SOLID-STATE NMR-STUDIES OF THE MOLECULAR-MOTION OF PYRIDINE IN ITS TRI-O-THYMOTIDE CLATHRATE

When crystallized from pyridine, tri-o-thymotide forms a clathrate compound with the solvent as guest molecule. In the confined space of the cage, the pyridine molecule exhibits considerable motional freedom. At 77 K the solid state H-2 NMR spectrum is consistent with the molecules undergoing a restricted in-plane motion. The 2H NMR data also show that at temperatures above 77 K the molecules undergo an out-of-plane libration as well as 180 degrees flips about their 2-fold axis. The barrier to this activated process was evaluated from line shape simulations. The C-13 CP/MAS NMR data obtained by using the dipolar dephasing technique confirm the models for the molecular motion established by H-2 NMR. The NMR data are compared to those from single crystal X-ray diffraction, which provide little evidence regarding molecular motion.