Deuterium solid-state NMR study of pyridine dynamics in intercalated 2H-TaS₂

A quadrupole echo deuterium NMR line-shape study on polycrystalline samples of perdeuteriated pyridine and selectively deuteriated pyridine-d₂ intercalated into 2H-TaS₂ was performed between 280 and 393 K. Above 333 K, a slow growth in intensity of three motionally narrowed components coincident with depletion in intensity of the rigid pyridine pattern was observed for the perdeuteriated sample. Deuterium spin-lattice relaxation times were also recorded over the same temperature range. A short T₁ component, which was attributed to the three motionally narrowed peaks, showed a sharp decrease in correspondence with their appearance in the line shapes. The narrowing motion was determined to be rapid rotational diffusion of the pyridine molecule about an axis perpendicular to the C₂ symmetry axis and parallel to the molecular plane. Since theoretically determined motionally reduced quadrupole coupling constants support this interpretation, it was possible to determine the orientation of the pyridine in the van der Waals gap.