Spinning-sideband patterns

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. 

In Table 2, the A chemical shifts of the carbon atoms of alkoxy species attached to zeolite framework oxygen atoms are summarized. In general, the spins of surface alkoxy species are characterized by relatively long T times (2-5 s), an efficient CP, and broad spinning sideband patterns. The adsorption and... 

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.

A comparatively long N- -H hydrogen bond length in the benzoxazine dimer (see below), measured using an advanced solid state NMR technique (DIP-HSQC),has been reported [52]. This technique employs REDOR-type recoupling under fast MAS to recouple the heteronuclear - N dipolar interaction, such that rotor-encoded spinning sideband patterns are obtained. 

Fig. 8. Calculated heteronuclear DQ MAS NMR spinning sideband patterns for rotating phenylene groups and different flip angles or as indicated (Nrcpl DIS/ coR= 1.67).

Fig. 14. (a) Solid-state l3C spinning sideband patterns (sum projections) for the aromatic ternary CH in polyphenylene dendrimers, obtained at a spinning frequency of vR = 25 kHz. The spectra were recorded for different temperatures and generations, (b) Corresponding simulated spectra, obtained by taking into account different models of phenyl ring reorientation processes on a microsecond-timescale. For details, see ref. 44. 

Fig. 16. H- H DQ spinning sideband patterns of discotic HBC-C 2 in the solid state (top) and in the columnar liquid crystalline phase (bottom), where the discs rotate around the column axis as indicated. For details, see ref. 38.

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.

---