|The most important aspect in solid-state NMR is the dominance
of orientation-dependent interactions, which are usually averaged out in
NMR spectra of solutions. They lead to substantial broadening of the spectra and
a loss of spectral resolution.
High resolution can be re-gained by rapid sample rotation about the magic angle (54.7°), which, much like in the case of solutions, effectively averages all orientation-dependent interactions.
|While almost liquid-like resolution is achievable for
13C already at moderate spinning frequencies,
very-fast MAS in excess of 25 kHz is needed in order to narrow
dipolar-broadenend proton spectra. This is due to the strong multi-spin
dipolar-coupling network among the abundant protons, which is also responsible
for the smearing of the typical features of powder (Pake) spectra in the static
|Dipolar coupling constants are dependent on the distance between the coupled spins. Yet, their measurement is precluded when MAS is applied, because this valuable information is simply averaged away. The solution to this paradox is the application of recoupling pulse sequences of a specific duration as part of a two- or higher dimensional experiment. The variation of the recoupling time while monitoring the signal intensity provides access to the dipolar coupling. The experiment might embody the temporary excitation of higher-quantum coherences. A special variant of this approach comprises a second Fourier transformation over an indirect time domain, which leads to the appearance of coupling-specific sideband patterns.|
|The spectra shown here are special in that they were obtained
on samples naturally abundant in 15N (!) by
using inverse detection. I.e., the detection of the signal takes place on
protons instead of the weakly sensitive heteronucleus.
The distances which are extracted from the spinning sideband patterns are in good agreement with results from other methods. In fact, only neutron scattering or X-ray scattering involving substantial refinement is able to provide distance information with comparable accuracy.
|While line shape analysis might not be very reliable, due to the problem of multiply coupled spins and featureless spectra, multiple-quantum spectroscopy is able to provide faithful information. The technique resembles the recoupling experiments mentioned above, but can also be applied in static samples. Information on dipolar couplings is extracted by observing the signal intensity as a function of the duration of a double-quantum pulse sequence.|
|From the variation of signal intensities as a function of the
mixing time, tm, or the recoupling time,
it is possible to obtain the correlation time of the slow process,
tc, and the jump angle, respectively.
The long-time intensity plateau is further dependent on the number of sites accessible to the jump motion. A decrease of this plateau intensity at higher temperatures is indicative of short correlation times reaching the experimental time scale (intermediate motional regime).