Dynamic properties of the guest molecules in the pyrazine/α-zirconium phosphate intercalation compound: a multi-nuclear solid state NMR study

Dynamic properties of perdeuterated pyrazine (pyrazine-d(4)) guest molecules in the layered a-zirconium phosphate host structure have been established from solid-state H-2 NMR lineshape analysis and H-2 NMR spin-lattice relaxation time measurements. Both techniques indicate that the intercalation compound contains two different types of pyrazine-d(4) guest molecules, which undergo different dynamic processes. The relative proportions of the two types of pyrazine-d(4) guest molecules are approximately 30% (component A) and 70% (component B). The pyrazine-d(4) molecules representing component B undergo two-site 180degrees jumps about the N...N axis of the molecule, whereas the pyrazine-d(4) molecules representing component A undergo an effectively isotropic motion. The activation energy for the two-site 180degrees motion of component B is estimated to be 39 kJ mol(-1) from the H-2 NMR spin-lattice relaxation time measurements and 49 kJ mol(-1) from the H-2 NMR line-shape analysis. Structurally, it may be inferred that component B comprises guest molecules for which the N...N axis has a restricted orientation in space, presumably because of direct interactions with the alpha-zirconium phosphate host structure, whereas component A comprises guest molecules that are held rather weakly in the host structure (or on its external surfaces) such that their reorientation is relatively unestricted. Thermogravimetric analysis provides direct evidence for the existence of two different types of pyrazine guest molecules in the alpha-zirconium phosphate host structure, with different strengths of binding and with populations in the approximate ratio 30:70. High-resolution solid-state N-15 NMR and C-13 NMR spectra provide further support for the existence of two different types of pyrazine guest species within the alpha-zirconium phosphate host structure and provide direct evidence that the pyrazine guest molecules of component B are N-protonated (from the host structure).