th  - 1987 - 51st ENC Conference

MF49
Solid State 2H NMR Studies of Biphenyl in the l~-Cyclodextrin Clathrate Complex
Alan D. Ronemus*, Robert L. Void and Regitze R. Void
Depamnent of Chemistry B-O14, University of California at San Diego, La JoUa, CA 92093
Cyclodexu'ins are cyclic oligosaccharides comprised of D-glucose units connected by alpha-l,4-
glycoside linkages, forming a hydrophobic cavity which readily binds a variety of guest molecules to
make inclusion eomplexesJ The nature of guest motion in such complexes is of interest because
cyclodextrins have been utilized as enzyme models and as stabilizing agents for pharmaceuticals,
pesticides, flavors, and essences. Biphenyl should form a channel clathrate with I]-cyclodextrin, which is
of particular interest as the guest motion may resemble processes in the core region of thermotropic
liquid crystals. Previous studies of the dynamics of guests in cyclodextrin clathrates in solution have used
EPR spin probes, 2 and high resolution l~'oton and lSC NMR? '4 while in the solid state lSC CP-MAS 5 and
static 2I-I NMR 6 have been employed.
Solid state static 2H NMR is particularly well suited to the study of motional dynamics as powder
lineshapes are sensitive to motional processes with rates in the range of 103-107 s "-1. Further, the
quadrupolar interaction dominates to the extent that other interactions generally may be neglected, while
being small enough as to be experimentally and theoretically tractable.
In order to obtain undistorted lineshapes it is necessary to use the quadrupolar echo sequence. 7 A
program has been developed to simulate spectra from quadrupolar echoes, including the effects of
exchange during the delays and prises for several independent motional processes and finite pulse length,
allowing the determination of motional parameters for appropriate models, s
Solid state 2I-I NMR powder spectra of 4,4"-d2-biphenyl and biphenyl-dl0 in ~-cyclodexlrin have
been obtained over the temperature range from 103 to 303 K at several echo delay times. The lineshape
is strongly dependent on temperature over the range measured, approaching the fast motion limit at 303
K and the rigid limit at 103 K. The stx~tra may be interpreted in terms of two independent motional
processes: libration of the long axis of the molecule over a restricted range of angles, and hopping of the
rings in the four well internal torsional potential with two different barrier heights. 9 Rotation about the
long axis appears to be severely hindered as it occurs at rates no greater than 100 s -~ at 303 K. The
libration was modeled as all-site jumps on uniform geodesics with various grid spacings to approximate
motion in a cone of half-angle 27.5 degrees. It was found that a minimum of nineteen sites was required
to reproduce the experimental lineshapes for the para-deuterated complex, with rates ranging from 1 ×
103 s -~ at 103 K to 4 x 106 s -~ at 303 K. The internal hop was modeled as a four site nearest neighbor
jumping process with different rates to each neighbor. The inter-ring torsion angle of 39 degrees agrees
with the angle determined in the vapor phase 1° and in solution? l and with theoretical calculations? At
303 K the fast rate was 4 x 107 s -1 while the slow rate was 4 x l0 s s -l. Further simulations are in
progress to define the motional parameters across the entire range of temperatures studied. Single-crystal
rotation spectra of biphenyl-d~0~-cyclodextrin have been produced which indicate that the structure is of
the "screw-channel" type.
1. J. Szejtli, "Cyclodextrins and Their Inclusion Complexes," Akademiai Kaido, Budapest, 1982.
2. K. Flohr, R. M. Patton and E. T. Kaiser, J. Am. Chem. Soc. 97, 1209 (1975).
3. J. P. Behr and J. M. Lehn, J. Am. Chem. Soc. 98, 1743 (1976).
4. R. J. Bergeron and M. A. Channing, J. Am. Chem. Soc. 101, 2511 (1979).
5. H. Ueda and T. Nagai, Chem. Pharm. Bull. 29, 2710 (1981).
6. L. D. Hall and T. K. Lira, J. Am. Chem. Soc. 106, 1858 (1984).
7. J. H. Davis, K. R. Jeffrey, M. Bloom, M. I. Valic and T. P. Higgs, Chem. Phys. Len. 42, 390 (1976).
8. M. Greenfield, A. Ronemus, R. L. Void, R. R. Void, P. Ellis and T. Raidy, J. Magn. Reson., 70 (1986).
9. G. Haefelinger and C Regelmann, J. Comput. Chem. 6, 368 (1985).
10. O. Bastiansen and S. Samdal, J. Mol. Struct. 128, 115 (1985).
11. M. Barret and D. Steele, J. Mol. Struct. 11, 105 (1972).