7120
J . Plzys. Clzem. 1995, 99, 7120-7129
NMR Study of Dynamics of Dimethyl Sulfoxide Molecules in Kaolinite/Dimethyl Sulfoxide Intercalation Compound Shigenobu Hayashi National Institute of Materials and Chemical -Research, I -I Higashi, Tsukuba, Ibaraki 305, Japan Received: November 15, 1994; In Final Form: February 20, 1995@
We have studied molecular motions in kaolinite/dimethyl sulfoxide (DMSO) compounds by means of solidstate NMR. Quantitative analyses of thermogravimetric analysis and 13Cand 29Simagic-angle-spinning (MAS) NMR spectra reveal the formation of A12Si205(0H)~(CH3)2SO. 13CMAS NMR spectra show two inequivalent methyl carbons for interlayer DMSO molecules over the temperature range 170-330 K, demonstrating that no exchange between the two methyl sites takes place. We have measured 13C spin-lattice relaxation times TI for kaoliniteDMS0, where 13Cspins relax by rotation of the CH3 group around the C3 axis. Activation energies for the methyl rotation have been estimated for the two sites, 13.0 and 16.5 W/mol for the keyed and unkeyed methyl groups, respectively. 2H spectra have been measured for kaolinite/DMSO-d6 between 160 and 330 K, which are composed of resolved doublet (RD) and narrow central (NC) components. NC is a surface-adsorbed species, while RD contains surface and interlayer molecules. Line shapes indicate that RC undergoes rotation of the CD3 group around the C3 axis and NC does isotropic rotation. 2H TI has been measured also in the above temperature range, and 2H spins are relaxed by the CD3 rotation around the C3 axis. 29Sispins in the kaoliniteDMSO compound relax by the CH3 rotation in addition to the relaxation by paramagnetic impurities, indicating the contribution of 13C-IH dipolar interaction between the guest molecule and the host. The lH chemical shift of hydroxyl groups in the host was determined by the CRAMPS technique, which is not changed by the intercalation.
-
Introduction Kaolinite, A12Si205(OH)4, is a layered aluminosilicate with a dioctahedral 1:l layer structure consisting of an octahedral aluminum hydroxide sheet and a tetrahedral silica sheet.1-5 Many compounds can intercalate between kaolinite layers: formamides, acetanilides, dimethyl sulfoxide (DMS0),6-8 fatty acids? alkali acetates,8 and alkali halides.1° Thompson1' has studied kaolinite/DMSO intercalation compounds by means of high-resolution solid-state 13CNMR and found that there were two methyl sites. Thompson and Cuff12 and Raupach et al.13 have analyzed X-ray and neutron powder diffraction data. In their starting model, DMSO molecules are ordered in the interlayer space and their orientation is perpendicular to the layer plane. One methyl group is keyed into the ditrigonal holes in the silicate sheet, whereas the other is approximately parallel to the sheet. The sulfonyl oxygen forms hydrogen bonds with the inner-surface hydroxyls of kaolinite. Using this model, they arrived at a refined structure, which is schematically shown in Figure 1. In a previous work, we have verified the orientation of the guest molecule by use of cross-polarization (CP) dynamics from 'H in the host kaolinite to 13C in the guest deuterated DMSO CP takes place through the dipole-dipole interaction between nuclear spins whose magnitude depends markedly on the internuclear distance. Locations of methyl groups in the interlayer space were clearly demonstrated. At the same time, the two 13C NMR signals for the interlayer DMSO were unambiguously assigned to the two methyl sites; the high-frequency signal is ascribed to the keyed methyl carbons, while the low-frequency signal is attributed to the methyl carbons parallel to the sheet. Duer et al.15316have studied molecular motions in kaolinite/ DMSO-& compounds by means of 2H NMR. They proposed @
Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-3654/95/2099-7 120$09.00/0
Aluminum hydroxide sheet .
0
0
d
U
@
__
@ +$j
@
-
0
@
Silicasheet
v
d
-
@
@
@
Aluminum hydroxide sheet
e
@
@
Silicasheet
Figure 1. Schematics of the structure of the kaoliniteDMSO intercalation compound: darkly shaded circle, H; lightly shaded circle, H or D.
another structure model, where one DMSO molecule is keyed into the kaolinite lattice and the other is oreinted differently. This model was based on only their 2H NMR data, and no discussion was made about the consistency with other structural data described above. In the present work, we have studied motions of the guest molecules in the kaolinite/DMSO (and kaolinite/DMSO-d6) compounds mainly by means of 13C and 2H NMR. We measured temperature dependences of spin-lattice relaxation times TI as well as spectra. The obtained results demonstrate only the rotation of methyl groups around their C3 axis for the interlayer DMSO (and DMSO-&) molecules. NMR data of 29Si, 27Al, and lH are also presented.
Experimental Section Materials. Kanpaku kaolinite was obtained from The Clay Science Society of Japan, which was coded as JCSS-1101, 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 18, 1995 7121
Kaolinite/Dimethyl Sulfoxide Intercalation Compound
I
A
-
0 tn
B
C IO 0
10.0
MO
50.0
40 0
MO
h 1 0
28
Figure 2. X-ray powder diffraction pattems of (A) kaolinite (sample I), (B) kaoliniteDMSO (sample 11), and (C) kaolinitelDMS0-ds (sample 111).
