J . Phys. Chem. 1988, 92, 5055-5058
5055
Deuterium NMR Investigation of the Dynamics of Pyridine-Intercalated CdPS, P. L. McDaniel, G. Liu, and J. Jonas* Materials Research Laboratory and Department of Chemistry, University of Illinois, Urbana, Illinois 61 801 (Received: December 23, 1987)
Quadrupole echo deuterium NMR line shapes of perdeuteriated pyridine intercalated into CdPS3 were obtained between 260 and 360 K. At the low temperatures, a powder pattern indicative of the absence of large-amplitude motions was recorded. At temperatures greater than 280 K, the slow growth of three motionally narrowed components was observed, and by 3 10 K little evidence of the rigid pyridine component remained. The large-amplitude narrowing motion was determined to be a rapid rotational diffusion about an in-plane axis perpendicular to the C2 molecular symmetry axis. Theoretically determined motionally reduced quadrupole coupling constants (qzz)support this interpretation. On the basis of the reorientationalinformation, a preferred orientation for the pyridine molecule in the van der Waals gap is proposed.
1. Introduction
Metal phosphorus trisulfides, MPS3 ( M = Cd, Fe, Ni), and their intercalation complexes have been the subject of much interest due to their potential use in battery technology.'J A variety of techniques have been used in the study of these systems, such as electron spin X-ray d i f f r a c t i ~ n , ~neutron * ~ - ~ diffraction,1° magnetic su~ceptibility,2~~ and Raman spectroscopy." It has been shown that FePS3 and NiPS, both easily intercalate Li+ and Na+, while CdPS, will not intercalate Li+.2 CdPS,, however, will readily intercalate metallocene~~*'~ and neat Lewis bases such as pyridine,"J3 unlike the Ni analogue. It has been suggested that an explanation for this behavior might be that ion intercalating hosts must have lower bandgaps.2 In the case of NiPS,, the bandgap is 1.6 eV, while that for CdPS3 is 3.5 eV.2 In an attempt to understand the unusual intercalation chemistry of CdPS,, we investigated the dynamics of intercalated pyridine in the cadmium compound. We will compare the guest behavior seen in CdPS3 to that reported in the transition-metal di~u1fides.l~ Our recent i n ~ e s t i g a t i o n ' ~into pyridine dynamics in (CSZH5N)o,sTaS2using deuterium solid-state N M R of polycrystalline samples showed the sensitivity of this technique to the pyridine motion. Using the deuterium powder pattern line shapes of intercalated pyridine, we plan to examine the influence of the host on the guest molecule. The lamellar CdPS3 host material is composed of octahedrally coordinated Cd and P-P pairs (see Figure 1) sandwiched between sulfur 1 a ~ e r s . l These ~ layers are further separated by a van der
(1) Whittingham, M. S. Prog. Solid State Chem. 1978, 12, 41. (2) Brec, R.; Schleich, D. M.; Ouvrard, G.; Louisy, A.; Rouxel, J. Inorg. Chem. 1979, 18, 1814. (3) Lifshitz, E.; Francis, A. H. J . Phys. Chem. 1982, 86, 4714. (4) Lifshitz, E.; Gentry, A. E.; Francis, A. H. J. Phys. Chem. 1984, 88, 3038. (5) Cleary, D. A.; Francis, A. H. J . Phys. Chem. 1985,89, 97. (6) Nitsche, R.; Wild, P. Mater. Res. Bull. 1970, 5, 419. (7) Taylor, B. E.; Steger, J.; Wold, A. J. Solid State Chem. 1973, 7,461. ( 8 ) Brec, R.; Ouvrard, G.; Louisy, A.; Rouxel, J. Ann. Chim. Fr. 1980,5, 499. (9) Clement, R.; Girerd, J. J.; Morgenstern-Badarau, I. Inorg. Chem. 1980, 19, 2852. (10) LeFlem, G.; Brec, R.; Ouvrard, G.; Louisy, A,; Segransan, P. J. Phys. Chem. Solids 1982, 43, 455. (11) Barj, M.; Lucazeau, G. Solid State Ionics 1983, 9/10, 475. (12) Mathey, Y.; Clement, R.; Sourisseau, C.; Lucazeau, G. Inorg. Chem. 1980, 19, 2773. (13) Lifshitz, E.; Vega, S.; Luz, Z.; Francis, A. H.; Zimmermann, H. J . Phys. Chem. Solids 1986, 47, 1045. (14) McDaniel, P. L.; Barbara, T. M.; Jonas, J. J. Phys. Chem. 1988,92, 626. (15) Klingen, V.W.; Ott, R.;Hahn, H. Z . Anorg. Allg. Chem. 1973,396, *.I,
411.
