NMR pulse sequence:
REDOR
Rotational Echo DOble Resonance
Schaefer and coworkers introduced REDOR experiment based on the heteronuclear dipole interaction for the measurement of the interatomic heteronuclear distance.
The REDOR experiment was developed for spin-1/2 nuclei but has been applied to quadrupole nuclei by Fyfe and coworkers. In this case, quadrupole nuclei are the non-observed spins.
Scheme (1): Because the sample is rotating around the magic angle, the heteronuclear dipole coupling becomes time dependent and averages to zero over one rotor period. This heteronuclear dipole interaction has no effect on the 31P echo signal in a normal rotor-synchronized echo experiment.
Scheme (2): To reintroduce the effect of the heteronuclear dipole coupling, selective 180° pulses are applied to 27Al nuclei which reverse the sign of the heteronuclear dipole coupling. This coupling is no longer averaged to zero over one rotor period. As a result, 31P echo signal is attenuated.
The selective 180° pulses are applied to 27Al spins at one-half and full rotor periods after the initial 90° pulse applied to 31P spins. The two 180° pulses are separated by one-half of the rotor period.
These selective 180° pulses on the 27Al spins will interchanged mainly the ¦1/2> and ¦-1/2> energy levels, leaving the other energy levels relatively unperturbed. This occurs for 27Al nuclei submitted to strong quadrupole interaction.
As in SEDOR experiment, a normalized REDOR difference signal
defined as
(delta S)/S0 = [signal from Scheme (1) - signal from
Scheme (2)]/signal from Scheme (1),
is plotted versus the rotor periods. The interatomic distance is
extracted by fitting these data with a theoretical curve.
Bertmer and Eckert proved that in the limit of short dipole
evolution times where (delta S)/So < 0.2 to 0.3,
the REDOR curve becomes geometry-independent:
(delta S)/So = (N Tr)2
M2/[IQ (IQ + 1) Pi2].
The normalized REDOR difference signal is proportional to the square of the dipole evolution time (N Tr) and the curvature depends on the van Vleck second moment M2. The spin IQ is that of the non-observed nuclei.