J . Phys. Chem. 1989, 93, 7609-7618
[Cu(dien)(bipyam)]X, in 4VYF'Y 20.00
1
A
I
I.""
0
20
PreFsure 6pkbary
100
Figure 11. Pressure dependence of the energies of the electronic transitions of 1 (filled symbols), 2 (half-filled symbols), and 3 (open symbols) in a poly(4-vinylpyridine) environpent.
in 1 and 2 are associated with the complexes. In any event, the similarity in behavior of 1 and 2 with 3 indicates that the waters are not important in determining the molecular configuration in the polymer. Consideration of the intensity data in the polymer (Figure 10) provides some insight into the wnfiguration of the complexes. At low pressure in the polymer the intermediate-energy (d9 d+y) band appears with greater intensity than the high-energy (dn, dyz d x + z )band. Based on the corresponding intensities in polycrystalline 1 and 2 (Figures 3 and 4) at low pressure, it is likely that the configuration in the polymer more closely resembles the less regular configuration found in polycrystalline 2. Furthermore,
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7609
since the magnitude of the difference in intensity between the two bands is greater in the polymer than in polycrystalline 2, it seems likely that the configuration in the polymer is even more distorted than in polycrystalline 2. The extent of distortion from regular square pyramidal geometry in the polymer must, however, be less than that found in polycrystalline 3 at low pressure since the low-energy band is of low intensity at all pressures in the polymer. Consequently, it is reasonable to believe that the configuration in the polymer can be characterized by a value of the angle a3 somewhere near 150. The pressure-dependent intensity behavior of the complexes in the polymer is similar to that observed in the polycrystalline complexes. A steady transformation to a more regular square pyramidal configuration is indicated as the intermediate- and high-energy bands lose and gain intensity, respectively, in better compliance with strict C, selection rules. The intensity data suggest, for instance, that the geometry in the polymer at 70-80 kbar is similar to the configurations in polycrystalline 1 and 2 at lo5. On the other hand, much lower precision is sufficient (CPC =2
may often obscure the physical picture by offering a fit with relatively few parameters that is only acceptable because of the low quality, statistically speaking, of the data.
lo4) to discard a single-exponential model a t 77 K, where the excimer formation appears fully suppressed. The use of the MEM method seems to be a more effective way of analyzing decay curves for which the governing kinetic model is not a priori known than the use of a discrete free-floating fitting function with a gradually increasing number of fitting parameters. The MEM has, within its resolvability limits, an ability of picking correctly the underlying pattern of lifetimes,13*14while the latter method X
Acknowledgment. We acknowledge the financial support of the National Science and Engineering Research Council of Canada. Registry No. 1,3-Di(1-pyrenyl)propane,61549-24-4.
Deuterium Nuclear Magnetic Resonance Measurements of Rotation and Libration of Benzene in a Solid-state Cyclamer Jong Hoa Ok, Regitze R. Vold, Robert L. Vold,* Department of Chemistry, University of California, San Diego, La Jolla, California 92093
and Margaret C. Etter Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: March 23, 1989)
Deuterium NMR spectroscopy has been used to study the molecular motion of benzene-d6in a 1,3-cyclohexanedionecyclamer with a host:guest ratio of 6:l and, for comparison, in pure solid benzene-d6. The quadrupole coupling parameters for pure benzene-d6 at 87 K were found to be $qQ/h = 183 f 1 kHz and 7 = 0.04 0.005, in accord with literature values. $qQ/h for pure benzene-d6is slightly temperature dependent, dropping to 177 f 1 kHz at 252 K. Values of $qQ/h for benzene-d6 trapped in the cyclamer were found to be the same, within *1 kHz experimental error, as those of pure solid benzene& at the same temperature. Quadrupoleecho line shapes were recorded at 40-and 100-ps pulse spacings and at several temperatures between 87 and 252 K for benzene-& and between 124 and 290 K for the cyclamer. Major features of the line shapes can be accounted for in terms of large angle jumps about the major symmetry axis. The activation energy of jump motion is 16.5 f 0.1 kJ/mol for benzene-d6and 24.9 f 0.4 kJ/mol for benzened6in the cyclamer. Anisotropic displacement parameters derived from room-temperature X-ray data, as well as the observed temperature dependence of the quadrupole coupling constant both for benZe.ne-d6 in the cyclamer and for pure benzene-d6,can be ascribed to fast vibrational motion. A simple model that describes this motion in terms of restricted wobbling of the ring normal can be used to fit both the NMR and X-ray data.
