Riekel, C.; Fischer, C. 0. J . Solid State Chem. 1979, 29, 181. (6) Fernandez-Moran, H.; Ohstuki, M.; Hibino, A,; Hough, C. Science 1971, 174, 498. (7) Meyer, S. F.; Howard, R. E.; Stewart, G. R.; Acrivos, J. V.; Geballe, T. H. J . Chem. Phys. 1975, 62, 4411. (8) Gamble, F. R.; Osiecki, J. H.; Pisharody, R.; DiSalvo, F. J.; Geballe, T. H. Science 1971, 174, 493. (9) Schollhorn, R.; Zageflca, H. D.; Butz, T.; Lerf, A. Mater. Res. Bull. 1979, 14, 369. (10) Parry, G. S.; Scruby, C. B.; Williams, P. M. Philos. Mag. 1974, 29, 601. (11) Lifshitz, E.; Vega, S.; Luz, Z.; Francis, A. H.; Zimmermann, H. J . Phys. Chem. Solids 1986, 47, 1045. (12),(a) Meirovitch, E.; Belsky, I . J . Phys. Chem. 1984, 88, 6407. (b) Meirovitch, E.; Belsky, I . J . Phys. Chem. 1984, 88, 4308. (13) Schwartz, L. J.; Meirovitch, E.; Ripmeester, J. A,; Freed, J. H . J . Phys. Chem. 1983, 87, 4453. (14) Rice, D. M.; Wittebort, R. J.; Griffin, R. G.; Meirovitch, E.; Stimson, E. R.; Meinwald, Y . C.; Freed, J. H.; Scheraga, H. A. J . Am. Chem. Sot. 1981, 103, 7707.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 627
Pyridine Dynamics in Intercalated 2H-TaS2 //////////
//////////
-(0)
(b)
A
(/N//////
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1
Figure 1. Proposed pyridine orientations in the van der Waals gap of 2H-TaS2: (a) the C, axis is perpendicular to the host layers; (b) a pyridine bilayer where the molecular planes and the C2 axes are parallel to the TaSz layer; and (c) the C2 axis is parallel and the molecular plane
200
0
-200
200
0
-200
is perpendicular to the host.
metric amounts of tantalum foil and sulfur powder (99.9+% purity, Aldrich Chemical C O . ) . ~The components were placed in a quartz ampule (16 cm in length and 2.2 cm in diameter) which had been heated to drive away water adsorbed on the glass surface. The tube was then sealed off leaving the reactants under vacuum. The quartz tube was heated at 900 "C for approximately 7 days which resulted in the production of lT-TaS2,a gold flecked black powder. Slow annealing over the course of 3 days transformed the 1T-TaS2 into a black powder having trigonal prismatic metal coordination (2H-TaSz). Production of 2H-TaSz was confirmed through powder X-ray diffraction and microanalysis in addition to visual inspection. The trigonal prismatic 2H form was chosen because of its proven stability at the temperatures investigated in this study and also because of the ease of intercalation as opposed to 1TTaS2.I5 Prior to intercalation into the 2H-TaS2 host, the perdeuteriated pyridine (98 atom % zH) was dried. The effects of the presence of water on bipyridine and pyridinium ion formation in these intercalation complexes have been previously reported by Schollhorn9 and Lomax.I6 To alleviate this complicating factor, pyridine was dried by refluxing with BaO, followed by distillation and collection of the appropriate fraction at the pyridine boiling point. These steps were carried out under a dry nitrogen atmosphere to prevent reintroduction of atmospheric water. The pyridine was then degassed by the traditional freeze-pumpthaw cycle to remove dissolved oxygen from the liquid prior to intercalation. To intercalate the polycrystalline host, excess deuteriated pyridine was added to TaS2 in an evacuated sealed flask. Heat was applied to the system by immersion of the flask in an oil bath at 120 "C. After 2 days at this temperature, the excess pyridine was removed and the resultant powder was heated slightly (35 "C) while under vacuum to remove any pyridine remaining on the powder surface. The resultant intercalation complex stoichiometry was found to be (C52H,N)o,43TaS2 by microanalysis. A second sample of the intercalation complex (referred to later as sample B) was prepared by the same methods described here with newly prepared 2H-TaS2 and dried perdeuteriated pyridine. In addition to the perdeuteriated pyridine intercalation compound, a selectively deuteriated sample was prepared. Pyridine-d,, deuteriated at the ortho positions on the ring, was synthesized in the manner described by Kintzinger and Lehn.I7 Successful synthesis was confirmed by microanalysis, mass spectrometry, and *H NMR. The pyridine-d2 was dried, degassed, and intercalated in the same manner as the other samples. The ZHN M R measurements were performed with a home-built spectrometer incorporating an Oxford Instruments superconducting magnet with a field strength of 4.2 T. The spectrometer has been described in more detail elsewhere.'* An Oxford Ins t r u m e n t s CF200 c r y o s t a t and DTC2 controller were used t o provide sample heating and temperature regulation. The temperature controlling unit was accurate to within A1 K as was
121, 145. (17) Kintzinger, J. P.; Lehn, J. M. Mol. Phys. 1971, 22, 273. (18) Barbara, T. M.; Sinha, S.; Jonas, J.; Tinet, D.; Fripiat, J. J. J . Phys. Chem. Solids 1986, 47, 669.