Hinckley indexI7 of 0.50. Kaolinite/DMSO and kaolinitel DMSO-d6 compounds were synthesized according to the procedures of Duer et DMSO and DMSO-& were obtained from Nacalai Tesque, Inc. (Kyoto, Japan) and E. Merck (Darmstadt, Germany), respectively. The deuterium enrichment in DMSO-& was higher than 99 atom %. Kaolinite powders were immersed in DMSO and DMSO-& and kept at room temperature for 2 months. Some samples were immersed at 333 K for 1 week. They were dried at 333 K for 2 days. X-ray powder diffraction pattems were measured by a Rigaku RAX-01 Geiger Flex diffractometer with Cu K a radiation. Thermogravimetric (TG) and differential thermal analyses (DTA) were performed by a Rigaku Thermoflex TGSl10. The sample temperature was raised at a rate of 5 Wmin in air atmosphere. Most of data presented below are concerned with the following three samples: the original kaolinite, kaoliniteDMSO synthesized at room temperature, and kaolinite/DMSO-d6 also synthesized at room temperature, which are coded as the samples I, 11, and 111, respectively. NMR Measurements. NMR measurements were carried out by Bruker MSUOO and ASX200 spectrometers with static magnetic field strengths of 9.4 and 4.7 T, respectively. MSL400 was used for measurements of magic-angle-spinning (MAS) spectra of I3C (Larmor frequency VL of 100.61 MHz), 'H (400.14 MHz), 29Si (79.50 MHz), and "Al (104.26 MHz), and 2H static spectra (61.42 MHz). ASX200 was used for measurements of I3C MAS (50.32 MHz) and 'H CRAMPS (200.13 MHz) spectra. I3C spectra were measured using CP and single pulse sequence with high power 'H decoupling (named HD) under MAS conditions. I3C spin-lattice relaxation time T I was measured with the inversion recovery method and Torchia's method18 under MAS conditions. 2H spectra were obtained for static samples by Fourier transformation of the latter half of the echo signal obtained with quadrupole echo pulse sequences. *H TI was measured with the inversion recovery method combined with the quadrupole echo pulse sequence for signal detection. 29Sispectra were measured using the CP technique under MAS conditions, since 29Si T I was very long. 29Si T I
was measured with Torchia's method. 27Al spectra were measured with the single pulse sequence and high power 'H decoupling under MAS. 'H MAS spectra were obtained with the single pulse sequence using a Bruker high-speed-spinningtype probehead. 'H CRAMPS spectra were measured with BR24 multiple pulse sequence in quadrature detection model9 using a Bruker CRAMPS probehead. Spectra were presented with the following signals being 0 ppm or 0 Hz: neat tetramethylsilane for I3C, 29Si, and 'H; D2O for 2H; and 1 M (mol/dm3) Al(N03)3 aqueous solution for 27Al. The higher frequency side of the spectra with respect to the standard signal was expressed as positive. Line shapes, relaxation curves, and TI values were analyzed by our computer programs. Intensities of the spinning sidebands in I3C MAS spectra were analyzed by our computer programs constructed according to Fenzke et aLZ0 Chemical shift interaction is defined as
(3) VC
= (d22 - d11)/(d33
- diso)
(4)
where 811 , 8 2 2 , and 8 3 3 are principal values of the chemical shift tensor, 8isois an isotropic chemical shift, Ad, is the magnitude of chemical shift anisotropy, and vc is an asymmetric factor.
Results and Discussion
X-ray Powder Diffraction. Figure 2 shows X-ray diffraction pattems for the original kaolinite and the intercalation compounds synthesized at room temperature. The basal spacing of the original kaolinite i s 0.716 nm,I4 whereas those of the intercalation compounds are 1.12 nm. The intercalated samples contain the original kaolinite. The samples synthesized at 333 K show pattems similar to B and C in Figure 2, except that they contain smaller amounts of the original kaolinite.
Hayashi
7122 J. Phys. Chem., Vol. 99, No. 18, 1995 TABLE 1: Quantitative Analyses of TG and I3C and 29Si NMR Spectra sample no. guest molecule
I1 DMSO
TG weight loss (380-460 K) (wt %) 7.1 x in A ~ ~ S ~ ~ O ~ ( O H ) ~ T ( C H ~ ) ~ S O0.25 I3CNMR fraction of interlayer DMSO (mol %) 76 x in A12Si205(0H)4*x(CH3)2SO 0.19 29SiNMR fraction of intercalation compounds 0.23
111
DMSO-& 10.7 0.37 77 0.28 0.31
