Spectral Spinning sideband

For kaolinite the sample permeability was very low and the solution was poorly removed. The spectra (Figure 3C) are consequently complex, containing peaks for inner and outer sphere complexes, CsCl precipitate from resMual solution (near 200 ppm) and a complex spinning sideband pattern. Spectral resolution is poorer, but at 70% RH for instance, inner sphere complexes resonate near 16 ppm and outer sphere complexes near 31 ppm. Dynamical averaging of the inner and outer sphere complexes occurs at 70% RH, and at 100% RH even the CsCl precipitate is dissolved in the water film and averaged. 

Fig. 15. Comparison between the static, MAS and MQMAS 23Na NMR spectra of multisite sodium salts. Contour levels in the 2D plots were taken at 70%, 35%, 16%, 8%, and 4% of the maximum spectral intensities asterisks indicate the spinning sidebands. (Reproduced, with permission, from Medek et a/.209)...

Spectral editing Removal of spinning sidebands Pulse sequences modulate... 

Figure 4(B) shows the simulated 17O MAS spectra as a function of i]q from 0 to 1. These calculations were carried out using the same conditions of the 170 stationary NMR spectra. In the similar manner as in the case of the stationary NMR spectra, the MAS spectra exhibit characteristic line shapes from which the information on t]q as well as Cq can be extracted. The anisotropy of CS tensors is removed by MAS and only <5iSO will be obtained if it exists. When the sample spinning frequency is not high enough, the effect of spinning sidebands spaced at the spinning frequency around the central peak needs to be considered in the spectral simulation. The frequency contribution from the second-order quadrupolar interaction under slow/intermediate MAS conditions is given in the literature 46,47 A complicated line shape is expected to appear in the MAS NMR spectrum so that a computer simulation is not trivial (see Figure 15).

Very high quality NMR spectra have been obtained by Jakobsen et al. (2001) for a number of metal nitrates using high-quality MAS probes to determine the complete manifold of spinning sidebands from which extremely reliable " N NMR interaction parameters for these compounds (Table 8.4) were extracted by spectral simulation. 

Figure 2-1 Effects of misadjusted shim settings, (a) Z3 misset. (b) Z5 misadjusted. (c) Z2 misset. (d) Z4 misadjusted. (e) First-order spinning sidebands X, Y, XZ, and YZ misset. (f) Second-order spinning sidebands XY and X2-Y2 misadjusted on lop of first-order sidebands, (g) High-order nonspin shims X3, Y3, Z3X, and Z3Y misset. (h) Zl, Z2, and Z4 misadjusted. These spectral effects have been exaggerated for the purpose of illustration. (Courtesy of Varian Inc. Technical Publications.)...

To obtain a high-resolution spectrum free from the unwanted CT CT peak, the third method proposed uses half-rotor synchronisation, instead of full-rotor period synchronisation, during [40]. The increment in (or kt for split-ti experiments) is set to half the rotor period, thereby doubling the isotropic spectral width (Fi=2Vj., where Vj. is the MAS rate). This means that two sets of STn CT resonances will be observed, a centreband and a spinning sideband. In principle, the latter will be well resolved from the CT CT diagonal peaks, which in general do not display any sidebands in Compared with a full-rotor synchronisation, the S/N ratio is reduced by a factor of two and the experimental time is doubled. As an example. Fig. 13a shows the STMAS spectrum of zeolite scolecite. 

Figure 7. Pulse sequence and coherence transfer pathway diagram for a H DQ MAS experiment using the BAB A recoupling sequence for the excitation and reconversion of DQCs. The rectangular blocks represent pulses of flip angle 90°, with the choice of the phases being described in, e.g., ref 25. If the q increment is set equal to a rotor period, a rotor-synchronized two-dimensional spectrum is obtained, while reducing q, and hence increasing the DQ spectral width, leads to the observation of a DQ MAS spinning-sideband pattern.

Rotor-synchronized H DQ MAS spectra can only deliver information about relative proton—proton proximities (except for cases where the DQ peak(s) due to a known internal or external standard are well resolved).83 The DQ MAS experiment (see Figure 7) can, however, be performed in an alternative fashion if the t increment is reduced, which corresponds to an increase in the DQ spectral width, a DQ MAS spinning-sideband pattern is observed35-36 (provided that a recoupling sequence which has an amplitude dependence on the rotor phase, e.g., BABA91 or DRAMA93, is used). 

By spinning samples simultaneously about the magic angles of 54°44 and 30.6°, appropriate to quadrupolar interactions, both these and the dipolar interactions can be removed through the elegant DOR technique due to Pines et al. [20], This technique, however, remains prone to some difficulty in spectral interpretation due to the proliferation of unwanted spinning sidebands. 

In the case of MAS, the spectra typically display a number of spinning sidebands spaced by the spinning frequency. The intensities of the spinning sidebands approximately represent the intensity of the static powder spectrum and hence the overall envelope of the sideband intensities represents the powder line shape. While analytical solutions have also been derived for the intensity of the sidebands as a function of the spinning frequency and anisotropic shielding parameters, it is typically much easier to determine these interaction parameters from numerical simulations - again considering that this method directly takes the experimental errors and spectral noise into account. 

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