D. Reichert, K. Saalwächter.
Dipolar Coupling: Molecular-Level
In: Encyclodedia of Nuclear Magnetic Resonance (NMR
Crystallography). Editors: R. K. Harris, R. E. Wasylichen, M. J. Duer. John
Wiley & Sons, Chichester 2009. DOI 10.1002/9780470034590.emrstm1020.
Though solids are sometimes thought of as completely rigid, it is by no means
true that molecules in solid-state materials are immobile and do not have any
appreciable internal mobility. To the contrary, such materials sometimes exhibit
an astonishing variety of segmental motions as well as molecular reorientations.
Many of these motions were previously found to take place in solution, usually
but not necessarily at higher rates. Molecular dynamic processes are well worth
studying, for they are crucial for the macroscopic properties of materials. They
cover a wide range of motional frequencies and involve dynamic modes that might
have different effects on the application properties. Moreover, they are
particularly important in NMR crystallography as solid–solid phase
transformations are often accompanied by a change or onset of molecular motion.
Furthermore, structure determination by solid-state NMR is significantly
affected if molecular motions are present.
To draw the connection between the
macroscopic properties and the structure and dynamics at the molecular level, it
is necessary to determine both the characteristic time (correlation time of
motion, τc) as well as the geometry (like
jump angles and population of sites) of the processes with a reasonable
spectral–and thus molecular–resolution. Although many recent papers deal with
dynamic studies in solution (in particular, in connection with biopolymers),
such studies gain particular importance in solid materials. NMR spectroscopy is
a particular convenient and versatile tool to search, investigate, and
characterize dynamic processes in the solid state. The dipolar coupling is
perhaps the most important NMR interaction for the study of dynamic process in
solids at the molecular level. The present article describes the history,
principles, recent developments, and some applications of dynamic solid-state
NMR spectroscopy for the investigation of molecular dynamic processes utilizing