Thermogravimetric Analysis. DMSO evolves at two steps when the sample is heated. The first step begins at 300 K and finishes at 370 K, while the second begins at 380 K and finishes at 460 K. They are centered at 330 K and 430 K, respectively. Both are accompanied by broad endothermic peaks. Samples I1 and I11 have only the second peak. Table 1 lists the weight loss and the number of DMSO molecules per A12Si205(OH)4 unit. Sample I has no peaks. For incompletely dried samples, both peaks are observed. I3C NMR. Figure 3 shows I3C MAS NMR spectra of kaoliniteDMSO (sample 11). Two intense peaks with almost equal intensities are observed at 44.2 and 43.1 ppm, and a third broad peak is observed at 40.0 ppm. Although the latter peak is not clearly shown in the CPMAS spectra (Figure 3A), it has considerable intensity in a spectrum measured with the single pulse sequence together with high power 'H decoupling (HD), as shown in Figure 3B. The third peak is assigned to DMSO molecules adsorbed on the outer surface. CP is ineffective for those molecules, since molecular motion is too fast. Figure 3C shows a CPMAS spectrum measured at the lower Larmor frequency, which has peaks at the same positions. Two inequivalent methyl carbons are confirmed to be present from the spectra. From the previous work, the 44.2-ppm peak is ascribed to the keyed methyl group and the 43.1-ppm peak to the methyl group located in parallel to the sheet.I4 Figure 4 shows I3C MAS spectra of kaolinite/DMSO-d6 (sample 111). Two peaks with almost equal intensities are observed again at 43.4 and 42.2 ppm. Each line width is broader than the corresponding peak in kaoliniteDMS0, since the dipole-dipole interaction between I3C and *H is not excluded completely. A third broad peak is observed at 39.7 ppm in the HDMAS spectrum and not in the CP/MAS spectrum. A lower frequency shift of about 0.9 ppm caused by deuteration has its origin in the isotope effect of DMSO itself. The chemical shifts of neat liquid samples are 41.7 and 40.8 ppm for DMSO and DMSO-&, respectively.21 Thus, deuteration is considered to have no effect on the structure of the guest molecules. Basal spacing, whose original value was 0.716 nm, becomes 1.12 nm for both kaoliniteDMSO and kaolinite/DMSO-& We have deconvoluted the HD/MAS spectra to estimate the amount of surface-adsorbed DMSO molecules. The fractions of the interlayer DMSO (or DMSO-&) molecules are 0.76 and 0.77 for the samples I1 and 111, respectively, as listed in Table 1. Using these fractions and the total amount of DMSO estimated from TG, the interlayer DMSO molecules per AlzSi205(OH)4 unit are 0.19 and 0.28 for the two samples, respectively. Although TG results have no lower temperature weight loss in samples 11 and 111, these samples contain considerable amounts of surface-adsorbed DMSO molecules. This fact demonstrates that the outer-surface DMSO molecules adsorb as strongly as the interlayer DMSO for those samples from the standpoint of TG. The incompletely dried samples have a large signal at 41.1 and 40.3 ppm for kaoliniteDMSO and kaoliniteDMSO-&
I
50
45
40 PPm
Figure 3. I3C MAS NMR spectra of kaoliniteDMSO (sample II), measured at room temperature: (A) C P N A S at vL = 100.61 MHz, (B) H D N A S at 100.61 MHz, and (C) CP/MAS at 50.32 MHz. The spinning rate was 3.00 kHz; the contact time for CP was 2 ms.
I
50
45
40 PPm
Figure 4. I3C MAS NMR spectra of kaoliniteDMSOd6 (sample III), measured at room temperature and at YL = 100.61 MHz. The spinning rate was 3.00 kHz: (A) CP/MAS with a contact time of 20 ms and (B) HD/MAS.
intercalation compounds, respectively. The positions are shifted to higher frequency by about 1 ppm when compared with samples I1 and 111. Although these peaks are assigned to outersurface DMSO molecules, the adsorption strength is weaker than those in samples I1 and 111, as indicated by TG. TG results show a lower temperature weight loss for these incompletely dried samples. We have measured I3C CP/MAS spectra of sample I1 at 100.61 MHz and at a low spinning rate of 700 Hz. Sideband intensity analysis gives us parameters of chemical shift interactions: Ada = -41 f 4 ppm, vc = 0.85 f 0.11 for the peak of d,,, = 44.2 ppm; Ad, = -38 jl 6 ppm, vc = 0.74 f 0.14 for the peak of d,,, = 43.1 ppm. Principal components of the chemical shift tensor are 611 = 69 & 3, 8 2 2 = 46 f 2, and d33 = 17 f 3 ppm for the former, and 611 = 65 f 3, 8 2 2 = 46 f 2, and d33 = 18 f 4 ppm for the latter. There seems to be little difference in the anisotropic chemical shift interactions
Kaolinitemimethyl Sulfoxide Intercalation Compound
J. Phys. Chem., Vol. 99, No. 18, 1995 7123
A
330K
3
4
5
6
(K9
1031~
Figure 7. Temperature dependence of I3C spin-lattice relaxation time of kaolinite/DMSO (sample 11), measured at V L = 100.61 MHz. The isotropic chemical shifts are (0)44 ppm and (A)43 ppm. The inversion recovery and Torchia's methods were used above and below 294 K, respectively. Both methods were used at 294 K. The lines in the figure show the calculated values.
Ih
190K I
6
50
TABLE 2: Observed 13C T1 Values at Different Magnetic Fields sample no.
40
I1
Figure 5. Temperature dependence of I3C CPNAS NMR spectra of the kaoliniteDMSO compound (sample 11), measured at Y L = 100.61 MHz. The spinning rate was about 1.7 kHz; the contact time was 2 ms.
44
h
E c *-w 42 v
1
, 200
T I (ms) and temperature at shift (ppm) 100.61'MHz 50.32 MHz
1
45 PPm
40
guest
250
300
3
T (K) Figure 6. Temperature dependence of I3C isotropic chemical shifts (0, A) and their difference (0) of kaoliniteDMSO (sample 11), measured at YL = 100.61 MHz.
