Molecular dynamics and ordering of pyridine in its cyclophosphazene inclusion compound as evaluated by solid state 2H NMRspectroscopy

Abstract

Variable temperature ²H NMR experiments (line shape analysis, relaxation studies) were carried out on the pyridine-d₅–tris-(1,2-dioxyphenyl)-cyclotriphosphazene inclusion compound in the temperature range between 110–300 K. It is found that the pyridine guests are highly mobile throughout the whole temperature range covered here. The observation of three superimposed ²H NMR signals can be understood in terms of a particular (motionally averaged) orientation of the pyridine molecules, which is a consequence of the molecular symmetry of the pyridine guests and the imposed channel restrictions. The experimental data are consistent with a combined rotation on cone–small angle fluctuation model, which assumes a fast molecular reorientation between two superimposed cones with an opening angle for the inner cone between 59–73° (angle of fluctuation between 1–3°). On the basis of this model assumption it is possible to reproduce both the experimental ²H NMR line shapes and the spin–lattice relaxation data in a quantitative way. The analysis of the partially relaxed spectra (inversion recovery experiments) yields the correlation times for this overall motional process. They follow an Arrhenius behavior from which an activation energy of 8.7 ± 0.4 kJ mol⁻¹ is derived. The results are discussed in the framework of the published data for related systems.