200
0
-200
I.p,
200
-200
0
1 Al&d
385K
200
0
-200
200
0
-200
kHz Figure 2. Deuterium NMR line shapes of (C52H5N)o.4,TaS2obtained by using a solid echo sequence incorporating composite 90' pulses. The echo delay used was 35 p s and 1024 scans were averaged with equilibrium delay times of 14-10 s (280 to 400 K, respectively). Apodization with an exponential function was performed providing 1 kHz of line broadening for signal-to-noise improvement. The line shapes were symmetrized in order to increase S/N by zeroing out the quadrature channel; no changes other than an increase in S/N were observed in the frequency domain signal after zeroing when compared to raw spectra.
confirmed through the use of a secondary copper/constantan thermocouple. A solid echo sequence (9Oo,-~-9O0,) was used in obtaining deuterium line shapes,lg and an inversion recovery sequence with a 90" echo read pulse was used in the longitudinal relaxation time ( T I )measurements. Solid echo formation overcomes the problem of probe and preamplifier recovery times by delaying the echo maximum formation by a time T , allowing data collection to begin after probe recovery. A typical 90" pulse length was 2.3 ps, but due to a total line-shape width of almost 300 kHz, finite pulse length effects are significant. Reduced intensity at the parallel edge of the line shape made it necessary to incorporate composite pulses to improve the uniformity of pulse excitation. The composite 90" pulse was composed of three rotations with no interpulse separations, 135°,900-,450,~0 while the composite inversion pulse used in the T 1 experiments was a quintet of pulses, 45°,1800,900~,1800,450,.2' Significant enhancement in parallel edge intensity was seen with the use of these composite sequences. The spin-lattice relaxation time data was analyzed by using an exponential fit routine suggested by Levy and Peat," which corrects for incomplete inversion as a result of inhomogeneities in the irradiating field. TIdata were reproducible to within &IO% of the reported values. Taking into account other experimental f a c t o r s s u c h as signal-to-noise, t h e overall error i n our T1d a t a is estimated to be f15%. ~~~~
(15) Whittingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic: New York, 1982. (16) Lomax, J. F.; Diel, B. N.; Marks, T. J. Mol. Cryst. Liq.Cryst. 1985,
359 K
~~
~
(19) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976,42, 390. (20) (a) Levitt, M. H.; Suter, D.; Emst, R. R. J . Chem. Phys. 1984,80, 3064. (b) Barbara, T. M. J . Magn. Reson. 1986, 67, 491. (c) Siminovitch, D. J.; Raleigh, D. P.; Olejniczak, E. T.; Griffin, R. G. J . Chem. Phys. 1986, 84, 2556. (21) Tycko, R. Phys. Reo. Lett. 1983, 51, 775. (22) Levy, G.; Peat, I. J . Magn. Reson. 1975, 18, 500.
McDaniel et al.
628 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
lh
297K
200
0
I
200
0
I40 I 5 O l
-200
1
0 d3-pyridine
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1
I2O
342K
A d2-pyridine
t
8ol 3
-200
70
w
I I
200
0
6?i60
280
393K
300
320
T
340 360 (K)
380 400 420
Figure 4. Variation of the motionally reduced quadrupole coupling constant (qJ with temperature. The circles represent the peaks observed in the perdeuteriated sample, while the triangles represent the peaks observed in the pyridine-d2 sample. The points ranging in qrr from 155 to 165 W z are attributed to the rigid pyridine component, while the other groupings of points are due to the three motionally narrowed components seen above 338 K. All qzz calculations assumed 9 = 0. -200
kHz Figure 3. Deuterium N M R line shapes of pyridine-d2in TaSz obtained under similar conditions as the perdeuteriated samples. A 35-ps echo delay was used to collect 16384 (300 and 410 K) or 8192 (350 K) spectra which were then averaged together. Again 1-kHz line broadening was applied. 111. Results and Discussion
Powder pattern line shapes of (CS2H5N)o.43TaS2 obtained in this study are shown in Figure 2. From 280 to 333 K the line shapes are characteristic of a rigid lattice, with a quadrupole coupling constant (QCC) of 164 f 1 kHz a t 280 K. This value is slightly smaller than the value of 178.0 f 1.2 kHz reported by Barnes et for neat pyridine at 77 K. Comparison between a neat pyridine line shape at 200 K and the 280 K pyridine/TaS2 line shape shows no difference in QCC. This suggests the presence of some residual motion in pyridine at 200 K. At 338 K, motion is detected through an increasing intensity a t frequencies close to 30 kHz, while the rigid pattern perpendicular edge remains essentially unchanged at approximately 60 kHz. By 342 K, two sets of peaks have begun to grow in height a t the expense of the rigid component. A third motionally narrowed peak is resolved a t 377 K and by 385 K most evidence of the rigid pyridine has disappeared. Two pyridine fractions having vastly different mobilities (rigid and motionally averaged) have been observed in this system previously by Riekel and F i s ~ h e r They . ~ ~ concluded that above 70 "C (343 K) two pyridine phases were present. They also suggested a deintercalation mechanism wherein the more mobile of these two phases would undergo deintercalation first. Our study shows that, at temperatures below those necessary for deintercalation, the rigid phase has disappeared leaving only the mobile pyridine phase. No spectral changes are observed above 385 K in the range studied. We also observed a zero-frequency peak which grew in height with increasing temperature. The intensity of the central peak was not reproducible for spectra collected at lower temperatures after having cycled through a higher temperature. This is evi-
