High-speed MAS

The REDOR experiment has formed the basis for a large number of ideal pulse type recoupling experiments, and later finite pulse variants, for heteronuclear dipolar recoupling. These include experiments such as frequency selective REDOR (FS-REDOR) [80], TEDOR (Transferred Echo DOuble Resonance) [25], and 3D variants of TEDOR [81, 82], which have found important applications, e.g., for measurement of intemuclear 13C-15N distances in biological solids. We should also mention that rotor-encoded variants of TEDOR, such as REPT, HDOR [83], and REREDOR [84], have been proposed for 1H13C dipolar recoupling under high-speed MAS conditions. 

As demonstrated in Fig. 3, even with high-speed MAS, spinning sidebands do occur. These sidebands may be confused with actual resonances in the NMR... 

Solid-state 13C NMR spectra were obtained on a General Electric Omega PSG 400 spectrometer operating at 100.6 MHz for l3C nuclei using a Doty Scientific 5 mm high speed MAS probe. The various NMR experiments performed in this study are shown schematically in Figure 1. The carbon chemical shift scale (11) and 90 degree... 

Restoring the interactions that are averaged out by (high-speed) MAS has been the most active research area in solid-state NMR in recent years. Novel pulse sequences for this purpose, including those can restore heteronuclear dipolar coupling, homonuclear dipolar coupling, chemical shift, etc., have been continually proposed. Some of these sequences were reviewed in the previous article,1 hence only new developments in this direction will be summarized in the following. 

Under high-speed MAS, CP might become inefficient because of weaker dipolar interaction results. Numerous solutions have been proposed in order to improve the CP efficiency, including phase inversion, amplitude modulation and frequency/amplitude modulation.1,267-274 The simplest technique is RAMP-CP,272-274 which is now widely adopted in practice. 

G. Jeschke, W. Hoffbauer and M. Jansen, Survival of dipolar splitting between equivalent nuclei in high-speed MAS-NMR—Interpretation of the 31P coupling patterns for tetraphosphorus deca-oxide. Chem. Eur.., 1998,4(9), 1755-1761. 

The resolution of conventional proton MAS spectra presented by Kaflak-Hachulska et al. [20] is much better than those discussed earlier (Figs. 19 and 20). For MAS at 40 kHz rotational sidebands are missing. Ultra-high speed MAS... 

H NMR spectra of Gly-containing peptides and polypeptides in the sohd state were measured at 800 MHz and at high-speed MAS of 30 kHz to... 

Continuing the theme of comprehensive studies, Woessner [23] recently carried out another major investigation of clay structure. The main improvement here is the use of high-speed MAS. Some of the spectra were collected at 9 kHz, which was significantly faster than for most spectra of clays published to that point. Several of the conclusions reached are in agreement with previous structural studies. One additional parameter that was extracted was the SOQE (second-order quadrupole effect), which is simply the isotropic part of the second-order quadrupolar interaction. The QCC (quadrupolar coupling constant) and T] (asymmetry parameter) are defined in this paper as follows... 

When the MAS experiment is appHed to quadrupolar nuclei such as Al, the quality of the NMR spectrum depends dramatically on the magnitude of compared with both the MAS spin rate and the magnetic field, Bq. Excellent spectra are obtained for small Cq sites when studied with high-speed MAS spin rates at high Bg. Conversely, Al sites can be invisible for high Cq values and modest MAS spin rates and Bo, note the evolution in line shapes for the Al MAS NMR spectra in Fig. 7.11. Likewise, the four-coordinate AIO4 sites in an aluminum isoproxide complex, with Cq = 12.3 MHz, are observable at 20 kHz and 19.6 T while three-coordinate aluminum sites in related complexes, with Cq > 30 MHz, are not observable [14],... 

Computations of the proton chemical shift by DFT for the intermolecular hydrogen bond in the white form of methylnitroacetanilide, together with the experimental value obtained by high-speed MAS NMR were used by Harris et al. to determine the N-H distance as 1.03 + 0.02... 

K. Yamauchi, S. Kuroki, I. Ando, T. Ozaki, A. Shoji, NMR chemical shifts and quadrupole coupling constants in solid poly(L-alanine)s determined using a high-speed MAS technique, Chem. Phys. Lett. 302 (1999) 331. 

F. Aguilar-PariUa, B. Wehrle, H. Braunling, H.-H. Limbach, Temperamre gradients and sample heating in variable temperature high speed MAS NMR spectroscopy,. Magn. Reson. 87 (1990) 584-591. 

A. Samoson, T. Tuherm, Z. Gan, High-field high-speed MAS resolution enhancement in H NMR spectroscopy of solids. Solid State Nucl. Magn. Reson. 20 (2001) 130-136. 

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