Monday, February 13, 2006

PCCP: VT 2H NMR; inclusion compounds

Physical Chemistry Chemical Physics, 2006, (Advance Article)

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

Jorge A. Villanueva-Garibay and Klaus Müller

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

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