(16) Covino, J.; Dragovich, P.; Lowe-Ma, C. K.; Kubin, R. F.; Schwartz, R. W. Mater. Res. Bull. 1985, 20, 1099.
0022-3654/88/2092-5055$01.50/0
Waals (vdW) gap into which pyridine can be intercalated. The orientation and dynamics of the pyridine in the gap are not well understood. The intercalated molecule could conceivably undergo a variety or combination of motions: 180° flipping, wobbling, low-amplitude librations, or rapid reorientational motion. All of these motions can be readily discerned from the 2H N M R line shape of a polycrystalline sample. There has been a previous study', on single crystals of (C?HSN)o,sCdPS3using 2H NMR. Lifshitz et a].', reported both the reorientational motion and orientation in the gap for perdeuteriated and selectively deuteriated pyridine at temperatures from 260 to 300 K. They concluded that the pyridine was undergoing a rapid rotational diffusion about an axis perpendicular to the molecular plane (see Figure 2a) and was oriented with the molecular plane and C, symmetry axis parallel to the host layers. This conclusion was drawn despite the presence of three distinct peaks which they identified through isotopic substitution as being due to the CY, p, and y deuterons on the pyridine. In the present study on polycrystalline (CS2H5N)o,41CdPS3, we were able to extend the temperature range over which line shapes were collected (260-360 K). In contrast to the previous singlecrystal study,', we interpret the motion as rapid rotational diffusion about an in-plane axis perpendicular to the C2 symmetry axis (see Figure 2c) in our powdered sample. This large amplitude rotational motion is supplemented by a low-amplitude libration. We also conclude the orientation in the gap to be such that the molecular plane is perpendicular to the CdPS, layers, while the C, symmetry axis is parallel to the host (see Figure 3). These results for the CdPS3 intercalation complex are very similar to that seen for the TaS2/pyridine system studied earlier in our lab.I4 This orientation was also seen for the MnPSe,(pyr),/, (pyr = pyridine) complex,Is while in the MnPSe3(pyr)1/4 complex, pyridine was found to have the parallel orientation, showing the sensitive nature of the orientation to guest stoichiometry. Experimental details are provided in section 11. Synthetic methods and 2H N M R techniques used are discussed. Section I11 contains experimental results, a detailed discussion of the data, and the resulting conclusions of our investigation. 11. Experimental Section
Polycrystalline CdPS3 was prepared by sealing stoichiometric amounts of high-purity (>99.99%), Aldrich Chemical Co.) cadmium sulfide, red phosphorus, and sulfur in a quartz ampule under vacuum. The quartz ampule had been heated prior to the introduction of the powders to drive off adsorbed water from the surface of the tube. The ampule (16 cm in length and 2 cm in diameter) was then placed in a preheated tube furnace at 800 O C (17) Foot, P. J. S.; Shaker, N. G. Mater. Res. Bull. 1983, 18, 173. (18) Otani, S.; Shimada, M.; Kanamaru, F.; Koizumi, M. Inorg. Chem. 1980, 19, 1249.
0 1988 American Chemical Society
5056
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 C*
t
McDaniel et al.