between the two methyl carbons. The unkeyed methyl carbons deviate from the axial symmetry more than the keyed methyl carbons. Figure 5 shows the temperature dependence of I3C CPNAS spectra of sample 11. Two intense peaks are always observed in the temperature range between 170 and 330 K. The isotropic chemical shifts are plotted in Figure 6. The peak positions shift toward the higher frequency side with increase in temperature. The magnitude of the frequency shift is larger for the lowfrequency peak than for the high-frequency peak. Therefore, the interval between the two peaks gradually decreases. As shown in Figure 5, the 43-ppm peak has a broader line width than the other peak at low temperature, and the width decreases
I11
44.2 390(294K) 400 (295 K) 43.1 270 (294 K) 230 (295 K) DMSO-& 43.4 4400 (294 K) 5700 (320 K) 42.2 2500 (294 K) 4100 (320 K) DMSO
as the temperature increases. The position and the line shape of the low-frequency peak are much more dependent on temperature than those of the high-frequency peak. If the exchange between the two methyl sites takes place, the interval between the two peaks should begin to decrease and finally collapse when the exchange rate is compatible with the peak interval. The fact that two peaks are observed over the temperature range studied indicates that the exchange between the two sites is slow enough compared to the peak interval of 50 Hz (see Figure 3C). We have measured I3C spin-lattice relaxation times TI in the temperature range from 170 to 320 K. The magnetization recovery curves are exponential. Figure 7 shows the temperature dependence of 7'1 for the two methyl peaks of the interlayer DMSO molecules. Minimum TI values are observed for both peaks, and the relaxation mechanism is considered to be motional origins. The dipole-dipole interaction between 13C and 'H is considered to be fluctuated by the motion. I3C 7'1 of kaolinite/DMSO-d6 (the sample 111) was measured at 100.61 MHz with the inversion recovery method. The TI values are listed in Table 2: 4.4 s at 294 K and 5.7 s at 320 K for the 43.4-ppm signal, and 2.5 s at 294 K and 4.1 s at 320 K for the 42.2-ppm one. Deuteration makes TI very long, indicating that I3C-IH dipolar interaction within the same CH3 group plays a dominant role in the 13Crelaxation of kaoliniteDMS0. I3C 7'1 of kaoliniteDMSO (sample 11) was measured at the lower Larmor frequency of 50.32 MHz and at room temperature with the inversion recovery method. TI values are 400 and 230 ms for the 44.2- and 43.1-ppm peaks, as listed in Table 2. These values agree with those measured at 100.61 MHz within the experimental errors. There are no field dependences in the relaxation, indicating that the room-temperature value is on the high-temperature side of the BPP-type relaxation. We have analyzed the temperature dependence of the
Hayashi
7124 J. Phys. Chem., Vol. 99, No. 18, 1995 TABLE 3: Parameters Obtained from T I Analyses samvle nd.
guest
shift (uvm)or nuc c;hp&ent
I3C
I1
DMSO
I11
DMSO-& I3C
44 43 43 42 RD NC
2H a
E, ( k ~ / i o l ) vo(rad/s) 13.0 16.5 13.3 16.9 15.0 10.3
1.2 x 10l3 2.8 x lOI3 0.85 x lOI3 2.0 x 1013 1.2 x lOI3 0.13 x 10I3
0.86 0.77 0.86b 0.17b 0.52 0.S2b
Correction factor: (l/T&bs = (l/T1)~~1&.Assumed.
relaxation time by BPP theory. According to Noggle,22I3C TI of a methyl group is described as
1
1 ( 3 cos2 - 1)2ro+ (3 sin2 e cos2 e)z,
J(w) =4
1 +W22,2
1
+ W2t,2
100
+
measured at Y L = 61.42 MHz. Quadrupole echo pulse sequence was used with a pulse interval of 25 ps.
(74 (7b)
l/z2 = 2R, i4R,,
(7c)
where R l and RIIare rotational diffusion constants perpendicular and parallel to the C3 axis. Methyl groups in the interlayer DMSO molecules undergo a rotation around the C3 axis, as will be demonstrated by 2H NMR spectra below. Assuming that only the C3-axis rotation contributes to the I3C relaxation, we have analyzed the T I data. The rotational diffusion constant perpendicular to the C3 axis (R,) is 0, so that l/zo = 0, l/zl = RII,and 1/22 = 4Rll. The Arrhenius relation is assumed for the temperature dependence of the correlation time: 1- yo exp( 21
-50
Figure 8. 2H NMR spectra of kaoliniteDMSO-& (sample 111),
+ R,,
l/z, = 5R,
1
0 k Hz
where yc and ys are nuclear gyromagnetic ratios of I3C and S (IH or 2H) spins, respectively, S is a nuclear spin quantum number of the S spin, rC-H is a distance between I3C and S spins, ocand ws are resonance frequencies in angular frequency units, and 8 is the angle between the C3 axis of the methyl group and the C-H(D) vector. For a tetrahedral angle, cos2 6 = 119. TO, z1, and 22 are correlation times, and are described as
l/zo = 6R,
I
50
-2)
where YO is a frequency factor at infinite temperature, E, is an activation energy, R is the Boltzmann constant, and T is the temperature in K units. Using eqs 5 and 8, we have analyzed the temperature dependence of I3C T I . The calculated T I values are shown by the solid lines in Figure 7, and the obtained parameters are listed in Table 3. The keyed methyl groups rotate faster than the unkeyed groups. The activation energy for the keyed methyl group (13.0 kJ/mol) is smaller than that of the unkeyed group (16.5 kJ/mol). A correction factor is introduced to fit the calculated TI values to the experimental ones. If the C-H
distance is assumed to be 0.109 nm, the correction factors are 0.86 and 0.77 for the two peaks, respectively. Considering experimental errors, the observed T I agrees with the calculated T I . Consequently, we can conclude that I3C spins relax only by methyl rotation around the C3 axis. We have analyzed I3C TI values for kaolini /DMSO-& similarly. The frequency factor is assumed to be v 2, where YO is the factor of kaolinite/DMSO. The correction ctors are assumed to be the same as those in kaolinite/DM '0. The obtained activation energies, listed in Table 3, are 13.3 and 16.9 kJ/mol for the keyed and the unkeyed methyl groups, respectively. Those values are in good agreement with the values in kaolinite/DMSO. ZHNMR. 2H NMR spectra are suitable to study molecular motions whose rate is of the order of 100 Mz. We have measured 2H NMR spectra of the kaolinite/DMSO-d6compound in the temperature range between 160 and 330 K. Figure 8 shows 2H spectra at several temperatures. A Pake doublet pattern is observed at 160 K. With increase in temperature, a narrow component grows up at the central region, and the Pake doublet component gradually changes its line shape. The total signal intensity decreases gradually and monotonically with increase in temperature. No intensity loss with motional origins was observed in the above temperature range. We have deconvoluted the spectra into two components: resolved doublet (called RD) and the narrow central (NC) components. Figure 9 shows an example at 300 K. Theoretical powder line shapes for a I = 1 nucleus without motion are assumed for the RD component, while a Lorentzian line shape is used for the NC component. Convolution of the two components reproduces the observed spectra very well over the temperature range studied. The RD component can be ascribed to CD3 groups rotating around the C3 axis only, while the NC component is attributed to the groups in isotropic motion. We have measured dependence of the line shape on the time interval between the two pulses in the quadrupole echo pulse sequence by changing the pulse interval from 5 to 300 ps at all the temperatures studied, and no dependence was observed except that the NC component decays faster than the RD component.
