Sideband intensities

The experimental data closely resemble the simulation based on the all-trans PSB values. The discrepancy between the two solid-state NMR studies on rhodopsin arises in part from a difference, in signal-to-noise ratio and in part from possible problems associated with a fatty acid resonance which overlaps with the centerband in the previous study. The simulations illustrate the sensitivity of the sideband intensities to changes in the chemical shift tensor, as well as the quality of data necessary to accurately determine the shift tensor values. 

Fig, 16. 50.1-MHz 13C MAS spectra of benzaldehyde-a-13C and benzene reacting on zeolite HY. The spectrum acquired at 120 K after the sample was heated at 448 K clearly shows an isotropic chemical shift at 207 ppm, consistent with the chemical shift of the trityl cation. Furthermore, the Herzfeld-Berger analysis of the sideband intensities reveals an axially symmetric tensor, thus providing unambiguous evidence for the trityl cation 16. 

Measurements of the static 13C line shape or sideband intensities of acetone on many solid acids at room temperature underestimate the chemical shift anisotropy due to motion, but the principal components of the chemical shift tensor can be accurately measured at reduced temperature. Table V reports these data for acetone on a wide variety of Brpnsted and Lewis acids (43, 45) note that the largest contribution to the isotropic shift is <5n The shift induced by A1C13 and other Lewis acids is rationalized by... 

It is assumed that shielding is the only magnetic influence on spinning sideband intensities. The isotropic chemical shift criso = —<5 Si/ppm is defined as equal to one-third of the trace of the shielding tensor (Trcr/3). The principal values of the shielding tensor (an,... 

Table 5 Calculated dipolar rotational sideband intensities for an aromatic CH pair undergoing various molecular motions, under MAS at 1.894 kHz (from [40])...

As proved by the authors, the 2D CSA-amplified PASS has a number of favourable features. First, a gain in signal intensity by a factor of 2 is achieved over experiments that require either a storage period or quadrature detection in the indirect dimension. Second, in common with the XCS (chemical-shift modulation) and CSA amplification experiments, isotropic shifts do not occur in the ct>i dimension, leading to efficient sampling of the indirect dimension. Finally, spinning sideband intensities are the same as in conventional MAS spectra allowing routine analysis. 

Herzfeld J, Berger AE (1980) Sideband intensities in NMR spectra of samples spinning at the magic angle J Chem Phys 73 6021-6030... 

Detailed calculations of the spinning sidebands in DAS spectra have been carried out using average Hamiltonian and irreducible tensor approaches (Sun et al. 1992). In DAS spectra the sideband intensities and their moments depend on the relative rotor phase between the two evolution periods. The sideband intensities additionally depend on the ratio of the time spent at each angle. The 2D O DAS spectrum of zeolite Sil-Y (Figure 3.22) shows three lines in the ratio 2 1 1 (Bull et al. 1998). Simulation of the anisotropic slices from the O 2D DAS spectrum for each peak allows extraction of xq and t] for each resonance. 

Figure 6.3. O MAS NMR spectrum of BaZrOa showing a sideband intensity distribution resulting from the CSA. From Bastow et al. (1996a) by permission of the copyright owner.

Figure 7 shows the relative increase in sideband intensities for CP over BD as HPO4" and CO3" ions are introduced into the lattice and the considerable broadening of the CHA-B peaks [42]. The effect of HPO " ions is obvious, since they give pronounced sideband patterns and cross-polarize very efficiently. This has already been explained during the discussion of BRU. Figure 8 proves that the appearance of sideband patterns is very sensitive to the structure of the material [20]. It was found [42] that decrease in crystallinity results in considerable increase of linewidths and sideband intensities, and in dramatic reduction or disappearance of DD peaks (Fig. 9) [42]. The centreband in the CP spectrum of poorly crystalline HA looked like a superposition of a sharp line on a broader background. The CP and BD spectra of amorphous calcium phosphate were virtually identical within the experimental error. Aue et al. [42] admitted that they found no satisfactory explanation for the latter observation. Likewise, the discussion of the CO3" effect (Fig. 7) did not go very far. It was stated that the CHA-B lines were 5-10 times broader than those from various HA samples and that this broadening showed rather small dependence on carbonate concentration. It was inferred that the introduction of CO3" increased the CP efficiency of some PO " ions in the crystal lattice and imparted some changes in the PO " shift anisotropies. The former is questionable, because no protons were introduced into intracrystalline sites proximate to PO ions and the latter acceptable, because the CHA lattice must be less ordered, at least in the...

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. 

Magnitudes and relative orientations of quadrupole coupling and chemical shift tensors have been determined from MAS NMR spectra at 14.1 T for seven divalent metal pyrovanadates using least-squares fitting of the integrated soinnine sideband intensities observed for the central and satellite... 

Fig. 47. Plots of the intensity ratio of the first and second-order sideband intensities to the centre signal of the simulated and experimentally obtained dipolar spectra...

Figure 46 shows the simulated spectra for a series of different N—H bond lengths at a rotor frequency of 2.0 kHz. The sideband pattern is sensitive to the N—H bond length. The relative intensity of SSBs to the centre peak decreases with longer N—H distance. It is therefore possible to determine the N—H bond length within an accuracy of 0.01 A by careful comparison of the integral ratio of the centre peak to the sideband intensities of the dipolar spectrum obtained experimentally with that of the simulated spectra. 

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