P. Speier et al., THE SITES AND DYNAMICS OF P-XYLENE GUEST MOLECULES IN DIANINS INCLUSION COMPOUND - A DEUTERON NMR-STUDY, Applied magnetic resonance, 9(1), 1995, pp. 81-102
Citations number
12
Categorie Soggetti
Spectroscopy,"Physics, Atomic, Molecular & Chemical
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.