0 Cd
o
T
os 'P
290K
I
I 6 58 I
1
200
100
0 kHz
-100
-2m
Figure 1. Structure of the lamellar host material CdPS3prior to intercalation.
h
3'0K
200
100
200
100
0 kHz
-100
-200
0
-100
-200
H
(a 1 (b) (C 1 Figure 2. Three possible rotational diffusion models for pyridine: (a) rotation about an axis normal to the molecular plane, (b) rotation about the C2 symmetry axis, and (c) rotation about an axis perpendicular to the symmetry axis. The angles shown in the figure (e-, Os, and e,) represent the angles that the C-2H bonds make with respect to the axis of rotation and are as follows: (a) 90°, 90°, and 90°, (b) 57.46', 62.13', and 180°, (c) 32.54', 27.87', and 90°."
(a 1 (b) (C 1 Figure 3. Proposed orientations for intercalated pyridine: (a) pyridine bilayer where the molecular planes and the C2 axes are parallel to the host layers, (b) the C, axis perpendicular to the host layers, and (c) the C2axis parallel and molecular plane perpendicular to the host.
for 6 days. Before removal from the furnace, the sample was slowly annealed over the course of 3 days. The resultant CdPS3 was a pale yellow polycrystalline material. Prior to intercalation, the perdeuteriated pyridine (99.5 atom % *H, Merck, Sharp & Dohme Isotopes) was rigorously dried. The possible formation of bipyridine and the pyridinium ion as a result of the presence of water has been previously reported in (C5H5N)o,sTaS2.19~20 For alleviation of the effect of this complicating factor on the dynamics of the intercalated molecule, the pyridine-d5was dried by refluxing over BaO followed by fractional distillation and collection of the appropriate fraction. These steps were carried out under a dry nitrogen atmosphere to avoid the reintroduction of water to the pyridine. The dry pyridine-d, was then degassed to remove dissolved oxygen before intercalation by using the traditional freeze-pump-thaw cycle. Intercalation of the CdPS, was accomplished by combining the powdered CdPS3 and an excess of dry degassed pyridine-d, in an evacuated sealed flask. The flask was heated at 80 OC for 3 days4 by immersion of the flask in an oil bath. Noticeable swelling of the powder was observed as the intercalation reaction progressed. The excess pyridine was then removed by a vacuum distillation method followed by gentle heating (60 "C) of the intercalated (19) Schollhorn, R.; Zagefka, H. D.; Butz, T.; Lerf, A. Mater. Res. Bull. 1979, 14, 369.
(20) Lomax, J. F.; Diel, B. N.; Marks, T. J. Mol. Cryst. Lip. Cryst. 1985, 1 2 1 , 145.
200
100
0 kH2
-100
-200
kH2
Figure 4. Temperature effects on the deuterium NMR line shapes of (C5ZH5N)o,,,CdPS3.The spectra were recorded by using a solid echo sequence incorporating composite 90' pulses. A 35-ps echo delay was used, and 8192 scans were averaged by using equilibrium delays between 11 and 8 s (260 and 360 K, respectively) to avoid saturation. Apodization with an exponential function was performed providing 1000 (260 K), 2000 (280-290 K), or 500 Hz (310-330 K) of line broadening for signal-to-noise improvement. The line shapes were symmetrized to increase the 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. CdPS3 powder to remove any residual pyridine adsorbed to the crystallite surfaces. The resultant deep yellow polycrystalline material was found to have a stoichiometry of (C,H5N)o,,,CdPS3 by microanalysis. The deuterium N M R line shapes were collected by using an Oxford Instruments wide-bore (1 3-cm diameter) superconducting magnet with a field strength of 4.2 T. The spectrometer and NMR probe have been described in detail elsewhere.21s22Temperature regulation was achieved by using a Model NB Lauda circulating/heating bath or a Lauda Ultra Kryomat, Model K70DW. A copper/constantan thermocouple located directly above the sample cell was used to determine the temperature to an accuracy of f l OC. A quadrupole echo sequence (9OoX-~-9Ooy)was used to collect the deuterium line shapes.23 The use of an echo avoids problems created by the preamplifier and probe recovery times by delaying the echo maximum formation by a time T . Typically a 90" pulse length was 3.7 ps. Due to the breadth of the deuterium powder patterns (>250 kHz), significant finite pulse length effects were observed. To enhance the intensity of the parallel edge of the lineshape, we employed composite pulses. The uniformity of pulse excitation was improved by using a trio of pulses, 135OX90°-,-450,,24-26 instead of the simple noncomposite 90° pulse. Spectra were also collected by using both simple and composite quadrupole echo sequences wherein the 90' pulse length was 2.2 hs. The resulting line shapes were identical, indicating that re(21) Barbara, T. M.; Sinha, s.;Jonas, J.; Tinet, D.; Fripiat, J. J. J . Phys. Chem. Solids 1986, 47, 669. (22) Jonas, J.; Hasha, D. L.; Lamb, W. J.; Hoffman, G. A,; Eguchi, T. J . Magn. Reson. 