Bruker BioSpin Solid State NMR, User Manual

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User Manual Version 002 BRUKER BIOSPIN 271 (327)
21
CRAMPS: General 21
CRAMPS is an acronym standing for “Combined Rotation And Multiple Pulse
NMR Spectroscopy”. Multiple Pulse Spectroscopy had long been thought not to
work under spinning around the magic angle, but in fact it does work, as long as
the pulse cycle times are substantially shorter than the rotation period.
CRAMPS suppresses homonuclear dipolar interactions between the abundant
spins (mostly protons) and chemical shift anisotropy simultaneously through the
combination of multiple pulse techniques and magic angle spinning. J-couplings
and large heteronuclear dipolar couplings are not suppressed.
Reference:
1. L.M. Ryan, R.E.Taylor, A. J. Patt, and B. C. Gerstein, An experimental study of resolution of proton
chemical shifts in solids: Combined multiple pulse NMR and magic-angle spinning , J. Chem. Phys. 72
vol.1, (1980).
Homonuclear Dipolar Interactions 21.1
Homonuclear dipolar interactions among spins with a strong magnetic moment
and high natural abundance - mainly
1
H or
19
F, and to a much smaller extent
31
P
- are usually very large unless averaged by high mobility. Especially in the case of
protons, spin exchange is usually rapid compared to routinely achievable rotation
periods, meaning that MAS alone cannot suppress the homonuclear dipolar
broadening. Even spin rates in the order of 70 kHz, which is no longer a mechani-
cal problem, cannot fully average this interaction in rigid solids. As chemical shift
differences among the coupled nuclei become larger, the interaction becomes
more heterogeneous and MAS can suppress it more efficiently. This is the reason
why fast spinning alone works much better on
19
F or
31
P than on protons. hetero-
nuclear dipolar coupling, such as between
13
C and
1
H, can in principle be spun
out, but only if the homonuclear coupling between protons is small, or averaged
by motion or a suitable pulse sequence. CRAMPS sequences therefore play an
important role also in experiments where X-nuclei are observed.
Multiple Pulse Sequences 21.2
Dealing with a heteronuclear dipolar coupling is easy: continuous high power irra-
diation of one coupling partner will decouple it from the other nucleus, as in the
case of
13
C observation while decoupling protons. However observing a nucleus
while decoupling it from like spins at the same time is obviously not trivial, since
the signal cannot be observed under the much higher decoupling RF. Observation
of the signal and decoupling pulses must therefore be alternately applied. Sup-
pression of a homonuclear dipolar interaction occurs when the magnetization vec-
tor of the coupled spins is tilted into the magic angle. This condition can be
achieved either by 4
л /2 pulses of suitable phase and spacing (multiple-pulse
methods), or by off-resonance irradiation of suitable offset and RF-field (Lee-