Bruker BioSpin Solid State NMR, User Manual

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17
Basic MQ-MAS 17
Introduction 17.1
The MQMAS experiment for half integer quadrupole nuclei is a 2D experiment to
separate anisotropic interactions from isotropic interactions. In the NMR of half in-
teger quadrupole nuclei the dominant anisotropic broadening of the c1/2
<-> –1/2 transition (CT), and symmetric multiple-quantum (MQ) transitions, is the
2
nd
order quadrupole interaction which can only partially be averaged by MAS.
The satellite transitions (ST, e.g. the ±3/2 <-> ±1/2 transitions) however, are
broadened by a 1
st
order interaction, which is several orders of magnitude larger
than the 2
nd
order broadening. Under MAS the 1
st
order interaction of the ST can
be averaged but since the spinning cannot be fast compared to the first order
broadening (of the order of MHz), a large manifold of spinning side bands re-
mains. The 2
nd
order broadening of the CT can only be narrowed by a factor of 3
to 4 by MAS so a signal is observed that still reflects this 2
nd
order broadening,
which may be of the order of kHz. Lineshapes resulting from nuclei in different en-
vironments are thus likely to be unresolved in a simple 1D spectrum.
The 2D MQMAS experiment exploits the fact that the 2nd order broadening of the
symmetric MQ transitions (e.g. +3/2 «-3/2 in a spin 3/2), is related to the 2nd order
broadening of the CT by a simple ratio. A 2D spectrum is recorded which corre-
lates e.g. a +3/2 «–3/2 3Q coherence involving the satellite transitions and the +1/
2 «–1/2 single quantum coherence of the central transition. This spectrum shows
a ridge line shape for each site, with slope given by the ratio of the second order
broadening of the two transitions (-7/9 in the case of the 3Q transition). A projec-
tion of the 2D spectrum perpendicular to this slope yields an isotropic spectrum
free from quadrupolar broadening.
Pulse sequences 17.2
Figure 17.1.
and
Figure 17.2.
show two of the basic sequences, a 3-pulse and a
4-pulse sequence with z-filter. Both sequences start with an excitation pulse
p1
that creates 3Q coherence which is allowed to evolve during the evolution period
d0
. In the 3-pulse sequence the subsequent conversion pulse
p2
flips magnetiza-
tion back along the z-axis, which after a short delay
d4
(to allow dephasing of un-
desired coherency) is read out with a weak CT selective 90° pulse
p3
. In the 4-
pulse sequence, however, the conversion pulse
p2
changes 3Q coherency to 1Q
coherency which then passes through a Z-filter of two CT selective 90° pulses in a
p3-d4-p3
sequence.