1981, 42, 169. (23) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390. (24) Levitt, M. H.; Suter, D.; Ernst, R. R. J . Chem. Phys. 1984,80, 3064. (25) Barbara, T. M. J . Magn. Reson. 1986, 67, 491. (26) Siminovitch, D. J.; Raleigh, D. P.; Olejniczak, E. T.; Griffin, R. G. J . Chem. Phys. 1986, 84, 2556.
Dynamics of Pyridine-Intercalated CdPS3
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 5057 and eliminates the possibility of pyridine experiencing 1 80° flips.28 The three models proposed for rapid rotational motion are shown in Figure 2. We can calculate the anticipated qrr (observed quadrupole coupling constant) and compare these values to those extracted from the line shape. With use of the 330 K line shape, the qzzvalues can be found by assuming q = 0 and by knowing that
200
100
0
-100
-200
k HI
(b) 300K
where vQ is the frequency of the perpendicular edge of the line shape.29 The three motionally narrowed peaks have the following qrr: 99.5, 85.6, and 78.8 kHz. To calculate the anticipated qzpfor a particular reorientational model, we can use the following relationship:
I
qzz = YzQCC(3 COS' 0 - 1 )
200
100
0
-100
-200
kHz
Figure 5. 2H NMR line shapes of (C5ZH5N)o.41CdPS3 at 300 K under similar conditions as the spectra in Figure 4: (a) Line shape of newly prepared sample and (b) same sample as in a after having been temperature cycled through 360 K.
orientational motion during the pulse did not have a major effect on the line shapes. 111. Results and Discussion
Powder pattern line shapes of pyridine-d5 intercalated CdPS3 are shown in Figure 4. At 260 K, the line shape is a typical rigid lattice powder pattern. The quadrupole coupling constant (QCC) is determined to be 163 i 1 kHz at this temperature. Comparison with the solid pyridine QCC of 178.0 f 1.2 kHz taken at 77 K reported in the literaturez7 hints at the presence of some additional averaging motion at 260 K in the intercalated environment. As the temperature is increased, the spectra indicate the presence of the onset of motion (see Figure 4). At 280 K motion is evident through the growth in intensity at about f40 kHz. By 285 K, it appears that there are at least two motionally narrowed peaks which are growing in intensity relative to the rigid perpendicular edges at approximately i 6 0 kHz. At 310 K, there remains very little evidence of rigid pyridine. For temperatures at and above 310 K, three motionally narrowed peaks are clearly evident. A zero-frequency peak is present at all temperatures. This has been attributed14 to isotropic pyridine that has not been entirely removed from the powder surface. We believe that slight deintercalation took place at the higher temperatures because initially there was a smaller isotropic pyridine peak at temperatures below 300 K. With high-temperature cycling, however, followed by the collection of data at a low temperature, the amplitude of the central peak had increased significantly (see Figure 5). This behavior was observed in our previous study but to a lesser extent.14 We believe this to be due to the low temperature of deintercalation for pyridine in CdPS3, which is reportedly 385 K.4 An interesting feature of this system is the sudden drop in signal-to-noise seen at the onset of motion (280 K). This would indicate that the system is experiencing irreversible dephasing typically seen in an intermediate exchange regime. In this exchange-rate region, the portions of the line shape having T2values smaller than the echo delay time, T , will not be totally refocused; therefore, a drop in signal intensity is to be e x p e ~ t e d . ~ ' . ~ ~ A comparison of the motionally narrowed line shapes with the motional models mentioned in section I allows us to determine the movement of the intercalated pyridine. The three motionally narrowed line shapes at and above 310 K have a zero asymmetry parameter ( q ) , which would suggest that the molecule was undergoing a rapid rotational diffusion of 3-fold or higher symmetry (27) Barnes, R.
