The sites and dynamics of p-xylene guest molecules in Dianin’s inclusion compound; a deuteron NMR study

Abstract

Single crystals of Dianin’s inclusion compound with methyl and ring deuterated p-xylene guest molecules were grown and studied by FT deuteron NMR. The spectra from the deuterated methyl groups reveal that these groups reorient rapidly down to 12K; thereafter they enter into the tunneling regime. The rings of the p-xylene guests become motionless whenT reaches 110K. By measuring the orientation dependence of the quadrupole splittings, determining from these data the quadrupole coupling 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 cages of Dianin’s inclusion compound. We find two sets of independent sites. Each contains three C₃ related individual sites. In each set the population of one of the sites is strongly depleted. The only large-anlge molecular motions are 180° rotational jumps about the long molecular axes. From measurements ofT ₁ we conclude that these jumps are thermally activated,τ ₀ = 5·10⁻¹⁴ s, ΔE=20 kJ/mol. Additional motions are rapid librations, also about the long molecular axes. Their amplitude increases with increasing temperature, at 300 K it reaches 20°. With 2D-exchange spectra we demonstrate that a p-xylene guest cannot change its site on a timescale of 100 ms and a tempering experiment suggests that this is true on a timescale of several days even atT=371 K.