THE SITES AND DYNAMICS OF P-XYLENE GUEST MOLECULES IN DIANINS INCLUSION COMPOUND - A DEUTERON NMR-STUDY

Single crystals of Dianin's inclusion compound with methyl and ring de uterated p-xylene guest molecules were grown and studied by FT deutero n NMR. The spectra from the deuterated methyl groups reveal that these groups reorient rapidly down to 12 K; thereafter they enter into the tunneling regime. The rings of the p-xylene guests become motionless w hen T reaches 110 K. By measuring the orientation dependence of the qu adrupole splittings, determining from these data the quadrupole coupli ng tensors of the ring deuterons and relating these tensors to the C-D bond directions we infer the sites of the p-xylene guests in the cage s of Dianin's inclusion compound. We find two sets of independent site s. Each contains three C-3 related individual sites. In each set the p opulation of one of the sites is strongly depleted. The only large-ang le molecular motions are 180 degrees rotational jumps about the long m olecular axes. From measurements of T-1 we conclude that these jumps a re thermally activated, tau(0) = 5.10(-14) s, Delta E = 20 kJ/mol. Add itional motions are rapid librations, also about the long molecular ax es. Their amplitude increases with increasing temperature, at 300 K it reaches 20 degrees. With 2D-exchange spectra we demonstrate that a p- xylene guest cannot change its site on a timescale of 100 ms and a tem pering experiment suggests that this is true on a timescale of several days even at T = 371 K.