Y
J. Phys. Chem., Vol. 99, No. 18, 1995 7125
KaoliniteDimethyl Sulfoxide Intercalation Compound 60
0. 4
55
0.3
50
0.2
45
0. 1
Obs.
h
z
5
P
6i
Calc.
c-4
*L *7 40 1
3O
Comp.
T
6)
Figure 11. Temperature dependence of the quadrupole coupling constant (0)and the asymmetric factor ( 0 )of the RD component in kaolinite/DMSO-d6 (sample 111).
N C i L 102
50
100
0
-50
1: Hz
Figure 9. Deconvolution of *HNMR spectra obtained at 300 K.
h
?!
v
h -
tI
*
I
10 I
I
I
3
4
5
6
1 0 3 1 ~( ~ - 1 )
t *
01150-
‘
200
Figure 12. Temperature dependence of *H77 of the RD (0)and NC (A)components in kaolinite/DMSO-d6 (sample III), measured at Y L = 61.42 MHz. The lines in the figure show the calculated values. 250
300
I
350
T (K) Figure 10. Temperature dependence of the fraction of the NC component in the 2H NMR spectra of kaolinite/DMSO-d6 (sample 111).
Figure 10 shows the fraction of the NC component. This component increases monotonically with temperature, and its fraction reaches 22% at 330 K. From I3C NMR, this sample was found to contain 23 atom% surface-adsorbed DMSO molecules. At 160 K the RD component contains both the interlayer and the outer-surface DMSO molecules. With increase in temperature, a part of the outer-surface DMSO molecules initiate isotropic motion, resulting in the growth of the NC component. At 330 K, most of the outer-surface DMSO molecules undergo isotropic motion, while the interlayer DMSO molecules remain in anisotropic motion. From the simulation of the line shape, we have obtained parameters of quadrupole interaction for the RD component. Figure 11 shows the quadrupole coupling constant (QCC = e2Qq/h)and asymmetric factor (VQ) of the RD component. At 160 K, QCC = 59 lEHz and VQ = 0. This means that the methyl groups rotate only around their C3 axis and that the fluctuation of the C3 axis is negligible. With increase in temperature, QCC decreases gradually up to 280 K and rapidly above 280 K. VQ is zero below 250 K and is not zero and increases above 250 K. Motions other than the C3-axis rotation might take place,
and those motions are fast enough not to give patterns in the intermediate motional regime; Le., they should be much faster than 100 kHz. We have measured 2H TI to study molecular motions of the order of several megahertz. The inversion recovery method was used together with the quadrupole echo pulse sequence. The recovery curve for the RD component is almost exponential at and above 260 K. Nonexponential behavior is not negligible below 260 K, in which T I is estimated from the time point that the magnetization recovery becomes l/e of the initial value. On the other hand, the NC component shows exponential decay over the temperature range studied. The nonexponentiality of the RD component is considered to be caused by the overlapping of the signal of the surface-adsorbed species. Figure 12 shows the temperature dependence of 2H T I . We have analyzed it theoretically. The relaxation mechanism is considered to be a fluctuation of the quadrupole interaction caused by motions. TI caused by quadrupole interaction for I = 1 spins can be expressed theoretically as follows:” -I-4J(2wD)]
(9)
The temperature dependence of the correlation time is shown in eq 8. The RD component undergoes rotation around the C3 axis, as suggested by its line shape. The rotational diffusion constant
Hayashi
7126 J. Phys. Chem., Vol. 99, No. 18, 1995 100 80
60 h .r
z.
I
20
\
0
40
I 0
,
, 10
,
, 20
30
t1/2 ( 4 1 2 )
Figure 14. 29Si spin-lattice relaxation curves of kaolinite (A), kaoliniteDMSO (0),and kaolinite/DMSO-& (O), measured at YL = 79.50 MHz and a spinning rate of 3.00 kHz and with Torchia’s method. The line is a least-squares fit of the kaolinite data.
-85
-90
-95 PPm
Figure 13. 29Si CP/MAS NMR spectra of (A) kaolinite (sample I), (B) kaoliniteDMSO (sample 11), and (C) kaoliniteDMSO-& (sample 111), measured at YL = 79.50 MHz and a spinning rate of 3.00 kHz. The contact time was 8 ms.