*
One may be left a t the end of an extensive calculation with the uncomfortable possibility that even though the simulated spectra fit the experimental data, the motional trajectories used in the simulation may not be unique, or even particularly realistic. In order to explore this possibility we have conducted careful studies of the *Hquadrupole echo line shape as a function of temperature and pulse spacing in benzene-d6 in a clathrate with 1,3-~yclohexanedione,and for comparison, in pure solid benzene-& For both materials, sufficient diffraction data exist to place strong constraints on the choice of motional trajectories. In particular, the observation of very small thermal ellipsoids for benzene carbon a t o m in the cyclamer" and for deuterons as well as carbons in benzene-d6I2 implies that any large-angle motion must occur by jumps between minima of deep potential wells, rather than by a mechanism of small step diffusion. Moreover, restricted whole-body libration in pure benzene accounts quantitatively for the thermal parameters obtained by fitting both X-rayI3J4 and neutron12.15 diffraction data. In addition to
Introduction The development of quadrupole echo techniques,'J together with procedures3+ for minimizing instrumental artifacts and sophisticated computer for line shape simulation, has established deuterium N M R spectroscopy as a powerful source of information about molecular motion in solid materials. Motional information is generally extracted from line shapes of polycrystalline powders by comparing experimental spectra with a series of simulated spectra in which one or more rate parameters are systematically varied. This indirect (though unavoidable) procedure requires that a priori assumptions be made about the set of orientations that are sampled by the molecules as they move. (1) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lert. 1976, 42, 390. (2) Boden, N.; Clark, L. D.; Hanlon, S. M.;Mortimer, M. Faraday Symp. Chem. SOC.1978, 1979, 109. (3) Griffin, R. G. Merhods Enzymol. 1981, 72, 108. (4) Spiess, H. W. Adv. Polymer. Sci. 1985, 66, 24. (5) Spiess, H. W. NMR, Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1978; Vol. 15, p 55. (6) Henrichs, P. M.; Hewitt, J. M.; Linder, M. J . Magn. Reson. 1985,60, 280. (7) Wittebort, R. J.; Olejniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1987, 86, 541 1. (8) Greenfield, M. S.;Ronemus, A. D.; Vold, R. L.; Vold, R. R.; Ellis, P. D.; Raidy, T. R. J . Magn. Reson. 1987, 72, 89. (9) Schwartz, L. J.; Meirovitch, E.; Ripmeater, J. A.; Freed, J. H. J. Phys. Chem. 1983,87,4453. (IO) Meier, P.; Ohmes, E.; Kothe, G.; Blume, A.; Weidner, J.; Eibl, H. J . Phys. Chem. 1983, 87, 4904.
(11) Etter, M. C.; Urbanczyk-Lipkowska, Z.; Jahn, D. A.; Frye, J. S. J . Am. Chem. SOC.1971, 108, 5871. (12) Jeffrey, G. A.; Ruble, J. Rr.; Ruble, R. K.;McMullan, R. K.; Pople, J. A. Proc. R . SOC.London, A 1987, A414, 47. (13) Motozato, Y . ;Nishihara, T.; Hirayama, C.; Furuya, Y . ;Kosugi, Y . Can. J. Chem. 1982, 60, 1959. (14) Cox. E. G.; Cruickshank, D. W. J.; Smith. J. A. S. Proc. R . SOC. London, A 1958, A247, 1. (15) Bacon, G. E.; Curry, N. A.; Wilson, S.A. Proc. R . SOC.London, A 1964, A279, 98.
0022-365418912093-7618SO1 SO10 0 1989 American Chemical Societv , , I
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