Magic spinning sidebands

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

Fig. 31. Representative 188-MHz l9F NMR spectra of p-fluoroaniline (top) andp-fluoroni-trobenzene (bottom) obtained in zeolites HY and HZSM-5. Spinning sidebands are denoted by asterisks. Spectra were acquired (several thousand scans) using magic angle spinning (4-mm rotors), cross polarization (2 ms), and proton dipolar coupling. (Reprinted with permission from Nicholas et al. (82). Copyright 1995 American Chemical Society.)...

The other general way of resolving powder patterns from different chemical sites is to generate multidimensional NMR spectra in which the desired powder patterns (or magic-angle spinning sideband patterns) are resolved in one dimension, separated according to (for instance) isotropic chemical shift in another dimension. These techniques are discussed below in the relevant section for each type of nuclear spin interaction. 

Another way of dealing with the resolution problem for powder lineshapes is to use multidimensional NMR techniques to separate powder pattern lineshapes (or magic-angle spinning sideband patterns) according to isotropic chemical shift, as mentioned previously. 

Fig. 14. The pulse sequence for recording the double-quantum 2H experiment.37 The entire experiment is conducted under magic-angle spinning. This two-dimensional experiment separates 2H spinning sideband patterns (or alternatively, static-like 2H quadrupole powder patterns) according to the 2H double-quantum chemical shift, so improving the resolution over a single-quantum experiment. In addition, the doublequantum transition frequency has no contribution from quadrupole coupling (to first order) so, the double-quantum spectrum is not complicated by spinning sidebands. Details of molecular motion are then extracted from the separated 2H spinning sideband patterns by simulation.37 All pulses in the sequence are 90° pulses with the phases shown (the first two pulses are phase cycled to select double-quantum coherence in q). The r delay is of the order 10 gs. The q period is usually rotor-synchronized.

FIGURE 6. 338.7-MHz19F magic angle spinning (MAS) spectra of Kel-F obtained as a function of MAS speed. Arrows indicate spinning sidebands. The asterisk indicates the signal from a (CF2—CF2)2 impurity . Reproduced by permission of Academic Press Inc., from Reference 60... 

Often, in liquid-phase as well as in single-crystal NMR work, the samples are deliberately spun, often at special orientations (magic angle) relative to B. This will average out dipolar interactions, often narrowing the observed lines by substantial amounts. For example, see Ref. 3, Section 7.10 and Ref. 4, Chapter 8. Under some circumstances, additional lines (spinning sidebands) are generated by the MAS technique.32-34... 

Fig. 3 The effect of slow magic-angle spinning. A set of spinning sidebands appears with a centre-band at the isotropic chemical shift and further lines spaced at the spinning frequency. The intensities of the sidebands change with spinning speed with higher-order sidebands (i.e., those further away from the centre-band) becoming less intense as the spinning speed increases. The chemical shift parameters used in the calculation of these sideband patterns are isotropic chemical shift offset 0 Hz chemical shift anisotropy 5 kHz asymmetry 0. Reproduced with permission from [16]...

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