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.