G.; Bloom, J. W. J . Chem. Phys. 1972, 57, 3082.
where QCC is the rigid lattice quadrupole coupling constant and 6 is the angle made by the C-2H bond with respect to the axis of r o t a t i ~ n . ~The ~ , ~angles ~ have been determined from microwave data3' and are shown in Figure 2. With use of eq 2, a rigid lattice QCC of 178.0 ~ H Zand , ~the~known values for the various e's, the following are the expected qrZ(8J, qrr(Ba),and qrz(Oy)for deuterons a,D, and y for the three reorientational models: (a) 89.0,89.0,89.0kHz;(b) 11.75, 30.65, 178.0kHz;(c) 100.8, 119.6, 89.0 kHz. An examination of these values allows us to rule out two of the models. In case a (shown in Figure 2), where the rotation is about an axis perpendicular to the molecular plane, all three C-2H bonds would make the same angle with respect to the axis of rotation; therefore, we would expect to see only one peak. We, however, see three distinct peaks. In case b,, we would expect to see three peaks. If rotation about the C2symmetry axis was occurring, the y deuteron would not undergo any motional narrowing, while the a and p deuterons would be extremely narrowed. The collapse of the line shapes due to the a and p deuterons is attributed to the fact that the angles that those CSH bonds would make relative to the axis of rotation are very close to the magic angle of 54.7O. This value of 0 causes the 3 cos2 0 - 1 term in eq 2 to go to zero. Comparing this model to our observed line shape, we see no residual evidence of the rigid peak with increasing temperature. Also, the motionally narrowed peaks do not suffer the narrowing effect expected in the presence of this motion, showing the failure of case b. We therefore conclude that the case c rotational model is the most likely. Each of the three deuterons would make slightly different angles with respect to the rotation axis; thus we would expect to see three overlapping line shapes. This same conclusion was reached for the TaSz/pyridine system studied earlier in our iab.14 If we directly compare the qrr values extracted from the 330 K line shape to those calculated by using eq 2 (experimental: 85.6, 99.5, 78.8 kHz; theory: 100.8, 119.6, 89.0 kHz), a significant discrepancy is seen. The smaller values determined experimentally could be caused by a small but nonzero value for q or slight deviations from the angles (ea,e,, e,) determined from microwave data relative to those in the intercalated environment. Another possibility is the presence of an additional motion that would further average the rigid lattice QCC. Rapid low-amplitude librational motion has commonly been cited as the reason for reduced rigid lattice QCC value^.^^^^^ Librational motion can be described as a rapid low-amplitude motion of the C-2H bond within a cone fixed in The motion is conic in nature, (28) Gall, C. M.; DiVerdi, J. A,; Opella, S. J. J . Am. Chem. SOC.1981, 103, 5039. (29) Meirovitch, E.; Krant, T.; Vega, S.; J . Phys. Chem. 1983,87, 1390. (30) Mehring, M.; Griffin, R. G.; Waugh, J. S. J . Chem. Phys. 1971,55, 746. (31) Bak, B.; Hansen-Nygaard, L.; Rastrup-Anderson, J. J . Mol. Specirosc. 1958, 2, 361. (32) Meirovitch, E. J . Phys. Chem. 1984, 88, 6411. (33) Meirovitch, E.; Belsky, I. J. Phys. Chem. 1984, 88, 6407.
