- I. Fischbach, H. W. Spiess, K. Saalwächter, G. R. Goward.
Solid-State NMR Spectroscopic Investigations of
Model Compounds for Imidazole-based Proton Conductors.
J. Phys. Chem. B 108, 18500-18508 (2004).
The temperature dependence and the exact geometry of slow molecular
reorientations in imidazolium methyl sulfonate are investigated using modern
one-dimensional MAS exchange spectroscopy. Earlier high-temperature studies have
evidenced a fast 180 degree flip motion of the imidazole ring, which is here
shown to slow down on cooling, and is believed to be a prototypical molecular
process involved in Grotthus-type proton transport in imidazole-based proton
conductors intended for fuel cell applications. It is further shown that
valuable information on the relative orientations of CH and NH dipolar coupling
tensors with respect to the chemical shift anisotropy tensors of the respective
heteronuclei can be obtained from the MAS exchange data as well as static
13C and 15N line
shapes, without the necessity of performing more involved single-crystal NMR
experiments. The principal axes of the CSA tensors are found to not coincide
with the CH or NH bond axes, in contrast to earlier assumptions involving
similar compounds. Imidazole itself is shown to be more complex than might be
expected, based on its simple structure. Implications on earlier studies of
pure imidazole, where ring flips were claimed to be absent, are discussed.