Dipolar powder pattern

It is well known from theory and experiment that the interaction of ethylene with silver is nearly repulsive (711. This weak adsorption can be investigated with solid-state C NMR (72], and an illustration of such spectra for 1- C-C2H4 and 1, 2- C2 C2H4 is depicted in Fig. 36. The value extracted from the chemical shift/dipolar powder patterns of these spectra is 1.34 A. This result is close to the value of 1.335-1.340 A for ethylene in the gas phase (731, a bond distance that reflects the weak interaction between the silver and the adsorbed ethylene. 

The H dipolar powder pattern has features which affect other NMR experiments. First, this is an inhomogeneously broadened line composed of many, narrow but homogenously broadened peaks. Second, the two subspectra generated by ms( H) = +1/2 are mirror images of each other. Third, powder-pattern averages of axially-symmetric units, such as H, give the characteristic Pake... 

T. Nakai and T. Terao, Measurements of heteronuclear dipolar powder patterns due only to directly bonded couplings, Magn. Reson. Chem. 30 42-44 (1992). 

Molecules in the solid state are in fixed orientations with respect to the magnetic field. This produces chemical shift anisotropic powder patterns for each carbon atom since all orientations are possible (Fig. 2). It was shown as early as 1958 that rapid sample rotation of solids narrowed dipolar-broadened signals [18]. Several years later, it was recognized that spinning could remove broadening caused by CSA yet retain the isotropic chemical shift [19]. 

Clore et al. [63] have noticed that for an isotropic distribution of dipolar bond-vectors the probabilities resemble the shape of a chemical shift anisotropy powder pattern... 

The Tte of the 3Fe-4S centre in succinate ubiquinone reductase between 4 and 8 K is decreased by interaction with paramagnetic cytochrome b.98 To mitigate the impact of spectral diffusion the relaxation times were measured by a picket-fence sequence with 100 pulses. Analysis of the powder pattern distribution of relaxation times indicated that the anisotropic dipolar interaction dominated over isotropic scalar interaction and a lower limit of 10 A was estimated for the distance between the iron-sulfur cluster and the heme. 

Efforts to understand the state of hydrogen in metals and metal hydrides have involved the use of NMR for many years. This study combines the conventional solid state NMR techniques with more recently developed high-resolution, solid state NMR techniques (5,6). Conventional NMR techniques furnish information on dipolar interactions and thus can furnish static geometrical information on hydrogen positions and information on proton motion within such solids. The newer multiple pulse techniques suppress proton-proton dipolar interaction and allow information on other, smaller interactions to be obtained. This chapter reports what the authors believe is the first observation of the powder pattern of the chemical shift tensor of a proton that is directly bonded to a heavy metal. 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

Fig. 3. Example spectra from the one-dimensional dipolar-shift experiment taken from reference 7. (a) (Top) Experimental l3C chemical shift anisotropy powder pattern for Ru(C5H5)2 and (below) for comparison, the dipolar shift l3C spectrum for the same compound, (b) Calculated dipolar-shift lineshapes for different angles (indicated) between the lH-13C dipolar and chemical shift anisotropy tensor principal z-axes.

A special case occurs if the solid contains nuclear spin pairs that are sufficiently far apart from each other such that intrapair coupling dominates interpair coupling. In this situation, instead of a featureless broad lineshape, we observe a well-defined powder pattern known as a Pake doublet (Figure 15.15). Since the dipolar coupling between two nuclei... 

If the nuclei are of the same species but are not magnetically equivalent (e.g., because of chemical shift differences), the four basis functions of Eq. 6.1 must be used. However, because the dipolar coupling is normally much larger than chemical shift differences, the resulting spectra and powder pattern are little changed from those in Fig. 7.6. 

Figure 2.5. A. Schematic powder pattern of a spin- /2 nucleus dipolar coupled to a second spin- /2 nucleus. B. Example of a H Pake powder pattern from NaAlSi206.H20 from Yesinowski, Eckert and Rossman (1988) with permission of the American Chemical Society.

More recently, the static NMR spectra of 16 potassium compounds have been obtained by Bastow (1991) using a solid pulse echo sequence. Most of these compounds have relatively small dipolar coupling, resulting in sharp powder pattern features of the observed central transition. Accurate simulations allowed the values of xq and T to be determined and also indicated very small CSA contributions. Most of these model compounds contain only 1 potassium site, but even those compounds with 2 inequivalent K sites could be sufficiently resolved to allow accurate and unambiguous simulation (Bastow 1991). The NMR interaction parameters for potassium compounds are collected in Table 8.8. 

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