perpendicular to the C3 axis (RJ is 0, so that l/zo = 0, l/zi = RII,and I / q = &?I. The calculated TI values are shown by solid lines in Figure 12, and the obtained parameters are listed in Table 3. The activation energy Ea is 15.0 kJ/mol. There are two inequivalent methyl sites, and the I3C 7‘1 leads to two activation energies, 13.3 and 16.9 kJ/mol, as described above. The present value of 15.0 kJ/mol is in good agreement with the average of the above two values. Assuming that the QCC value for rigid lattice is 180 kHz, which is a typical value for C-D bonds, a correction factor 0.52 is necessary to adjust the calculated values to the observed ones. Overlapping of the signals of the two inequivalent sites might result in the necessity of the correction. The line shape of the NC component might suggest isotropic rotation of the methyl groups. Strictly speaking, this means that RII>> 100 kHz and RL >> 100 kHz, and not that RII= RI >> 100 kHz. We use the word “pseudo-isotropic’’ to express the motion for the NC component. T I data in Figure 12 suggest that the motion of the NC component is faster than that of the RD component. It is unlikely that tumbling of whole molecule is faster than the methyl C3-axis rotation. Thus, the C3-axis rotation contributes to the relaxation of the NC component as well. Assuming only the C3-axis rotation and the same correction factor as the RD component, we get an activation energy of 10.3 kJ/mol, which is smaller than those of the interlayer DMSO molecules. 29Siand 27AlNMR. Figure 13 shows 29SiCP/MAS spectra of kaolinite and its intercalates. The original kaolinite has two peaks at -90.8 and -91.5 ppm. They are ascribed to two inequivalent Si sitesz3 A new peak appears at -92.6 ppm by the intercalation, and the two peaks in the original kaolinite are collapsed in the intercalates. Similar spectra have been reported by Thompson’’ and Duer et a1.I5 Their chemical shifts are -93.1 and -93.8 ppm, respectively. The shift change in the present work is smaller than those values, which may be caused by the origin of the host kaolinite. We have prepared several samples, and the fraction of the intercalated kaolinite was at most 50%.
A considerable amount of unreacted kaolinite is present in the synthesized samples. Deconvolution of spectra gives us an approximate amount of the reacted kaolinite. Note that we should be careful of the cross-polarization efficiency. Our previous work indicates that at a contact time of 8 ms we can estimate the amount.I4 The estimated fractions of the intercalation compounds are 0.23 and 0.31 for samples I1 and 111, respectively. These values are in good agreement with the numbers of interlayer DMSO molecules, as shown in Table 1. This demonstrates that the compound has a formula Si2Al205(0H)4-(CH3)2SO. This means that one DMSO molecule is keyed in every ditrigonal hole of the silica sheet. 29Si spins relax through the interaction with electron spins on paramagnetic impurities, as reported in our previous work.24 Spin diffusion is negligible under magic angle spinning of about 3 kHz. Figure 14 shows magnetization recovery curves plotted as a function of square root of time. The solid triangles show data for the original kaolinite, which relaxes directly through the dipole-dipole interaction with electron spins. An average value of TI is estimated from the slope, which is 1500 s. Open circles show the magnetization recovery for the -92.6-ppm signal in sample 11, Le., kaolinite/DMSO. The recovery curve deviates downward from the straight line. The CH3 rotation contributes to the relaxation additively. This means that the dipole-dipole interaction between ’H in the guest molecules and 29Si in the host contributes to the relaxation. TI derived from the ‘H-29Si dipole-dipole interaction is estimated from the long-time region when the recovery curve is plotted as a function of time, which is about 700 s. On the other hand, open squares are data for the -92.6-ppm signal in sample In, i.e., kaolinite/DMSO-d6. Since the dipole moment of is much smaller than that of ‘H, contribution of the CD3 rotation to the 29Sirelaxation is small. The initial part coincides well with the data of the host kaolinite, which means that paramagnetic impurities play a dominant role. The long-time region is, however, governed by the dipole-dipole interaction between 29Siand *H. Figure 15 shows 27Al spectra. The line shape is governed by the second-order quadrupole i n t e r a c t i ~ n . The ~ ~ peak becomes a little broader by the intercalation, which means the enlargement of quadrupole coupling constant QCC. The present results are similar to Duer et a1.I5 Since the spectra are convolution of the original and the intercalated kaolinites, the quadrupole interaction parameters are extracted from simulation of the spectral pattern. Although the original kaolinite has two inequivalent A1 sites, we simulated its spectral pattern using
KaoliniteDimethyl Sulfoxide Intercalation Compound
J. Phys. Chem., Vol. 99, No. 18, 1995 7127
I
B
L
20
I
I
10
0
-10
-20
PPm
Figure 15. 27AlHDlMAS NMR spectra of (A) kaolinite (sample I), (B) kaoliniteDMSO (sample 11), and (C) kaoliniteDMSO-d6(sample 111), measured at Y L = 104.26 MHz and a spinning rate of 4.00 kHz.
20
10
0
-1 0
PPm
Figure 16. 'H MAS NMR spectra of (A) kaolinite (sample I), (B) kaoliniteDMSO (sample 11), and (C) kaoliniteDMS0-ds(sample 111), measured at Y L = 400.14 MHz and a spinning rate of 10.00 kHz.