denced by its unexpected large amplitudes at both 377 and 338 K, because these line shapes had been acquired after having taken data through 393 K. We believe that this irreversibility is due to deintercalation taking place at the higher temperatures. This behavior allows us to attribute the peak a t zero frequency to isotropic pyridine adsorbed on the surface of the polycrystalline sample. The appearance of a central peak has been observed in other systems where the authors reached a similar c o n c l ~ s i o n . ~ ~ The results of a study of pyridine-d2 intercalated into 2H-TaS2 are shown in Figure 3. The line shapes show only one set of narrowed peaks at the high temperatures. This indicates that only one dominant motion is occurring, where each C-2H bond makes a unique angle with respect to the axis of rotation (vide infra). Otherwise, multiple Pake doublets would be observed in the pyridine-d2 line shape. The large-amplitude central peak (see Figure 3) indicates the presence of more surface pyridine than was seen in the perdeuteriated sample, which we believe to be a result of insufficient drying. Figure 4 shows an assignment of the peak observed in the pyridine-d2 spectra with respect to the three motionally narrowed peaks observed in the perdeuteriated analogue. We can then conclude that the ortho deuteron corresponds to the intermediate Pake doublet of the three motionally narrowed peaks. Spin-lattice relaxation time measurements as a function of temperature were recorded for the same perdeuteriated sample used in the line-shape experiments. The TI data acquired at any one temperature proved to be multiexponential. Conventional TI data analysis methods using a plot of In [ 1 - M ( r ) / M O versus ] T (where M ( r ) and Mo were measured from the echo maximum) revealed three lines of different slope which represent three nonequivalent Ti's in the sample. Traditionally the Fourier transformation method or the natural logarithm of intensity versus T plot are used in evaluating TI data in systems with multiple relaxation rates. These methods are not applicable to this system because of the intensity distortions introduced as a result of overlapping powder patterns. An exponential fit was, therefore, ~~~~~~
(23) Barnes, R. G.; Bloom, J. W. J . Chem. Phys. 1972, 57, 3082.
~~~
(24) Majors, P. D.; Raidy, T.E.; Ellis, P.D.J A m Chem. SOC1986, 108, 8123
Pyridine Dynamics in Intercalated 2H-TaS,
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 629
T (K) m o m h m m 0 Nmh, d
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Figure 6. Three possible rotational diffusion models suggested for pyridine: (a) rotation about the C, axis, (b) rotation about an axis per-
0
IO
H
pendicular to the symmetry axis, and (c) rotation about an axis normal to the molecular plane. The angles shown in the figure, 01,02, and 03, represent the angles that the C-2H bonds make with respect to the axis of rotation and are as follows: (a) 180°, 62.13', and 57.46'; (b) 27.87', 32.54', and 90'; (c) 90°,90°, and 90°.26
2,8
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32
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Figure 5. Relationship between short deuterium T I component and inverse temperature. The triangular points represent data taken on sample B of (C52H,N)043TaS2 to check for sample dependence of TI values.
performed for each T1region to obtain the actual relaxation times reported. In the temperature regime studied, the long component was generally on the order of 1-2 s, while the short component was under 200 ms, and the intermediate value was generally on the order of twice the size of the smallest T I value. Assignment of these different T,'s to the line-shape components through Fourier transformation of the T , data reveals that the long T , component is associated with the rigid pyridine, the intermediate T1is attributed to the isotropic pyridine adhering to the surface of the polycrystalline sample, and the shortest TI component belongs to the three motionally narrowed peaks (no distinction can be made between the Tl's of the three narrowed peaks). Fourier transformation allows us to conclude that the three components cannot be attributed to an orientational dependence of TI across the powder pattern. The presence of all three components indicates the coexistence of rigid solidlike pyridine and motionally narrowed pyridine. Figure 5 shows a graph of the short T I component versus reciprocal temperature. Note that a rapid increase in relaxation rate is observed at the same temperature that motionally narrowed peaks first appear in the pyridine-d5 line shapes. The slope of the T I curve indicates that the pyridine c 1, even is not in the rapid isotropic motion regime, where w o ~
