1525
J . Phys. Chem. 1991, 95, 1525-1527
Conformation and Stability of Alkane/Urea Inclusion Compounds: Evidence from Deuterium NMR Spectroscopy Gina M. Cannarozzi, Ghirmai H. Meresi, Robert L. Vold, and Regitze R. Void* Department of Chemistry, University of California, San Diego, La Jolla, California 92093 (Received: October 10, 1990)
n-Alkanes contained as guests in urea inclusion compounds (UIC) and once thought to exist exclusively in the all-trans conformation have recently been proposed to contain relatively large amounts of gauche conformers. Those suggestions have subsequently been challenged in a recent Letter to this Journal by Casal, who suggested that the observed gauche conformations were due to free alkane formed by decomposition of the UIC’s. Deuterium quadrupole echo NMR spectra have now been obtained of the urea inclusion compounds containing the normal alkanes with n = 8, 10, 12, 16, 19, and 36, samples of which were subjected to magic angle spinning or ultrasonic agitation. The deuterium spectra show that while the c8 and CloUIC’s are indeed unstable at the temperatures prevailing during these treatments, their decomposition does not affect conclusions drawn from the 13CNMR spectra. The deuterium quadrupolar splittings reported here are most readily explained in terms of trans-gauche isomerization near the ends of the n-alkanes, but the gauche content is less than that suggested by MM2 and M D calculations.
Since their accidental discovery by Bengenl in 1949 n-alkane/urea inclusion compounds (UIC) have fascinated chemists and spectroscopists alike. The belief that in these channel or tubulato clathrates2 the normal alkyl derivatives exist in an alltrans conformation has spawned numerous spectroscopic studies with the aim of either correlating structure with spectral features or elucidating alkyl chain dynamics in this particular environment-infinitely long, hexagonal channels with an inside diameter of ca. 5.25 A. Much has been written in support of a planar, all-trans conformation of the included and the guests have been shown to rotate rapidly about the channel axis.”” Deuterium N M R spectroscopy, in particular, has been useful in establishing that the terminal CD3CD2groups possess extra internal degrees of freedom, assumed to be just large angle torsional and bending motions.11-13 Recently, Imashiro et aI.l4 reported I3C CPMAS spectra of several ( n = 7-20) n-alkane UIC’s and observed a well-defined ”y-effect”15 which they associated with the substantial concentrations of gauche conformations near the ends of the alkanes. In a subsequent study, lmashiro et a1.I6 measured C H dipolar splittings in SASS (switched angle sample spinning) 13C spectra of the AUIC’s with 7, 8, 9, and 10 carbon atoms and compared their data with the results of MM2 molecular mechanics calculations. They concluded that significant amounts (25-35%) of gauche conformations are present near the end of the chains, and smaller amounts in the center, and that trans-gauche isomerization is responsible for the observed reduction in dipolar splittings. At the same time, Vold et al.” suggested on the basis of Newtonian ( I ) Bengen, M. F. Ann. Chem. 1949, 565, 204. (2) Weber, E.; Josel, H.-P. J . Inclusion Phenom. 1983, I , 79. (3) Smith, A. E. Acra Crysr. 1952, 5, 224. (4) Chatani, Y . ;Anraku, H.; Taki, Y . Mol. Crysr. Liq. Crysr. 1978, 48, 219. (5) Fawcett, V.; Long, D. A. Ado. Ramon Spectrosc. 1972, I , 570. (6) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J . Phys. Chem. 1984,88, 334. (7) MacPhail, R. A.; Snyder, R. G.; Strauss, H. L. J . Chem. Phys. 1982, 77, 1118. (8) Casal, H. L.; Cameron, D. G.; Kelusky, E. J . Chem. Phys. 1984, 80, 1407. (9) Umemoto, K.; Danyluk, S. S. J . Phys. Chem. 1967, 71, 3757. (IO) Bell, J . D.; Richards, R. E. Trans. Faraday SOC.1969, 65, 2529. ( I I ) Gilson, D. F. R.; McDowell, C. A. Mol. Phys. 1961, 4, 125. ( I 2) Greenfield, M. S.;Vold, R. L.; Vold, R. R. J . Chem. Phys. 1985,83, 1440. (13) Greenfield, M. S.;Vold, R. R.; Vold, R. L. Mol. Phys. 1988.66, 269. (14) Imashiro, F.; Maeda, T.; Nakai, T.; Saika, A.; Terao, R. J . Phys. Chem. 1986. 90, 5498. (15) Grant, D. M.; Paul, E. G. J . Am. Chem. Soc. 1964, 86, 2984. (16) Imashiro, F.; Kuwahara, D.; Nakai, T.; Terao, T. J . Chem. Phys. 1989, 90, 3356.
