MAS spinning sidebands

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... 

KD2PO4 (lower panel). indicates MAS spinning sidebands [25]... 

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). 

Figure 9. Simulated homonuclear DQ MAS spinning-sideband patterns generated in the time domain using eq 6, with the powder average being performed numerically, for different values of the product of D and Trcpi.

Gregory et al. were able to determine the relative orientation of 13C CSA tensors from an analysis of 13C DQ MAS spinning-sideband patterns obtained using the DRAWS recoupling method.117... 

In Figure 21 a and c, H (700.1 MHz) DQ MAS spinning-sideband patterns obtained for the lactam (at 10.8 ppm) NH resonances of bilirubin, with rrcpi equal to (a) two and (c) three rotor periods at a vr = 30 kHz are shown. In the rotor-synchronized H DQ MAS spectrum in Figure 20b, in addition to the intense NH—NH DQ peaks, weaker DQ peaks due to DQCs involving the OH and aliphatic protons are observed. An inspection of the spectra in Figure 21 reveals the existence of spinning sidebands due to all these different DQCs note that the DQ peak for the NH—NH pair is at the second-to-left position. 

Figure 31 presents experimental H DQ MAS spinning sideband patterns for the aromatic protons in (a) the crystalline and (b) the LC phases of a-deuterated HBC—C12.22 The MAS frequency was 35 and 10 kHz in (a) and (b), respectively, with two rotor periods being used for excitation/reconversion in both cases, such that rrcpi equals 57 and 200 /us in the two cases. The dotted lines represent best fit spectra simulated using the analytical time-domain expression for an isolated spin pair in eq 6. As noted in section VIIB, the aromatic protons exist as well isolated pairs of bay protons, and, thus, an analysis based on the spin-pair approximation is appropriate here. As is evident from the insets on the right of Figure 31, the DQ MAS spinning sideband patterns are very sensitive to the product of the D and rrcpi. The best-fit spectra for the solid and LC phases then correspond to DI(Zji)s equal to 15.0 0.9 and 6.0 0.5 kHz, respectively. 

Importantly, since r pi is known, the absolute value of D can be extracted by an analysis of DQ MAS spinning-sideband patterns. As a specific example. Fig. 9.27 presents experimental DQ MAS spinning sideband patterns for the aro-... 

Fig. 9.30 H—heteronuclear MQ MAS spinning-sideband patterns, obtained at a Vr = 25 kHz, using the REPT-HMQC experiment. The patterns correspond to the sum projections over the resonance due to the aromatic core CH in the 2D spectra of HBC-C12, and HBC-PhCi2. The spectra for the room temperature (solid) and high temperature LC phases were...

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