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The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
TABLE I: S , and Be as a Function of Temperature temp, K S, e., deg temp, K S,
Be, deg
22 22 25
25 28 29
300 310 330
0.89 0.89 0.86
340 350 360
0.86 0.83 0.82
having semiangle e, and the order parameter, S,, used to describe the effect of the motion on the QCC is given by
qrr = 1/2QCC.Sc.(3 cos2 0 - 1)
(3)
= 1/2QCC*(3 COS' 0 - 1 )
(4)
e,( 1 + cos e,)
(5)
where S, =
1/2
cos
QCC* is the librationally reduced QCC, and all other parameters have been described. The order parameter is temperature dependent, so the angle 1 9 will ~ change with sample temperature. From eq 4, QCC* can be calculated, and with the fact that S, = (QCC*)/(QCC), 0, can be determined for a particular temperature. At 330 K S, was found to be 0.86, which gives 0, 25 OC. Table I shows the change in S, and 0, with variation in temperature. The amplitude of the librational motion indicates that the intercalated pyridine has more motional freedom in the CdPS3 host when compared to 2H-TaS2 as seen in our previous study.I4 This conclusion is also supported by the lower onset temperature for the large-amplitude rotational diffusion in the CdPS3 system. The lower intercalation and deintercalation temperatures further suggest that the pyridine is less tightly held in the CdPS3 vdW gap relative to 2H-TaS2. On the basis of the similarity of the (C52HSN)o,41CdPS3 system to (C52H5N)o,5TaS2 in terms of the reorientational and librational motions and vdW gap expansions, we speculate that the Orientation of the pyridine in the gap is such that the molecular plane is perpendicular to the host layers and the C2 symmetry axis is
=
McDaniel et al. parallel (see Figure 3c). A reported vdW gap expansion of 5.85 A in CdPS3 upon intercalation with pyridine would suggest that a monolayer of pyridine laying flat in the gap would be unlikely, as suggested by Lifshitz et al.I3 For the bilayer configuration (Figure 3a), any large-amplitude motion would be hindered. We might expect rotation about an axis perpendicular to the molecular plane or a wobbling motion, neither of which are supported by our line-shape temperature study. Cases b and c (Figure 3) are both possible purely on the basis of gap expansion, and the determination of the reorientational motion alone does not allow us to absolutely determine a preferred orientation. Both orientations in the gap would allow for rotation about an in-plane axis perpendicular to the C, symmetry axis, but we believe case c to be the likely orientation simply by analogy with other pyridine intercalation c o m p l e x e ~ . ' ~ J ~ In summary, our 2H N M R study has allowed us to determine the reorientational motion of pyridine intercalated into CdPS3. At 285 K, evidence of motional narrowing was detected through the appearance of three overlapping powder patterns. With increasing temperature, the intensity of the motionally narrowed peaks grew, passing through an intermediate exchange regime, until finally above 310 K, little evidence of the rigid pyridine remained. Comparison of experimental qrr values with calculated motionally reduced quadrupole coupling constants showed that the pyridine was undergoing rapid rotational diffusion about an in-plane axis perpendicular to the C2symmetry axis in addition to low-amplitude librational motion. By analogy with both the TaS2/pyridineI4and MnPSe3(pyr)l/3systems, we concluded that the preferred orientation of the pyridine in the gap is such that the nitrogen lies equidistant between the layers as shown in Figure 3c.
Acknowledgment. This work was supported in part by the National Science Foundation under Grants NSF DMR 86-12860 and N S F C H E 85-09870. We also thank the referees for their helpful comments. Registry No. CdPS3, 60495-79-6; CSZHSN,7291-22-7.
