Static quadrupolar patterns

Physical background. MAS will narrow the inhomogeneously broadened satellite transitions to give a series of sharp sidebands whose intensity envelopes closely follow the static powder pattern so that the quadrupole interaction can be deduced. The work of Samoson [25] gave real impetus to satellite transition spectroscopy by showing that both the second-order quadrupolar linewidths and isotropic shifts are functions of I and m. Some combinations of I and produce smaller second-order quadrupolar effects on the satellite lines than... 

In the case of MAS conditions, the spectrum has a sufficient resolution to resolve distinct deuteron sites with different chemical shifts and the SSB pattern reflects the envelope of the static quadrupolar lineshapes. 

This compound has two crystallographically distinct vanadium sites. While the static spectrum is a superposition of two powder patterns of the kind shown in Figure 3, MAS leads to well-resolved sharp resonances. Weak peaks denoted by asterisks are spinning sidebands due to the quadrupolar interaction. 

MAS is routinely used in solid-state NMR spectroscopy for eliminating the elfects of chemical shift anisotropy, heteronuclear dipolar interactions and first-order quadrupolar interactions. In this method the sample is rotated abont the axis inclined at 54.74° with respect to the external magnetic field Bo, so that the average of the geometric term in nnclear spin interactions (3cos 0 — 1) = Except for some specific applications of static wide-line solid-state NMR spectroscopy (i.e., for or for the determination of CSA patterns), the MAS is an essential feature of solid-state NMR spectroscopy and can also be nsed in variable temperature mode. The MAS methodology can also be nsed for high-resolntion studies of viscous hquids and soft solids. 

An inhomogeneous line is one in which separate parts of the line can be identified as due to separate contributions. The most common example is a powder pattern due to anisotropies such as quadrupolar or chemical shifts in which crystallites with different orientations with the static magnetic field contribute to different parts of the line. 

Signal enhancements were obtained in H — O cross-polarization experiments without spinning, and reliable second-order quadrupolar powder patterns were observed [1291. Relaxation parameters involved in cross-polarization transfer were shown to be characteristic of the various sites, so that they can be used for signal assignment. In addition, in some cases the differences in crosspolarization rates were used to edit spectra by a selective enhancement of protonated oxygen resonances, such as those from surface hydroxyl groups in amorphous silica. The latter method can be applied to complicated systems, provided dipolar H — O interactions for the various sites are different. We illustrate this procedure by using a static H — O CP spectrum [126] of talc. 

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