molecular dynamics calculations that the CZ-C3 bond in the nnonadecanelurea inclusion compound might spend as much as 40% time in the gauche conformation with minor amounts of gauche conformations in the interior of the chain, usually as part of “kinks”, the g+tg- conformations. These observations were met with scepticism by Wood et a1.I8 and Casal19 who argued on the basis of Raman and infrared spectroscopic data that the amount of gauche conformations proposed to be present in the AUIC‘s was much too high. CasalI9 suggested that the gauche conformations found by Imashiro et a1.I6 in C7-CIo UIC’s might be an artifact, caused by the decomposition of N M R samples subject to high levels of rf irradiation and rapid sample spinning in both CPMAS and SASS experiments. Similar decomposition of UIC’s has earlier been demonstratedz0to take place under conditions of ultrasonic agitation and large (3-18 “C) temperature rises detected during rapid (4-8 kHz) magic angle spinning NMR experiments2I were quoted by Casal in support of this idea. In this Letter we address the questions of alkane conformation and UIC stability by presenting recent deuterium quadrupole echo NMR spectra of AUIC samples subjected to magic angle spinning and ultrasonic agitation. Powder samples of the urea inclusion compounds with perdeuterated n-alkanes (MSD Isotopes) were prepared by precipitation from solutions of hot methanoll2propanol. The products were filtered, washed with benzene, allowed to dry, and divided into three portions. Two were put in short, capped 5-mm sample tubes, one of which was used as a standard and the other was sonicated for 3 h in a Fisher Scientific ultrasonic cleaning bath. The third portion was weighed, spun at 3 kHz for 3 h in a 7-mm Doty sapphire magic angle spinner, and reweighed, and then transferred to a 5-mm capped sample tube for recording of the deuterium quadrupole echo spectrum. No weight loss was observed except passibly for the octane sample. All NMR data were processed by using Dennis Hare’s software package FTNMR, customized for solid-state NMR spectroscopy. Effects of Sonication and Magic Angle Spinning. Deuterium powder spectra of untreated c8 and Clourea inclusion compounds are shown in Figure la and, with labeled splittings, in Figure 2a. As reported earlier8 for the CI9UIC, three distinct powder patterns can be discerned. The innermost one with a splitting between the perpendicular edges, A I 18 kHz, arises from the CD, groups,
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(17) Vold, R. L.; Vold, R. R.; Heaton, N . J. Ado. Magn. Res. 1989, 13, 17. (18) Wood, K. A.; Snyder, R. G.; Strauss, H. L. J . Chem. Phys. 1989,91, 5255. (19) Casal, H. L. J . Phys. Chem. 1990, 94, 2232. (20) Casal, H. L. Appl. Spectrosc. 1984, 38, 306. (21) Bjorholm, T.; Jacobsen, H. J. J . Magn. Reson. 1989, 84, 204.
0022-365419112095-1525$02.50/0 0 1991 American Chemical Societv
1526 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
Letters
+ -
61-
IC+
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57-
+
n
#
CS
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v)
CD,(inner)
I
2-CD2
L
1
+ 2ot 49
100
80
60
40
20
0 -20 -40 -60 -80 -100
Frequency (kHd
Figure 1. Deuterium quadrupole echo spectra of n-octane/urea inclusion compound of a freshly prepared sample (a) and one that had been spun at 3 kHz at ambient temperature for 3 h in a 7-mm Doty M A S probe (b). The two spectra (0.1 g, 5-mm sample) were recorded under identical conditions at 25 OC and 38.4 MHz on a Chemagnetics CMX spectrometer, averaging 1024 quadrupole echoes with a pulse spacing of 40 ws, a 90' pulse length of 1.8 ps, and a recycle delay of 10 s. The temperature was maintained at 25.0 "C to within 0.01 O C using a homebuilt tem-
perature controller.
k, 100 80
60
40
20 0 -20 -40 -60 -80 -100 Frequency (kHz1
Figure 2. Deuterium quadrupole echo spectra of n-decanelurea inclusion compound before (a) and after (b) being subjected to magic angle spinning as described in the caption to Figure 1. The bottom spectrum (c) was obtained from a sample exposed to sonication for 3 h in an ultrasonic cleaning bath with a final temperature of 44 "C.The three spectra were recorded under identical conditions at 25 O C and 55.3 MHz on a homebuilt spectrometer interfaced to a Nicolet 1280 data system. The tem-
perature was controlled to better than 0.1 "C by a Lakeshore Cryotronics controller.
