Dynamics of benzene and pyridine guest molecules in their tri-ortho-thymotide inclusion compounds. Solid-state 2H NMR Studies

Abstract

Dynamic properties of perdeuteriated benzene (C₆D₆) and perdeuteriated pyridine (C₅D₅N) guest molecules in their tri-ortho-thymotide (TOT) inclusion compounds have been studied via variable-temperature wide-line solid-state ²H NMR spectroscopy. The temperature dependences of the quadrupole echo ²H NMR lineshape and the ²H NMR spin–lattice relaxation time for C₆D₆/TOT in the temperature range 96–293 K are interpreted in terms of in-plane rotation (nearest-neighbour 2π/6 jump model) of the benzene molecule, together with restricted wobbling of the molecular plane. The ²H NMR spin–lattice relaxation time measurements suggest that there are two dynamically inequivalent types of benzene molecule (included in structurally different types of tunnel within the TOT host structure), with their activation energies for in-plane rotation estimated to be 4.0 and 5.9 kJ mol^–1. For C₅D₅N/TOT in the temperature range 90–363 K, three types of motion have been identified: (i) in-plane libration; (ii)π jumps of each pyridine molecule about its two-fold symmetry axis; (iii) restricted wobbling of the molecular plane. On the basis of ²H NMR lineshape analysis of ²H NMR spin–lattice relaxation time measurements, the activation energy for motion (ii) is estimated to be ca. 17-20 kJ mol^–1.