S. M. De Paul, K. Saalwächter, R. Graf, H. W. Spiess.
Sideband Patterns from Rotor-Encoded Longitudinal Magnetization in
MAS Recoupling Experiments.
J. Magn. Reson.
146, 140-156 (2000).
Recent multiple-quantum MAS NMR experiments have shown that a change in the
rotor phase (and, hence, in the Hamiltonian) between the excitation and
reconversion periods can lead to informative spinning-sideband patterns.
However, such “rotor encoding” is not limited to multiple-quantum experiments.
Here it is shown that longitudinal magnetization can also be rotor-encoded. Both
homonuclear and heteronuclear rotor encoding of longitudinal magnetization
(RELM) experiments are performed on dipolar-coupled spin-1/2 systems, and the
corresponding sideband patterns in the indirect dimension are analyzed. In both
cases, only even-order sidebands are produced, and their intensity distribution
depends on the durations of the recoupling periods. In heteronuclear experiments
using REDOR-type recoupling, purely dipolar sideband patterns that are entirely
free of effects due to the chemical-shielding anisotropy can be generated.
Advantages and disadvantages of the heteronuclear RELM experiment are discussed
in the context of other methods used to measure heteronuclear dipolar couplings.