one component approximately; the obtained parameters are QCC = 3.15 MHz and VQ = 0.65. The intercalated part of samples I1 and I11 has QCC = 3.90 MHz and VQ = 0.65. The QCC values in this work are somewhat smaller than those of Duer et a1.'5
'H NMR. The original kaolinite has two components in IH static spectra, as reported p r e v i o ~ s l y .The ~ ~ broad component with a full width at half-maximum (fwhm) of about 30 kHz is ascribed to hydroxyl groups in the kaolinite host, and the narrow component with an fwhm of 1 kHz is attributed to H20 molecules adsorbed on the outer surface. MAS can narrow the resonance line considerably. Figure 16 shows only the central portion of 'H spectra measured under high-speed MAS conditions, although spinning sidebands have considerable intensities. The center peak of sample I (kaolinite) consists of two components; one has a peak at 2.2 ppm, and the other is a shoulder at 4.4 ppm. The former is ascribed to hydroxyl groups, and the latter to H20 molecules. Sample I1 (kaoliniteDMS0) also has two components: a hydroxyl peak at about 2 ppm and a DMSO peak at 2.8 ppm, as shown in Figure 16B. The chemical shift of liquid DMSO is 2.73 ppm,20 and there is no difference between the intercalates and the liquid state. Sample III (kaolinite/DMSO-d6) has only one component ascribed to hydroxyl groups, which is observed at 2.0 ppm. The line shape is asymmetric, similarly to the original kaolinite. We have carried out CRAMPS experiments to get higher resolution spectra. Figure 17 shows 'H CRAMPS spectra. The peak positions are 2.9, 3.1, and 3.0 ppm for kaolinite (Figure 17A), kaoniteDMS0 (Figure 17B), and kaoliniteDMS0-d6 (Figure 17C) samples, respectively. In Figure 17, A and C show similar spectra, both of which are attributed to hydroxyl groups of the kaolinite host. Those peaks have a tail on the higher frequency side, which is considered to be the origin of the asymmetric line shape in the MAS spectra. This tailing depends on the sample, as Kanpaku kaolinite with another origin does not show this asymmetry. The fact that kaolinite/DMSO-d6 has a spectrum similar to the original kaolinite indicates that state of the hydroxyl group is not changed by the intercalation. Weak hydrogen bonds are formed between the hydroxyl group and
I
20
I
10
1
0
-1 0
PPm
Figure 17. 'H CRAMPS spectra of (A) kaolinite (sample I), (B) kaoliniteDMSO (sample 11),and (C) kaoliniteDMS0-ds(sample 111), measured at Y L = 400.14 MHz and a spinning rate of 1.60kHz. The d 2 pulse width was 1.5 ,us. The negative peak at about 12 ppm is an artifact produced by radio frequency irradiation.
the sulfonyl oxygen, which is similar to the original kaolinite where weak hydrogen bonds connect layers. The sharp peak in the kaoliniteDMSO sample is assigned to methyl groups of DMSO. Motions of Guest Molecules in the Interlayer Space. In the present work, we have confirmed only the methyl rotation around the C3 axis as the motion of the guest molecules. There is no clear evidence suggesting other specific motions. Duer et have proposed other motions, which are based on the narrow central peak in 2H static spectra of kaoliniteDMS0-ds. We have ascribed the narrow central (NC) component to the surface-adsorbed species. Even when a clear peak of the surface-adsorbed DMSO molecules is not observed in I3C spectra, we might observe the NC component in 2H spectra.
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7128 J. Phys. Chem., Vol. 99, No. 18, 1995
The surface DMSO molecules adsorb at various sites with various strengths. The I3C line width of these molecules is broader than that of interlayer DMSO molecules. The surface DMSO molecules initiate the pseudo-isotropic motion at various temperatures, as is observed in 2HNMR spectra. The line width of the NC component is narrow enough to detect it with high sensitivity. The quadrupole interaction parameters of the RD component depend on temperature. Motions other than the C3 axis rotation take place, which are in the fast-limit region. Two possible motional modes are proposed: two-site jump and three-site jump with unequal potential wells. In the former model, the DMSO molecules change direction within the plane parallel to the sheet with the 0 atom and one of two CH3 group pinned. With increase in temperature, the fluctuation of the direction increases. In the latter model, the C3 axis is fixed, and the depths of the potential wells for the three H atoms become unequal with increase in temperature. If the potential well depends on temperature, the apparent activation energy might depend on temperature as well. We did not find any activation energy changes in this temperature range. Therefore, the first model is considered to be more likely. Duer et ul,15316 have analyzed their 2H spectra by assuming two inequivalent sites. A convolution process might produce experimental errors. On the other hand, I3C NMR can clearly distinguish the two sites, the keyed and the unkeyed methyl groups. Taking this advantage, we have obtained the rate of the methyl rotation individually. The activation energies are 13.0 and 16.5 kUmol for the keyed and the unkeyed groups, respectively, in kaolinite/DMSO. It seems to be peculiar at first sight that the keyed group has a smaller activation energy than the unkeyed group. We have estimated interatomic distances between the methyl carbons and their surrounding atoms, using the structure of Thompson and Cuff.I2 The keyed methyl group is surrounded by six oxygen atoms of the silica sheet. The C-0 interatomic distances are 0.278, 0.308, 0.318, 0.323, 0.370, and 0.375 nm. On the other hand, the methyl group located in parallel to the sheet is surrounded by three oxygen atoms in the silica sheet and one hydroxyl group in the aluminum hydroxide sheet. The C - 0 interatomic distances are 0.332, 0.360, 0.364, and 0.371 nm. If only the C-0 interaction is taken into account, the unkeyed methyl group has a larger space and is expected to do faster rotation. The unkeyed methyl group is faced on one H atom in the aluminum hydroxide sheet, and the C-H interatomic distance is 0.313 nm. This H atom might play an important role in determining the methyl motion. Molecular dynamics calculations might be useful to ascertain these consideration. The keyed methyl group has a higher frequency I3C chemical shift value than the unkeyed group. Surface-adsorbed DMSO molecules and neat liquid sample have chemical shifts of 40.0 and 41.7 ppm, respectively, which are much lower frequencies than those of the interlayer molecules. The interaction with the kaolinite host is considered to cause the high-frequency shift. The keyed group has a greater interaction with the host than the unkeyed one. Previously, we have studied tetramethylammonium ions trapped in zeolites by means of high-resolution solid-state 13CNMR.25,26The I3C chemical shift of the methyl group is shifted to higher frequency with decrease in the cage size trapping the ion. Derouane and Nagy have explained the chemical shift by correlating it with the surface curvature effects influencing the physisorption energy.27 The present results are consistent with their interpretation, since the higher frequency shift is observed for the keyed methyl group which is surrounded by a sphere with the smaller surface curvature.
