Sidebands spinning

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. 

Figure 2. Cs MAS NMR spectra of Cs-exchanged hectorite sample dehydrated at 500°C before analysis at temperatures from 80 to -80°C. The labeled peaks near -120 and +30 ppm are true center bands, and the other peaks are spinning sidebands. After reference 22.

Figure 3. CS MAS NMR spectra of Cs- exchanged (A) silica gel, (B) illite, (C) kaolinite, and (D) boehmite collected at H = 11.7 T, room temperature and the indicated relative humidities. The peaks marked by are spinning sidebands. After reference 27.

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. 

We have referred to the various interactions which can cause line broadening in the solid state. One of these, which is normally not a problem in liquid state NMR, is due to the fact that the chemical shift itself is a tensor, i.e. in a coordinate system with orthogonal axes x, y and z its values along these axes can be very different. This anisotropy of the chemical shift is proportional to the magnetic field of the spectrometer (one reason why ultra-high field spectrometers are not so useful), and can lead in solid state spectra to the presence of a series of spinning sidebands, as shown in the spectra of solid polycrystalline powdered triphenylphosphine which follows (Fig. 49). In the absence of spinning, the linewidth of this sample would be around 75 ppm ... 

Fig. 51 A—C Phosphorus-31 MAS spectra of compounds A-C. Signals marked with an asterisk are due to spinning sidebands...

Figure 5 11B MAS NMR spectra (a) pristine phenyl modified elastomer (b) heated at 480°C for 2 h (c) heated at 580°C for 2h (d) after 1 MGy gamma exposure. ( represents spinning sidebands.)...

Fig. 7 Comparison of the 170 DOR and MAS NMR (30 kHz) spectra of Ba2P207 (a), Mg2P207 (b), and Na4P2Oy (c) at B0 = 17.4 T. Spinning sidebands (asterisks, plus symbol) arise due to the limited spinning speed of the outer rotor ( 1,400 Hz). (Reproduced with permission from [146])...

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

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... 

Fig. 13 Determination of 31P Tx values for fosinopril sodium using the inversion-recovery method. The spectra represent approximately a 50/50% w/w mixture of polymorphic forms A (8 = 52 ppm) and B (8 = 55 ppm). Spinning sidebands are represented by asterisks. 

There are a lot of other things in a typical NMR. There are spinning sidebands, small duplicates of stronger peaks, evenly spaced from the parent peak. They fall at multiples of the spin rate, here about 30 Hz. Spin the sample tube faster and these sidebands move farther away slow the tube and they must get closer. 

Figure 5. Li MAS NMR spectrum of the Mn(IV) spinel (Lio.5Zno.5)tet(Lio.5Mni 5)oct04 and the typical hyperfine shifts observed for lithium in a series of local environments. Hyperfine shifts are given next to the two isotropic resonances in the Li spectrum all other peaks are spinning sidebands, which are predominantly caused by the electron—nuclear dipolar coupling.

Figure 6. Li MAS NMR spectrum of the layered compound Li2MnOs acquired at a MAS frequency, Vr, of 35 kHz. Spinning sidebands are marked with asterisks. The local environment in the Mn +/Li+ layers that gives rise to the isotropic resonance at 1500 ppm is shown. Spin density may be transferred to the 2s orbital of Li via the interaction with (b) a half-filled t2g orbital and (c) an empty d/ Mn orbital to produce the hyperfine shifts seen in the spectrum of Li2MnOs. The large arrows represent the magnetic moments of the electrons in the t2g and p orbitals, while the smaller arrows indicate the sign of the spin density that is transferred to the Li 2s and transition-metal d orbitals.

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