K. Saalwächter, K. Schmidt-Rohr.
Relaxation-Induced Dipolar Exchange with Recoupling - A Novel
MAS NMR Method for Determining Heteronuclear Distances without Irradiating the
J. Magn. Reson.
145, 161-172 (2000).
A new magic-angle spinning NMR method for distance determination between unlike
spins, where one of the two spins in question is not irradiated at all,
is introduced. Relaxation-Induced Dipolar Exchange with Recoupling (RIDER)
experiments can be performed with conventional double-resonance equipment and
utilize the familiar pi-pulse trains to recouple the heteronuclear dipolar
interaction under magic-angle spinning conditions. Longitudinal relaxation of
the passive spin during a delay between two recoupling periods results in a
dephasing of the heteronuclear coherence and consequently a dephasing of the
magnetization detected after the second recoupling period. The information about
the dipolar coupling is obtained by recording normalized dephasing curves in a
fashion similar to the REDOR experiment. At intermediate mixing times, the
dephasing curves also depend on the relaxation properties of the passive spin,
i.e. on single- and double-quantum transverse relaxation times for the case of
I=1 nuclei, and these relaxation times can be estimated with this new method. To
a good approximation, the experiment does not depend on possible quadrupolar
interactions of the passive spin, which makes RIDER an attractive method when
distances to quadrupolar spins are to be determined. The new method is
demonstrated experimentally with 14N and
2H as heteronuclei, and observation of
13C in natural abundance.