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the next with A2 51 kHz from the 2-CD2 group, and the widest spectrum, A3 57 kHz from the remaining methylene groups. The reduced signal/noise in Figure 1b is due to a 60-70% loss of octane from the host lattice during the three hours the sample was spun at 3 kHz in a Doty MAS probe. We were not able to measure the temperature inside the sapphire rotor itself, but the temperature of the stator, measured with a thermocouple taped to it, was 21 "C. From the results of Bjorholm and Jacobsen,21 who used a probe similar to ours, we estimate a possible tem-
19
151
,
0
4
,
,
,
8
,
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,
,
,
,
,
,
,
,
1 6 20 24 28 Chain Length in)
,
,
32
,
,
36
,
.I 40
Figure 3. Deuterium quadrupolar splittings vs chain length. The splittings were measured between the perpendicular edges of powder patterns as shown in Figures 1 and 2.
perature increase to be of the order of 4-6 "C. Agitation of the CBUIC (results not shown) for 3 h in the ultrasonic cleaning bath, during which time the temperature rose to 44 OC, resulted in complete loss of the alkane. The results of magic angle spinning and sonication of a powder sample of C l oUIC are illustrated in Figure 2. The C l oUIC is stable for at least 3 h at 3.5 kHz (see Figure 2b) but does decompose in the ultrasonic bath, as shown in Figure 2c. The 80-90% loss of alkane presumably occurs because the bath temperature rose to 44 "C. Similar treatments of the CI2,CI6,C19, and c36 UIC's resulted in no decomposition. These results are in complete agreement with the thermogravimetric results of McAdie22which showed loss of Cl0 above 40 "C and no loss of alkane below 60 OC for AUIC's with n L 12. Without knowing more about the conditions of the CPMAS and SASS experiments of Imashiro et al.lS9l6it is impossible to determine whether their samples of AUIC's with n = 7-10 decomposed partially during their measurements. However, the spectra presented by these authors can only arise from the inclusion compounds. If free alkane is present in their samples, it is not apparent from their spectra, and as far as the CPMASIs and SASSI6spectra being the result of an exchange process as proposed by Casal,I9 we consider this interpretation to be in error. For spectral narrowing to occur it is necessary to postulate that exchange of alkane between free liquid (or gas) and trapped guest takes place at a rate comparable to the width of the I3C spectra, Le., lo3 s-I or higher. Not only is translational motion strongly hindered in the inclusion compound because the channels are full, but as shown previously by CasalI9 urea reverts from the hexagonal to the tetragonal form upon decomposition of the inclusion compound. Thus, the equilibrium between alkane + urea and the inclusion compound is likely to be very slow in the heterogeneous systems considered here. Alkane Conformation. For rigid, aliphatic deuterons with (nearly) symmetric electric field gradient tensors one expects the quadrupolar splittings A to fall in the range 123-144 kHz.23,24 For an extended alkyl chain of ideal geometry and rotating rapidly about its long axis A will be reduced by a factor of -1/2 for the CD2 groups and -1/6 for the methyl groups, which also rotate (22) McAdie, H. G. Can. J . Chem. 1962, 40, 2195. (23) Rinne, M.; Depireux, J. Adu. Quad. Reson. 1976, I , 327. (24) Wofsy, S. C.; Muenter, J . S.; Klemperer, W. J . Chem. Phys. 1970, 53, 4005.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1527
Letters rapidly about the Cl-C2 bond. The splittings obtained from the AUIC powder patterns (see Figure 3), 50-60 kHz for A2 and A3 and 16-1 8 kHz for A I , are close to, and slightly below, the range expected for such rapidly rotating, extended alkyl chains. The reduced splittings may be explained in terms of vibrational motions~'1-12*25 or by invoking the presence of gauche conformat i o n ~ . ' If~ vibrational deformations alone were responsible one should expect an overall decrease in all ~ p l i t t i n g s since , ~ ~ more and more low-frequency modes get populated as the chain length increases. Instead we observe that the splitting A3, from the innermost CD2 groups, increases with increasing chain length n, while A2, from the 2-CD2 group, is independent of n within experimental error (at least for the mostly even-numbered subset studied here), and the CD3 splitting decreases with increasing values of n. The last observation is difficult, if not impossible, to rationalize on the basis of vibrational motion alone. Instead, a decrease in methyl splitting with increasing n follows straightforwardly from a structural model which includes gauche conformations near the ends of the alkane molecule. It is beyond the scope of this Letter to present the full mathematical treatment leading to general expressions for the quadrupolar splittings observed here; similar treatments are available elsewhere.26 Instead we use a very simple approach in which the assumptions are as follows: (a) ideal tetrahedral geometry; (b) all quadrupole coupling constants (QCC) are equal in the absence of motion; (c) the rotations about the channel axis and the C,-C2 and the trans-gauche isomerizations are all fast compared to the quadrupolar splittings; (d) gauche conformations are allowed only about the C2