The I3C isotropic chemical shifts increase with temperature, which means the increase in the host-guest interaction. The temperature dependence is larger in the unkeyed methyl group than in the keyed group. Since the DMSO molecule is pinned at both the 0 atom and the keyed methyl group, fluctuation of the atomic positions is the largest in the unkeyed methyl group, which leads to the larger temperature dependence of this group. The I3C anisotropic chemical shift interactions for the two methyl carbons are almost the same. The unkeyed methyl group, however, deviates from the axial symmetry slightly more than the keyed group. The hydroxyl group surrounding the unkeyed methyl group might affect the conformation.
Summary We have measured NMR spectra and T I of I3C,2H, 29Si,27Al, and 'H for kaolinite, kaoliniteDMS0, and kaolinite/DMSO-ds and have made the following conclusions. (1) I3C NMR distinguishes interlayer and surface-adsorbed DMSO molecules, whereas 29Si NMR distinguishes original kaolinite and intercalates. Combining those NMR results with TG, the formation of AhSi205(OH)4*(CH3)2SO is confirmed. (2) Two methyl groups are observed for the interlayer DMSO (DMSO-&) molecules. No exchange between the two methyl groups is observed in the temperature range -170-330 K. 13C spins of interlayer DMSO and DMSO-& relax by only the methyl rotation around the C3 axis. The keyed methyl group rotates faster than the group locating in parallel to the sheet. (3) 2H spectra are composed of two components, the RD component undergoing CD3 rotation around the C3 axis and the NC component rotating around an axis perpendicular to the C3 axis as well as around the C3 axis. Interlayer DMSO molecules are included in the RD component. Surface-adsorbed DMSO molecules belong to the RD component at 160 K, and with increase in temperature a part of them initiate rotation around the axis perpendicular to the C3 axis. 2H spins relax through the fluctuation of quadrupole interaction caused by the CD3 rotation around the C3 axis. (4) 29Si spins in the intercalated kaolinite relax by the CH3 (or CD3) rotation in addition to the effect of paramagnetic impurities. The dipole-dipole interaction between 'H of the guest molecules and 29Sicontributes to the relaxation. ( 5 ) The 'H chemical shift of the hydroxyl groups in the interlayer space is not affected by the intercalation of DMSO.
Acknowledgment. The author is grateful to Dr. E. Akiba for the measurements of the X-ray powder diffraction patterns. References and Notes (1) Adams, J. M. Clays Clay Miner. 1983, 31, 352. (2) Young, R. A,; Hewat, A. W. Clays Clay Miner. 1988, 36, 225. (3) Bish, D. L.; Von Dreele, R. B. Clays Clay Miner. 1989, 37, 289. (4) Bish, D. L. Clays Clay Miner. 1993,41, 738. ( 5 ) Akiba, E.; Hayakawa, H.; Hayashi, S.; Miyawaki, R.; Tomura, S . ; Shibasaki, Y.; Izumi, F.; Asano, H. Clays Clay Miner., submitted for publication. (6) Olejnik, S.; Aylmore, L. A. G.; Posner, A. M.; Quirk, J. P. J . Phys. Chem. 1968, 72, 241. (7) Olejnik, S.; Posner, A. M.; Quirk, J. P. Clay Miner. 1970, 8, 421. (8) Costanzo, P. M.; Giese, Jr., R. F. Clays Clay Miner. 1990,38, 160. (9) Sidheswaren,P.: Bhat, A. N.; Ganguli, P. Clays Clay Miner. 1990, 38, 29. (10) Thompson, J. G.; Uwins, P. J. R.; Whittaker, A. K.; Mackinnon, I. D. R. Clays Clay Miner. 1992, 40, 369. (11) Thompson, J. G. Clays Clay Miner. 1985, 33, 173. (12) Thompson, J. G.; Cuff, C. Clays Clay Miner. 1985, 33, 490. (13) Raupach, M.; Barron, P. F.; Thompson, J. G. Clays Clay Miner. 1987, 35, 208. (14) Hayashi, S.; Akiba, E. Chem. Phys. Lett. 1994, 226, 495.
KaoliniteDimethyl Sulfoxide Intercalation Compound (15) Duer, M. J. J.; Rocha, J.; Klinowski, J. J . Am. Chem. Soc. 1992, 114, 6867.
Duer, M. J.; Rocha, J. J . Magn. Reson. 1992, 98, 524. Hinckley, D. N. Clays Clay Miner. 1963, 11, 229. Torchia, D. A. J . Magn. Reson. 1978, 30, 613. Burum, D. P.; Cory, D. G.;Gleason, K. K.; Levy, D.; Bielecki, A. J . Magn. Reson., Ser. A 1993, 104, 347. (20) Fenzke, D.; Maess, B.; Heifer, H. J . Magn. Reson. 1990, 88, 172. (21) Hayashi, S.; Yanagisawa, M.; Hayamizu, K. Anal. Sci. 1991, 7, 955. (22) Noggle, J. H. J . Phys. Chem. 1968, 72, 1324.
(16) (17) (18) (19)
J. Phys. Chem., Vol. 99, No. 18, 1995 7129 (23) Hayashi, S.; Ueda, T.; Hayamizu, K.; Akiba, E. J . Phys. Chem. 1992, 96, 10922.
(24) Hayashi, S.; Ueda, T.; Hayamizu, K.; Akiba, E. J . Phys. Chem. 1992, 96, 10928.
(25) Hayashi, S.; Suzuki, K.; Shin, S.; Hayamizu, K.; Yamamoto, 0. Chem. Phys. Lett. 1985, 113, 368. (26) Hayashi, S. K.; Suzuki, K.; Hayamizu, K. J . Chem. Soc., Faraday Trans. 1 1989, 85, 2973. (27) Derouane, E. G.; Nagy, J. B. Chem. Phys. Left. 1987, 137, 341. JP943067F
