DQ spinning sidebands

One way of studying molecular motions involves monitoring the reduction of dipole-dipole couplings probed by DQ spinning sidebands. The site selectivity is particularly high for heteronuclear DQ coherences. In Fig. 8, simulated sideband patterns are plotted for the C-H group, which is a sensitive probe of phenylene rotational motions, often met in practice. At low temperatures, one would expect... 

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.

Fig. 37. Experimental C NMR results, applying the pulse sequence shown in Fig. 36 to determine the H— H torsion angles in (a) mono-ammonium maleate (AHM) and (b) diammonium fumarate (DAF). Plots of DQ-filtered signal amplitudes a(ti) for AHM (c) and DAF (d). DQ spinning sideband patterns for AHM (e) and DAF (f), obtained after Fourier transformation of the time-domain data shown in (c, d) as described in ref. 163. (Reproduced from ref. 163, 1996, with permission from Elsevier Science.)...

Fig. 9. 27 Extracted columns from H (500.1 MHz) DQ MAS spectra of HBC-Qj, showing the DQ spinning sideband patterns for (a) the aromatic protons at 8.3 ppm in the solid phase (T = 333 K), and (b) the aromatic protons at 6.2 ppm in the LC phase (T = 386 K). In each case, best-fit spectra, generated according to the analytical expression for a spin pair, are shown (shifted to the left to allow a better comparison) as dotted lines. A spinning frequency Vr, equal to 35 and 10 kHz was used for...

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).

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.

The two-dimensional DQ MAS experiment can be performed in two distinct ways. We consider first spectra recorded in a rotor-synchronized fashion in t, i.e., the t increment is set equal to one rotor period, tr. In this way, all spinning sidebands in F can be considered to fold back onto the centerband position. The appearance of a rotor-synchronized 2D H DQ MAS spectrum is illustrated in Figure 8. Since the DQ frequency corresponding to a given DQC is simply the sum of the two SQ frequencies, DQCs between like (AA) and unlike (AB) spins can, in general, be distinguished in that, in the former case, a single peak at (2v, rA) is observed, while, in the latter case, two peaks at (rA + vb, v ) and (rA + vb, vb) are observed. [The notation (v, v2) refers to a DQ peak centered at rq and v2 in the / ) and F2 dimensions, respectively.]... 

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. 

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